Sucrose_ Properties and Applications

SucroseProperties and Applications

Sucrose Properties and Applications

Edited by

M. MA THLOUTHI Faculte des Sciences

Universite de Reims Champagne-Ardenne

and

P. REISER CEDUS

Paris

SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.

First edition 1995

© 1995 Springer Science+Business Media Dordrecht Originally published by Chapman & Hali in 1995 Softcover reprint of the hardcover 1 st edition 1995

Typeset in 1O/12pt Times by Cambrian Typesetters, Frimley, Surrey

ISBN 978-1-4613-6150-3 ISBN 978-1-4615-2676-6 (eBook) DOI 10.1007/978-1-4615-2676-6

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers at the Glasgow address printed on this page.

The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or Iiability for any errors or omissions that may be made.

A catalogue record for this book is available from the British Library

Library of Congress Catalog Card Number: 94-79052

e Printed on permanent acid-free text paper, manufactured in accordance with ANSIINISO Z39.48-1992 (Permanence of Paper).

Preface

This book has as its origin the wish of the sugar profession in France, asrepresented by CEDUS (Centre d'etude et de documentation du sucre), togather together the widespread information on sucrose into a singlesource-a volume directed at researchers, producers and users of sucrose,which would also provide useful background material for students. Theidea received an enthusiastic response from all the authors invited tocontribute to this work, who are known not only for their expertise in thefield but also for their enthusiasm for the subject.Although sucrose is often described as a unique commodity with the

advantages of high purity, low price, ready availability and optimumsweetness, and although it is used as a model for the study of carbohydratesand biological molecules, our knowledge of its properties has advancedonly slightly in recent years. Most industrialists and researchers still usedata known for more than 60 years. The recent literature on sucrose, whichis relatively abundant, consists either of repetitions of what has alreadybeen published, or of sophisticated computations rather distant fromeveryday concern.This situation is not new. In the preface to his excellent handbook,

Principles of Sugar Technology (1953), Pieter Honig states: "There is onlyone way in which the sugar industry can make real progress, and that is bysystematic research and by collecting facts in a critical and objective way.This is the only real foundation on which improvements can be made."This statement is still true. In writing this book, we have tried to collectdata, to consider the most important properties of sucrose in detail, and toprovide a new insight into recent aspects of sugar studies and applications.After a short discussion of the economic aspects of sucrose, recent

studies of sucrose structure in its crystalline form and in aqueous solutionusing modern tools like 13C NMR are presented, along with calculations ofmolecular mechanics. Chapter 3 provides an overview of sucrose crystallization, starting with a basic understanding of the driving force in thecrystallization processes, viz. supersaturation of solution, and ending witha new and promising technology which is even environmentally compatible:the cooling crystallization of raw juices. In chapter 4 the structure andpractical aspects of amorphous sugar are considered. Amorphous sucrosemay be found both at high temperatures and below OCC, and its studyproves to be informative as a model for most amorphous food systems.Following the discussions of crystalline and amorphous sucrose, its

vi PREFACE

solution properties are treated in the subsequent four chapters. In chapter5, solubility-one of the most important properties for both research andindustrial work-is detailed, with particular emphasis on the most recentequations for the calculation of solubility as a function of temperature andin the presence of other saccharides. Rheological properties are covered inchapter 6, which discusses the equations available for the calculation ofviscosity as a function of concentration or temperature, and the effect ofviscosity on molasses formation and exhaustion and on the running ofindustrial machinery. Methods of analysis of sucrose in syrups and liquidsugars are described in chapter 7, and a critical compilation of the mostimportant physical properties of sucrose and sucrose solutions is presentedin chapter 8.Applications of sucrose are dealt with in the last four chapters. Some ofthe properties of sucrose, mainly due to hydration, taste and high purity,are important for food technology and are described in chapter 9. Thecompatibility of sucrose with other food ingredients and the enhancementsof food flavour are covered in chapter 10. Chapter 11 is devoted to the useof sucrose as a raw material for chemical and enzymatic reactions. Thisopens the door for the possible diversification of the sugar industry, whichis the real challenge for the near future. Finally, new technology for thedrying of fruits and vegetables, based on one of the numerous interestingproperties of sucrose, namely osmotic pressure, is described in chapter 12.We do not claim to cover all aspects of the properties and applications of

sucrose, because of the ubiquity of this molecule and the diversity ofinformation currently available. We only wish to stimulate further theinterest of sugar scientists and technologists and to ensure that none oftheir fervour is lost.

M. MathlouthiP. Reiser

Contributors

Professor G.G. Birch Department of Food Science and Technology,University of Reading, Reading RG6 2BX,UK

Dr Z. Bubnik Institute of Chemical Technology, Technika1905, 16628 Prague 6, Czech Republic

Dr M.A. Clarke Sugar Processing Research Institute, Inc., 1100Robert E. Lee Blvd, New Orleans, LA 70124,USA

Mr J. Genotelle Ingenieur Arts et Manufacture, 5 Rue Frevillele-Vingt, 92310 Sevres, France

Dr S. Guilbert CIRAD, 24 Avenue du Val de Montferrand,B.P. 5035, 34032 Montpellier, France

Dr M.A. Godshall Sugar Processing Research Institute, Inc., 1100Robert E. Lee Blvd, New Orleans, LA 70124,USA

Professor P. Kadlec Institute of Chemical Technology, Technika1905, 16628 Prague 6, Czech Republic

Dr R. Khan POLYBios, LBT, Area di Ricerca, Padriciano99, 34012 Trieste, Italy

Dr J.P. Lescure S.N.F.S. Services Techniques, 369, Rue JulesGuesde, B.P. 39, 59651 Villeneuve d'AscqCedex, France

Professor G. Mantovani Universita di Ferrara, Dipartimento di Chimica,Via L. Borsari 46, 44100 Ferrara, Italy

Professor M. Mathlouthi Laboratoire de Chimie Physique Industrielle,Faculte des Sciences, Universite de ReimsChampagne-Ardenne, B.P. 347, 51602 ReimsCedex, France

Dr S. Perez INRA Rue de la Geraudiere, BP 527, 44072Nantes Cedex, France

viii

Dr A.L. Raoult-Wack

Mr P. Reiser

Dr G. Rios

Professor G. Vaccari

Professor A.J. Vlitos

CONTRIBUTORS

CIRAD, 24 Avenue du Val de Montferrand,B.P. 5035, 34032 Montpellier, France

Ingenieur, CEDUS, 30 rue de Lubeck, 75116Paris, France

University of Montpellier 11,3402 Montpellier,France

Universita di Ferrara, Dipartimento di Chimica,Via L. Borsari 46, 44100 Ferrara, Italy

World Sugar Research Organisation, Universityof Reading Innovation Center, Philip LyleBuilding, PO Box 68, Reading RG6 2BX, UK

Contents

1 Economical aspects of sugarA.I. VLITOS

1.1 Introduction1.2 Nutritional and energetic aspects1.3 Sugar in world trade: statistical data1.4 ConclusionBibliography

2 The structure of sucrose in the crystal and in solutionS. PEREZ

1

112910

11

2.1 Introduction 112.2 Nomenclature 122.3 Structural characteristics of crystalline sucrose 132.3.1 Conformation of sucrose 142.3.2 Hydrogen bonding in crystalline sucrose 162.3.3 Packing features of crystalline sucrose and relation to morphology 182.3.4 Solid-state cross-polarization magic angle spinning NMR

spectroscopy of sucrose 202.4 Exploration of sucrose conformations by computational methods 212.4.1 Conformational variability of the fructofuranose ring 222.4.2 Conformational variability of the sucrosyllinkage 22

2.5 The solution conformations of sucrose 262.5.1 High-resolution NMR spectroscopy 262.5.2 Chiro-optical measurements 27

2.6 Conclusions and perspectives 28Acknowledgements 30References 30

3 Sucrose crystallisationG. VACCARI and G. MANTOVANI

3.1 Introduction3.2 Morphology3.2.1 Single crystal3.2.2 Twins3.2.3 Conglomerates3.2.4 Effect of impurities

3.3 Solubility3.3.1 Pure solution3.3.2 Impure solution

3.4 Crystallisation3.4.1 Nucleation3.4.2 Crystal growth

33

333434353639474848494953

x CONTENTS

3.5 Crystallisation technique3.5.1 Evaporation crystallisation3.5.2 Cooling crystallisation

3.6 Crystal quality3.6.1 Inclusions3.6.2 Colour of the crystals

3.7 ConclusionsReferences

5860636669707172

4 Amorphous sugarM. MATHLOUTHI

75

4.1 Introduction 754.2 Structure of amorphous sucrose 764.2.1 Order and disorder in concentrated amorphous solution 764.2.2 Structure of freeze-dried and spray-dried sucrose 804.2.3 Dry-milled and extruded sucrose 82

4.3 Thermal properties of amorphous sugar 844.3.1 Behaviour of concentrated amorphous solutions 854.3.2 Glass transition of amorphous solid sucrose 86

4.4 Rearrangement in amorphous sugar 894.4.1 Moisture dependence of the amorphous-crystalline transformation 894.4.2 Temperature dependence of amorphous sugar transformation 91

4.5 Practical importance of amorphous sugar 944.5.1 Structure modification of sugar and the shelf-life of food products 954.5.2 Agglomeration, caking and the stability of crystalline sugar 95

4.6 Conclusion 97References 97

5 Sucrose solubilityz. BUBNIK and P. KADLEC

101

5.1 Introduction 1015.2 Expression of concentration and composition of sucrose solutions 1015.2.1 Relationships for expression of concentration of sucrose

in pure and impure water solutions 1035.3 Sucrose solubility in water 1055.3.1 Effect of temperature on the sucrose solubility 1055.3.2 Phase equilibrium diagram for the system sucrose-water 1075.3.3 Supersaturated solutions 108

5.4 Sucrose solubility in impure sugar solution and other solvents 1105.4.1 Three-component triangle diagram 1105.4.2 Influence of beet and cane non-sugars on the solubility of sucrose

solutions in technical sugar solutions 1145.4.3 Equation for solubility of sucrose in impure solutions 115

5.5 Solubility of sucrose in other solvents 1185.5.1 Ternary systems: sucrose-water-organic liquid solvent 118

5.6 Solubility of some saccharides 1185.7 Conclusion 121List of symbols 123References 124

6 Rheological properties of sucrose solutions and suspensionsM. MATHLOUTHI and J. GENOTELLE

6.1 Introduction6.2 Theoretical basis of viscosity relations

126

126127

CONTENTS

6.2.1 Einstein's equation6.2.2 Viscosity--{;oncentration relations6.2.3 Viscosity-temperature relations6.2.4 Results and interpretation

6.3 Viscosity of impure solutions6.3.1 Relations applicable to homogeneous phases6.3.2 Relations applicable to heterogeneous phases6.3.3 Results and interpretation

6.4 Methods for determining viscosity and flow properties6.4.1 Laboratory methods6.4.2 Factory methods

6.5 Applications6.5.1 Effect of viscosity on crystallization6.5.2 Effect of viscosity on molasses formation and exhaustion6.5.3 Effect of viscosity on machines running

6.6 ConclusionReferences

7 Analysis of sucrose solutionsJ.P. LESCURE

7.1 Introduction7.2 Sucrose identification by vibrational spectroscopy7.2.1 Infrared7.2.2 Raman spectroscopy7.2.3 NMR spectroscopy

7.3 Methods of titration of sucrose7.3.1 Physical methods7.3.2 Refractometry7.3.3 Polarography7.3.4 NIR spectrophotometry7.3.5 Isotope dilution

7.4 Chemical methods of analysis7.4.1 Reducing sugars7.4.2 Chromatography

7.5 Enzymatic methods7.5.1 Methods description7.5.2 Situation of enzymatic methods

7.6 Determination of the syrups quality7.6.1 Purity7.6.2 Ash7.6.3 Colour7.6.4 S027.6.5 Heavy metals

7.7 Microbiology7.8 Standards and regulations7.8.1 Codex Alimentarius7.8.2 The European Regulation 79/7967.8.3 Pharmacopoeia7.8.4 Sugar regulation

References

8 Physical propertiesP. REISER, G.G. BIRCH and M. MATHLOUTHI

8.1 Introduction8.2 Properties of the crystal

Xl

127128132135138138139141143144145147147149151151152

155

155156157159159160160164166166167168168169173173176176176178179179179180181182182182183183

186

186186

xii CONTENTS

8.2.1 X-ray crystallinity8.2.2 Melting point8.2.3 Density8.2.4 Compressibility8.2.5 Granulometry8.2.6 Electrical properties8.2.7 Specific heat

8.3 Properties of amorphous sucrose8.3.1 Density8.3.2 Specific heat8.3.3 Glass transition, recrystallization and melting8.3.4 13C NMR spectra of amorphous sucrose

8.4 Aqueous solutions8.4.1 Concentration units8.4.2 Solubility8.4.3 Density of sucrose solutions8.4.4 Density and apparent specific volume8.4.5 Refractive index8.4.6 Polarimetry8.4.7 Thermal properties of aqueous sucrose solutions8.4.8 Increase in volume8.4.9 Boiling point8.4.10 Freezing point8.4.11 Water activity8.4.12 Osmotic pressure8.4.13 Surface tension8.4.14 Viscosity of sucrose solutions

References

187187188188188188189190190190190190191191191200200202206206211211212213213217217221

9 Technological value of sucrose in food productsM.A. CLARKE

9.1 Introduction9.1.1 Sources, production and consumption of sucrose9.1.2 Comparative sweetness of sugar

9.2 Chemical properties of sucrose9.2.1 Purity9.2.2 Solution reactions: inversion, degradation, Maillard and

browning reactions9.2.3 Sensory properties9.2.4 Color9.2.5 Antioxidant properties

9.3 Physical properties of sucrose9.3.1 Colligative properties

9.4 Applications: effects of sucrose in food processing9.4.1 Breads9.4.2 Cakes9.4.3 Cookies and sweet biscuits9.4.4 Icings and frostings9.4.5 Beverages9.4.6 Jams, jellies and preserves9.4.7 Confectionery9.4.8 Dairy products9.4.9 Ready-to-eat breakfast cereals9.4.10 Meats9.4.11 Frozen and tinned vegetables

9.5 Biochemical properties of sucroseReferences

223

223223224225225

226230231232232232240240241242243243243243244244244244245246

CONTENTS Xlii

10 Role of sucrose in retention of aroma and enhancing the flavoroffoods 248M.A. GODSHALL

10.1 Introduction10.2 Sucrose and the other basic tastes

10.2.1 Interactions with salty taste10.2.2 Interaction with bitter taste10.2.3 Interaction with acid-sour taste10.2.4 Interaction with other sweeteners

10.3 Retention of aromas10.3.1 Fixing volatiles with sucrose10.3.2 Co-crystallization10.3.3 Headspace effects-aromas in solution

10.4 Modifying the taste of sucrose10.4.1 Enhancing the sweetness of sucrose10.4.2 Hydrocolloids and perception of sweetness10.4.3 Temperature effects on sweet perception10.4.4 Masking the sweetness of sucrose10.4.5 Interaction of sucrose-color-flavor10.4.6 Iron-sucrose interactions

10.5 Effect of sucrose in selected food systems10.5.1 Coffee10.5.2 Fatty systems10.5.3 Chocolate confectionery10.5.4 Fruit flavors10.5.5 Effect of crystal size on mouthfeel in confections10.5.6 Sucrose and cake crumb, crust and quality

References

11 Sucrose: its potential as a raw material for foodingredients and for chemicalsR. KHAN

11.1 Introduction11.2 Chemical reactivity

11.2.1 Reactivity towards tritylation reaction11.2.2 Cyclic acetalation reactions11.2.3 Selective esterification11.2.4 SN 2 displacement reactions

11.3 Enzymic reactions11.3.1 Lipase-catalysed acylation reactions11.3.2 Selective deacylation reaction

11.4 Food ingredients11.4.1 High-intensity sweetners11.4.2 Emulsifiers and surface active compounds11.4.3 Low-calorie fat11.4.4 Non-cariogenic, reduced calorie, low-intensity sweetners11.4.5 Bulking ingredients

11.5 Chemicals from sucrose11.5.1 Synthetic polymers based on sucrose11.5.2 Detergents

References

248248249249249250251252252253255255256256256257257258258258259259259260260

264

264265265265266268268268269269270270270271273274274275276

XtV CONTENTS

12 Sucrose and osmotic dehydration 279A.L. RAOULT-WACK, G. RIOS and S. GUILBERT

12.1 Introduction12.2 General presentation of osmotic dehydration12.3 Operating variables related to the sucrose concentrated solution12.4 Influence of sucrose impregnation on the end-product quality12.5 Control of the sucrose concentrated solution12.6 ConclusionReferences

Index

279279282284286287288

291

1 Economical aspects of sugarA. J. VLITOS

1.1 Introduction

Sugar is a major commodity in world trade. Although competition fromalternative sweeteners and starch-derived isoglucose is substantial, over100 million tons of sugar have been produced worldwide annually in recentyears. Approximately a similar amount is consumed. Unlike many otherfoods, sugar can be produced in the temperate zones as well as in thetropics and subtropics. It is, in every sense, an 'international' commoditywith a well-established infrastructure. Sugar-beet accounts for most of thesugar produced and consumed in Europe, although cane sugar is stillimported from the so-called Lome Convention nations (Africa, Caribbean,Fiji, etc.). Sugar-cane is still the major source of sugar in Africa, SouthAmerica, Asia, Hawaii, Florida, Louisiana, Fiji, Mauritius and in theCaribbean islands. In North America, other than Florida, Louisiana andTexas, sugar-beet is the leading sugar-producing crop. The lead producerof sugar in the world is now the European Community (Ee).Although mechanisation of agricultural operations is quite common inareas producing sugar-beet, sugar-cane cultivation relies more heavily onhand-labour especially in Third World nations. However, notable exceptions are Australia, Hawaii, Florida, Louisiana and Texas where mechanised sugar-cane cultivation is the rule rather than the exception. Thequestion which often arises is which country is the most efficient producerof sugar in the world, and whether sugar-cane represents the more efficientsource? The answer is that Australia is probably the most efficientproducer of sugar (in economic terms) from sugar-cane and France themost efficient producer from sugar-beet. The level of efficiency is roughlyequal.

1.2 Nutritional and energetic aspects

Nutritional trends will play a role in future markets for sugar. Sinceoverconsumption of fats is now considered a more serious health hazardthan overconsumption of carbohydrates, many food processors will find itmore convenient to replace fats with sugars in many foods. The physical

M. Mathlouthi et al. (eds.), Sucrose© Springer Science+Business Media Dordrecht 1995

2 SUCROSE

and chemical properties of sugar are still considered vital in adding bulkand taste to certain foods (i.e. fibre) which otherwise would prove blandand unappetising. Thus, the markets for sugar both for non-food and forfood uses are likely to remain buoyant in the long term. One of theimportant reasons for the continued popularity of sugar both for food andother uses is its relatively low and stable price and the ability of suppliers torespond to market demand rapidly and reliably. The infrastructure built tosupply sugar to the market is second to none worldwide. Production,storage, shipping and packaging of sugar have been developed to the veryhigh level of efficiency demanded by an international commodity ofcommerce and trade.An important economic aspect of sugar production has to do with the

utilisation of by-products and with the use of sugar and its by-products,molasses, to produce ethanol (a liquid fuel) and other chemicals. In Brazilthe production of ethanol as a liquid fuel represents a major market forsugar; this alternative use for sugar is likely to become increasinglyimportant in nations lacking fuels but which have a capacity to producesugar crops.

It should be noted that the photosynthetic efficiency of sugar cane and ofsugar beet offers hope that in future more nations will become aware of thepotential value of these crops as sources of renewable energy supplies andas sources of products presently relying on petroleum and other fossil fuels.The economics of sugar-based chemical products will change as supplies offossil fuels become scarce and more expensive. Environmental pressuresmay also favour, in the long term, the use of sucrochemicals which may bemore biodegradable and less damaging to the environment than traditionalproducts.

1.3 Sugar in world trade: statistical data

To demonstrate more quantitatively the economic aspect of sugarproduction a series of figures follow, showing the world production ofsugar-beet and sugar-cane on a scale rivalled by very few other food crops.The following data reproduced from the United States Department ofAgriculture's Economic Research service demonstrate the scale of sugar'seconomic prominence in world trade. As mentioned above, worldconsumption and production (Figure 1.1) roughly keep pace, but from1990 to 1993 production outpaced consumption. This situation is likely tochange in 1994 given shortfalls of production in Cuba and elsewhere.Consumption in some parts of the world is increasing as may be noted fromthe data in Figure 1.2. Consumption in Asia is rising and is likely tocontinue in the future. It is also interesting to note the relatively highconsumption in Eastern Europe from 1988 to 1991 (Figure 1.3). This is

ECONOMICAL ASPECfS OF SUGAR 3

92/93'90/9186/87 88/89

Marketing year

84/85

95

90.982/83

100

105

Millio n metnt: tons, raw va lue

120

110

115

'Forecast.

Figure 1.1 World production (---) and consumption (- - -).

Million metric tons, raw value40 -,----------------------,

30

20

10

o1982 83 84 85 86 87 88 89 90 91 92

Year

Figure 1.2 Consumption in Asia.

4 SUCROSE

(d)

o 5 10 15 20 25

Million metric tons, raw value

Figure 1.3 Consumption in selected regions (average of 1988-1991 marketing years).(a) North America; (b) Latin America (includes Central America, Caribbean and SouthAmerica (excluding Mexico) ); (c) Western Europe; (d) Eastern Europe (includes former

Soviet Union); (e) Africa; (f) Middle East.

likely to continue and may accelerate. Of special interest is the projectedconsumption of sugar in the People's Republic of China (Figure 1.4) whichis approaching that of the US. Of course, on a per capita basis, USconsumption is much greater than that in the People's Republic of China asmay be noted in Figure 1.5. Table 1.1 lists consumption in the EC andother major consuming nations.Sugar and its pricing often confuses laymen. The world price is muchlower than the prices in the USA and other nations and the differences inprice are often quite large as seen in Figure 1.6. Similarly, there aredifferences between 'world refined' and 'raw sugar' prices (Figure 1.7).Price indices for refined beet and cane sugar in the USA are reflected inFigure 1.8 where it will be noted that the refined cane sugar index wasmuch greater than that for refined beet sugars. Similarly, the differences insugar price (retail, wholesale refined and domestic raws in the USA) canvary considerably (Figure 1.9). Of particular relevance in assessing

ECONOMICAL ASPECTS OF SUGAR

Million metric tons. raw value

16

5

",.,-"

14

12

i 0

-- ---

Former Soviet Union,/ -,

,/ "--.'

-----'India

8

6

4

2

--'-. ----,~

~U.:S=-.~_.-==-=::::-:=- ;;;>

China __ ---Me xico

o1982/83 84/85 86/87 88/89

•90/ 91

'Forecast Marketing year

Figure 1.4 Projected consumption in China compared to other nations.

Table 1.1 Sugar consumption (metric tons X 106) in the EC and other major consumingnations

Country or area

Former Soviet Union'ECI

IndiaUnited StatesChinaBrazilMexicoJapanIndonesiaPakistan

OthersWorld total

'Includes 12 former USSR republics, excludes Baltic states.IIncludes unified Germany, excludes French overseas.

1990-91

13.0412.8212.327.967.507.094.262.792.422.40

37.57110.17

1991-92

12.512.813.18.07.67.24.32.82.52.5

39.0112.8

(f)

o 10 20 30 40 50 60

Kilograms refined

Figure 1.5 Consumption of sugar (per capita) - world and selected areas. (a) World; (b)China; (c) India; (d) US; (e) EC) (f) Cuba.

Cents/lb.

25

20

15

10

./ -,./ .....

./

------

5

o1982 84 86

Year

88 90 92

Figure 1.6 World and US sugar prices. (- - -) USA: Contract no. 14 New York. (--) theworld: Contract no. 11 Stowed Caribbean ports.

Cents/lb.

16

12

8

4

o1982 84 86

II

//

//

//

I

88

Year

\\

\\\.

90 92

Figure 1.7 World refined and raw sugar prices. (--) Refined: London daily price, Europe.(- - -) raw: price no. 11 Caribbean.

June 1982 100140 I

130

120

110

1001983 84 85 86 87 88 89 90 91 92

Year

Figure 1.8 US produced price index for refined beet (.) and cane sugar C,w).

8 SUCROSE

Cents lib.

50

40Retail 11

Domestic row, N.Y. spot 31

30

20

Wholesale refined 21 /- ./ - \... / \ I"

1.1/ -, ...... /',-~/_-~/ ......

1"" ... ----- .... - ... .. - .... -- ... _ .... __ .. ,-------I,

10

o1982 84 86 88 90 92

Yea r

Figure 1.9 US sugar prices. 1, US average; 2, midwest; 3, starting June 1985 prices are fornearly futures.

Cents/lb .. dry weight

35

25Sug or. refined

HFCS-42

",., '" HFCS-55 ,.-- ~ __ ',..- ...... ,----... .. --- .............. ', ',-~' /' '-- ...................:..:_//

//15

5

o1,982 84 86 88 90 92

Year

Figure 1.10 Wholesale prices for HFCs and sugar - US midwest market.

ECONOMICAL ASPECTS OF SUGAR

19112 =100160

150

110 ~

130 ,

901903 84 85 86 87 88 89 90 91 92

Year

9

Figure 1.11 US consumer price index for (~) sugar and (.) selected sweetener-containingproducts.

competition to sugar from other isoglucose or high fructose corn syrups(HFCS-55 and -42) are the relative prices compared with refined sugar. Itmay be seen in Figure 1.10 that price advantages are presently in favour ofboth HFCS-55 and HFCS-42. A US consumer price index for sugar andselected sweetener-containing products is reproduced in Figure 1.11. It isinteresting to note that from 1983 to the present sweetener-containingproducts have maintained a much higher price index than sugar andartificial sweeteners.

1.4 Conclusion

Sugar is a major commodity in world trade. An infrastructure has beenbuilt over the past 100 years to maintain production, shipping and storageof sugar to keep pace with consumption and in some years to outpaceconsumption. In the foreseeable future, sugar will continue to be indemand both for food and non-food uses. It is a food commodity producedat relatively low cost in temperate, subtropical and tropical areas of the

10 SUCROSE

world and as a regenerable source of energy represents an importanteconomic asset.

Bibliography

ISO (1992) International Sugar Organisation Sugar Yearbook, ISO, London, UK.USDA (1993) Sugar and Sweetener Situation and Outlook Report. Economic ResearchService. United States Department of Agriculture, Washington DC.

2 The structure of sucrose in the crystal and insolutions. PEREZ

2.1 Introduction

The term 'structure' defines the arrangement of all the parts of a whole.For a molecule, this requires the identification and the relative spatialorientation of all the constituting atoms. Needless to stress that such aknowledge is of fundamental importance for the basic understanding of themolecular properties and functions. The evolution of the concept ofstructure has been parallel to the progress in structural chemistry.Obviously, the first constitutional representation of sucrose, advanced byTollens in 1883 (Tollens, 1883), represented the synthesis from the mostadvanced tools available at the time. It took, however, 10 years beforeFischer came up with the correct formulation for sucrose as a glucofuranosyl fructofuranoside (Fischer, 1893). This was followed by severalkey discoveries including the representations given by Haworth (1929),Pigman (1948), Morrison and Boyd (1959). In a recent review about theevolution of the structural representation of sucrose, Lichenthaler et al.(1991) covered with elegance some of the major facts of the structuralrepresentations of sucrose including those provided by modern computergraphics.In many chemical, biological and technological processes, the threedimensional structure of a molecule may be of considerable significance.At the present time, there are two main experimental methods availablefor determining three-dimensional structures, at the atomic level: X-raycrystallography of single crystals and nuclear magnetic resonance (NMR)spectroscopy of solutes. In the crystalline state, the descriptor 'structure'has a static connotation. A crystal structure analysis will provide accuratedescription of the individual three-dimensional arrangement. In the liquidstate, the descriptor 'structure' is associated to a statistical one-dimensionalprobability. Therefore, a dynamic rather than a static description must besought, especially when geometries are changing rapidly. Hence, the term'structure' must also encompass the dynamic fluctuations that themolecule may undergo.The present chapter describes the essential concepts and tools whichmay be required to apprehend fully the current state of knowledge about


12 SUCROSE

the structures and the different levels of structural organizations ofsucrose. It is the author's hope that these tools will also be useful to thereader to decipher the structural works and modifications that will dealwith such a unique molecule.

2.2 Nomenclature

Most monosaccharides exist in the form of heterocyclic rings or cyclichemiacetals, such as five-membered furanoses or six-membered pyranoses.The centre of chirality generated by hemiacetal ring closure is called theanomeric centre which is the only carbon bound to two oxygen atoms. It islabelled C-1, and the others are numbered sequentially around the ring.The two stereoisomers are referred to as anomers, designated a or fJaccording to the configurational relationship between the anomeric centreand a specified anomeric reference atom. The D or L designation of theconfiguration refers to the position of the hydroxyl group on theasymmetric carbon farthest from the C-1, i. e. the C-5 of hexoses and the C4 of pentoses.The official name of sucrose, according to the IUPAC-IUB Commissionof Biochemical nomenclature is fJ-D-fructofuranosyl-a-D-glucopyranoside.It is abbreviated to fJ-D-Fruf-(2-1)-a-D-Glcp. The numbering of the atomsis shown in Figure 2.1. A disaccharide is a compound in which two

Figure 2.1 Sucrose and its atomic labels of interest. = 0-5g - C-Ig - O-Ig - C-2f.1jJ = C-Ig- 0- Ig - C-2f - 0-5f. OJg = 0-5g - C-5g - C-6g - 0-6g. OJr = 0-5f - C-5f - C-6f - 0-6f. Xr = 0-5f

- C-2f - C-lf - O-If.

THE STRUCTURE OF SUCROSE 13

monosaccharide units are joined by a glycosidic linkage, i.e. the anomericcentre is always linked to the hydroxyl of another sugar. It can be regardedas formed by the reaction of one glycosidic (anomeric) hydroxyl group withanother hydroxyl group liberating one water molecule. When there is nohemiacetal group free, the resulting disaccharide is known as a nonreducing disaccharide. This is the case for sucrose.The conformation of a molecule is best described by angular rotationsabout bonds which are given by torsion angles. A torsion angle in thesequence of atoms A-B-C-D is measured by the angle which the bond AB makes with the bond C-D when projected down B-C. The angle is 0°when the bonds A-B and C-D are eclipsed (or cis), whereas it is 180° whenthe bonds A-B and C-D are trans; it is counted positive when C-D isrotated clockwise with respect to A-B.The conformations about the glycosidic linkage bonds are described bythe following torsion angles:

ct> = 0-5g - C-1g - 0-lg - C-2f'\jJ = C-1g - 0-lg - C-2f - 0-5fThe orientation of the three hydroxymethyl groups are described by thetorsion angles, wg , Wf and Xf:

Wg= 0-5g - C-5g - C-6g - 0-6gWf= 0-5f - C-5f - C-6f - 0-6fXf = 0-5f - C-2f - C-lf - O-lfThe orientations of the 0-6g, 0-6f and O-lf primary hydroxyl groups arereferred to as either gauche-gauche (GG), gauche-trans (GT) or transgauche (TG) depending on whether the values of the above torsion anglesare closest to -60°, 60° and 180°. The sign of the torsion angles is defined inagreement with the IUPAC-IUB Commission of Biochemical Nomenclature (1971).

2.3 Structural characteristics of crystalline sucrose

In 1947, an X-ray determination of the structure of sucrose sodiumbromide dihydrate (Beevers and Cochran, 1947) confirmed the chemicallyassigned relative configuration of the asymmetric carbons of the molecule.The X-ray determination of the structure of sucrose was performed byBeevers et al. (1952) but for technical reasons the accuracy of thedetermination was not satisfactory. Subsequently, Brown and Levy (1963),carried out a highly precise refinement using neutron diffraction whichallowed a thorough description of the essential structural features offeredby crystalline sucrose. Becauseof the scale of the computational problemat the time, the refinement of the structure was not continued to complete

14 SUCROSE

Table 2.1 Crystal data of sucrose as revealed by neutron and X-ray diffraction

Space group

z

d(calc)

Neutron

10.8633 (5)8.7050 (4)7.7585 (4)102.945 (6)

X-ray

10.8648 (15)8.7028 (12)7.7578 (11)102.956 (15)

715

2

1.590 Mgm-3

convergence. Ten years later were published, in a back to back fashion,two further refinements of the structure of sucrose. The first refinement byBrown and Levy (1973) was performed using neutrons at a wavelength of1.078 A, whereas the refinement was based on X-ray data collected usingMoka at 0.71069 A (Hanson et at., 1973). These works gave unit cellparameters and space group symmetry in good agreement (see Table 2.1).Whereas X-ray diffraction determines the maxima of electron densitydistribution, neutron diffraction determines nuclear atomic coordinates.Except for hydrogen or deuterium atoms, the difference between thenuclear positions and the electron density peaks is noticeable only in veryhigh precision structure analysis. Differences in bond lengths from X-ray(dx) and neutron diffraction (dN) analysis of the same crystal structure ofsucrose are small for dx-<iN for C-OH bonds (0.0048 (12) A), whereas forC-H and O-H bonds the differences appear to be much larger andsignificant, i.e. -0.13(1) A and -0.17(2) A. Therefore, for the rest of thepresentation, the fractional coordinates determined from the neutrondiffraction data will be used. The location of the coordinates of the 45atoms of sucrose in its crystalline unit cell was determined with such a highaccuracy because the number of experimental structure factors used was2813.

2.3.1 Conformation of sucrose

From this set of fractional coordinates bond lengths can be readilycomputed, as can valence angles and torsional angles of interest. Thetorsion angles of interest are reported in Table 2.2, whereas a molecularrepresentation of sucrose in its crystalline conformation is shown in Figure2.2. The glucose residue adopts a 4C j conformation, whereas that of thefructofuranose residue is a 4T) twist (<p = 265.2, q = 0.353) (Cremer and

THE STRUCTURE OF SUCROSE

Table 2.2 Torsion angles C) of interest for crystalline sucrose

Torsion angles about the glycosidic linkage0-5g - C-Ig - 0-lg - C-2f 107.8H-Ig - C-1g - 0-lg - C-2f -8.0C-Ig - 0-lg - C-2f - 0-5f -44.7C-1g - O-Ig - C-2f - C-If 73.7C-Ig - O-Ig - C-2f - C-3f -159.8

15

Endocyciic torsion angles0-5g - C-1g - C-2g - C-3gC-1g - C-2g - C-3g - C-4gC-2g - C-3g - C-4g - C-5gC-3g - C-4g - C-5g - 0-5gC-4g - C-5g - 0-5g - C-I gC-5g - 0-5g - C-Ig - C-2g

Exocyciic torsion anglesC-5g - 0-5g - C-Ig - 0-lg0-lg - C-1g - C-2g - 0-2g0-2g - C-2g - C-3g - 0-3g0-3g - C-3g - C-4g - 0-4g0-4g - C-4g - C-5g - C-6g0-5g - C-5g - C-6g - 0-6gC-4g - C-5g - C-6g - 0-6g

55.0-56.056.3-54.955.2-55.0

67.754.762.8-64.364.6-56.464.3

0-5f - C-2f - C-3f - C-4fC-2f - C-3f - C-4f - C-5fC-3f - C-4f - C-5f - 0-5fC-4f - C-5f - 0-5f - C-2fC-5f - 0-5f - C-2f - C-3f

C-5f - 0-5f - C-2f - 0-lg0-5f - C-2f - C-If - O-IfC-3f - C-2f - C-If - O-If0-lg - C-2f - C-If - O-If0-2f - C-2f - C-3f - 0-3f0-3f - C-3f - C-4f - 0-4f0-5f - C-5f - C-6f - 0-6fC-4f - C-5f - C-6f - 0-6f

-31.2161.6-27.38.014.7

-102.4171.3-72.150.6

-157.5-78.4-69.549.3

Figure 2.2 Representations of sucrose in its crystalline conformation; the intramolecularhydrogen bonds are shown as dashed lines.

Pople, 1975). The orientations of the primary hydroxyl groups are gauchegauche for both OJg and OJf and is trans-gauche for Xf' They all correspond tolow-energy arrangements usually observed in crystal structures of carbohydrates. It should also be noted that sucrose has overlapping sequences ofC-O bonds that cause anomeric and exo-anomeric effects C-5g - 0-5g - CIg - 0-lg - C-2f - 0-5f - C-5f.

16 SUCROSE

2.3.2 Hydrogen bonding in crystalline sucrose

The description of the crystalline structure of sucrose cannot be madewithout considering the influence of hydrogen bonding. A hydrogen bondis the attractive force that arises between the donor covalently pair X-Hand other non-covalently nearest neighbour electronegative acceptoratoms A, A', ... The hydrogen bonding in the crystalline state is knownfrom crystal structure analysis of almost 100 mono-, di-, tri- andtetrasaccharides, amino acids, and many peptides (Jeffrey and Saenger,1991). General rules have been assessed along with general hydrogenbonding patterns. Chief among them are the occurrences of two- threeand four-centre hydrogen bonds. The two-centre hydrogen bond scheme:X-H ... A; is by far the most common, but there is strong evidence thatthe three-centred bonds occur frequently. A systematic study of the threecentre hydrogen bond in carbohydrate crystal structures indicated a widerange of situations going from the almost symmetrical cases with r-r ~ 2.1A, 8-8 ~ 135° to very unsymmetrical configurations where r-r' ~ 0.6 Aand 8-8' ~ 70°. The occurrence of four-centre hydrogen bonding is byfar less frequent. Its geometrical definition requires only that X-H , .. A,X-H .. , A' and X-H .. , A" angles are greater than 90°,Starting from the set of fractional coordinates and using the neighbouring

Table 2.3 Description of the hydrogen bonds in crystalline sucrose

O-H ... O Symmetry O-H H ... 0 0 ... 0 O-H ... OCO)operations'

Intramolecular hydrogen bondO-If-H ... 0-2g 1,000 0.974 1.851 2.781 158.60-6f-H ... 0-5g 1,000 0.972 1.895 2.850 167.1

Putative intramolecular hydrogen bond0-3f-H ... 0-lg 1,000 0.969 2.506 2.744 93.6O-If-H ... 0-lg 1,000 0.974 2.440 2.772 100.0

Intermolecular hydrogen bond0-2g-H ... 0-6f 1,001 0.972 1.892 2.855 170.20-3g-H ... 0-3f II, 001 0.959 1.907 2.862 172.80-6g-H . , . 0-3g II,1 -1 1 0.956 1.921 2.848 162.90-3f-H ... 0-4f II, 000 0.969 1.908 2.864 168.5

0-4f-H ... O-If 1,00-1 0.976 1.760 2.716 165.4

0-2f 1,010 2.309 2.838 116.60-4g HO-3g 1,000 0.912 2.534 2.879 103.0

0-6g II, 1 0 1 2.539 3.373 152.1

'The symmetry transformations generate the coordinates of the acceptor oxygen atoms fromthe basic coordinates taken from the work of Brown and Levy (1973). The first digitrepresents either (I) the symmetry operation x,y,z, or (II) the symmetry operation -x,112+y, -z. The last three digits specify a lattice translation along a, band c.

molecules derived from combinations of space group symmetry and unitcell symmetry operations the set of intra- and intermolecular hydrogenbonds can be unravelled. All the oxygen atoms in the molecule areinvolved in hydrogen bonding. The crystalline structure is characterized byan unusually large number of weak hydrogen bonds (see Table 2.3). Aschematic representation of the hydrogen bond features found in crystallinesucrose is shown in Figure 2.3(a).The two inter-residue intramolecular hydrogen bonds, O-lf-H ... O-2g

O-If

(a) ~ ~0-5--~----H--D -6f----1..--H--1)-2g O-Ig

/1/2.85 1.90 1.89 5>~:~5 /<')165 / H:V

0-6g-H: ~63'\

2.54 I ~: , 1.92 /

! \ &1.76

15~O_~g~_O_3f~_O_4f-._6~/17 " 2.53 ~: 1.91• 1.91 I

\II

: 2.51I

0-4g

0-2f O-Ig

Figure 2.3 (a) Schematic representation of the hydrogen bond features found in crystallinesucrose. (b) The four-centre hydrogen bond network in crystalline sucrose.

18 SUCROSE

and 0-6f-H ... 0-5g, are both from the fructofuranose to the glucopyranose residue. As a consequence, the ring oxygens lie on the same sideof the midline of the molecule. These bonds are arranged in an eightmembered ring fashion including (i) the minor intramolecular componentof a three-centre bond from the glucose primary hydroxyl group 0-6g-H tothe ring oxygen 0-5g; and (ii) 0-5g accepts two hydrogen bonds, one intra,the other inter-residue.The intermolecular hydrogen bonding involves an unusual four-centrebond. A four-centre hydrogen bond is defined as one in which the protonmakes three first neighbour contacts to potential hydrogen bond acceptoratoms in the forward direction with respect to the donor X-H bond. Theglucose 0-4g-H displays such a four-centres feature as shown in Figure2.3(b). The 0-4g-H of the glucopyranose residue forms two intermolecularbonds, to the fructofuranose ring oxygen 0-2f, and the glucopyranose 0-6gand an intramolecular hydrogen bond to 0-3g. All the OH ... 0 anglesare greater than 90°. Two of these bonds form an unusual triangle with thetwo-centre bond from 0-6g-H to 0-3g-H.

2.3.3 Packing features of crystalline sucrose and relation to morphology

In order to arrive at a full understanding of the packing arrangement, theintermolecular energy between a given molecule (i.e. the reference

Table 2.4 Description of the packing features in crystalline sucrose

Ref. Symmetry Numberoperation 'short

contacts"

H, 1 -1 I 122H, I 0 I 122

H,O -I I 92H, 001 92

1,00 I 851,00 -1 85

H, 0 -10 69H, 000 69

1,0 -10 491,01 0 49

H,I -10 24H, 1 00 24

H-bonds

0-6g-H 0-30-4g-H 0-6

0-3g-H 0-3f

0-2g-H 0-6g0-4f-H O-If

0-3f-H ... 0-4f

0-4g-H ... 0-5f

~ Energy(kcal mol")

-5.20-5.20

-5.14-5.14

-6.01-6.01

-3.60-3.60

-2.21-2.21

-0.73-0.73

'The symmetry transformations generate the coordinates of the acceptor oxygen atoms fromthe basic coordinates taken from the work of Brown and Levy (1973). The first digitrepresents either (I) the symmetry operation x,y,z, or (H) the symmetry operation -x,1/2+y, -z. The last three digits specify a lattice translation along a, band c.

molecule) and all its neighbours is evaluated by taking into account theintermolecular hydrogen bonds as well as the non-bonded interactions.The results obtained from a packing analysis of sucrose are given in Table2.4. In the molecular arrangements found in the crystal structure ofsucrose, the packing is highly dense, since each molecule is surrounded by12 neighbours; this of course is in agreement with the high density of 1.59of the crystals. All these 12 neighbours occur in pairs but only some giverise to strong intermolecular associations. From these, preferred molecularlayers can be defined. The strongest layer arises from a combination of thetwo-fold screw symmetry and a translational symmetry. The two-fold screwaxis element is parallel to the b axis, and it generates a chain where themolecules are very tightly interacting. The interactions involve intermolecular hydrogen bonds and many van der Waals contacts which express thecomplementarity of the molecular shapes in contact (Figure 2.4(a». On

(a) (b)

(<) ' ' '''''' ,~'."t:.

(d)

Figure 2.4 (a) The molecular chain around the two-fold screw axis in crystalline sucrose.Projection of the crystal structure of sucrose normal to: (b) the a crystallographic axis; (c) the

b crystallographic axis; (d) the c crystallographic axis.

20 SUCROSE

the basis of energy this chain may be viewed as a 'nuclei' for furtherarrangements during the crystal growth. Associating such chains through asimple translation in the (1 0 1) direction makes molecular layers having ahigh density of atoms. The lateral cohesion between these layers isachieved by a simple translation. For this reason there is no cancelling ofthe intrinsic polarity arising from a given molecule first and the molecularchain, second. Therefore, an overall polarity is maintained at the threedimensional level of each sucrose crystal. Figures 2.4(b)-(d) are projectionsof the crystalline structure of sucrose normal to the a, band c axis,respectively.

2.3.4 Solid-state cross polarization magic angle spinning NMRspectroscopy of sucrose

NMR chemical shifts are extremely sensitive to the electronic environmentof the nuclei. They provide probes for identifying chemical structures. IHand l3C NMR are the most common methods used for the structuralelucidation of organic molecules in solution. Technical advances ofmultiple-pulse and spin-enhancement methods have extended the application of NMR spectroscopy to solids. On crystalline powder samples it ispossible to measure the isotropic equivalent chemical shifts experimentally,using magic angle spinning (MAS). One commonly applied method is BCcross-polarization (CP-MAS), since this method addresses the effect ofcrystal environment on the electronic structure around the carbon nucleus.In principle, the comparison between solution and solid-state NMRspectroscopy provides a probe for studying the effect of solvation andcrystal lattice on the molecular structure.Sucrose is one of the rare cases for which solid-state NMR and crystalstructure analysis have been performed on the same molecule. The solidstate l3C (CP-MAS) of partially deuterated crystalline samples of sucrosehas been reported (Pfeffer et al., 1990) and its assignment compared withthe solution spectra (Figure 2.5). From this work it appears that there aresome resonances whose relative chemical shift positions do not correspondto each other. The resonance at C-lf is observed at a much lower field inthe solid state (6 = 67.9) than in solution. As for resonances at C-6g andC-6f they are both found at higher fields in the solid state. In addition, C-3fresonance is out of place relative to its position in solution (6 84.4 versus6 77.5, respectively). These differences are not unexpected since theelectronic surrounding of each atom differs on going from the solid state tothe solution. They may be due to different conformations of the threeprimary hydroxyl groups in the solid and solution, as well as a change in thepuckering of the fructofuranosyl moiety.


C-2f

100I

C-lg

90I

C-lg

C-st

80I

C-3t

C-SgC.3g\, C-2g

'l/ C-4g

C_3tC-4t

70I

C-4g

C-4tC-2g

c~t

C·l'C~

60I

C-6tC·lt C~g

100 90 80 70 60I

Figure 2.5 Solid-state 13CP-MAS NMR spectra of sucrose, along with the correspondingresonances as recorded in the solution state.

2.4 Exploration of sucrose conformations by computational methods

Conformational analyses of sucrose have been attempted in many instancesusing methods based on atoms or electrons. Electron-based modelling ormolecular orbital methods fall into two classes: the ab initio method whichincludes all electrons and requires only minimal parametrization and thesemi-empirical methods. Their application is still limited to somehow smallmolecules which can be considered as analogue to carbohydrates. One ofthe most recent applications in the field deals with a tetrahydropyrantetrahydrofuran analogue of sucrose (Van Alsenoy et at., 1994). The semiempirical methods include only the valence electrons, and must thereforebe calibrated extensively. Although requiring significant computing resources, calculations can be performed on molecules of the size of sucroseusing such a program as PCILO (Perturbation Configuration Interactionusing Localized Orbitals; Diner et at., 1969). Atom-based methods (alsocalled molecular mechanics), predict the properties of molecules in theground state by a system wherein atoms are connected by 'springs'(represented by potential energy functions such as simple harmonic

22 SUCROSE

oscillators). When calibrated (i.e. parametrized) on related model molecules, molecular mechanics often gives good structures and energies forcarbohydrates. The molecular mechanics program, MM3 (Allinger et al.,1989) which includes tools to deal with exo-anomeric effect and hydrogenbonding has been used to investigate many neutral disaccharides.Although MM3 has special functions to accommodate the anomericsequences, the effect of overlapping anomeric sequences of sucrose, havenot been included in the parametrization. Disaccharide computations areusually performed in the space of glycosidic torsion angles and tV wherethe residues can adjust internally at each increment. These calculations,so-called 'flexible' or 'relaxed residue' analysis, are complicated by theexistence of a very large number of local minima on the multi-dimensionalpotential energy surface, especially those connecting with pendant groupssuch as primary and secondary hydroxyl groups. Despite their inherentdifficulties, these methods are gradually replacing the previously usedmethods which treated each residue with a fixed rigid geometry and wereassessing the space available to the molecule by rotations about theglycosidic torsion angles and tV.

2.4.1 Conformational variability of the fructofuranose ring

The energy surface for fructofuranose (Figure 2.6) calculated in the spaceof puckering parameters (, q) has been calculated with MM3 (Allinger etai., 1989). These calculations account for the influence of the conformationof both the primary and the secondary hydroxyl groups. The map agreeswell with the experimental data, showing all crystallographically observedconformations to have energies within 2 kcal mol-1 of the lowest energystructure. All but one of the 20 structures are located in the large'northern' minimum. Observed shapes range from the 2E to the 4Ts form,spanning five northern conformations. Conversion between the northernand southern minima is possible by either western or eastern routes.

2.4.2 Conformational variability of the sucrosyllinkage

The relative orientations of the fructofuranose and glucopyranose rings ofsucrose were first modelled using the HSEA (Hard Sphere Exo-anomericEffect; Thogersen et al., 1982) program. That work, starting with thecrystalline conformation, predicted a structure very close to that of thestarting sucrose structure. At that time, it was concluded (Bock andLemieux, 1982) that sucrose was conformationally rigid, partly as a resultof the two intramolecular inter-residue hydrogen bonds: O-lf-H ... 0-2gand 0-6f-H . . . 0-5g. Subsequent work with the PFOS (PotentialFunction for Oligosaccharide Structures) program (Tvaroska and Perez,1986) yielded five local minima and suggested that the glucopyranose-

2

3~52To 10

3 4 5 .

~Eoo

THE STRUCTURE OF SUCROSE

o

°E2J34

5

23

Figure 2.6 Conformational wheel for a fructofuranose. The conformations of the fructofuranose ring are described as deviations from an improbable structure in which all atoms areco-planar. In any case three atoms always describe a plane, so possible low energy shapes willhave either one or two atoms out of plane. The structures in which only one atom deviates arecalled envelopes (E), with the deviant atom either subscripted or superscripted to show thedirection of its deviation from the plane. When two atoms are out of plane, they lie onopposite sides of the ring and the shape is described as a twist (7). Altogether there are 20 suchcharacteristics E and T shapes which can be placed on a circle called a 'conformational wheel'.Such a representation allows to follow the path of conformational interconversion that avoidsall-planar structures. To describe structures in which the two deviant atoms are not equallydisplace from the plane of the ring, the Creme-Pople puckering parameter <j> is used, alongwith the parameter q which defines the amplitude of puckering. For a particular structure, qcan be shown as the length along the radius of the conformational wheel, with an all-planar

structure residing at the centre.

fructofuranose linkage can assume several different and well distinctconformations (Herve du Penhoat et ai., 1991). The first 'relaxed-residue'modelling study of sucrose (Tran and Brady, 1990a, b) was done with thecomputer program CHARMM (Brooks et ai., 1983) using parametersappropriate for carbohydrates (Ha et ai., 1988). Three low-energy regionswere found to contain five local minima. A recent study with another forcefield (PIMM88) (Lichtenthaler et ai., 1992) gave comparable results. Theconformational behaviour of sucrose has been analysed with MM3, usingslightly different sets of parameters (French and Dowd, 1993; Perez et ai.,1993).

24 SUCROSE

0VN

0co

0N

0 !&to

0

0toI

0N

I

The relaxed-residue potential energy surface of sucrose using MM3 with16 structures (arising from different combinations of side group orientations) is shown in Figure 2.7. In agreement with the results obtained for the'rigid residue' approximation (Herve du Penhoat et ai., 1991) and the'relaxed residue' modelling of sucrose (French and Dowd, 1993; Perez etai., 1993) there is a limited conformational flexibility for the torsionangle (40-140°) compared with the 'IjJ torsion which covers all, or almostall, the angular range from -240 to 120°. Such a behaviour is typical ofdisaccharides having axial-equatorial linkages. Nevertheless, a reason forthe 'IjJ rotation to be more facile is that the fructofuranose ring can easilydeform. Relaxing the geometries in modelling studies does not drasticallyalter the location of minima, but it lowers or removes the barriers amongminima found with 'rigid' residues and it lowers the relative energies of theremaining alternate minima. Three low-energy domains are found on thismap. The larger domain is located around = 80° and 'IjJ = -60° containsthe global energy minimum Sl ( = 110.0, 'IjJ = -60° (GT-GT-TG).Another low-energy conformer, S2, occurs at = 70° and 'IjJ = 90 (GTGT- TG). The crystalline conformation of sucrose (S*) is found at theperiphery of this domain. The two other low-energy domains have energies2-3 kcal mol-1 higher. Typical low-energy representatives are S3 and S4which occur respectively at = 75° and 'IjJ = 170° (GT-GG-TG) and =90° and 'IjJ = 55° (GT-GT-TG). The scheme of intramolecular hydrogenbonds found in crystalline sucrose (O-lf-H ... 0-2g and 0-6f-H ... 05g) is not present in any of the low-energy conformers described hereupon.Actually, only Sl displays a hydrogen bond based on 0-2g and O-lf butwith an inversion of the donor-acceptor scheme. As for S3 family ofconformers, several inter-residue hydrogen bonds can be formed such asO-lf-H ... 0-5g, O-lf-H ... 0-6g, and 0-2g-H ... 0-6f; their occurrences were already proposed by Tran and Brady (1990a, b). The S4conformers are characterized by a 0-6g ... 0-3f hydrogen bond whichcould exhibit a 'flip-flop' in its donor-acceptor scheme.There are intermediate conformations of the interresidue linkage whichpermit the formation of an intramolecular hydrogen bond. However,

Figure 2.7 Adiabatic map of sucrose, along with the molecular representations of the fourlow energy conformers. The conformational energy map for sucrose has been made withMM3(92) with a dielectric constant" = 4. The map shows the lowest relative energy at each10° increments from one of the 16 starting models. The lower axis corresponds to the rotationabout the C-1g - 0-lg bond (torsion angle <1» of glucose, which is axial. The vertical axiscorresponds to the rotation about C-2 - 0-lg bond (torsion angle "IjJ) of fructose which ispseudo-equatorial. Iso-energy contours are drawn at increment of 1 kcallmol with respect tothe lowest energy conformation at 51; the outer limit is at 10 kcal/mol. The coding of greygoes from dark grey for the lowest energy area, to light grey for conformations having anenergy higher than 10 kcallmol. At a given energy value, greater rotation is permitted aboutthe "IjJ torsion angle. The conformation observed in the crystal structure of sucrose is shownby '. For each representative of the low energy conformers (51-54) a molecular drawing isshown. They are inserted into an electrostatic potential molecular surface as obtained by

MNDO calculations.

26 SUCROSE

numerous low-energy conformations do not allow the formation of anysuch stabilizing features. This is actually found in the crystal structure ofnumerous other sucrosyl-containing molecules. A survey of all the sucrosemoieties in crystalline oligosaccharides (French and Dowd, 1993; Perez etal., 1993) yields a 40° range in the <j> torsion angle as well as a 90° range in'4'. These numerical data from crystal structure determinations provideessential references for investigating the behaviour of the molecule insolution (Jeffrey, 1973).

2.5 The solution conformations of sucrose

Average properties, such as those measured by NMR spectroscopy, chirooptical methods, infrared and Ramam spectroscopy, can be calculatedusing an ensemble representation derived from the calculated potentialenergy surfaces by standard mechanical methods.

2.5.1 High-resolution NMR spectroscopy

Internal flexibility displayed at the glycosidic linkage, as well as at theprimary hydroxyl groups is clearly evident from calculations of molecularstructures using the different protocols described in the previous section.But it is only recently that such a feature has been integrated in theinterpretation of the solution conformations of sucrose as inferred fromhigh-resolution NMR spectroscopy. In the past decade, there has been anupsurge in the application of NMR in the assessment of molecularconformations in solutions. Basically, two types of effects can be put towork: those transmitted through bonds (such scalar effects as couplingconstants) and those transmitted through space (dipolar effects, such as inthe case of nuclear overhauser enhancements, NOE). If the distancedependence is assumed, as in the case of NOE, through space effects maybe used to estimate internuclear distances. Determining three-dimensionalstructures and conformations of carbohydrate molecules from NMRsolution is a rather complex process, and this field still needs to be firmlyestablished (Carver, 1991; Meyer et al., 1993; Perez, 1993). Until recently,almost all applications of NOE to conformational analysis of carbohydrateshas assumed, explicitly or implicitly, monoconformational behaviour, i.e.the occurrence of a single conformation in solution. Bock and Lemieux(1982) argued, supported by modelling using the HSEA method anddetailed 13C, T1 and NOE measurements, that the overall conformation ofsucrose is approximately the same in water and in dimethyl sulfoxide(DMSO) solutions, and this conformation is similar to that observed in thecrystalline state. These conclusions were supported by analysis of 13CNMR spin-lattice relaxation times of aqueous sucrose over a range ofconcentrations and temperatures (McCain and Markley, 1986a, b). Basedon IH NMR measurements in DMSO, (Christofides and Davies, 1985;

Davies and Christofides, 1987) it was suggested the occurrence of acompetition between two hydrogen bonds, i.e. O-lf-H ... 0-2g and 0-3fH ... 0-2g. This later hydrogen bond can only exist in the area of =100° and 'IjJ = -160°. Obviously such a scheme of dual hydrogen bondscannot be envisaged for explaining the experimental data in DMSO.A reinvestigation of the conformational behaviour of sucrose and its 2deoxy-analogue in diluted aqueous solution was performed with the fullconsideration of internal flexibility of the molecules. For both moleculesIH steady-state NOE and NOESY data were measured along with thelong-range 13C_1H coupling constants (Herve du Penhoat et al., 1991). Theconformational equilibrium between the many low-energy conformers wasdescribed using the data derived form energy calculations using the PFOSapproach. Typically, the overall population was made of several thousandsof conformers. For each predicted low-energy conformer, the theoreticalsteady-state method and the 3JC_H data were correlated with the glycosidictorsion angles. Agreement between experimental and theoretical datacould not be reached by any single conformation model, and onlyconformational averaging about all the potential energy surface canreproduce the experimental data. The inclusion of hydrogen bonding in theforce field did not affect the statistical weight of calculated NOE, and thesimilar values of observed NOEs for sucrose and the 2-deoxy analogueargue against the importance of hydrogen bonding in sucrose conformationin aqueous diluted solution. The lack of conformational rigidity of sucrosewas confirmed by an independent investigation (Poppe and Van Halbeck,1992) using several temperatures and magnetic field strengths. Someintriguing questions remain to be solved about the type of motion thatsucrose undergoes in solution, such as the rate of internal motionscompared to the overall tumbling rate of the molecule and compared to thedynamics of water molecules.

2.5.2 Chiro-optical measurements

Optical rotation has long been used to study the conformation ofcarbohydrates. A model based on interacting oscillators has recently beenextended to the calculations of disaccharides. The calculations refer tomolecules in vacuum, but a solvent correction can be applied. Opticalrotations are calculated for the linkage geometry representing energeticallyfavoured regions of the <1>, 'IjJ space. The statistically averaged opticalrotation of a disaccharide can be calculated using the energy surface withthe detailed conformational dependence of the disaccharide on opticalrotation. Sucrose has been the subject of a recent investigation (Stevensand Duda, 1991) where a rigid-residue conformational study was used toassess the low-energy conformers. It was concluded that the furanose ringform which predominates in solution has the phase angle located in thenorthern conformer ranging from 4£ over 4T5 to 5£' The optical rotation of

28 SUCROSE

sucrose was best accounted for in terms of an equilibrium mixture of twolinkage conformers, one being similar to the crystalline structure and theother one representing another energy minimum having the 0-3f ... 02g intramolecular hydrogen bond.

2.6 Conclusions and perspectives

The aim of the present chapter was to describe the three-dimensionalstructures that sucrose can display. Many experimental and theoreticalcharacterizations of differeiif levels of structural investigations have beenreported. In order to help rationalize the body of fragmented evidences,computational molecular modelling was used thereby providing a continuous depiction of the molecular behaviour of sucrose. Modelling isparticularly useful for sucrose which is less obvious than structures of otherdisaccharides. This is because sucrose has a flexible fructofuranose ringlinked to the fairly rigid glucopyranose ring. This flexibility is enhanced bythe presence of three primary hydroxyl groups, one of which is next to theglycosidic linkage.Experimental and theoretical works agree and show the sucrose linkageto be nearly as flexible as any other disaccharide linkage. A survey of allthe sucrosyl moieties in crystalline oligosaccharides yields a 40° range in the<j> torsion angle as well as a 90° range in 'ljI. In its crystalline conformation,the sucrose molecules 'uses' most of its conformational degrees of freedomto pack in a highly dense fashion.As intra- and intermolecular hydrogen bonding in the solid state isinfluenced by packing effects, it cannot be expected that the fixed overallshape is retained in solution. Somehow conclusively, a 13C CP-MASanalysis of crystalline sucrose indicated that the three primary hydroxylgroups could have different bond rotamers distributions in the solid and inthe liquid states. They can indeed satisfy their hydrogen bonding withprotic solvents. Recent NMR studies in water have provided furtherevidences indicating that the intramolecular hydrogen bonds were notpermanently maintained (Herve du Penhoat et ai., 1991). The conformational behaviour could be best described as a dynamical average of severalinterconverting conformers in solution at least in the case of dilutedsolution. Of course the population of low-energy conformers may varysignificantly as a function of the concentration, temperature and the natureof the solvent. In DMSO solution, it is believed that two main low-energyconformations are prevailing, which occur with a 2:1 equilibrium.Mathlouthi and collaborators (Mathlouthi and Luu, 1980; Mathlouthi et

ai., 1980) observed a concentration dependence of Raman frequenciesassigned to the CHz groups of the C-lf and C-6f hydroxymethyl groups ofsucrose. They interpreted this observation as indicating that both intra-


molecular hydrogen bonds exist at high concentration, but are successivelybroken upon dilution, with a single hydrogen bond persisting at intermediate concentration. The disappearance of the O-If ... 0-2g hydrogenbond, at low concentration, may be accompanied by a change in glycosidiclinkage conformation. A concentration dependence in the low-angle X-raydiffraction behaviour of sucrose solution was suggested to be compatiblewith such model. Infrared data on solution, quenched melt, and freezedried samples and CPMAS 13C NMR data on freeze-dried samples werealso analysed on the basis of such a model (Mathlouthi, 1981; Mathlouthi etal., 1986). It is clear that all these data are consistent with the lack ofconformational rigidity of sucrose as inferred from the extensive set oftheoretical characterizations.Pioneering work on the field of sweet-taste chemoreception (Shallenberger and Acree, 1967; Kier, 1967; Shallenberger et al., 1969) proposedthat there should be complementarity between a hydrogen bond donor,AH, and acceptor, B, on both receptor and sweetener molecules. Thedistance between AH and B should be between 2.5 and 4.0 A. Thistripartite AH-B-X theory originated from a consideration of the crystallineconformation of sucrose. It is worth mentioning that the occurrence of sucha glucophoric feature can also be identified on many of the lowest energyconformations that sucrose can adopt. Besides, such a criterion is satisfiedby almost every sugar and many amino acids in many ways irrespective ofthe property of the sugar molecule. There must be other stereochemicalrequirements based on molecular shape and the electrostatic potentialassociated with that shape that must be involved in the elicitation ofsweetness (Jeffrey, 1993). Actually, the wealth of significant informationprovided by elucidating and analysing crystalline complexes betweenprotein and carbohydrates has shown that all the potential sources ofconformational flexibility may be utilized by the carbohydrate molecule tomould itself to fit the protein combining site and to allow furtherinteractions with the neighbouring protein surface.It has been suggested that hydrogen bonding, displaying both co

operativity and polarizability could play an important role in molecularrecognition (Jeffrey, 1993). Electrostatic information could be transmittedin the vicinity of a receptor site. The transfer of electrostatic informationthrough polarization of the hydrogen bonded water structure at thereceptor site could be effective in several ways such as guiding thesweetener molecule on to the receptor site in the correct orientation. Inperspective the necessary structural characterizations of the structures ofsucrose in water solutions are underlined. While the role of water structurein sweet taste chemoreception has been examined (Mathlouthi et al., 1993)no detailed investigation of the electrostatic potential has been performedyet. Not only do we need molecular descriptions of the preferred

30 SUCROSE

conformations that sucrose can adopt once solvated, but also theperturbation that the structure of water undergoes in presence of suchsolute.

Acknowledgements

The author wishes to thank the many collaborators who have contributedto the exploration of some of the aspects of structural sucrochemistrypresented in this chapter; in particular, Florence Casset, Soizic Cros, SorenEngelsen, Catherine Herve du Penhoat, Stephan Houdier, Anne Imbertyand Christophe Meyer.

References

Allinger, N.L., Yuh, Y.H. and Lii, J-H. (1989) Molecular Mechanics. The MM3 force fieldfor hydrocarbons. J. Am. Chem. Soc., 111,8551-8566.Beevers, e.A. and Cochran, W. (1947) Proc. Royal Soc. London, Ser. A., 190,257.Beevers, e.A., McDonald, T.R.R., Robertson, J.H. and Stern, F. (1952) Acta Crystallogr.,5, 68~90.Bock, K. and Lemieux, R.U. (1982) The conformational properties of sucrose in aqueoussolution: intramolecular hydrogen-bonding. Carbohydr. Res., 100,63-74.Brooks, C.L., Bruccoleri, R.E., Olafson, B.D., States, D.J., Swaminathan, S. andKarplus, M. (1983) CHARMM: A program for macromolecular energy, minimization anddynamics calculations. J. Compo Chem., 4, 187-217.Brown, G.M. and Levy, H.A. (1963) Sucrose: Precise determination of crystal andmolecular structure by neutron diffraction. Science, 141,921-923.Brown, G.M. and Levy, H.A. (1973) Further determination of the structure of sucrosebased on neutron-diffraction data. Acta Crystallogr., B29, 790-797.Carver, J.P. (1991) Experimental structure determination of oligosaccharides. Curro Opinion

Struct. Bioi., 1,716-722.Christofides, C. and Davies, D.B. (1985) Co-operative and competitive hydrogen bonding insucrose determined by SIMPLE 'H N.M.R. spectroscopy. J. Chem. Soc., Chem.Commun., 1533-1534.Cremer, D. and Pople, J.A. (1975) A general definition of ring puckering coordinates. J.

Am. Chem. Soc., 97, 1354-1358.Davies, D.B. and Christofides, e. (1987) Comparison of intramolecular hydrogen-bondingconformations of sucrose-containing oligosaccharides in solution and the solid state.Carbohydr. Res., 163, 269-274.Diner, S., Malrieu, J.P., Jordan, F. and Gilbert, M. (1969) Localized bond orbitals and thecorrelation problem. Energy up to the third-order in the zero differential overlapapproximation. Application to a-electron system. Theor. Chim. Acta, 15, 110-110.Fischer, E. (1893) Ber. Deutsch Chem. Ges., 26, 2400-2412.French, A.D. and Dowd, M.K. (1993) Exploration of disaccharide conformations bymolecular mechanics. J. Mol. Struct. (Theochem), 286, 183-201.Ha, S., Giammona, A., Field, M. & Brady, J.W. (1988) A revised potential-energy surfacefor molecular mechanics of carbohydrate. Carbohydr. Res., 180,207-221.Hanson, J.e., Sieker, L.e. and Jensen, L.H. (1973) Sucrose: X-ray refinement andcomparison with neutron refinement. Acta Crystallogr., B29, 797-808.Haworth, W.N. (1929) The Constitution of Sugars. Arnold and Co., London, UK, pp. 70-71.Herve du Penhoat, e., Imberty, A., Roques, N., Michon, V., Mentech, J., Descotes, G. andPerez, S. (1991) Conformational behaviour of sucrose and its deoxy analogue in water asdetermined by NMR and molecular modelling. J. Am. Chem. Soc., 113,3720-3727.


IUPAC-IUB Commission of Biochemical Nomenclature (1971) Arch. Biochem. Biophys.,145,405-421.Jeffrey, G.A. (1973) Conformational studies in the solid state: Extrapolation to molecules insolution. In Carbohydrate in Solution (Adv. Chern. SeT., 32) American Chemical Society,Washington, pp. 177-196.Jeffrey, G.A. (1993) Hydrogen bonding with sugars and the role of hydrogen bonding inmolecular recognition. In Sweet-Taste Chemoreception (eds Mathlouthi, M., Kanters, J.A.and Birch, G.). Elsevier Applied Science, London, UK, pp. 1-10.Jeffrey, G.A. and Saenger, W. (1991) Hydrogen Bonding in Biological Structures. SpringerVerlag, Berlin, Germany.Kier, L.B.A. (1967) A molecular theory of sweet taste. 1. Pharm. Sci., 61,1394-1397.Lichtenthaler, F.W., Immel, S. and Kreis, U. (1991) Evolution of the structural representation of sucrose. Starch/Starke, 43, 121-132.Lichtenthaler, F.W., Immel, S., Martin, D. and Miiller, V. (1992) Some disaccharidederived building blocks of potential industrial utility. Starch/Starke, 44, 445-456.Mathlouthi, M. (1981) X-ray diffraction study of the molecular association in aqueoussolution of o-fructose, o-glucose, and sucrose. Carbohydr. Res., 91,113-123.Mathlouthi, M. and Luu, D.V. (1980) Laser-Raman spectra of o-glucose and sucrose insolution. Carbohydr. Res., 81, 203-212.Mathlouthi, M., Luu, c., Meffroy-Biget, A.M. and Luu, D.V. (1980) Laser-Raman studyof solute-solvent interactions in aqueous solutions of o-fructose, o-glucose, and sucrose.Carbohydr. Res., 81, 213-223.Mathlouthi, M. Cholli, A.L. and Koenig, J.L. (1986) Spectroscopic study of the structure ofsucrose in the amorphous state and in aqueous solution. Carbohydr. Res., 147, 1-9.Mathlouthi, M., Bressan, C., Portmann, M.a. and Serghat, S. (1993) Role of waterstructure in sweet taste chemoreception. In Sweet- Taste Chemoreception (eds Mathlouthi,M., Kanters, J.A. and Birch, G.). Elsevier Applied Science, London, UK, pp. 141-174.McCain, D.C. and Markley, J.L. (1986a) The solution conformation of sucrose: Concentration and temperature dependence. Carbohydr. Res., 152,73-80.McCain, D.C. and Markley, J.L. (1986b) Rotational spectral density functions for aqueoussucrose: Experimental determination using l3C NMR. J. Am. Chem. Soc., 108,4259-4264.Meyer, C., Perez, S., Herve du Penhoat, C. and Michon, V. (1993) Conformational analysisof 4,1' ,6'-trichloro-4, l' ,6'-trideoxy-galacto-sucrose (Sucralose) by a combined molecularmodelling and NMR spectroscopy approach. 1. Am. Chem. Soc., 115, 10300-10310.Morrison, R.T. and Boyd, R.M. (1959) Organic Chemistry. Allyn and Bacon, Inc., Boston,MA, USA, p. 789.Perez, S. (1993) Theoretical aspects of oligosaccharide conformation. Current Opinion Struct.

Bioi., 3, 675--680.Perez, S., Meyer, c., Imberty, A. and French, A.D. (1993) Molecular features andconformational flexibility of sucrose. In Sweet- Taste Chemoreception (eds Mathlouthi, M.,Kanters, J.A. and Birch, G.). Elsevier Applied Science, London, UK, pp. 55-73.Pfeffer, P.E., Odier, L. and Dudley, R.L. (1990). Assignment of l3C CPMAS NMR spectraof crystalline methyl jl-o-glucopyranoside and sucrose using deuterium labelling andinterrupted proton decoupling. J. Carbohydr. Res., 9, 619-{i29.Pigman, W.W. (1948) Chemistry of Carbohydrates. Academic Press, New York, USA,p.446.Poppe, L. and Van Halbeek, H. (1992) The rigidity of sucrose: Just an illusion? J. Am. Chem.

Soc., 114, 1092-1094.Shallenberger, R.S. and Acree, T.E. (1967) Molecular theory of sweet taste, Nature, 216,480-482.Shallenberger, R.S., Acree, T.E. and Lee, c.Y. (1969) Sweet taste of 0- and L-sugars andamino-acids and the steric nature of their chemo-receptor site. Nature, 221, 555-556.Stevens, E.S. and Duda, C.A. (1991) Solution conformation of sucrose from opticalrotation. J. Am. Chem. Soc., 113,8622-8627.

Thogersen, H., Lemieux, R.U., Bock, K. and Meyer, B. (1982) Further justification for theexo-anomeric effect. Conformational analysis based on nuclear magnetic resonancespectroscopy of oligosaccharides. Can. J. Chem., 60, 44-57.Tollens, B. (1883) Ber. Deutsch Chem. Ges., 26,921-924.

32 SUCROSE

Tran, V.H. and Brady, l.W. (1990a) Disaccharide conformational flexibility. I. Anadiabatic potential energy map for sucrose. Biopolymers, 29,961-976.Tran, V.H. and Brady, l.W. (1990b) Disaccharide conformational flexibility. II. Moleculardynamics simulations of sucrose. Biopolymers, 29, 977-997.Tvaroska, I. and Perez, S. (1986) On the conformational energy calculations of oligosaccharides. Carbohydr. Res., 149, 389-410.Van Alsenoy, C., French, A.D., Cao, M., Newton, S.Q. and Schafer, L. (1994) Ab initioand molecular mechanics studies of the distorted sucrose linkage in raffinose. (Submittedfor publication)

3 Sucrose crystallisationG. VACCARI and G. MANTOVANI

3.1 Introduction

One of the main characteristics of commercial sugar is its high purity(>99.9%) which is comparable to that of the purest organic substanceswhich are produced on an industrial scale. To obtain such a product, bothfrom beet or cane, rather complex processing schemes are followed whichdepend on the quality and quantity of non-sucrose compounds present inthe solution at the end of the extraction stage. Before reaching thecrystallisation stage, the solution leaving the extractor is usually subjectedto various purification steps in order to eliminate as many non-sucrosecompounds (non-sugar) as possible. Nevertheless, a remarkable amount ofsuch compounds remains in solution so that it will be the crystallisationprocess which will complete the purification operation by obtaining crystalswhich, normally through recrystallisation process, give a commercialproduct with the desired characteristics.The scope of the present chapter is to give some basic ideas on thecrystallisation process and to correlate the presence of non-sucrosecompounds and crystallisation management with the characteristics of thefinal product. For a better understanding of the phenomena occurringduring crystallisation, we mention some basic concepts about sucrosemolecules and sugar solutions even though this subject is widely researchedand described in detail in other chapters of this book.The sucrose molecule has eight hydroxyl groups which can be involved inhydrogen bond formation. In sufficiently diluted aqueous solutions all thehydroxyl groups form hydrogen bonds with water molecules. If theconcentration increases, the great flexibility of the bond joining the twoglucose and fructose molecules promotes the formation of a structurepresenting, at first, only an intramolecular bond and then two intermolecular bonds (Mathlouthi, 1981). If sucrose concentration in the solutionincreases, aggregation phenomena occur between sucrose molecules whichpromote step by step the formation of aggregates which will create thestable three-dimensional nucleus (see below) (Schliephake, 1963).For crystal formation, the various hydroxyl groups of the sucrosemolecule are available for hydrogen bonding. Even if the two intramolecular hydrogen bonds are not taken into consideration, it must be pointed


34 SUCROSE

out that one of the hydroxyl groups cannot be involved in the formation ofhydrogen bonds in the single crystal for steric reasons. On the other hand,such a group plays a determining role in twin formation (see below)(Aquilano et al., 1983; Mantovani et al., 1983).

3.2 Morphology

3.2.1 Single crystal

Vavrinecz (1965) described in detail the sucrose crystal morphologyconsidered as all the faces which determine the sucrose external habit.Sucrose belongs to the sphenoidic class of the monoclinic crystal systemand is characterised by 15 simple forms whose relative positions are shown inFigure 3.1(a). The eight most important faces are shown in Figure 3.1(b).The crystal is polar along the binary b-axis belonging to space group P21and its faces are in general identified via the Miller's indices or throughconventional letters (see Figure 3.1).The relative development of the different faces, depending upon thedifferent growth rate, determines the exterior shape of the crystal. Inparticular, the most rapid faces tend to disappear from the finalmorphology where we can find, in general, the eight faces mentionedabove. In pure solution the faces of the right pole (+b) grow more rapidlythan those of the left pole (-b). As far as the development along the threecrystallographic axes is concerned, the crystal becomes more elongatedalong the b-axis although we have to take into account that, in pure

(b)

c (001)

-r (101)

q (011)i (503) q' (011)

0(111) .. )\:I 0' (111)

f-k--

f (210) P (110)

"- f (210)"'.

w (111) w' (111)

C

(a) Lb

Ua (100)

d (101)

............... p' (110)

Figure 3.1 (a) Sucrose crystal showing all the 15 forms. (b) The eight most important faces.(The Miller's indexes of the different faces are indicated in parentheses.)(After Vavrinecz,

1965.)

SUCROSE CRYSTALLISATION 35

solution also, the different faces grow, and then also the elongation alongthe three axes, depends upon temperature and supersaturation (see below).The presence of impurities in the mother liquor can then deeply modify theexterior shape of the crystal.The way sucrose molecules are packed inside the crystal has beenstudied since the 1960s (Mantovani et al., 1967; Smythe, 1967, 1971). Morerecent investigations (Aquilano et al., 1983), which were based on thestudy of the formation of periodic bond chains (PBCs), allowed this subjectto be analysed thoroughly by defining more carefully the crystal morphological aspect (Figure 3.2).By means of such analysis it is possible to define the F (flat), S (stepped)or K (kinked) character of the different faces (Hartman and Perdok, 1955).Without going into detail on such items we can point out that S, and inparticular K, faces tend to disappear from the crystal morphology sincethey are more rapid than F faces. This is the reason why only eight out ofthe 15 forms described above are present in the crystal: six of such faces areF faces (a, p, p', C, r, q) and two (0, d) have an S character. The presenceof these two last faces in the final morphology, and in particular the dfaces, is to be related to the effect of the solvent which stabilises the facesthemselves (Aquilano et al., 1986).

3.2.2 Twins

The hydroxyl group which, as mentioned above, is not involved in thehydrogen bond formation of the single crystal becomes essential in the twin

Figure 3.2 Projection of sucrose crystal structure along the c-axis. At the centre of the crystalis drawn the projection of one of the PBes. (From Aquilano et al. (1983), reproduced with

permission.)

36 SUCROSE

formation. The twins are composed of two single crystals, the relativeposition of which is rational and constant from the crystallographic point ofview. According to Vavrinecz (1965) they are formed by rotating one ofthe single crystals 1800 around the c-axis to obtain junction with the othersingle crystal which remains in its original position. Also, according toVavrinecz (1965), we can distinguish three types of twin crystals:

(1) twin crystals which have their left poles turned towards each otherwhile the right poles point outwards (Figure 3.3(a»;

(2) twin crystals which have their right poles grown together while theleft poles are pointing outwards (Figure 3.3(b»; and

(3) twin crystals which represent a transition between the two abovetypes where both single crystals are placed behind each other andare grown together along the a face (Figure 3.3(c».

From the crystallographic point of view (Mantovani et al., 1983) only twinsof type (1) can grow in pure solution; the growth of twins of types (2) and(3) is to be ascribed to the presence in the solution of impurities whichmodify the morphology of twins of type (1). From the technological pointof view, the presence of twins represents a negative aspect either relatingto strictly technological problems or crystal quality. Moreover, twins tendto be larger in comparison to single crystals not only because they originatefrom the junction between two crystals but also because they present, atthe two poles of the b-axis, two couples of pi faces (which grow morerapidly in comparison with p faces). Further, in the zone of junctionbetween the two crystals, growth centres are activated (dislocations) whichfavour the crystal growth along the c-axis. Consequently, larger crystals areobtained compared to single crystals, with constant growth conditions (seeFigure 3.4), which worsen the sugar commercial quality.Moreover, the junction zone between the two single crystals becomes aweak point from the mechanical point of view. Therefore, the two crystalscan separate after growth, generating fragments which are technologicallynoxious. Again, in the junction zone between the two crystals, motherliquor can be retained and hardly eliminated through the usual crystalswashing inside the centrifuge, so even worsening the final crystal quality.The formation of twins is strictly related to solution supersaturation; thehigher the supersaturation the higher the probability of twins' formation.

3.2.3 Conglomerates

Whereas the junction of two crystals forming twins obeys well-definedcrystallographic rules, the formation of conglomerates during the crystallisation process is due to a completely random junction of two or moresingle crystals growing together (Figure 3.5).Although the formation of conglomerates has not been completely

SUCROSE CRYSTALLISATION

c (001)

I ......, .. /..

a (100)

J"'-

-r (101)

(a)

c (001)

d (101)

. p' (110)

37

d (101

-P (110) .

) ..............

•\. .•....... ,

GV-- ..

a (100)

~ l\

-r (101)

(b)

(c)

Figure 3.3 Twins of sucrose crystals. (a) Twin of type (1); (b) twin of type (2); (c) twin oftype (3). (After Vavrinecz, 1965.)

38 SUCROSE

Figure 3.4 Single and twin crystals present in the same crop.

Figure 3.5 Conglomerates present in commercial sugar.

explained, it seems that it can be related to the crystallisation conditions(Pot, 1980; Kuijvenhoven, 1983). High supersaturations, which in theirturn favour high growth rate and spontaneous nucleation phenomena,certainly promote the formation of conglomerates. Even the traditionalcrystallisation in the pans, which promotes local supersaturations, favoursthe formation of conglomerates. Other important parameters are seedcharacteristics, stirring, crystallisation duration and crystal size. In particular it may be noted that the number of conglomerates is lower for better


seed characteristics, higher stirring, lower crystallisation duration andsmaller crystal size. Of course, the negative consequences on sugarcharacteristics, quoted above with regard to the twins, become moreserious in the case of conglomerates, in particular, as far as crystal washingduring centrifugation is concerned.

3.2.4 Effect of impurities

Omitting, at this moment, the effect of impurities on the solutioncharacteristics, and in particular on sucrose solubility, we take intoconsideration the possibility of interaction between such impurities and thedifferent crystal faces. If such impurities modify the relative growth rates ofthe different faces, the crystal habit is also modified. Such modificationscan be such that certain faces, which normally are not present, can appear.Powers (1970) pointed out that, if structural affinity with a single faceexists, or there is the possibility of bond formation, the molecule ofimpurity can statistically remain on the surface for a period of time. If thelocal conditions are favourable, a localised layer of impurity molecules candevelop, so hindering further attachment of sucrose molecules fromsolution; in such a case we can observe an even drastic decrease of thegrowth rate of the face itself up to its total blocking. If, soon after, the localconditions change, the layer-by-Iayer growth of sucrose molecules can startagain overgrowing the impurity layer. On the other hand, if the interactionbetween the impurity and the layer is weak, the weakly adsorbed impuritymolecule can be easily removed by the sucrose molecule coming from themother liquor phase. In this case there is competition between impurityand sucrose molecules, also influenced by temperature, which causes amodification of the growth rate of the face itself.Before going into details on the specific effect of particular impurities onsucrose crystal morphology, we consider the different morphologies ofsugar crystals originating from cane or beet processing. Obviously, if it isrefined sugar, the morphological differences are unimportant because inboth cases the concentration of impurities in the final growth solutions isvery low. On the contrary, if we consider raw sugar, and in particular theone coming from low-grade boiling, crystals originating from beetprocessing are morphologically very different from the ones coming fromcane processing. Furthermore, if db is the ratio between the crystalelongation along c- and b-axes (see Figure 3.1), such a ratio for beet rawsugar is lower than 1, whereas it is higher than 1 for cane raw sugar. Inother words, the impurities as a whole present in beet juices favour theelongation of crystals along b-axis whereas in the case of cane raw sugar theimpurities as a whole favour the elongation along the c-axis. We have totake into account that high concentrations of raffinose in beet juices and ofmono-, oligo- or polysaccharides in cane juices remarkably intensify such

40

(a)

(b)

SUCROSE

Figure 3.6 Morphological characteristics of raw sugar originating from (a) beet, and(b) cane processing.

elongation along b- and c-axes, respectively. In Figure 3.6 cane and beetraw sugars are compared.

In order to analyse the specific role of the different impurities as habitmodifiers, they can be subdivided into the following four groups:

(a) monosaccharides,(b) oligosaccharides,(c) polysaccharides, and(d) inorganic non-sugars.


It is understandable that impurities relating to the groups (a), (b) and (c)can interact with some sucrose crystal faces since they are, at leastpartially, formed by structural units which belong to the sucrose molecule. Infact (see Figure 3.2) the various faces of the sucrose crystal present at theirsurface, under peculiar steric conditions, glucose or fructose units.Depending on the number and strength of bonds established by thedifferent sugars with the various sucrose crystal faces, we can observepoisoning phenomena of some faces also at low impurity concentration(i.e. raffinose). Moreover, we can observe competition effects with sucrosemolecules at high impurity concentrations (i.e. monosaccharides).

3.2.4.1 Monosaccharides. Van Hook (1983a, 1988) studied carefully theeffect of glucose, fructose and invert (this he considered as an equimolecularsum between glucose and fructose) on sucrose crystal morphology. Hepointed out that the most evident effects are observed at high concentrations of monosaccharides (100-200 g per 100 g water). Such concentrationscharacterise cane processing juices and, in particular, low-grade boilingproducts.Vaccari et al. (1990, 1991a) taking as a starting point the peculiarmorphology observed in cane crystals (Figure 3.7), which show, togetherwith an elongation along c-axis, the appearance of rare faces on the rightpole and a peculiar D shape, studied deeply the influence of glucose andfructose on sucrose crystallisation, drawing the following conclusions:

• the most significant effects were observed at the highest concentrationof the impurities;

• glucose interferes in particular with the left pole so far as tocompletely block the p faces growth;

• fructose interferes in particular with the right pole drastically slowingdown the growth of the 0' and q' faces (which, in general, are notobserved in the crystal), whose size becomes particularly important;and

• the combined effects of glucose and fructose causes an unusual crystalform as shown in Figure 3.8.

Such results were interpreted by Aquilano et at. (1990) as a competitionmechanism in the surface absorption between the monosaccharides andsucrose molecules. Such a mechanism becomes as much remarkable as theimpurities concentration becomes higher.

3.2.4.2 Oligosaccharides. With the general term oligosaccharides wemean molecules of sugars formed by three or more monosaccharide units.The most important from the processing point of view are the ones presentin the raw material and, in particular, raffinose (present in beet) and thethree kestoses and theanderose (present in cane). We can see in Figure 3.9

42 SUCROSE

•

Figure 3.7 Cane sucrose crystal showing the characteristic D shape.

r------------'"I •.••..••.•••.•••.••.•• _........

.. " ~.~."! ••.

: I: I: I: I

1}:~ ;;iii>I /

c

•.. b

Figure 3.8 Schematic drawing of the morphological change of sucrose crystal grown in thepresence of high concentrations of glucose and fructose.


. GALACTOSE - GLUCOSE - FRUCTOSE~

(raffinose)

CDGLUCOSE - FRUCTOSE - FRUCTOSE

(1 - Kestose)

GLUCOSE - ®FRUCTOSE GLUCOSE - FRUCTOSE - FRUCTOSE

(sucrose) (6 - Kestose)

- FRUCTOSE - GLUCOSE

(Neo-Kestose)

- FRUCTOSE

GLUCOSE - GLUTOSE

(Theanderose)

- FRUCTOSE

Figure 3.9 Composition of the main oligosaccharides.

that such compounds are made by a molecule of monosaccharide(galactose, fructose or glucose) bonded to a sucrose molecule. It istherefore understandable, at least qualitatively, that there is the possibilityof an interaction between such molecules and that of the sucrosecrystal faces so modifying their growth morphology.Although many papers have been published (see the references in Moreldu Boil, 1991, 1992) about the effect of oligosaccharides on the elongationalong the c-axis of crystals originating from cane processing, this subjecthas been not studied yet in depth from the structural point of view as wasdone for raffinose. The macroscopic effect of this trisaccharide, whichpromotes the formation of very elongated crystals along the b-axis (Figure3.10), has been carefully studied by Vim Hook (1981a,b, 1983). Taking intoaccount that such morphological changes can be observed even atrelatively low impurity concentrations, it could be supposed that, unlikewhat occurs in the presence of glucose and fructose, some particular crystalfaces were selectively poisoned.A complete review on the influence of raffinose on sucrose crystal

44 SUCROSE

Figure 3.10 Sucrose crystal grown in the presence of raffinose. (From Vaccari et al. (1986),reproduced with permission.)

kinetics and morphology has been published by Vaccari et al. (1986). It ispointed out that p' faces growth is completely blocked whereas no slowingdown effect can be observed on the p faces. Relating to the other faces,there is a rapid disappearance of the d face morphology (the growth rate ofwhich is not influenced by the presence of raffinose), a slowing down of thec faces and a drastic blocking of the a and r face growth. The finalmorphology then becomes very simplified and crystals obtained growonly along the -b-axis so presenting a needle-shape aspect as shown inFigure 3.10.From the technological point of view, these facts can cause big problemswhich can be summarised as follows:

• decrease, as a whole, of the crystal growth rate in the presence of ahigh concentration of the trisaccharide such as can occur in theSteffen plants for molasses desugarisation (Hartmann, 1974);

• fragmentation of the thin crystals grown and the possibility that a partof these fragments pass through the liners of the centrifuge.

We can incidentally point out that raffinose can cause a relatively unusualphenomenon-'tapering'-which promotes the peculiar morphology shownin Figure 3.11. Such a phenomenon has been studied utilising the interferometric holography, and the relevant results have been discussed(Bedarida et al., 1988).Another rather surprising effect caused by the presence of raffinose inthe growth solution is the change of the type (1) twins into types (3) and(2). Such a phenomenon (Mantovani et al., 1983), presented as a scheme inFigure 3.12, is directly promoted by the blocking of p' and a faces whichfavours the overlapping of the a faces from p faces.

3.2.4.3 Polysaccharides. The presence of polysaccharides, and inparticular dextrans (glucose polymers), is particularly important in caneprocessing. Many researchers have studied the morphological effects due


r- b

a

Figure 3.11 Sucrose crystal tapering effect caused by the presence of raffinose in the growthsolution.

p'l~t ·Ip,p'~

pp'

pP~p

a

Figure 3.12 Morphology modification of sucrose twins type (I) in the presence of raffinosein the growth solution.

to the presence of such polymers (Bruijn, 1966; Kamoda et ai., 1968;Sutherland, 1968; Leonard and Richard 1969; Sutherland et ai., 1969;Imrie and Tilbury 1972; Covacevich and Richard, 1974) and, although theconclusions are often conflicting, there is agreement that such compoundsplay an important role. The reason for possible discrepancies can be due tothe different experimental conditions, different characteristics of thepolymers (i.e. molecular weight), and eventual synergistic effects which

46 SUCROSE

Figure 3.13 Sucrose crystal, grown in the presence of dextrans, elongated along the c-axis.(From Mantovani et al. (1993), reproduced with permission.)

will be discussed below. What can be observed is an elongation along the caxis which can give crystals with a particularly high db ratio.As in the case of raffinose, these drastic morphological effects have

detrimental influence from the technological point of view. In fact, theelongation along the c-axis is not to be considered as an increase in thegrowth rate along this axis but as a decrease of the growth rate of both pand pi faces along the b-axis. Sucrose crystal grown in the presence of highconcentrations of dextrans are shown in Figure 3.13.

3.2.4.4 Inorganic non-sugars. Although it is rather difficult to suppose adirect interaction between inorganic compounds and the different faces ofthe sucrose crystal, many authors have studied this subject not alwaysreaching coincident results. However, the most studied effect concerns theanomalous development of the d face of crystals grown in the presence ofpotassium chloride (Aquilano et al., 1986, 1987). This phenomenon isexplained through an epitactic effect between one of the potassiumchloride crystal faces and the sucrose crystal d face. Sucrose crystal grownin the presence of potassium chloride is shown in Figure 3.14.


Figure 3.14 Sucrose crystal, grown in the presence of potassium chloride, showing a large dface.

The simultaneous presence of a big number of compounds in theindustrial growth solutions can cause crystal morphological modificationswhich cannot be ascribed to only one of such substances, eithercarbohydrates with a low or high molecular weight, or different organic orinorganic compounds. In fact, negative or positive synergistic effects canoccur so that such impurities can help to enhance a certain habitmodification or limit the effect that each of them, being alone, can exert onthe final crystal shape. This fact can, at least within certain limits, justifythe apparently conflicting effects described by different researchers on thegrounds of tests carried out on crystals grown from industrial solutions.

3.3 Solubility

Although this subject has been covered in chapter 5 of this book, we wouldlike to discuss some points with the aim of relating them to the followingparagraph concerning nucleation and crystal growth. Sucrose is includedamong the compounds presenting high solubility in water and such asolubility is highly influenced by the presence of both organic and inorganiccompounds which are normally present in industrial solutions. To deal withthe problems related to nucleation and growth it is extremely important toknow the solubility curve as a function of temperature and how such acurve can vary in the presence of other compounds in the solution which, inthe following, will be generically named 'non-sugars'.

48

S/W

6

5,5

5

4,5

4LABILE

3,5

3

2,5

SUCROSE

+III

1 II

I

~------

UNSATURATED

2'----"-----'--L---'-----'-_.L---'-----'-_.L---'-----'----l30 35 40 45 50 55 60 65 70 75 80 85 90

Temp.oC

Figure 3.15 Schematic sucrose solubility diagram. Solid line, ft = 1.00; dotted line, ft = 1.35.ft = (s/w)/(s/w),at ,oln'

3.3.1 Pure solution

A schematic sucrose solubility diagram in water is shown in Figure 3.15where the supersaturation zone is subdivided into the parts related tonucleation and growth (see below). It is pointed out that, starting from anunsaturated solution, it is possible to enter the supersaturation zone viaisothermal evaporation (line 1) or simple cooling (line 2) or adiabaticevaporation (line 3). In this last case, cooling is promoted by waterevaporation under vacuum. This solubility curve can be shifted to lower orhigher SIW ratio values (S, sugar; W, water) depending on the presence ofnon-sugars in the solution.

3.3.2 Impure solution

The effect of non-sugars on sucrose solubility depends on both their typeand concentration. In the case of traditional beet processing, a low nonsugar concentration in the solution (purity >90%), causes a sucrosesolubility decrease: when the NIW ratio increases (N, non-sugar; W,water) sucrose solubility linearly increases. Figure 3.16 shows the solubilitycurve calculated according to Gmt data (curve A) (Vavrinecz, 1978-79)bearing in mind that the trend of such a curve can vary depending on the


(S/W) impure solution

1,4 (S/W) pure solutionA

1,3

1,2B

1,1

0,9

32,521,50,5

0,8 L-__---JL-__---l ---l.. "'-'- --...L 4

oN/W

Figure 3.16 Effect of non-sugar composition and concentration on the saturation function.

non-sugar composition. In particular, curve B shows the trend of thesolubility curve for Quentin molasses (Vavrinecz, 1978-79) where sodiumand potassium ions have been partially replaced by magnesium ions. CurveC shows the saturation function concerning a raw juice direct crystallisationvia the elimination of the whole traditional purification process as will bedescribed below (Vaccari et al., 1993a). The big effect exerted on sucrosesolubility by both the non-sugar concentration and composition is quiteclear.By moving from beet to cane processing, it is clear that the change of thenon-sugar composition can promote sucrose solubility variation. Takinginto account the high percentage of invert which more and more increasesin the cane sugar juices, a special attention has been devoted to thispeculiar non-sugar (or, better, non-sucrose) on sucrose solubility(Vavrinecz, 1978-79). Figure 3.17 emphasises how sucrose solubilityprogressively decreases when invert concentration increases.

3.4 Crystallisation

3.4.1 Nucleation

We have already pointed out (see section 3.1) that in sufficientlyconcentrated solution there is formation of aggregates among sucrose

50 SUCROSE

(8/W) impure solution(8/W) pure solution

1.0

.: .'. '::. 70·C....

':.<;;:'::::':'::::':.~':.'. '. 40

'.''::'" ' .. 30

'<:. 25. 23

. 20

0.7'----------------1-0.0 to

InvertWater

Figure 3.17 Sucrose solubility variation in the presence of invert. (after Vavrinecz, 1978-79.)

molecules. By reference to Figure 3.15, as we enter the supersaturation zonethere is an even higher probability that these aggregates reach suchdimensions to originate a stable three-dimensional nucleus which, increasing, will form the sucrose crystal. From the thermodynamic point ofview, the stable three-dimensional nucleus is formed only if a certainenergy barrier is overcome so that the process can go on towards the crystalformation. Without going into detail of the theoretical discussion (Mullin,1972) we can remember that, in the formation of a nucleus, there is arequest of energy for the surface formation, whereas a part of energy isreleased by molecules owing to the decrease of the molecular motioncaused by transition from solution to crystalline state (crystallisation is, asis known, an exothermic process). The total energetic balance as a functionof the molecules aggregate dimension has the trend depicted in Figure 3.18where the critical radius rc and critical free energy !1Gc are pointed out.

If the dimension of the molecules aggregate is less than rc the aggregatewill tend to collapse so releasing sucrose molecules, whereas if it is more


Free Energy 6. G

+

--------------------_.,-~-~--

Radius01---=::::=----------------1--------'.-----+

dissolution regrowth

Figure 3.18 Total free energy variation in the nucleation process.

51

than rc the process will go on towards the progressive increase of thenucleus dimension, that is the crystal formation occurs. The probabilitythat aggregates present in the solution reach the critical radius dimensionswill be as much higher as the solution supersaturation. If supersaturation isnamed 13, and defined as the ratio of sucrose/water of the solution andsucrose/water in saturated solution at the same temperature, the followinghas been shown (Van Hook, 1959; Mullin, 1972; Maurandi, 1981):

rc = A/(T X In f3) (3.1)

where A is a constant depending on the interface tension and molecularvolume and T the absolute temperature. From equation (3.1) we canobserve that the critical radius is as much smaller as temperature andsupersaturation are higher. Moreover, the critical free energy value can beobtained through the following formula:

(3.2)

where B is a coefficient again depending on the interface tension andmolecular volume. We can observe that the dependence of I:!.Gc ontemperature and supersaturation is still more important compared withcritical nucleus radius.The nucleation rate J, e.g. the number of nuclei formed per unit time

52 SUCROSE

and unit volume, which can be expressed via a type Arrhenius equation,becomes:

J = K X exp - (C/(T3 X (In fJ)2)) (3.3)

where K and C are constants. It can be observed that the nucleation rate isas much higher as higher are temperature and supersaturation.What is reported above relates to pure solutions which do not containcrystals or other solid impurities able to favour nucleation by lowering theenergy barrier leading to the stable nuclei formation. In such a case,nucleation (indicated as heterogeneous nucleation) can occur under softersupersaturation conditions as shown in Figure 3.19.

It should be remembered that nucleation rate cannot increase to infinitywith supersaturation increase. In fact, beyond a certain fJ value, thesolution viscosity can reach such values that the system assumes a glassycharacter so drastically decreasing the nucleation rate.With regard to Figure 3.15, it should be taken into account that thepresence of non-sugar in the solution will shift not only the saturationcurve, as pointed out above, but also the nucleation zone. In particular,increasing the non-sugar concentration (and consequently decreasing thesolution purity quotient) the metastable zone will spread so moving away

Nucleation rate

heterogeneousnucleation

homogeneousnucleation

Supersaturation

Figure 3.19 Nucleation rate variation as a function of supersaturation for homogeneous andheterogeneous nucleation.


from the saturation curve up to the nucleation zone. In other words, in thepresence of higher concentrations of non-sugar, more drastic supersaturation conditions will be necessary in order that the spontaneous nucleationphenomenon can occur (Maurandi, 1981).

3.4.2 Crystal growth

3.4.2.1 Pure solution. The growth process of crystals from solution ingeneral, and of sucrose crystal in particular, is a very complex one. Thesolute molecules, which in solution can be more or less hydrated oraggregated, must have the possibility, as already pointed out, of placingthemselves on the crystal surface. Here they must be free to move andsuitably orientated to be able to find a suitable position where they may befixed after having lost their hydration water.Bearing in mind the complexity of this subject, it is understandable thatit has been studied from different points of view with the aim of explainingits various steps. It is well known that the surface diffusion theory has beenstudied through the two-dimensional nucleation mechanism (Kossel, 1934)and dislocations, which led to the Burton, Cabrera and Frank (BCF)theory (Burton et al., 1951), the more complex Gilmer, Gez and Cabreratheory (Gilmer et al., 1971) and the volume diffusion growth mechanismelaborated by Chernov (Chernov, 1961). Bennema (1968) studied thesetheories also using data of sucrose crystal growth. However, the study ofthe crystallisation process as a whole is here limited by referring to thediffusion process theory (Smythe, 1971; Mullin, 1972).

If dm is the amount of sucrose which crystallises in the time dt and e andCO are the concentration of supersaturated and saturated solutions,respectively, the growth rate can be given by a general equation such as

dm/dt = K A (e - eot (3.4)

where A is the total surface of the crystals and K is the kinetic coefficient ofthe crystallisation total process. If R is the specific growth rate and a therelative supersaturation as (cleO - 1), equation (3.4) can be written asfollows:

R = (dm/dt) (VA) = K'an (3.5)

where K' = K (eot. Taking this into account the whole process can beregarded as the sequence of two principal steps: the first of which(diffusion process) transfers molecules from the bulk solution to thecrystals surface and the second one concerns the insertion of molecules intothe surface (reaction process). By reference to Figure 3.20, we can divideequation (3.4) into the following ones which concern the two subsequentsteps, respectively:

54

R = kd (C - C')R=kr(c'-coy

SUCROSE

diffusion processreaction process

(3.6)(3.7)

where kd and k r are the coefficient of mass transfer by diffusion and therate constant for the surface reaction, respectively; c' is the sucroseconcentration at the crystal-solution interface and z is the order of thesurface reaction process. In the simplest case where z = 1, we caneliminate the term c' in equations (3.6) and (3.7) so obtaining the followingexpression:

(3.8)

which, if compared with equation (3.4) where n = 1, points out that theoverall crystal growth coefficient K depends upon the two constants k d andk r • Since the two diffusion and reaction processes occur in series it isobvious that the slower controls the total growth rate. This fact means thatif k r « k d the overall growth process is ruled by surface reaction whereasif kd « k r it is the diffusion process which rules the whole process. Thesetwo extreme situations can be obtained under particular conditions ofgrowth temperature or solution stirring (Maurandi et ai., 1988).With regard to the growth temperature it has been widely shown that atlow temperature it is the surface reaction which dominates, whereas at hightemperature it is the volume diffusion which prevails. If we would like toexperimentally determine the values of k r and kd , it is sufficient to calculatethe K value of the overall process at low and high temperatures,

c

CRYSTAL

stagnant

film

•Icrystal/solution

interface

diffusion

reaction

bulk of solution

c'c.Q

~CCDoc

8

Figure 3.20 A model representation of concentration driving force for a two-stepcrystallisation process.


respectively. Taking into account that the k i coefficients are related totemperature through a normal Arrhenius relationship:

k i = Z exp - (E/R1) (3.9)

where E i is the activation energy, R is the gas constant, T is the absolutetemperature and Z is a constant, we can calculate the activation energy ofthe surface reaction (at low temperature) and the diffusion process (at hightemperature). In Figure 3.21 the data of activation energy reported by VanHook (1981a) for surface reaction, volume diffusion and the overallprocess, are shown.As regards solution stirring, we have to take into account that kd = D/bwhere D is the diffusion coefficient and b the thickness of the boundarylayer around the crystal where mass transfer occurs only by diffusion (seeFigure 3.20). The value of b can be obtained bearing in mind theexpression involving the Sherwood (Sh), Schmidt (Sc) and Reynolds (Re)numbers (Mullin, 1972; Maurandi, 1989)

Sh = 2/3 Re l /2 Sc1l3 (3.10)

where Sh = Lib; Sc = 1']/Q D; Re = u L Q/1']; L is crystal size; 1'] isthe viscosity coefficient of the solution; Q is the density of the solution; Dis the diffusion coefficient of the growth unit in the solution; b is thethickness of the boundary layer; and u is the crystal-solution relative rate.

K Joule/mole80

(a)

60

(c)

40

20 (b)

80604020

°COL..---------'--------'------L..------.-J.+o

Figure 3.21 Activation energy for (a) surface reaction; (b) volume diffusion; and (c) overallprocess. (After Van Hook, 1981a).

56 SUCROSE

By obtaining the value of () we have

() = 3/2 X L I/2 X (T]/Q)1/6 X D 1I3/U 1/2 (3.11)

From equation (3.11) we can observe that the boundary layer thicknessdecreases with the increase of u, that is of the solution stirring, so causingthe increase of kd value. For sufficiently high values of solution stirring, thekd value becomes much greater in comparison with the value of k r and theoverall process will be ruled by the surface reaction and then K = kr . InFigure 3.22 the variation of the growth rate R as a function of the relativemotion crystals/solution is shown: for sufficiently high values of stirring thek r value can be calculated and then, taking into account the relationshipamong K, k r and kd , the kd value under different crystallisation conditions,can be obtained.What was described above relates to the simplest case where z ofequation (3.7) = 1 and, consequently, the value of n in the generalequations (3.4) and (3.5) = 1. For z values higher than 1, the expression ofthe rate equation becomes obviously more complicated so that thevariation of R with supersaturation is of parabolic type at low supersaturation and becomes linear at high supersaturation values as shown in Figure3.23.Moreover, we have to bear in mind that for solutions having highviscosity, such as concentrated sucrose solutions with high impuritiescontents, k d varies with the solution viscosity which, in turn, depends upon

growth rate

stirring

Figure 3.22 Effect of stirring on crystal growth rate.


growth rate

supersaturation

Figure 3.23 Growth rate as a function of supersaturation if z > 1.

57

the supersaturation; therefore, the R value can reach a maximum and thendecrease at sufficiently high supersaturation values (Smythe, 1971;Maurandi et al., 1988).

3.4.2.2 Impure solutions. The presence of organic or inorganic impurities in the growth solution promotes a decrease of the overall growthrate of crystals together with the change of their morphology. With regardto this latter aspect, we have previously observed that certain impuritiescan poison certain faces slowing down their growth until their completeblocking. This obviously corresponds to a decrease of the overall growthrate of the crystal. However, even if the impurity is not involved in a directinteraction with the crystal faces, it can play an important role in the crystalgrowth by modifying the solution characteristics and, in particular, itsviscosity. This is without taking into account the effect of the impurity onsucrose solubility as quoted above.We have already pointed out (see section 3.4.2.1) that the diffusion

constant k d depends on the diffusion coefficient through the relationship k d

= Dlo. The diffusion coefficient is, in turn, related to the solutionviscosity, l), through the Stokes-Einstein equation (Mullin, 1972):

D = A X (TIl)) (3.12)

where A is a constant and T is the absolute temperature. If viscosityincreases, the diffusion constant, and consequently k d , decrease.As previously reported for pure solutions (Mathlouthi and Kasprzyk,

58 SUCROSE

1984) viscosity varies with both temperature and supersaturation,decreasing if T increases and increasing if 0 increases. In particular,concerning supersaturation (0), viscosity exponentially increases at constant temperature according to the following relationship:

l] = a exp j3 (0 + 1) (3.13)

Therefore, we can infer that kd decreases if supersaturation increasesaccording to

kd = B exp - (j3 (0 + 1» (3.14)

where B is a coefficient depending on temperature and boundary layerthickness which, in turn, as previously mentioned, depends upon therelative crystal-solution rate and, consequently, the magma stirringconditions.In the presence of impurities, in particular for solutions having a low

purity quotient, the relationships between viscosity, concentration andtemperature become consequently more complicated. Many researchershave studied this problem (see the references in Mathlouthi and Kasprzyk,1984) taking into account the importance it has also from the technologicalpoint of view (see also chapter 6). Without going into such details whichcan be known through the data in the literature, we can sum up byemphasising that impurities cause a decrease of the growth rate as a whole.This decrease becomes particularly drastic for the low purity quotient ofjuices as pointed out in Figure 3.24 (Nakhmanovich and Zelikman, 1928;Van Hook, 1959; Maurandi et al., 1988).Obviously, the presence of impurities also affects the crystallisationprocess activation energy and, in particular, the diffusion process activationenergy (equation (3.9». It was shown (Maurandi et al., 1982, 1984; VanHook, 1983b) that impurities exert an effect which is analogous to the oneexerted by an increase of temperature: a concentration increase promotesan increase of the volume diffusion influence on the overall crystal growthprocess.

3.5 Crystallisation technique

All the points discussed above from the theoretical point of view have to betaken into consideration in the sugar factory where the main goal is toproduce the maximum amount of sugar to be put in the bags. Of course,theory must fit with practical situations encountered in both beet and caneprocessing.

If we consider again the sucrose solubility curve shown in Figure 3.25,we have to point out that, from the industrial point of view, we must avoidconcentrating the solution (line 1) up to the spontaneous nuclei formation.


Growth rate (91m 2 min.)8

6

4

59

2

70 75 80 85 90 95 100Purity

Figure 3.24 Sucrose crystallisation rate variation as a function of juice purity. Values of /3:(.) 1.03; (+) 1.06; (*) 1.09; and (.) 1.12.

In fact, we must avoid the creation of an uncontrolled number of seeds sothat it is impossible to predict the final crystals size. Therefore, after havingreached the zone A we add a predetermined amount of nuclei having fixedsizes. Such nuclei, obtained by grinding the sugar itself, are suspended inan organic solvent before addition to the supersaturated solution. Afterthis addition, crystals can grow either constantly remaining within the zoneA by supplying undersaturated solution (evaporation crystallisation) orthrough a real cooling process (cooling crystallisation). So described, thisoperation seems very simple; on the contrary, the process is verycomplicated taking into account the high number of variables whichgradually modify the physico-chemical parameters to be observed and thenecessity of obtaining a final crystal product having well-defined characteristics.As for physical variables, we have to consider that, during crystallisation,the following parameters vary: mother liquor purity quotient; solubility;crystallisation rate; crystals surface; supersaturation; solution viscosity;crystallisation temperature; ebullioscopic rise; crystal to solution ratio;mother liquor Brix; magma Brix.The physico-mechanical limits are determined by considering that thecrystals to mother liquor ratio cannot be increased beyond a certain valuebecause the step following crystallisation is the physical separation of

60

S/W

SUCROSE

:

temperature

Figure 3.25 Sucrose solubility curve showing the course to be followed in industrialcrystallisation.

crystals from mother liquor through centrifugation. Moreover, the timeallowed for crystallisation cannot be too long, in particular at hightemperatures, either for energetic and plant engineering reasons or inorder to maintain the solution chemical characteristics, and, in particular,its colour. As regards the final crystal characteristics to be obtained, wehave to bear in mind that, though crystallisation by itself is an effectivepurification process, further recrystallisation steps can be needed afterdissolution of the crystals so carrying out, directly or indirectly, a trueraffination process.

3.5. J Evaporation crystallisation

In Figure 3.25 the course to be followed for an evaporation crystallisationis tentatively shown from the general point of view. Under particularvacuum conditions, in order to avoid too high ebullition temperatures, thesolution concentration is increased following the line 1. Then, the suitableamount of seed is added after having reached the metastable zone.However, this point should not be too close to the nucleation area in orderto avoid false grain formation and not too close to the saturation curve so


that there is a sufficient concentration range (c - CO) to allow a relativelyhigh growth rate (see equation (3.8)).

If we know the seed average size, which can be supposed spherical andhaving a volume v, and the density d of the sucrose crystal (1.55 g/cm3) , wecan obtain the average weight (P = v X d) of each seed and then, knowingthe total amount of seeds, we can calculate the number of seeds. After theaddition of seed and the starting of crystallisation, the supersaturation hasto be maintained around the point A by eliminating at the same time waterthrough evaporation and feeding undersaturated solution. During thisprocess we observe the following.

• crystal growth so that the crystals total area A progressively increases;• solution purity quotient decrease due to the non-sugar (N) concentration increase and the consequent decrease of the SIN ratio;

• solution viscosity increase;• specific growth rate R decreases on the grounds of that described inthe section 3.4.2.2;

• ebullition temperature increase;• magma Brix increase; and• crystals to solution ratio increase.

The amount, M, of sucrose (g/min) crystallised at a certain step of thecrystallisation process under certain operation conditions of temperature,supersaturation, juice purity and stirring, can be calculated by (Maurandi,1975)

M=RXA (3.15)

where R and A are the growth rate (g/m2 min) and crystals' total area (m2)at that moment, respectively.The total area, A, can be given by A = s X n where n is the number ofcrystals previously calculated in the seeding step (by supposing that neithernew nuclei formation nor crystals dissolution occur), and s is the averagearea of each crystal. Such area can be calculated by the followingrelationship:

s = 4·12 m2/3 (3.16)

where 4.12 is the statistical shape factor (Bubnik and Kadlec, 1992) and mis the average weight of each crystal given by the ratio between the amountof sugar crystallised and the total number of crystals. Taking into accountthat during crystallisation, which involves a progressive purity quotientdecrease, the growth rate decreases and the surface increases, we canfollow the variation of M through the progressive quotient decrease. Sucha curve presents a maximum and can be utilised for calculating the growthaverage rate ofM (g/min) during the whole crystallisation process. A curvecalculated using the data processed by Maurandi (1975) is shown in Figure3.26.

62 SUCROSE

A (m2) ; R (g/m2 min) ; M (g/min)8

6

4

2

M

P. 90unty8580

OL---------'-------------l...-----------*75

Figure 3.26 Variation of the amount of crystallised sugar (g/min) as a function of the solutionpurity quotient variation during crystallisation.

The value of M can be utilised for the calculation of the crystallisationtime given by the ratio between the amount of sugar to be crystallised andthe average rate of M calculated as described above. It is obvious that theaverage value of M will be as much smaller, and then the crystallisationtime as much longer, as much lower is the purity quotient of the juice to becrystallised (low R values) and as much smaller is the crystals number. Infact, crystallised sugar being the same, the lower the crystals number, thehigher the individual size of each crystal and the lower the total area A.Coming back to the point A depicted in Figure 3.25, we have to take intoaccount that during crystallisation the saturation curve as well as themetastable zone area might vary. Of course, this fact causes a furtherdifficulty if we have to maintain certain supersaturation and crystal growthrate conditions. At this point the problem is how we can prolong thecrystallisation process by eliminating water through evaporation andadding undersaturated solution. The key is the crystals to solution ratiowhich cannot overcome certain values related to the mother liquorviscosity characteristics and magma consistency. At the time scheduled forending the crystallisation process, at first the solution feeding and thenevaporation are interrupted; the final magma is then discharged at theprefixed Brix value into the cooling crystallisers where, following line 2,crystals will further grow up to point B. In the case of low boiling, thecooling step can be prolonged for 40-50 h. Under particular conditions ofsolution viscosity (low boiling), before centrifuging magma it can be


necessary to increase slowly temperature along the line 3, alwaysremaining within the supersaturation zone up to the point C correspondingto the centrifugation step.Depending on the purity characteristics of the juice to be submitted tothe first crystallisation step as well as the plant engineering choices whichcan be done also depending on the final sugar characteristics to beobtained, the mother liquor, after the first crystallisation stage, is thensubmitted to a second or eventually a third crystallisation before producingmolasses.Figure 3.27 shows a scheme in three stages giving white sugar as the firstproduct whereas sugar of second and third products is dissolved. On theground of what was discussed above, it is quite understandable that thecrystallisation time increases moving from first to second and thirdcrystallisations (the ratios are about 1:3:9), whereas, due to the viscosityincrease of the mother liquor, the crystal to solution ratio decreases fromthe first (about 1) to the low boiling crystallisation (about 0.6).Obviously, in the cane sugar crystallisation, the basic ideas remain the

ones discussed above even if, due to the different characteristics of themother liquor, the sugar end schemes can vary. In general, cane sugarfactories produce raw sugar which is converted into white sugar in therefineries where, after affination, dissolution and purification, the sugar isrecrystallised through various crystallisation steps (Figure 3.28).

3.5.2 Cooling crystallisation

Whereas during the evaporation crystallisation most of the sugar iscrystallised at nearly a constant temperature by maintaining constantsupersaturation conditions through solvent evaporation, in cooling crystallisation a different sucrose solubility variation in function of temperature isutilised. In the case of the true cooling crystallisation, that is without waterevaporation, the maximum amount of sugar to be obtained is given by thesolubility difference between temperatures at the starting and the end ofcrystallisation. By reference to Figure 3.25, it is then necessary that thecooling range be as large as possible including the high temperatures rangewhere the solubility variation is greater. Also in this case, we need tocarefully follow the variation of the operation parameters which normallyoccur during crystallisation both for avoiding nucleation phenomena andmaintaining the crystallisation rate within the programmed conditions. Infact, the rate of cooling should be adjusted taking into account thefollowing:

• the growth kinetics tend to decrease both due to the temperaturedecrease and progressive decrease of the mother liquor purityquotient;

TH

ICK

JUIC

E

I1stCRYSTALLISATION

I I

motherliquor

II

II

ISI

raw

sug

ar

II

MELTING

I-I

2ndCRYSTALLISATION

,~

motherliquor

,

II

2n

dra

wsu

gar

II

AFFINATION

II

3rdCRYSTALLISATION

I

Fig

ure3.27Crystallisationschemeinthreestages.

WH

ITE

SU

GA

R

MO

LA

SS

ES

0\

.j::..

til c:: (") ::c o CIl t'1

refineryrawsugar

RA

WSU

GA

R

motherliquor

AFFINATION

MELTING

LOWBOILING

MO

LA

SSE

S

motherliquor

PURIFICATION

3rdCRYSTALLISATION

RE

FIN

ED

SUG

AR

CIl c:: (j '"o CIl tT1 (j '" -< CIl ~ t"" t"" Vi ;l> -l o Z

Fig

ure3.28Crystallisationschemeofacanesugarrefinery.

0\

VI

66 SUCROSE

• the crystal surface progressively increases with the growing of thecrystals; and

• viscosity is influenced by two conflicting factors: temperature decreasepromotes viscosity increase whereas the progressive decrease of theS/W ratio, due to the crystallisation process, tends to decreaseviscosity.

Also in this case we can draw a profile of the variation of the sucroseamount M crystallised in the unit time along the range of the mother liquorpurity quotient variation, that is along the cooling range. We can thencalculate the average value of M and determine the cooling time.However, we have to take into consideration that for optimising thegrowth conditions, we need to vary the cooling rate during the wholecrystallisation process.Crystallisation through cooling only, starting from traditional thick

juices, has been studied both in the laboratory (Mantovani et at., 1988) andon pilot plant (Vaccari et at., 1988). The authors started from the point thatthe soft conditions which can be maintained during cooling crystallisationcan allow crystals of commercial quality to be obtained even starting fromparticularly coloured juices.A cooling crystallisation scheme in three stages has been proposed which

is shown in Figure 3.29.Recently, the subject has been reconsidered by Schliephake et at. (1992)who have pointed out the advantages of the multiple-stage coolingcrystallisation also from the point of view of the energy saving. Recentlyagain, the cooling crystallisation principle has been applied to the directcrystallisation of raw juice such as eliminating any type of purificationprocess (Vaccari et at., 1991b,c, 1992, 1993b). The basic scheme shown inFigure 3.30 has been verified at the level of pilot plant and further studiesare in course in order to evaluate the industrial applicability of the process.The simultaneous presence of evaporation crystallisation and coolingcrystallisation can be found in the so-called continuous crystallisationsystems (Austmeyer, 1986; Schliephake et at., 1987) and in the MET plants(Maurandi et at., 1986). In this case, the main goal does not concern crystalquality but rather the crystallisation process rapidity.

3.6 Crystal quality

From the commercial point of view, sugar crystal quality is determinedboth by internal and external crystal characteristics. Whereas the internalcharacteristics depend on both the crystallisation conditions and solutionquality, external characteristics mainly depend upon the efficiency of thewashing conditions during centrifugation step and storage conditions.

TH

ICK

JUIC

E

CONCENTRATION

3rdCOOLING

CRYSTALLISATION

motherliquor

MELTING

II

1stCOOLING

CRYSTALLISAnON

V> c::motherliquor

1I

('"')

:<l0 V>

CONCENTRATION

II

tTl('"') :<l -<: V>

AFFINATION

II

I-l ;I> L

'L

'2ndCOOLING

WH

ITE

SUG

AR

CiiCRYSTALLISATION

;I> -lrawsugar

I0

motherliquor

Z

I

MO

LA

SS

ES

...I

LOWBOILING

ICONCENTRATION

Figure3.29Basicschemeofacoolingcrystallisationprocessinthreestages.

2j

RA

WJU

ICE

,"

Il

CONCENTRATION

IAFFINATION

l,

I1stCOOLING

I1stRAWSUGAR

II

CRYSTALLISATION

IMELTING

motherliquor

II

CONCENTRATION

IFILTRATION

lmotherliquor

if

I2ndCOOLING

I2ndRAWSUGAR

II

CRYSTALLISATION

ICRYSTALLISATION

",

000

VJ c: (j '"o VJ tTl

MO

LA

SSE

S

Fig

ure3.30Basicschemeofthedirectcrystallisationofrawjuice.

RE

FIN

ED

SUG

AR


3.6.1 Inclusions

The crystal internal characteristics can be conditioned by the presence ofimpurities inside the crystal lattice. These impurities can be coloured orcolourless and they can have high or low molecular weights. Taking intoaccount that the presence of such substances can be very important relatedto the crystal quality and, in particular, colour and ash content, it is clearthat such parameters have been deeply studied often yielding conflictingresults.One point supported by a number of researchers emphasises that certaincompounds, which are coloured and have a high molecular weight, can bebonded to the sucrose molecules at the different crystal faces so more orless uniformly finding a place inside the sucrose crystal. On the contrary,other authors ascribe major importance to the phenomenon of physicalcapture of mother liquor microdroplets (sometimes also macrodroplets)inside the crystal. The presence of such microdroplets (inclusions) has beenwidely studied using photography by Powers (1970). It is clear that, if suchinclusions are made by mother liquor containing impurities, and inparticular coloured ones, they affect crystal quality.The interpretation put forward by Powers mainly relates to the etch pitsformation which occur inside the crystallisers during the evaporation. Infact, due to the stirring, crystals reaching a zone at higher temperature(near heating surfaces) where the solution is undersaturated, tend todissolve so creating on their surface small or big cavities which are filledwith mother liquor. The latter becomes included when crystals, moving bystirring to a no more undersaturated zone, rapidly grow so healing theirsurface. However, recently it has been pointed out that the capture ofmother liquor can be simply caused by particular growth kineticsconditions which, by destabilising the crystal surface, can create hollowsable to capture solution (Mantovani et at., 1985a).Taking into account that the various crystal faces have different growthrates it has been proposed, and then experimentally confirmed, that themost rapid faces can include mother liquor more rapidly than the otherones independently of the crystals originating from beet or cane processing(Mantovani et at., 1985b, 1986). In particular, in the case of beet sugarcrystals, which grow more rapidly along the +b-axis direction, the pi facesare involved in the capture of mother liquor (Figure 3.31(a)). As far ascane sugar crystal is concerned, which is elongated along the c-axis asalready mentioned, it is along this direction that the phenomenonpreferentially occurs (Figure 3.31(b».The capture of mother liquour can be limited through a control of thegrowth kinetics conditions and, in particular, taking care of supersaturation.

70 SUCROSE

(a)

(b)

Figure 3.31 Coloured inclusions in crystals grown from (a) beet; and (b) cane juices.

3.6.2 Colour of the crystals

This parameter is obviously related to the previous section but we need totake into account the contribution given to the whole colour by the layer ofmother liquor surrounding the crystals. The washing carried out in thecentrifuge by water and/or vapour might minimise such a problem;however, the higher the washing the higher the amount of sugar dissolved.On the other hand, even if the washing is prolonged, the quality of sugarremains constant due to the presence of colouring matter inside the crystal


(3)

-----... re-entrant angle

(b)

Figure 3.32 Re-entrant angles in (a) twins; and (b) conglomerates.

lattice (Shore et aI., 1984). Moreover, the presence of conglomerates andtwins, irregular crystal size and mother liquor viscosity remarkably affectthe washing effect (Broughton, 1986; Van der Poel et al., 1986):In particular, the presence of re-entry angles, which can occur both intwins and conglomerates (Figure 3.32), makes difficult the elimination ofmother liquor even dissolving a high amount of sugar. In this case, we canonly avoid the formation of such crystals by adopting suitable growthconditions.Crystals having very high size heterogeneity can cause packing phenomena inside the centrifuge basket slowing down the washing water flowand creating preferential channels so that the massecuite is only partiallywashed. Also in this case the problem should be solved in advance whichmeans during the crystallisation process in order to avoid false grainformation. The mother liquor viscosity increases by moving from highpurity syrups crystallisation to low boiling. As is known, for the latter thewashing during centrifugation is scarcely useful so that an artificial magmais normally prepared by mixing sugar with a saturated solution having afixed purity. Such a magma is then again centrifuged and the crystalswashed as pointed out in Figure 3.27. Also in raw sugar refining (Figure3.28) the mother liquor layer outside the crystal is at first eliminatedthrough the preparation and centrifugation of an artificial magma.

3.7 Conclusions

From the preceding pages of this account it can be inferred how complexare the problems involved in crystallisation. Taking into account that the

72 SUCROSE

latter basically is a purification process, the study in depth of its variousaspects becomes fundamental relating with the final product quality. Onthe other hand, it needs to be borne in mind that, when all is said and done,the final sugar characteristics must fit the customer's requirements.

References

Aquilano, D., Franchini-Angela, M., Rubbo, M., et al. (1983) Growth morphology of polarcrystals: a comparison between theory and experiment in sucrose. 1. Crystal Growth, 61,369-376.Aquilano, D., Rubbo, M., Mantovani, G., et al. (1986) Equilibrium and growth forms ofsucrose crystals in the {hOI} zone. 1. Theoretical treatment of {lOI}-d form. J. CrystalGrowth, 74, 10-20.Aquilano, D., Rubbo, M., Mantovani, G., et al. (1987) Equilibrium and growth forms ofsucrose crystals in the {hOI} zone. 2 Growth kinetics of the {hOI }-d form. J. CrystalGrowth, 83, 77-83.Aquilano, D., Rubbo, M., Mantovani, G., et al. (1990) Sucrose crystal growth: theory,experiment and industrial application. In Crystallization as a Separation Process (edsMyerson, A.S. and Toyokura, K.). ACS Symposium Series, American Chemical Society,Washington, DC, USA, pp. 72-84.Austmeyer, K.E. (1986) Analysis of sugar boiling and its technical consequences, Part III.Continuous processing. Int. Sugar 1., 88, 50-55.Bedarida, F., Zefiro, L. Boccacci, P., et al. (1988) Growth of sucrose crystals and taperingeffect studied by holographic interferometry. J. Crystal Growth, 89, 395-404.Bennema, P. (1968) Surface diffusion and the growth of sucrose crystals. J. Crystal Growth,34,331-334.Broughton, N.W., Houghton, B.J. and Sisson, A. (1986) Factors affecting white sugarcolour. Part II: the sources of white sugar colour. Zuckerindustrie, 111, 1039-1046.Bruijn, J. (1966) Deterioration of sugar cane after harvesting: Investigation of thepolysaccharide formed. Int. Sugar 1., 71, 356-368.Bubnik, Z. and Kadlec, P. (1992) Sucrose crystal shape factors. Zuckerindustrie, 117, 345350.Burton, W.K., Cabrera, N. and Frank, F.e. (1951) The growth of crystals and theequilibrium structure of their surfaces. Phil. Trans. Royal Soc. London, A243, 299-358.Chernov, A.A. (1961) The spiral growth of crystals Soviet Phiysics Uspeki, 4, 116-148.Covacevich, M.T. and Richard G.N. (1974) The determination of the structure of 'dextrans'isolated from cane sugar. Proceedings Queensland Society Sugar Cane Technologists,Brisbane, Queensland, Australia, pp. 171-177.Gilmer, G.H., Ghez, R. and Cabrera, N. (1971) An analysis of combined surface andvolume diffusion processes in crystal growth. J. Crystal Growth, 8, 79-93.Hartman, P. and Perdok, W.G. (1955) On the relation between structure and morphology ofcrystals, I, II, III. Acta Crystallographica, 8, 49-52, 521-524, 525-529.Hartmann, E.M. (1974) The calcium saccharate process. Sugar Technol. Rev., 2, (3), 213252.Irmie, F.K.E. and Tilbury, R.H. (1972) Polysaccharides in sugar cane and its products.

Sugar Technol. Rev., 1,291-361.Kamoda, M., Onda, F., Ito, H. et al. (1968) Formation of needle-shaped sugar crystals.

Proceedings 13th Congress of ISSCT, Taiwan, Elsevier, Amsterdam, pp. 362-273.Kossel, W. (1934) Energy aspects of surface processes. Annalen der Physik, 21, 457-480.Kuijvenhoven, L.J. (1983) Aspects of Continuous Sucrose Crystallization (WTDH No. 156).Delft University of Technology, Delft, The Netherlands.Leonard, G.L. and Richard, G.N. (1969) Polysaccharides as casual agents in production ofelongated sucrose crystals from cane juices. Int. Sugar J., 71, 263-267.Mantovani, G., Gilli, G. and Fagioli, F. (1967) Influence of the crystalline structure ofnonsugars on habitus variations of sucrose. Zucker, 12, 663-{j68.


Mantovani, G., Vaccari, G., Accorsi, C.A., et al. (1983) Twin growth of sucrose crystals. J.Crystal Growth, 62, 595-602.Mantovani, G., Vaccari, G., Sgualdino, G. et al. (1985a) Colorants in sucrose crystal.

Industria Saccarifera Italiana, 78, 7-14.Mantovani, G., Vaccari, G., Sgualdino, G. et al. (1985b) Sucrose crystal colour as a functionof some industrial crystallisation parameters. Industria Saccarifera Italiana, 78, 79-86.Mantovani, G., Vaccari, G., Sgualdino, G. et al. (1986) Colouring matter inclusions insucrose crystals. Proceedings XIX Congress of I.S.S.C. T., 1986, vol. II, pp. 663-669;Industria Saccarifera Ttaliana, 79, 99-107.Mantovani, G., Vaccari, G. and Sgualdino, G. (1988) Unconventional cooling crystallization,

Zuckerindustrie, 113, 137-140.Mantovani, G., Vaccari, G., Marignetti, N. et al. (1993) Relationship between crystalelongation and the presence of some impurities in cane sugar processing. Sugar Journal, 56(1), 16-19,25.Mathlouthi, M. (1981) X-ray diffraction study of the molecular association in aqueoussolutions of D-fructose, D-glucose, and sucrose. Carbohydr. Res., 91,113-123.Mathlouthi, M. and Kasprzyk, P. (1984) Viscosity of sugar solutions. Sugar Technol. Rev.,11, 209-257.Maurandi, V. (1975) Theory and practice of sugar boiling. Sucrerie Beige, 94, 105-120, 141160, 163-169, 189-196.Maurandi, V. (1981) Nucleation kinetics in supersaturated solutions-a review. Zucker

industrie, 106, 993-998.Maurandi, V. (1989) Mass transfer in pure sucrose solutions. Zuckerindustrie, 114,976-979.Maurandi, V., Mantovani, G. and Vaccari, G. (1982) Influence of non-sugar on sucrosecrystallisation in impure beet syrups. Sucrerie Beige, 101,243-253.Maurandi, V., Mantovani, G. and Vaccari, G. (1984) Sucrose crystal growth activationenergies from pure and impure solutions. Zuckerindustrie, 109, 734-739.Maurandi, V., Paganelli, B. and Rossi, A. (1986) Sucrose crystal growth after the vacuumpans. Zuckerindustrie, 111,55-58.Maurandi, V., Mantovani, G. and Vaccari, G. (1988) Kinetic studies on low grade boiling.

Sugar Technol. Rev., 14, 29-118.Morel du Boil, P.G. (1991) The role of oligosaccharides in crystal elongation. Proceedings of

the South African Sugar Technologists' Association, Mount Edgecombe, Natal, SouthAfrica, pp. 171-178.Morel du Boil, P.G. (1992) Theanderose-a contributor to c-axis elongation in cane sugarprocessing. Int. Sugar J., 94,90-94.Mullin, J.W. (1972) Crystallization. Butterworths, London, UK.Nakhmanovich M.T. and Zelikman T.F. (1928) Methods of determining the speed ofcrystallization. Sakharnaya Prornyshlennost, 6, 32-53.Pot, A. (1980) Industrial sucrose crystallization. PhD thesis, Delft University of Technology,Delft, The Netherlands.Powers, H.E.C. (1970) Sucrose crystal: inclusion and structure. Sugar Technol. Rev., 1,85190.

Schliephake, D. (1963) Structure of aqueous sucrose solutions. Zucker, 16,523-527.Schliephake, K., Austmeyer, K.E. and Hempelmann, R. (1987) Crystallization of higherpurity massecuites by cooling. Zuckerindustrie, 112, 269-273.

Schliephake, D., Ekelhof, B. and Sittel, G. (1992) Prospects for sugar technology.2 Technical prospects for crystallization. Zuckerindustrie, 117,549-556.Shore, M., Broughton, N.W., Dutton, J.V., et al. (1984) Factors affecting white sugarcolour. Sugar Technol. Rev., 12, 1-99.Smythe, B.M. (1967) Sucrose crystal growth. TTl. The relative growth rates of faces and theireffect on sucrose crystal shape, Aust. J. Chern., 20, 1115-1131.Smythe, B.M. (1971) Sucrose crystal growth. Sugar Technol. Rev., 1, 191-231.Sutherland, D.N. (1968) Dextran and crystal elongation. Int. Sugar 1.,70,355-358.Sutherland, D.N. and Paton, N. (1969) Dextran and crystal elongation: further experiments.

Int. SugarJ., 71,131-135.Vaccari, G., Mantovani, G., Sgualdino, G., et al. (1986) The raffinose effect on sucrosemorphology and kinetics. Sugar Technol. Rev., 13, 133-178.

74 SUCROSE

Vaccari, G., Mantovani, G. and Sgualdino, G. (1988) The CCC system of continous coolingcrystallization. International Sugar Journal, 90, 213-220.Vaccari, G., Mantovani, G., Morel du Boil, P.G., et al. (1990) Colour inclusion and habitmodification in cane sugar crystals. Zuckerindustrie, 115, 1040-1044.

Vaccari, G., Mantovani, G., Sgualdino, G., et al. (1991a) Effect of glucose and fructose oncane sugar crystal morphology. Zuckerindustrie, 116,610-613.Vaccari, G., Mantovani, G. and Sgualdino, G. (1991b) Cooling crystallisation of raw juice.

Int. Sugar J., 93, 71-75.Vaccari, G., Mantovani, G., Sgualdino, G., et al. (1991c) Cooling crystallization ofraw juice:further testing and achievements. Int. Sugar J., 93, 213-216.Vaccari, G., Mantovani, G., Sgualdino, G., et al. (1992) Cooling crystallisation of raw juice:pilot plant testing. Zuckerindustrie, 117,724-728.

Vaccari, G., Mantovani, G., Sgualdino, G., et al. (1993a) Cooling crystallisation of raw juice:determination of sucrose solubility. Zuckerindustrie, 118, 780-782.Vaccari, G., Mantovani, G., Sgualdino, G., et al. (1993b) Cooling crystallization of raw juice:results of the second year's pilot plant experimentation. Int. Sugar J., 95, 381-390.Van der Poel, P.W., Struijs, J.L.M., Vriends, J.P.M., et al. (1986) Colour formation andcolour elimination from crystals. Zuckerindustrie, 111, 1032-1038.Van Hook, A. (1959) Kinetic of crystallisation - Growth of crystals. In Principles of Sugar

Technology (ed. Honig, P.) Vol. II, Elsevier, Amsterdam, The Netherlands.Van Hook, A. (1981a) Growth of sucrose crystals: a review. Sugar Technol. Rev., 8, 41-79.Van Hook, A. (1981b) Habit modifications of sucrose crystals. J. Am. Soc. Sugar Beet

Techno/., 21, 130-135.Van Hook, A. (1983a) Habit modifications of sucrose crystals. J. Am. Soc. Sugar Beet

Technol., 22, 60-72.Van Hook, A. (1983b) Mechanism of growth of sucrose crystals - a reassessment.

Proceedings 17th General Assembly CITS, Copenhagen, CTTS Tienen, Belgium, pp. 615631.Van Hook, A. (1988) A note on the hemimorphism of sucrose. Industria Saccarifera Italiana,81,1-4.Vavrinecz, G. (1965) Atlas of Sugar Crystals. Verlag Dr. A. Bartens, Berlin, Germany.Vavrinecz, G. (1978-79). The formation and composition of beet molasses. Sugar Technol.

Rev., 6, 117-306.

4 Amorphous sugarM. MATHLOUTHI

4.1 Introduction

The crystallinity of a substance depends on the method of measurement(Weyl and Marboe, 1962). When the crystals are too small theircrystallinity is hidden from the X-rays. It may sometimes be detected byelectron or neutron diffraction. However, the microcrystalline state is anunsteady state. It is just a non-equilibrium stage testifying the continuitybetween the crystalline and amorphous states. Amorphous means withoutshape. It refers to a visual observation. For the information on noncrystallinity to be complete, the technique revealing this will be discussed,i.e. X-ray amorphous.What characterizes amorphous solids in contrast to the liquids is theirdeparture from equilibrium. They have very long relaxation times and mayremain almost indefinitely in a metastable state. Amorphous is notsynonymous with 'absence of order'. There is always a certain structurewhich depends on the way the glassy or amorphous solid was prepared.While only one crystal structure of sucrose is known from X-ray (Hanson etal., 1973) or neutron diffraction (Brown and Levy, 1973), whatever theconditions of crystallization the information on the amorphous state maynot be obtained from a single method, and almost each technique ofpreparation leads to a different type of organization.Amorphous sugar can result from dry-milling, the quenching of the melt,the rapid drying of solutions (spray-drying), the freeze-concentration ofaqueous solutions and their freeze-drying as well as from the boiling or theextrusion of highly concentrated solutions followed by cooling. It may bedescribed as glassy, vitreous, rubbery or concentrated amorphous system.The characterization of its structure can be derived from the interpretationof the broad one or two bands diffuse X-ray diffractogram, the vibrationalspectra, the viscosity and the thermal behaviour manifested by temperatures and concentrations typical of glass transition, recrystallization,collapse and melting. Most of these physical and structural properties ofamorphous sugar are moisture dependent.Water content and water activity together with temperature are

determining factors of the mobility and reorganization of sucrose molecules


76 SUCROSE

in the amorphous matrix. These factors are at the ongm of stickiness,caking and recrystallization of amorphous sugar. Such an instability isdifficult to predict even though critical values of water activity, viscosity orglass transition temperatures were proposed (White and Cakebread, 1966)for structure transitions of different products containing amorphoussugars. The stability of amorphous sugar in foods and pharmaceuticals is ofpractical and economical importance. It affects quality attributes of foodssuch as texture, aroma retention and shelf-life. In particular, the stabilityof crystalline sugar, the duration of curing and the prevention of cakingrequire molecular interactions in the concentrated film of syrup around thecrystals to be controlled (Bressan and Mathlouthi, 1994).Determination of crystallinity and molecular structure of amorphoussugar are considered here. The interaction of non-crystalline sucrose withwater and the thermal properties including glass transition and recrystallization are also reviewed. The applications of amorphous sugar, undesirable(caking, stickiness) or desirable (fondant, fudges) are also described.

4.2 Structure of amorphous sucrose

Depending on the method of preparation, amorphous sucrose may beconsidered as a supercooled liquid (in the case of quenched melt, drymilled or extruded sucrose) or a supersaturated solution, sometimes calledconcentrated amorphous solution (CAS) for freeze-concentrated and, tosome extent, freeze-dried samples (Bellows and King, 1973). Thermodynamic conditions of crystallization of such media are filled up whereasthe kinetics requirements are far from being obtained. Both supercooledand supersaturated non-equilibrium states correspond to non-crystallinesucrose. They differ in the amount of moisture, generally higher forconcentrated amorphous solution, and in the degree of order of suchcomplex structures.

4.2.1 Order and disorder in concentrated amorphous solution

Highly supersaturated solutions of sucrose are relatively easy to obtaineither by freeze-concentration or by boiling. Discussion of the solutesolvent interactions in these metastable systems may be informative on thephase transitions that may occur under varying temperature conditions. Itwas remarked that a certain continuity exists between amorphous andcrystalline states. Likewise, there is a linkage between the structures ofdilute and concentrated solutions including concentrated amorphoussolutions and freeze-dried sucrose which is just a freeze-concentratedsample from which water (ice) was removed by sublimation.Thus, it is of relevance to recall the solute-solvent interactions in

AMORPHOUS SUGAR 77

aqueous solutions of sucrose at different concentrations. Several experimental evidences (Mathlouthi, 1980; Hooft et al., 1993; Perez et al., 1993)showed that the structure of the sucrose molecule in dilute aqueoussolution is not that of a hard sphere with two intramolecular H-bonds. It ishighly hydrated, lacking intramolecular hydrogen bonds and inducing along-range order of the bulk water sometimes called 'structure makereffect' (Walrafen, 1966). As the concentration is increased, the flexibility(Mathlouthi, 1981) around the glycosidic bond allows the establishment ofan intramolecular bond at intermediate concentrations and two intramolecular bonds at high concentrations. The limits of dilute, intermediate andhigh concentrations may roughly be situated at one-third of saturation,two-thirds and saturation concentration, respectively, i.e. 22.3, 44.6 and66.8% (w/w). Below a concentration of 22.3% (w/w) no inter- orintramolecular interactions are found. The organization of sucrosemolecules is comparable to that of the quenched melt as may be derivedfrom Fourier-transform infrared (FT-IR) spectra (Mathlouthi et al., 1986).A better resolution of IR bands in the 1200-1800 cm- I region was observedfor freeze-dried and concentrated solutions and assigned to a higher degreeof order in these media (see Figure 4.1). The absence of order in dilutesolutions and their comparison with the glassy molten state is known froma classical work on crystallization (Tammann, 1926).Most of the work on physico-chemical properties of sucrose in water hasdealt with dilute solutions, or for nucleation and growth of crystals, withsupersaturated solutions. Factor analysis of the FT-IR spectra of sucrosesolutions of 10, 23, 33, 40, 50, 66 and 70% (w/w) permitted differentiationof at least two species of sucrose molecules, one with intramolecular bondsand a relatively higher order comparable with the crystal and the other oflower order comparable with the quenched melt (Mathlouthi et al., 1986).The concentration 40% (w/w) sucrose in water was found either fromviscosity measurements (Schneider et al., 1963) or NMR results(Richardson et al., 1987) to correspond to a transition from solvated(sucrose-water association) to the associated state (sucrose-sucroseassociation). The change of behaviour of sucrose solutions at around 40%(w/w) was also demonstrated (Mathlouthi et al., 1980) from the ratio ofintensities of Raman bands characteristic of the sugar (6(CHz» on theone hand and of water (b(HzO)) on the other. The evolution of this ratioin terms of mass concentration displays a discontinuity at 30-40%.Prior to crystallization, the existence of swarms of sucrose molecules,called protonuclei, in the apparently homogeneous solution was demonstrated (Tikhomiroff, 1965) by means of various techniques (density,viscosity, thin-layer chromatography and interferometry). Thus, its seemsthat the degree of order (sucrose-sucrose association) increases as theconcentration increases. Depending on the thermal history of theconcentrated solution (freezing or boiling) and the rate of cooling or

78 SUCROSE

V ,k. I I

1400 1100 800 Cm-; 1400 800 Cm-'

Figure 4.1 FT-TR spectra of sucrose solutions with (a) 22, (b) 66, and (c) 70% (w/w)concentration and (a) vitreous, (b) lyophilized and (c) crystalline, sucrose.

heating, the molecular interactions in concentrated sucrose solutions aredifferent and so is the structure of the amorphous product.The freezing of a sucrose solution may be considered as a way ofremoving water from the solution. The rate of cooling influences the sizeand orientation of ice crystals. A rapid deep-freezing may increase theamount of supercooled unfrozen water. The overall structure of the freezedried product depends on the freezing step (Flink, 1983). The concentrationin concentrated amorphous solution (CAS) obtained by freezing is nothomogeneous (Luyet and Rasmussen, 1968). It is generally high (above80%) situated in the metastable region (supersaturation >1.35). No

AMORPHOUS SUGAR 79

nucleation or crystal growth of sucrose can occur and the hydrates found inconcentrated solutions at low temperatures are only short-lived agglomerates of sucrose and water molecules locally associated with a certain order(Young and Jones, 1949; Steinbach, 1977). These associates are morecomparable to the swarms described by Tikhomirof than to stablecrystalline species as is known for a-D-glucose, monohydrate or othercarbohydrate hydrates.The structure of the concentrated frozen solution is responsible foraccidents like the collapse during the freeze-drying of sucrose. Themobility of water and sucrose molecules when temperature is raised maylead to a dense glazed film in the surface which hinders the mass transferand affects the porosity of lyophilized sugar. These temperature-dependenttransformations will be discussed further below.Apart from freezing, concentrated amorphous solution may be obtainedby boiling. The glassy state is, therefore, favoured by a rapid cooling whichcontributes to increase the viscosity and by the presence of other molecularspecies like glucose, fructose, maltodextrins, or gelatine as is the case forconfectionery products. The structure of such a medium may be consideredas a supersaturated viscous solution. Supersaturation in boiled sweets isextremely high, above the range where nucleation and growth of sucrosecrystals are possible. Nevertheless, small crystals are found (Lewis, 1990)in confectionery products either because of a poor formulation or duringthe storage. The size of crystals plays a major role in the textural propertiesand quality of the product (Hartel and Shastry, 1991). However, theinstability of the grained confections is not only due to supersaturation. Itis also dependent on viscosity, temperature, agitation and the compositionin non-sucrose solids. The effect of each of these parameters on thestructure of the product especially as concerns the rate of nucleation andgrowth of crystals was only approached qualitatively (Hartel and Shastry,1991) except for a detailed study of the rates of progress of sucrosenucleation in sugar glasses with varied amounts of dextrose, fructose andwater (Herrington and Branfield, 1984). Even in these glasses, thedetermining factor seems to be the availability of water which allows or notthe transition from viscous to viscoelastic or rigid behaviour. A viscousphase was always present between the glassy and crystalline layers(Herrington and Branfield, 1984). The 'threshold' water content andpossibility of crystallization was found (Herrington and Branfield, 1984) todepend on the type of additive. In particular, a very slow rate ofcrystallization was found with fructose, which may be due to the increasedsolubility of sucrose in sucrose-fructose-water systems (Kelly, 1954), or tothe higher affinity of fructose for water. Although water is only present atvery low amounts (2-6%) in sugar glasses, its interactions with thedifferent solutes and its mobility are at the origin of the structure and thestability of the product.

80 SUCROSE

4.2.2 Structure of freeze-dried and spray-dried sucrose

Because of the difficulty of obtaining a reproducible glassy structure bymelting or evaporation of concentrated solution, freeze-dried sucrose wasvery often taken as a reference of the amorphous state. However,amorphous sugar is difficult to produce by lyophilization. A very rapidcooling in liquid nitrogen of individualized drops of sugar solution runningfrom a burette is necessary to obtain a non-crystalline sample by freezedrying (Carstensen and Van Scoik, 1990). The lyophilized drops were 'xray amorphous' with a large specific surface area (3.9 m2/g) whichincreased their reactivity with water vapour. As already mentioned, thestructure of freeze-dried sucrose depends on the method of measurement.X-ray diffraction permitted (Mathlouthi, 1973) differentiation of lyophilized sucrose from quenched melt glassy sucrose and to show theirdeparture from the crystalline state (see Figure 4.2). However, it is known(Legrand, 1967) that diffuse X-ray diffractograms may be obtained with a

(a)

10'0

(b)

'0

, , , I " 0015 14 13 1.2 1t 10 9 8

Figure 4.2 X-ray diffractograms of (a) lyophilized, (b) crystalline and (c) vitreous sucrose.

AMORPHOUS SUGAR 81

small number of small crystalline particles. Application of radiation withshorter wavelengths than X-rays, i.e. electron diffraction, was found(Mathlouthi, 1973) to be more sensitive to the detection of crystallinity infreeze-dried sucrose. Although lyophilized sugar is mainly amorphous, thepresence in its lamellar non-crystalline structure (see Figure 4.3) of zonescontaining microcrystals was ascertained by scanning electron microscopyand electron diffraction (Figure 4.4). The microcrystalline state of freezedried sucrose is responsible for its instability (Mathlouthi, 1973, 1975).Indeed its recrystallization in presence of water vapour or after annealingin the freeze-drier has as its origin the presence of microcrystals in theamorphous bulk. The morphology and structure of divided solids werefound to depend on the temperature and pressure of dehydration

Figure 4.3 Scanning electron microphotograph of lyophilized amorphous lamellar sucrose.

Figure 4.4 Microcrystals found in lyophilized sucrose and their electron diffractogram.

82 SUCROSE

(Rouquerol, 1964). Maintaining the freeze-dried sucrose in the freezedrier and applying heat was found to increase its crystallinity. Thecrystalline sample obtained after an annealing may be more or less porousdepending on the heat applied.Amorphous sugar can also be obtained by spray-drying. Its crystallizationwas obtained (Palmer et al., 1956) either by exposure to humid air (RH =32%) or more rapidly by exposure to humid air and seeding with 5% finecrystals. Rapid air-drying of sucrose generally takes place at such high ratesof water removal that the solution quickly reaches a high concentration.Under such conditions, the molecular association needed for crystallinestructure is not obtained and the product remains in the glassy state (Flink,1983). Spray-drying and drum-drying mainly concern lactose in whey ormilk and generally lead to amorphous lactose except if a pre-crystallizationstep was applied.

4.2.3 Dry-milled and extruded sucrose

Milling of crystalline sugar was found (Weichert, 1976) to produceamorphous sugar. The amorphization was attributed (Weichert, 1976) toan increase of temperature up to 2000°C for less than a microsecond, whichprovokes the melting of a layer of sucrose molecules at the fracture area.The structure of dry-milled sucrose was derived directly from themicrophotographs or indirectly from water vapour sorption and the heat ofsolution (Roth, 1976). However, as the origin of the transformation orderdisorder during the milling is the melting of superficial layers in the crystal,it may be of relevance to control the crystallinity of the glassy moltensugar. This was achieved using X-ray (Mathlouthi, 1973), FT-IR and CPMAS (cross-polarization magic angle spinning) 13C NMR spectra(Mathlouthi et al., 1986) of the amorphous quenched melt sample ascompared to lyophilized sugar on the one hand and to the crystallinesucrose on the other. The spectrum of the quenched melt was found(Mathlouthi et al., 1986) to be more comparable to the dilute solution thanto the solid freeze-dried sample. The effect of heat on the melted sugar wasto break inter- and intramolecular hydrogen bonds. While the structure ofcrystalline sugar is stabilized by hydrogen bonds, giving narrow resonancelines in the 13C NMR spectrum, excessive line-broadening in the spectrumof freeze-dried and molten sucrose makes it difficult to distinguish them onthe basis of this technique (see Figure 4.5). The use of differential thermalanalysis seems more appropriate for the differentiation of the two solidamorphous states. As may be seen in Figure 4.6, the thermogram of thefreeze-dried sample involves a peak of recrystallization at 110°C and amelting endotherm at 180°C like the crystalline sugar, beside the glasstransition at about 60°C. The glassy molten sample shows an importantglass transition endotherm at 60°C followed by a devitrification and a

AMORPHOUS SUGAR

(a)

83

(b)

120 100 80 60

(c)

Figure 4.5 CP-MAS l3C NMR spectra of (a) vitreous, (b) lyophilized and (c) crystallinesucrose.

staggered endothermic transformation which probably has as its origin anincrease of fluidity rather than a clear melting. Such a mobility of sugarmolecules depends on temperature, moisture and composition of the glass.Hence, for the dry-milled sugar, the few molecules thick layer which meltsby crystal breakage clumps as a glassy sugar which recrystallizes in thepresence of humid air.The traces of moisture are very likely at the origin of the discrepanciesbetween glass transitions temperatures found for small carbohydrates(Levine and Slade, 1988; Finegold et al., 1989; Ollett and Parker, 1990). Inboiled sweets and extruded sugar (Tabouret, 1977), the temperatures are

84

20

(a)

(c)

60

SUCROSE

100 140 180 'C

Figure 4.6 Differential thermograms of (a) crystalline, (b) lyophilized and (c) vitreoussucrose.

as high as 13o-200°C in the extruder under pressure. In these conditionsboth the dissolution and the melting may be put forward to explain theformation of the glassy state. Indeed, about 2-7% water remains in theextruded sugar. The dissolution of sucrose in presence of dextrose and10% water at 130°C was reproduced (Tabouret, 1977) in the laboratoryand given as an argument for the decrystallization of sucrose in a glucosesyrup (DE = 36-39) after extrusion. Therefore, the border betweensupercooled liquid (molten glass) and extremely supersaturated solutions isill defined as concerns the nature of the amorphous sugar when traces ofwater (2-7%) are present.

4.3 Thermal properties of amorphous sugar

The change in mechanical properties of the glassy sugar are determined byuse of different techniques amid which differential scanning calorimetryseems the more appropriate to account for the mobility of moleculesduring the glass transition. This structure modification invariably occursbelow the freezing point and is accompanied by an important change inphysical properties like viscosity, specific heat, entropy and specificvolume. The question whether glass transition is a kinetic phenomenon,

AMORPHOUS SUGAR 85

mainly dependent on viscosity and relaxation times or a thermodynamicprocess better accounted for by thermodynamic properties like entropies,heat contents and volumes, was not clearly answered in the early studies(Kauzman, 1948). It was elucidated later by Gibbs (1971) who demonstratedthat the flattening of the E-T, H-T, S-T and V-T curves derives from thefact that the number of configurations available in the glass transitionregion is small, which gives small entropies in polymeric glasses. Then,both the kinetic and thermodynamic approaches are valid and aresupported by the dearth of configurations in the activated state corresponding to the non equilibrium glassy structure. Thermal changes of noncrystalline sucrose were investigated either for CAS or for the amorphoussolid.

4.3.1 Behaviour of concentrated amorphous soiutions

Aqueous carbohydrate solutions are known to exhibit a non-equilibriumfreezing behaviour. The sucrose-water system was taken (MacKenzie,1977) as a model system for biological media and carefully studied duringthe cooling and rewarming by use of differential thermal analysis.Depending on the concentration of sucrose solution and the rate ofwarming, different thermal events are observed (MacKenzie, 1977).Among these are glass transition, antemelting, incipient melting, devitrification (recrystallization of ice) and an equilibrium melting, respectively,characterized by the temperatures Tg , Tam, Tim, Td and Tm' Thesupplemented phase diagram of sucrose-water system was established(MacKenzie, 1977). The crystallization of sucrose was never observed inthese studies nor in ours (Mathlouthi, 1973). The eutectic point is just anextrapolated intersection of sucrose solubility curve and ice melting curveand has no real experimental existence. Investigating the thermalbehaviour of the sucrose-water system reveals the existence of a sucroseconcentration above which ice cannot be formed (MacKenzie, 1977;Maltini, 1977). This maximally freeze-concentrated solution was found(Maltini, 1977) at 67-72% of soluble solids. It was recently determined tobe 81.2% sucrose (C'g) and shown by a novel experimental approach tooccur at a temperature (T'g) of -40°C (Ablett et ai., 1992).The discrepancies between the values of C'g' the maximally freezeconcentrated sucrose solution, were discussed (Ablett et ai., 1992; Sladeand Levine, 1994). The method of calculation of C'g based on thedetermination of the amount of ice from the melting peak area using thelatent heat of ice at O°C seems to underestimate this amount and also C'g'A consensus was proposed by Karel et ai. (1994) for a value of 80 ± 5% forall small carbohydrates. For the author, it seems logical to relate resultsfrom different origins. A wealth of data (Allen et ai., 1979; Akhumov,1975; Suggett, 1976; Franks, 1977; Culp, 1981) exists concerning the

86 SUCROSE

hydration numbers of sugars. Therefore, taking a hydration number, n =5, for sucrose and supposing these five molecules are unfrozen leads to aconcentration of 79.17% which might be C'g for sucrose. A hydrationnumber of three for glucose leads to a value of C'g= 76.92%. These valueslie within the range of 80 ± 5% proposed by Karel et al. (1993). At lowtemperatures all thermal events concern water rather than sugar. It isbeyond the scope of this chapter to discuss the structure of water insupercooled systems. As concerns sucrose, it may be noticed that whereasthe removal of water by heat allows very high concentrations (above 95%to be reached), the cooling has a limit in disassociating water from sugar.This limit is C'g = 80%, which corresponds to n = 5. Moreover, althoughthe thermodynamic conditions of crystallization are reached (the solutionis supersaturated), sucrose does not crystallize. This is due to the kinetichindrance with viscosity reaching the critical threshold of 1012 Pa s and tothe impossibility of establishing direct sucrose-sucrose bonds, the sugarmolecules being fully hydrated. However, if we consider, the sucrosemolecule hydrated with five molecules of water as the real solute species,the maximally freeze concentration (79.17% (w/w)) becomes a saturationsolution, with regard to the hydrated solute, a pseudo-equilibrium ratherthan a non-equilibrium. In these conditions, one can derive from previousphase diagrams (MacKenzie, 1977; Roos and Karel, 1991; Ablett et al.,1992) a new one that may be reconciled with classical (Van Hook, 1961)solubility and supersolubility curves delimiting the undersaturation,metastable and labile zones of crystallization of sucrose (see Figure 4.7).Our hypothesis of a pseudo-equilibrium of hydration reached by freezeconcentration was evoked by Roos and Karel (Roos and Karel, 1991) as anunknown alternative to the kinetic inhibition of crystallization. Theparallelism between solubility and supersolubility curves and the almostconstant value for supersaturation (0 = (C/Csat ) = 1.35) along thesupersolubility curve together with the correspondence of the glasstransition of 'amorphous sugar' Tg = 60°C with the supersolubility of thefully hydrated sucrose are further arguments for the existence of thepseudo-equilibrium state. This state of organization of sucrose-watersolutions constitutes the limit for a supersaturated or supercooled systemto cross before reaching the equilibrium conditions.

4.3.2 Glass transition of amorphous solid sucrose

The continuity between the behaviour of concentrated amorphous solutionsand non-crystalline solid sucrose may be observed from the continuity inTg-supersolubility curve (see Figure 4.7). The glass transition temperatureof amorphous sugar was found (Roos and Karel, 1990) to depend on themoisture content. It decreases on increasing water content. Water plays itsrole of plasticizer. A small amount of moisture (1%) provokes a decrease

60

20

Toe

AMORPHOUS SUGAR

UNDERSATURATION

,,,,,,\

\\

\\\

\\

60

Zone

LABILE

Zone

87

Figure 4.7 Phase diagram of sucrose-water binary system showing (- - - -) the ice-meltingcurve; (E) the hypothetical eutectic point and the maximally concentrated amorphous

solution with C'g = 80% and T'g = -40°C.

of about 20ce of the Tg value of the freeze-dried sample (Roos and Karel,1990). Increased amounts of moisture decrease the glass transitiontemperature below room temperature and provoke stickiness, collapse andrecrystallization. The glass transition temperature seems to be a goodindicator of the transitions that may occur in an amorphous sugar, likecollapse, stickiness and caking during storage.

88 SUCROSE

(4.1)

Application of equation (4.1) originally reported by Gordon and Taylor(1952) to fit the prediction of glass transition temperature, Tg , in binarypolymer mixtures, was found (Roos, 1993) suitable for water-smallcarbohydrates mixtures:

WI Tg , + kW2 Tg,

WI + kW2

where WI and W2 are weight fractions of solute and water, respectively, Tg"

and Tg, the Tg values for solute and water.With Tg, = -135°e and using the experimental values of Tg , Roos(1993) obtained a linear expression of k:

k = 0.0293 Tg + 3.61 (4.2)

The value of k = 4.7 ± 0.2 obtained (Roos, 1993) for sucrose solutions(65-80%) permitted (Roos and Karel, 1991) the establishment of a statediagram for amorphous sucrose with experimental results fitting with thosecalculated.The glass transition (Tg ) and melting (Tm) temperatures were oftenrelated, despite the fact that glass transformation is a kinetic process andmelting a thermodynamic process. An empirical relation, called the 'twothirds' rule (Tg = 2/3 Tm), was verified (Sanditov, 1976) from the relativechange in volume around glass transition supposed constant and thedifference between the coefficients of thermal expansion at Tg . Values ofTglTm = 0.69-0.70 were obtained for mannose, glucose and galactose andinterpreted as a sign of a constant change in specific volume during theglass-crystal transition. This 'two-thirds' rule is more or less in agreementwith the Tm/Tg values given in more recent work (Roos, 1993) to beconstant around 1.40. In particular, for sucrose a Tm/Tg ratio = 1.38 and amelting point at 187°e (460 K) yields a Tg of 60.3°e which is in agreementwith experimental results.The change in specific volume and other temperature dependentmodifications show that the liquid-glass transformation is a relaxationprocess. The jump in specific heat, dielectric constant and other coefficientsat Tg tend to compare the glass transition with a second-order phasetransition. Another noticeable change at the glass point is that of viscosity.A large increase in viscosity at Tg which reaches 10

12 Pa s was found byextrapolation of experimental data (Parks et al., 1934). Values of Tg werealso derived from empirical relations such as the Vogel-Tamman-Fulcher(VTF) (Angell et al., 1982) or Williams-Landel-Ferry (WLF) (Williams etal., 1955) equations. Their validity and departure from DSe experimentalresults were discussed (Parks et al., 1934; Slade and Levine, 1994). Thestructural reasons for the large increase in viscosity at Tg for associatedliquids like water-sugar systems may be explained by use of the theory of

AMORPHOUS SUGAR 89

rate processes (Glasstone et at., 1941) applied to liquids with free volumes.Indeed, for a molecule to flow, it is necessary that it jumps from one holeto another. For this it has to acquire a certain activation energy. Such anactivation energy is much higher for associated liquids than for otheranalogous substances because, in addition to the normal work required tomake a hole, it is necessary to break the hydrogen bonds surrounding themolecule. Increasing the temperature above Tg provokes a rapid decreasein viscosity which lead to the collapse, stickiness and recrystallization ofsucrose especially in presence of traces of water.

4.4 Rearrangement in amorphous sugar

Glass transition, collapse and sticky point temperatures were reported(Levine and Slade, 1986; Roos and Karel, 1990) to be closely related. Thedissimilarities of collapse and glass transition as proposed by To and Flink(1978a) were criticized by Levine and Slade (1986, 1989). The irreversibilityof the collapse of a freeze-dried product was considered as a macroscopicevent having a morphological effect (loss of porosity) while at molecularlevel the phenomenon is reversible. Although a certain continuity existsbetween concentrated amorphous solution and amorphous dry sugar thereversibility at Tg might concern water rather than sugar. Both the flexibilityaround the glycosidic bond and diffusion leading to a possible homogenousnucleation above Tg and in dry amorphous sucrose plead in favour of anirreversibility at a molecular level of the rearrangement (collapse ,stickiness).The structure modifications in non-crystalline sucrose mainly depend onmoisture, temperature and the composition of the medium.

4.4.1 Moisture dependence of the amorphous-<.:rystaliine transformation

The early studies (Makower and Dye, 1956; Palmer et at., 1956) ofamorphous sucrose submitted to relative humidities ranging from 4.6 to33.6% at 25°C showed that above 20% RH amorphous sucrose recrystallizes (in a reasonable time) after adsorption and desorption of water. Acharacteristic kinetics of water vapour adsorption and desorption wasobserved and since this earlier work the observation was repeated for otheroligosaccharides (Guilbot and Drapron, 1969), for dry-milled sucrose(Roth, 1976) and lyophilized sucrose (Mathlouthi, 1973). We report ourresults in Figure 4.8. They show an increase in sample weight exposed to 38and 44% RH followed by a decrease and a stabilization. Analysis of thesesamples by X-rays confirms the recrystallization after desorption of watervapour. The quenched melt glassy sucrose exposed to the same atmospheres remained amorphous and its adsorption of water was much lower

90 SUCROSE

than that of the lyophilized sugar (Mathlouthi, 1973). The rapid recrystallization of freeze-dried sucrose after sorption and desorption of water wasattributed to its microcrystalline structure (Mathlouthi, 1973). Particularattention was paid to the nucleation kinetics during the disorder-{)rdertransformation of amorphous sucrose (Van Scoik and Carstensen, 1990).A retarding effect on nucleation was observed with additives like gelatine,raffinose, invert sugar, fructose and other carbohydrates. These additiveshave in common their hydrophilicity and their binding of water whichprevents availability of this solvent and lowers its plasticizing effect.Increasing the temperature from 23 to 40°C for an amorphous sampleexposed to 33% RH decreases the lag time (time to reach maximumadsorption) from 240 to 10 h. The amorphous material was supposed tochange from anhydrous amorphate to hydrous amorphate form in a firststep upon adsorption of water. This allows molecules to change fromimproper orientations and distances to proper orientations and improperdistances. Then, the sucrose molecules achieve their bringing together(proper intermolecular distances) before crystallization (Carstensen andScoik, 1990).

water intake %

6

4

2

o

-2 hours

o 20 40 60 80 100 120

Figure 4.8 Kinetics of water adsorption and desorption by freeze-dried sucrose submitted todifferent relative humidities at 20°C. Relative humidities: (.) 20%, (D) 27%, (T) 38% and

(e) 44%.

AMORPHOUS SUGAR 91

It seems, however, difficult by the use only of visual, microphotographyand X-ray techniques (Van Scoik and Carstensen, 1990) to ascertain theclear differentiation of nucleation from crystallization. As already mentioned, the degree of order depends on the method of observation. 'X-rayamorphous' sucrose may be crystalline to electron diffraction. The 'X-raycrystallinity' of amorphous sucrose mixed with waxy maize starch andsubmitted to varied water activity values was found (Chinacoti andSteinberg, 1986) to be extremely slow or inhibited when a wet mixture wassubmitted to desorption between 0.33 and 0.84 aw . This might be due tothe mobility of water (Chinacoti, 1993) which depends on the physicalstate of sucrose and a certain competition for water between ingredients.In presence of other polysaccharidic components, the recrystallization ofamorphous sucrose, in humidified freeze-dried model systems, was alwaysdelayed when compared to that of pure sucrose (Iglesias and Chirife,1978).

4.4.2 Temperature dependence of amorphous sugar transformation

Increasing the temperature of non-crystalline sucrose provokes an increasein mobility of molecules especially in presence of water. Such a mobilityleads to the collapse behaviour either in concentrated amorphous solutionsat temperatures above Tg ' or in freeze-dried and spray-dried sugar athigher temperatures. Rewarming of the CAS was used as an annealingtreatment to favour the recrystallization of ice. If all crystallizable water isnot transformed into ice, it will recrystallize during the freeze-drying toform very fine crystals which melt below the melting point of the sampleand provoke a 'collapse' during the lyophilization (Flink, 1983). Such acollapse yields a non-porous freeze-dried sugar which is partially crystallineand partially sticky. This miscarried freeze-drying is comparable to otherdefects that happen during the dehydration and storage of amorphoussugar.The collapse of freeze-dried sugar was found (To and Flink, 1978b) tooccur over a range of temperature (20-40°C), which corresponds to theobserved (Roos and Karel, 1990) Tg when moisture was increased from 0to 1%. The reported values of Tg for amorphous sugar varied from 72 to40°C probably because the unavoidable traces of humidity in the samples.This especially occurs because during the drying process, a rapid removalof water from the surface originates in the formation of a glassy dried layerover a viscous concentrated solution. Mobility of water in this solutionduring the storage provokes stickiness and caking of the sugar.Annealing of CAS at a temperature slightly above the glass transition,

called homogeneous nucleation (Tn) or crystallization (Tc) or antemelting(Tam) and equal to -32°C proved to be efficient in preventing the collapseduring lyophilization. Likewise, a thermal treatment of 'annealing' of the

92 SUCROSE

dry amorphous sugar was applied (Mathlouthi, 1973) at different temperatures above 60°C, the Tg value of freeze-dried sucrose. Such a treatmentwas achieved in the freeze-drier for 3-10 h. Analysis of crystallinity of thefreeze-dried annealed samples was made by X-ray diffraction. The results

Figure 4.9 X-ray diffractograms of freeze-dried sucrose submitted to different treatments ofannealing: (a) 3 h at 60°C; (b) 8 h at 80°C; (c) 3 h at 95°C; and (d) 2 h at 120°C.

AMORPHOUS SUGAR 93

are reported in Figure 4.9. They show that with thermal treatment at 60°C theTg temperature did not improve crystallinity of the sample, whereas byheating for 3 h at 120°C the recrystallization temperature yielded a samplecompletely crystalline. The samples treated at 80 and 95°C show anincreased degree of crystallinity as temperature is increased. The samekind of information was obtained from the FT-IR spectra (in the region2800-3800 cm- I ) of freeze-dried sucrose submitted to temperaturesbetween 60 and 120°C (see Figure 4.10).Recrystallization was also obtained after sorption and desorption of

water. Comparison of the X-ray diffractograms (see Figure 4.11) of thelyophilized samples recrystallized after sorption of water at 20°C andannealing at 120°C show slight differences in the intensities of peaks atcharacteristic angles (12-13°8, 9-10°8 and 6-7°8). Comparison of therate of dissolution of the two samples showed that the annealed sucrosewhich kept a certain porosity dissolves more rapidly than the samplerecrystallized after sorption of water vapour which was a little bit sticky(Mathlouthi, 1973).

3400 3200 2800 Cm-1

Figure 4.10 Ff-IR spectra of freeze-dried sucroses submitted 1 h to annealing at differenttemperatures: (a) 60°C; (b) 80°C; (c) 100°C; and (d) 120°C.

94 SUCROSE

14

12

(3)

JJ10

J' ,W 8J'-" '.j

6

(b)

2

4

\

o-rj----,--r,---,.....---r,--,---rj--..--...,,--.-----., ()

15 13 II 9 7 5

Figure 4.11 Comparison of the X-ray diffractograms of freeze-dried sucrose recrystallizedafter annealing at 120°C (a) and after sorption of water vapour (b).

4.5 Practical importance of amorphous sugar

The non-crystalline state of the organization of sucrose in foods andconfectionery may have important consequences as concerns their stabilityand shelf-life. It may also alter the taste, the texture and aroma retention inthese products. Adsorption of water and increase in temperature aregenerally at the origin of phase transitions that may occur in foods

AMORPHOUS SUGAR 95

containing amorphous sugar. The presence of other compounds, especiallypolyols and carbohydrates that may share water with sucrose, constitutesan additional factor of instability.

4.5.1 Structure modification of sugar and the shelf-life of food products

The transformation of amorphous sugar may be either a softening, giving aviscous solution in which major defects are stickiness, collapse and a loss offlavour, or a recrystallization which can be detrimental to the quality ofboiled sweets, ice creams or porous dehydrated foods. The viscosity at thesticky point of a powder composed of maItodextrin, sucrose and fructosewas measured (Wallack and Judson King, 1988) and found equal to thepredicted (according to the Frenkel model) value of 106_108 Pa s. It wasconcluded that increasing the particle size decreases the surface powderenergy, hence the predicted viscosity and can prevent stickiness (Wallackand Judson King, 1988). Collapse and stickiness are related. They generallyoccur at glass transition temperature Tg• The effect of viscosity on thestability of fondants was discussed (Cakebread, 1972). It is needed in thiscase that an adequate proportion of corn syrup be added to sucrosesolution, so that viscosity is increased enough to maintain fine grains insuspension for a long time. Shelf-life of confectionery also needs thatequilibrium relative humidity be maintained below 80% to ensure safetyfrom risk of microbial spoilage. Change in structure, such as a recrystallization of sugar, may lead to a loss of flavour, a decrease in viscosity and anincrease in water activity which may cause the degradation of the product.Fudges are other sweets appreciated for their smoothness and fineness ofgrain. They have in common with chocolate that the grain should havea size not larger than 25 !-tm. It was noticed (Niediek, 1991) that the higherthe amount of amorphous sugar in chocolate, the higher its water contentand its aroma retention. Probably because its large specific area,amorphous sugar adsorbs large amounts of volatiles, whether they arehydrophilic or hydrophobic (Niediek, 1988). Aroma retention was foundto increase when the pressure is increased. For amorphous sucrose andamorphous lactose the heat of adsorption of volatile compounds is greaterthan that of evaporation. However, the loss of structure of amorphoussugar provokes a loss of volatile whether this collapse is caused by moistureuptake or heating (Flink, 1983). Flavour enhancement by sucrose inselected food systems is detailed in Chapter 10 of this book.

4.5.2 Agglomeration, caking and the stability of crystalline sugar

Amorphous sugar is in a metastable state. It is very hygroscopic. Itstendency to adsorb water even at low water activities (below 0.20 aw )

predisposes it to caking, agglomeration and the loss of free-flowing ability.

96 SUCROSE

This high reactivity with water is manifested in the exothermal heat ofsolution (Van Hook, 1981) (-8.8 cal/g for glassy slightly caramelized orlyophilized sucrose) while the heat of solution (Culp, 1946) of crystallinesucrose is 4.73 cal/g. Depending on the kind of amorphous sugar, the heatof solution may vary. The 'cotton candy' amorphous sugar was exothermalto the extent of 11 cal/g but this fell rapidly upon exposing to air (VanHook, 1981). The flowability and tendency to stick and cake concernsa number of powders like instantanized soluble coffee, dehydrated fruitjuices and powdered sucrose. This behaviour depends on wateractivity, size of particles and temperature. These parameters determinethe packaging and shelf-life of the powders. Considering the scopeof this book, particular interest is given to the storage of powderedsucrose and sugar lumps.Either for crystalline sugar or for the cubes, a more or less long period of

curing is necessary to prevent such accidents as the caking of powderedsugar in the bags or the silo or the sticking of cubes to the carton package.This period may last as long as 5 days for packaged cubes being ventilatedwith air at 25-30°C. The origin of caking and instability of crystalline sugaris very likely the non-equilibrium behaviour of the film surrounding thecrystals. Investigation of this behaviour resulted (Bressan and Mathlouthi,1994) in the proposition of a thermal treatment of sugar at 60°C prior tomoistening, moulding and drying of the cubes and ventilating them with airat ambient temperature. What complicates the problem of the stability ofcrystalline sugar is a tendency in the factories to dry at high temperatures(above 80°C) with large rates of air. This treatment rapidly yields a glassylayer at the surface of the crystal under which remains a layer ofsupersaturated solution in contact with the crystal. Recrystallization of thesupersaturated solution together with the dissolution of the glass implydiffusion phenomena of water and sucrose which may be accelerated inpresence of humidity in the air or impurities in the amorphous layer. Suchrearrangements always lead to equilibrium, which means that thecrystallizable sugar recrystallizes by incorporation to the existing crystalsand a saturated solution remains at a level of moisture of about 0.02%(w/w). However, reaching this equilibrium requires that the heat ofcrystallization and the water released be evacuated. If a suitable aerationwith fresh air having a dew point far below storage temperature isachieved, then the risks of caking, agglomeration and stickiness arereduced. As was already noticed, the high reactivity with water especiallyfor microcrystalline sucrose may lead to a spontaneous recrystallizationand the agglomeration of the powders. Caking may be inhibited by additionof very finely divided highly hygroscopic powders (Irani et ai., 1959) suchas starch or tricalcium phosphate which adsorb water and prevent sucroserecrystallization. The effectiveness of anticaking agents depends on thesize of particles and on relative humidity.

4.6 Conclusion

AMORPHOUS SUGAR 97

Amorphous sugar is present as a thin layer at the surface of crystallinesugar. It may also be produced by melting, spray-drying, freeze-drying orconcentration of solutions either by heating or cooling. Understanding ofthe modifications of its structure especially around the glass transitiontemperature requires that the interactions between water and sucrose beelucidated. A physico-chemical parameter which seems to be determiningas concerns the mobility of molecules and the rearrangements of structureis viscosity. Likewise, hydration water, specific area and the heat ofsolution are informative on the reactivity of amorphous sugar with water.Such a reactivity is manifested by a characteristic sorption and desorptionbehaviour when water activity is increased. The preponderance of water,even when it is present as traces, suggests the continuity betweenamorphous solid and aqueous solution. Amorphous sugar is present innumber of food products. Its structure is a determining factor for suchproperties as the texture, flavour and shelf-life of these products.Powdered sugar involves at its surf:lce an amorphous thin layer (glassy and/or supersaturated solution). Controlling the kinetic and thermodynamicconditions of recrystallization in the thin layer during the drying andcooling processes may be favourable to the stability and reduction of theduration of curing.

References

Ablett, S., Izzard, M.J. and Lillford, P.J. (1992) Differential scanning calorimetric study offrozen sucrose and glycerol solutions. J. Chem. Soc. Faraday Trans., 88, 789-794.Akhumov, E.I. (1975) Hydration of sucrose in solution. Zhurnal Prikkladnoi Khimii, 48,458-460.Allen, A.T., Wood, R.M. and MacDonald, M.P. (1974) Molecular association in thesucrose-water system. Sugar Techno/. Rev., 2, 165-180.Angell, C.A., Stell, R.C. and Sichina, W. (1982) Viscosity-temperature function forsorbitol from combined viscosity and differential scanning calorimetry studies. 1. Phys.Chem., 86, 1540-1542.Bellows, R.J. and King, c.J. (1973) Product collapse during freeze-drying of liquid foods.

AIChE Symp. Ser., 132.69, 33-41.Bressan, C. and Mathlouthi, M. (1994) Thermodynamic activity of water and sucrose and thestability of crystalline sugar. Zuckerindustrie, 8, 652-658.Brown, G.M. and Levy, H.A. (1973) Sucrose: Further determination of the structure ofsucrose based on neutron-diffraction data. Acta CrystaUogr., B29, 790-97.Cakebread, S.H. (1972) Grained confectionery, viscosity: shelf-life: effects of manipulation.

Confect. Prod., 3,132-149.Carstensen, J.T. and Van Scoik, K.G. (1990) Amorphous to crystalline transformation ofsucrose. Parm. Res., 7, 1278-1281.Chinacota, P. (1993) Water mobility and its relation to functionality of sucrose-containingfood systems. Food Technol., 1, 134-140.Chinacota, P. and Steinberg, M.P. (1986) Crystallinity of sucrose by X-ray diffraction as

98 SUCROSE

influenced by absorption versus desorption, waxy maize starch content, and water activity.J. Food. Sci., 51, 456-459, 463.Culp, E.J. (1946) Heat of solution: Sucrose in water, Sugar Yazucar, 41, 44-46.Culp, E.J. (1981) The magic of small numbers, molecules of water per molecule of sucrose.

Sugar J., 44, 7-10.Finegold, L., Franks, F. and Hatley, R.H.M. (1989) Glass rubber transitions and heatscapacities of binary sugar blends. J. Chern. Soc., Faraday Trans. 1,85,2945-2951.Flink, J.M. (1983) Structure and structure transitions in dried carbohydrate materials. In

Physical Properties of Foods (eds Peleg, M. and Bagley, E.B.). AVI, Westport, CT, USA,pp. 473-521.Franks, F. (1979) Solvent interactions and the solution behaviour of carbohydrates. In

Polysaccharides in Foods (eds Blanshard, J.M.V. and Mitchell, J.R.). Butterworths,London, UK, pp. 33-50.Gibbs, J.H. (1971) Nature of the glass transition and vitreous stante. In Modern Aspects of

the Vitreous State (eds MacKenzie, J.D.). Butterworths, London, UK, pp. 152-187.Glasstone, S., Laidler, K.J. and Eyring, H. (1941) Viscosity and diffusion. In The Theory of

Rate Processes. McGraw Hill, New York, USA, pp. 477-549.Gordon, M. and Taylor, J.S. (1952) Ideal copolymers and the second-order transitions ofsynthetic rubbers, I. Non-crystalline copolymers. 1. Appl. Chern., 2,493-500.Guilbot, A. and Drapron, R. (1969) Evolution en fonction de I'humidite relative de I'etatd'organisation et de I'affinite pour I'eau, de divers oligosides cryodeshydration. Bull. Inst.Int. Froid, Annexe, 9, 191-196.Hanson, J.e., Sieker, L.e. and Jensen, L.H. (1973) Sucrose: X-ray refinement andcomparison with neutron diffraction. Acta Crystallogr., 829, 797-808.Hartel, R.W. and Shastry, A.V. (1991) Sugar crystallization in food products. Crit. Rev.

Food Sci. Nutr., 1,49-112.Herrington, T.M. and Branfield, A.e. (1984) Physico-chemical studies on sugar glasses. I.Rates of crystallization. J. Food Technol., 19, 409-425.Hooft, R.W.W., Kanters, J.A. and Kroon, J. (1993) Molecular mechanics and dynamics.Calculations on sucrose and some derived artificial sweeteners. J. Food Technol., 19, 1119.

Iglesias, H.A. and Chirife, J. (1978) Delayed crystallization of amorphous sucrose inhumidified freeze dried model systems. 1. Food Technol., 13, 137-144.Irani, R.R., Callis, C.F. and Liu, T. (1959) Flow conditioning and anticaking agents. Ind.

Engng Chern., 51, 1285-1288.Karel, M., Roos, Y. and Briera, M.P. (1994) Effect of glass transition on processing andstorage. In Glassy State in Food (eds Blanshard, J.M.V. and Lillford, P.J.). NottinghamUniversity Press, Loughborough, UK, (in press).Kauzman, W. (1948) The nature of the glassy state and the behaviour of liquids at lowtemperatures. Chern. Rev., 43, 219-256.Kelly, F.H.C. (1954) Phase equilibria in sugar solutions. 1. Appl. Chern., 4, 405-409.Legrand, e. (1967) La Radiocristallographie. Presses Universitaires de France, Paris, France.Levine, H. and Slade, L. (1986) A polymer physicochemical approach to the study ofcommercial starch hydrolysis products (SHPs). Carbohydr. Polyrn., 6, 213-244.Levine, H. and Slade, L. (1988) Thermomechanical properties of small carbohydrate-waterglasses and rubbers: kinetically metastable systems at subzero temperatures. 1. Chern. Soc.Faraday Trans. 1,84,2619-2633.Levine, H. and Slade, L. (1989) Interpreting the behaviour of low-moisture foods. In Water

and Food Quality (ed. Hardman, T.M.). Elsevier Applied Science, London, UK, pp. 71134.

Lewis, D.F. (1990) Structure of sugar confectionery. In Sugar Confectionery Manufacture(ed. Jackson, E.B.). Blackie, Glasgow, UK, pp. 331-350.Luyet, B. and Rasmussen, D. (1968) Study by differential thermal analysis of thetemperatures of instability of rapidly cooled solutions of glycerol, ethylene, glycol, sucroseand glucose. Biodynarnica, 10, 167-192.MacKenzie, A.P. (1977) Non-equilibrium freezing behaviour of aqueous systems. Phil.

Trans. R. Soc. London, 8278, 167-189.Makower, B. and Dye, W.B. (1956) Equilibrium moisture content and crystallization ofamorphous sucrose and glucose. Agric. Food Chern., 4, 72-77.

AMORPHOUS SUGAR 99

Maltini, E. (1977) Studies on the physical changes in frozen aqueous solutions by DSC andmicroscopic observations. In Freezing, Frozen Storage and Freeze-Drying. IIF-IIRCommissions CI-C2, Karlsruhe, Germany, pp. 43-51.Mathlouthi, M. (1973) Contribution 11 l'etude de I'etat physique du saccharose aprescryodessication. Dr Eng Thesis, University of Dijon, France.Mathlouthi, M. (1975) Etude de I'etat physique du saccharose apres Iyophilisation. Ind. Alim.

Agric. (Paris), 92, 1279-1285.Mathlouthi, M. (1980) Contribution 11 l'etude physicochimique comparative du o-fructose, duo-glucose et du saccharose en solution aqueuse. Dr Sci Thesis, University of Dijon, France.Mathlouthi, M.(1981) X-ray diffraction study of the molecular association in aqueoussolutions of o-fructose, o-glucose and sucrose. Carbohydr. Res., 91, 113-123.Mathlouthi, M., Luu, e., Meffroy-Biget, A.M. and Luu, D.V. (1980) Laser-Raman studyof solute-solvent interactions in aqueous solutions of o-fructose, o-glucose and sucrose.Carbohydr. Res., 81, 213-223.Mathouthi, M., Cholli, A. L. and Koenig, J.L. (1986) Spectroscopic study of the structure ofsucrose in the amorphous state and in aqueous solution. Carbohydr. Res., 147, 1-9.Niediek, E.A. (1988) Effect of processing on the physical state and aroma sorptionproperties of carbohydrates. Food. Technol., 11,81-84.Niediek, E.A. (1991) Amorphous sugar, its formation and effect on chocolate quality.

Manufact. Confect., 6, 91-95.Ollett, A.L. and Parker, R. (1990) The viscosity of supercooled fructose and its glasstransition temperature. J. Text. Stud., 21, 355-362.Palmer, K.J., Dye, W.B. and Black, D. (1956) X-ray diffractometer and microscopicinvestigation of crystallization of amorphous sucrose. Agric. Food Chern., 4, 77-81.Parks, G.S., Barton, L.E., Spaght, M.E. and Richardson, J.W. (1934) The viscosity ofundercooled glucose. Physics, 5, 193-199.

Perez, S., Meyer, e., Imberty, A. and French, A. (1993) Molecular featur~s andconformational flexibility of sucrose. In Sweet Taste Chemoreception (eds Mathlouthi, M.,Kanters, J.A. and Birch, G.G.). Elsevier Aplied Science, London, UK, pp. 55-73.

Richardson, S.J., Baianu, I.e. and Steinberg, M.P. (1987) Mobility of water in sucrosesolutions determined by deuterium and oxygen-17 nuclear magnetic resonance measurements. J. Food Sci., 52, 806-809.Roos, Y. (1993) Melting and glass transitions of low molecular weight carbohydrates.

Carbohydr. Res., 238, 39-48.Roos, Y. and Karel, M. (1990) Differential scanning calorimetry study of phase transitionsaffecting quality of dehydrated materials. Biotechnol. Prog., 6, 159-163.Roos, Y. and Karel, M. (1991) Amorphous state and delayed ice formation in sucrosesolutions. Int. J. Food Sci. Technol., 26, 553-566.Roth, D. (1976) Amorphisierung bei der zeikleinerung und rekristallisation als ursachen deragglomeration von puderzucker und verfahren zuderen vermeidung. Thesis, Karlsruhe,Germany.Rouquerol, J. (1964) Etude de I'eau de constitution de plusieurs oxydes 11 grande surfacespecifique. Dr Sci Thesis, Paris, France.Sanditov, D.S. (1976) The liquid-glass transition. Russian J. Phys. Chern., 50, 1001-1004.Schneider, F., Schliephake, D. and Klimmek, A. (1963) Uber die Viskositat von reinenSaccharoselossungen. Zucker, 17,465-473.Slade, L. and Levine, H. (1994) Glass transitions and water-food structure interactions. In

Advances in Food and Nutrition Research (ed. Kinsella, J.E.). Academic Press, SanDiego, CA, USA, (in press).Steinbach, G. (1977) Phase equilibria in frozen solutions from refractometric measurementsof freezing curves. In Freezing, Frozen Storage and Freeze-Drying. IIF-IIR CommissionsCI-C2, Karlsruhe, Germany, pp. 53-66.Suggett, A. (1976) Molecular motion and interactions in aqueous carbohydrate solutions. III.A combined nuclear magnetic and dielectric-relaxation strategy. J. Sol. Chern., 5, 33-46.

Tabouret, T. (1977) Contribution 11 l'etude fondamentale de la decristallisation par extrusion.Industr. Alim. Agric. (Paris), 94, 383-392.

Tammann, G. (1926) The States of Aggregation, Van Nostrand, New York, USA.Tikhomiroff, N. (1965) Association moleculaire au cours de la periode de precristallisationdes solutions aqueuses sursaturees de saccharose. Indust. Alim. Agric. (Paris), 82,755-772.

100 SUCROSE

To, E.C. and Flink, J.M. (1978a) 'Collapse' a structural transitIOn in freeze-driedcarbohydrates II. Effect of solute composition. 1. Food Technol., 13,567-581.

To, E.C. and Flink, J.M. (1978b) 'Collapse' a structural transition in freeze-driedcarbohydrates, III. Perequisite of recrystallization. 1. Food Technol., 13,583-594.Van Hook, A. (1961) Crystallization, Theory and Practice. Reinhold, New York, USA.Van Hook, A. (1981) Growth of sucrose crystals, a review. Sugar Technol. Rev., 8, 41.Van Scoik, K.G. and Carstensen, J.T. (1990) Nucleation phenomena in amorphous sucrosesystems. Int. J. Pharm., 58, 185-196.Wallack, D.A. and Judson King, C. (1988) Sticking and agglomeration of hygroscopicamorphous carbohydrate and food powders. Biotechnol. Prog., 4, 31-35.Walrafen, G.E. (1966) Raman spectra studies of the effect of urea and sucrose on waterstructure. 1. Chern. Phys., 44, 3726-3726.Weichert, R. (1976) Untersuchungen zur temperatur an der bruchspitze. Thesis, Karlsruhe,Germany.Weyl, W.A. and Marboe, E.C. (1962) The Constitution of Glasses, A Dynamic Interpretation,

Vol. 1: Fundamentals of the Structure Inorganic Liquids and Solids. Interscience Publishers,New York, USA, pp. 118-127.White, G.W. and Cakebread, S.H. (1966) The glassy state in certain sugar containingproducts. J. Food Technol., I, 73-82.Williams, M.L., Landel, R.F. and Ferry, J.D. (1955) The temperature dependence ofrelaxation mechanisms in amorphous polymers and other glass forming liquids. J. Am.Chern. Soc., 77, 3701-3706.Young, F.E. and Jones, F.T. (1949) Sucrose hydrates, the sucrose-water phase diagram. 1.

Phys. Chern., 53, 1334-1350.

5 Sucrose solubilityZ. BUBNIK and P. KADLEC

5.1 Introduction

Sucrose solubility means the concentration of sucrose in a saturatedsolution which is in equilibrium with sucrose in the solid state. Solubility ofsucrose in water is of fundamental importance in defining the supersaturation, or driving force of sucrose crystal growth. Solubility of sucrose inmixtures of water with different organic solvents has important uses insome branches of the chemical and pharmaceutical industries, in analytics,etc. It is especially the case for ethanol, methanol, propyleneglycol,glycerol, acetone and pyridine.A molecule of sucrose has eight hydroxyl groups, three hydrophilicoxygen atoms (bound in a circle) and 14 hydrogen atoms. This enables theformation of hydrogen bonds with water molecules, hydration of sucrosemolecules and therefore easy dissolution of sucrose in water. In nonaqueous solvents, sucrose solubility is significantly lower than in water andsucrose does not dissolve in non-polar solvents. Sucrose shows muchhigher values of solubility in ammonia, dimethylsulphoxide, aminoethanoland methylamine. Lower values exist for sulphur dioxide, formic andacetic acid, dimethylformamide, pyridine, glycol, methanol, ethanol anddioxan.Solubility of sucrose is influenced by temperature and by the amount andtype of other dissolved molecules (impurities, non-sugars). Only a few ofthe impurities do not affect sucrose solubility or decrease it. Mostimpurities increase solubility. This is especially true of non-sugars whichremain in sugar juices after their purification by standard procedures ofsugar technology.

5.2 Expression of concentration and composition of sucrose solutions

Concentration of sugar solutions is expressed in different ways dependingon current application. Among the usual ways we can count weight percent(formerly Brix), mass and mole fraction, weight, sucrose to water ratio,molality, molar concentration and partial density. The cited quantities aredefined for pure and impure solutions in the following text.


5 Sucrose solubilityZ. BUBNIK and P. KADLEC

5.1 Introduction

Sucrose solubility means the concentration of sucrose in a saturatedsolution which is in equilibrium with sucrose in the solid state. Solubility ofsucrose in water is of fundamental importance in defining the supersaturation, or driving force of sucrose crystal growth. Solubility of sucrose inmixtures of water with different organic solvents has important uses insome branches of the chemical and pharmaceutical industries, in analytics,etc. It is especially the case for ethanol, methanol, propyleneglycol,glycerol, acetone and pyridine.

A molecule of sucrose has eight hydroxyl groups, three hydrophilicoxygen atoms (bound in a circle) and 14 hydrogen atoms. This enables theformation of hydrogen bonds with water molecules, hydration of sucrosemolecules and therefore easy dissolution of sucrose in water. In nonaqueous solvents, sucrose solubility is significantly lower than in water andsucrose does not dissolve in non-polar solvents. Sucrose shows muchhigher values of solubility in ammonia, dimethylsulphoxide, aminoethanoland methylamine. Lower values exist for sulphur dioxide, formic andacetic acid, dimethylformamide, pyridine, glycol, methanol, ethanol anddioxan.

Solubility of sucrose is influenced by temperature and by the amount andtype of other dissolved molecules (impurities, non-sugars). Only a few ofthe impurities do not affect sucrose solubility or decrease it. Mostimpurities increase solubility. This is especially true of non-sugars whichremain in sugar juices after their purification by standard procedures ofsugar technology.

5.2 Expression of concentration and composition of sucrose solutions

Concentration of sugar solutions is expressed in different ways dependingon current application. Among the usual ways we can count weight percent(formerly Brix), mass and mole fraction, weight, sucrose to water ratio,molality, molar concentration and partial density. The cited quantities aredefined for pure and impure solutions in the following text.

......0 N

Tab

le5.

1Sucroseconcentrationexpressedindifferentunits(at20°C)

Dry

SW

WS

CCm

CM

XM

substance,

(gsucroseper

(gwaterper

(gsucroseper

(molsucrose

(molsucrose

Molarfraction

DS(%)

gwater)

gsucrose)

litresolution)

perkgwater)

perkgsolution)

(molpermol)

00

-0

00

05

0.0526

19.00

50.89

0.1538

0.1487

0.00276

100.1111

9.00

103.80

0.3246

0.3033

0.00581

150.1765

5.67

158.90

0.5155

0.4641

0.00919

200.2500

4.00

216.20

0.7304

0.6316

0.01298

250.3333

3.00

275.90

0.9738

0.8060

0.01723

'"30

0.4286

2.33

338.10

1.252

0.9878

0.02204

c::35

0.5385

1.86

403.00

1.573

1.177

0.02754

(j '"

400.6667

1.50

470.60

1.948

1.375

0.03387

0 '"45

0.8182

1.22

541.20

2.390

1.581

0.04125

trl

501.0000

1.00

614.80

2.921

1.796

0.04996

551.2220

0.82

691.70

3.571

2.021

0.06039

601.5000

0.67

772.00

4.382

2.255

0.07311

651.8570

0.54

855.80

5.425

2.500

0.08897

702.3330

0.43

943.20

6.817

2.756

0.1093

753.0000

0.33

1034.50

8.764

3.022

0.1363

804.0000

0.25

1129.80

11.69

3.301

0.1738

855.6670

0.18

1229.10

16.55

3.591

0.2296

909.0000

0.11

1332.50

26.29

3.893

0.3212

9519.0000

0.05

1440.10

55.51

4.207

0.4998

100

-0.00

1551.90

-4.534

1.ססoo

......

0 N

Tab

le5.

1S

ucro

seco

ncen

trat

ion

expr

esse

din

diff

eren

tun

its

(at

20°C

)

Dry

SW

WS

CC

mC

MX

M

subs

tanc

e,(g

sucr

ose

per

(gw

ater

per

(gsu

cros

ep

er(m

olsu

cros

e(m

olsu

cros

eM

olar

frac

tion

DS

(%)

gw

ater

)g

sucr

ose)

litr

eso

luti

on)

per

kgw

ater

)p

erkg

solu

tion

)(m

olp

erm

ol)

00

-0

00

05

0.05

2619

.00

50.8

90.

1538

0.14

870.

0027

610

0.11

119.

0010

3.80

0.32

460.

3033

0.00

581

150.

1765

5.67

158.

900.

5155

0.46

410.

0091

920

0.25

004.

0021

6.20

0.73

040.

6316

0.01

298

250.

3333

3.00

275.

900.

9738

0.80

600.

0172

3'"

300.

4286

2.33

338.

101.

252

0.98

780.

0220

4c::

350.

5385

1.86

403.

001.

573

1.17

70.

0275

4(j '"

400.

6667

1.50

470.

601.

948

1.37

50.

0338

70 '"

450.

8182

1.22

541.

202.

390

1.58

10.

0412

5tr

l

501.

0000

1.00

614.

802.

921

1.79

60.

0499

655

1.22

200.

8269

1.70

3.57

12.

021

0.06

039

601.

5000

0.67

772.

004.

382

2.25

50.

0731

165

1.85

700.

5485

5.80

5.42

52.

500

0.08

897

702.

3330

0.43

943.

206.

817

2.75

60.

1093

753.

0000

0.33

1034

.50

8.76

43.

022

0.13

6380

4.00

000.

2511

29.8

011

.69

3.30

10.

1738

855.

6670

0.18

1229

.10

16.5

53.

591

0.22

9690

9.00

000.

1113

32.5

026

.29

3.89

30.

3212

9519

.000

00.

0514

40.1

055

.51

4.20

70.

4998

100

-0.

0015

51.9

0-

4.53

41.

0000

SUCROSE SOLUBILITY 103

The values of the concentration of pure sucrose solutions are expressedusing the suitable equations and reported in Table 5.1. As seen in Table 5.1,the evolution of the values deriving from the various expressions issignificantly different, especially for concentrated solutions. Usage ofweight percent deforms the correct idea about the ratio between thequantity of moles of sucrose and water. Application of weight percent isadvantageous for technical practice, e.g. for balance calculations. This unitis not always suitable for expressing physico-chemical parameters whichgenerally are simple functions of molality or mole fractions.Activity of the dissolved species should be substituted to concentration

in the relations used for accurate physico-chemical calculations. Activity,a, is expressed as a product of concentration Cm (mol sucrose per kg water)and activity coefficient fm:

a = fm X Cm (5.1)

(5.3)

According to eli and Valter (1967) it is possible to calculate activitycoefficient, fm, from the following relationship:

log fm = 0.06105 X Cm + 0.01203 X C;, - 0.002343X C~ - 0.000124 X em (5.2)

Equation (5.2) is valid for Cm < 5.8 mol/kg (i.e. 66.7%).

5.2.1 Relationships for expression of concentration of sucrose in pureand impure water solutions

These relationships are listed and defined below:

(a) Sucrose content is the weight percent of sucrose in the solution, S(%).

(b) Mass fraction X w (g sucrose per g solution)

SX =-w 100

(c) Sucrose to water ratio, SW (g sucrose per g water)For pure solutions:

SSW=-

100 - S

For impure solutions:

SSW=--

100 - DS

where DS is dry substance (%)

(5.4)

(5.5)


The values of the concentration of pure sucrose solutions are expressedusing the suitable equations and reported in Table 5.1. As seen in Table 5.1,the evolution of the values deriving from the various expressions issignificantly different, especially for concentrated solutions. Usage ofweight percent deforms the correct idea about the ratio between thequantity of moles of sucrose and water. Application of weight percent isadvantageous for technical practice, e.g. for balance calculations. This unitis not always suitable for expressing physico-chemical parameters whichgenerally are simple functions of molality or mole fractions.

Activity of the dissolved species should be substituted to concentrationin the relations used for accurate physico-chemical calculations. Activity,a, is expressed as a product of concentration Cm (mol sucrose per kg water)and activity coefficient fm:

a = fm X Cm (5.1)

(5.3)

According to eli and Valter (1967) it is possible to calculate activitycoefficient, fm, from the following relationship:

log fm = 0.06105 X Cm + 0.01203 X C;, - 0.002343X C~ - 0.000124 X em (5.2)

Equation (5.2) is valid for Cm < 5.8 mol/kg (i.e. 66.7%).

5.2.1 Relationships for expression of concentration of sucrose in pureand impure water solutions

These relationships are listed and defined below:

(a) Sucrose content is the weight percent of sucrose in the solution, S(%).

(b) Mass fraction X w (g sucrose per g solution)

SX =-

w 100

(c) Sucrose to water ratio, SW (g sucrose per g water)For pure solutions:

SSW=-

100 - S


SSW=--

100 - DS

where DS is dry substance (%)

(5.4)

(5.5)

104 SUCROSE

(d) Water to sucrose ratio, WS (g water per g sucrose)For pure solutions:

100-SWS=-

SFor impure solutions:

100-DSWS=--

S

(e) Concentration, c (g sucrose per litre solution), partial density

S X Qc=---

100

(5.6)

(5.7)

(5.8)

where Q is the specific gravity (kg/m3) of the sucrose solution.(f) Molality, Cm (mol sucrose per kg water)

For pure solutions:

1000 X SCm = (100 _ S) X M

s(5.9)


1000 X SC =m (100 - DS) X M s

where M s is sucrose molecular weight (342.30 g/moI)(g) Molar concentration CM (mol sucrose per litre solution)

S X Q

100 X M s

(5.10)

(5.11)

(h) Molar fraction, X M

For pure solutions: mol sucrose per (mol sucrose + mol water)S

(100 - S) X M s

S 1+-

(100 - S) X Ms Mw

(5.12)

For impure solutions: mol sucrose per (mol sucrose + mol water+ mol non-sucrose)

S

(100 - DS) X MsX M =----------------

S 1 DS - S+--+-----

(100 - DS) X Ms Mw (100 - DS) X MNS

(5.13)

104 SUCROSE

(d) Water to sucrose ratio, WS (g water per g sucrose)For pure solutions:

100-SWS=-

SFor impure solutions:

100-DSWS=--

S

(e) Concentration, c (g sucrose per litre solution), partial density

S X Qc=---

100

(5.6)

(5.7)

(5.8)

where Q is the specific gravity (kglm3) of the sucrose solution.

(f) Molality, Cm (mol sucrose per kg water)For pure solutions:

1000 X SCm = (100 _ S) X M

s(5.9)


1000 X SC =

m (100 - DS) X M s

where M s is sucrose molecular weight (342.30 glmoI)(g) Molar concentration CM (mol sucrose per litre solution)

S X Q

100 X M s

(5.10)

(5.11)

(h) Molar fraction, X M

For pure solutions: mol sucrose per (mol sucrose + mol water)

S

(100 - S) X M s

S 1+-

(100 - S) X Ms Mw

(5.12)

For impure solutions: mol sucrose per (mol sucrose + mol water+ mol non-sucrose)

S

(100 - DS) X MsX M =----------------

S 1 DS - S+--+-----

(100 - DS) X Ms Mw (100 - DS) X MNS

(5.13)


(5.14)

(5.15)

where M s is the sucrose molecular weight (342.30 g/mol); Mw isthe water molecular weight (18.016 g/mol); and MNS is the nonsucrose molecular weight.Remark: Kadlec and Sarka (1977) and Kadlec and Hyndnik(1979) have presented for beet non-sugars an average value fornon-sucrose molecular weight M NS = 114 g/mol which correspondsapproximately to one-third of the molecular weight of sucrose.Average value MNS = 205 g/mol was experimentally found fornon-sugars contained in cane solutions (Kadlec and Hyndrak,1979). It corresponds in this case to about three-fifths of sucrosemolecular weight.

5.3 Sucrose solubility in water

5.3.1 Effect of temperature on the sucrose solubility

As stated in the introduction, sucrose is very soluble in water because of itsstructure. The solubility increases significantly with temperature. Thisdependence is called the solubility curve and can be generally described bythe equation of the reaction isochore for ideal systems. It was found thatsucrose solutions do not comply to ideal systems and therefore differentempirical equations were suggested for the dependence of solubility ontemperature. They fit with experimental data. It is especially the case forpolynomial functions. The fitting was made with a number of the publishedexperimental data (Herzfeld, 1892a, b; Grut, 1937-38); Taylor, 1948;Charles, 1960; Smelik and Vasatko, 1970a, b, 1971,1972).Vavrinecz (1962) carried out a critical comparison of several functionalequations (results of 25 authors) and showed that the power seriesequation of the fourth degree gave the best results:

DS = 64.447 + 0.08222 X t + 1.6169 X 10-3 X f2 - 1.558X 10-6 X ~ - 4.63 X 10-8 X t4

where OS is dry substance (in the case of pure sucrose water solutions thevalues of OS equal that of sucrose content S(%)) and t is temperature (0C).Using this equation the mean probable error in the evaluation of

solubility is only ± 0.05%. The equation is valid in a range of temperaturesfrom -13 to 100°C and was adopted by the 15th Session of ICUMSA(Heitz, 1974).At the same time the equation according to Charles (1960) wasrecommended which is valid in the range of temperature from 0 to 86°C.

OS = 64.397 + 0.07251 X t + 2.0569 X 10-3 X f2 - 9.035X 1Q-6 X ~

106 SUCROSE

For the calculations which do not require high accuracy a simpleequation can be applied expressing the linear dependence of the water tosucrose ratio, WSsat,pure, on the temperature.

WSsat,pure = a X (b - t) (5.16)

By the modification of equation (5.16) we can obtain the relations forfurther, more frequent quantities - the sucrose to water ratio, SWsat,pure,and dry substance content, OSsat.pure, of saturated solutions.

ASWsat,pure =

b - t(5.17)

100 X AOSsat,pure =

B - t(5.18)

For the range of technically important temperatures from 30 to 80°C thecoefficients of equations (5.17) and (5.18) were evaluated for the dataaccording to Vavrinecz (1962) and Charles (1960) shown in Table 5.2.In the above temperature range the agreement of the values of solubilitycalculated from equations (5.14) and (5.18) (for Vavrinecz's values) alsofrom equations (5.15) and (5.18) (for Charles's data) is very good. Thestandard deviation of calculated values for dry substances according toboth types of equations is only 0.048% for Vavrinecz's data and 0.033% forCharles's values.For temperatures above 100°C the data are importantly different andinaccurate especially in earlier studies. Therefore, the study of Smelik et al.(1970-72) was selected because it fits best with the equations of Vavrineczand Charles and it permits by extrapolation to reach the sucrose meltingpoint:

os = 71.0615 + 5.3625 X 10-2 X t + 6.55303 X 10--4 X f (5.19)

Equation (5.19) is valid for the temperatures ranging from 100 to 125°C.Table 5.3 was calculated using equations (5.14) and (5.19) where thesucrose solubility is expressed by dry substance content and the ratio ofsucrose to water. The values above 125°C were obtained by extrapolationand therefore have a limited validity.

Table 5.2 Evaluation of the coefficients shown in equations(5.17) and (5.18)

Coefficient Vavrinecz (1962) Charles (1960)

a 0.0039102 0.0039682b 148.86 147.78A 255.74 252.01B 404.60 399.78


Table 5.3 Solubility of sucrose in water

Temperature g sucrose per Dry MolareC) g water substance, fraction, X M

DS(%) (mol per mol)

-10 1.7615 63.79 0.08484-5 1.7837 64.08 0.085820 1.8127 64.45 0.087105 1.8489 64.90 0.0886810 1.8926 65.43 0.0905915 1.9443 66.04 0.0928320 2.0047 66.72 0.0954425 2.0741 67.47 0.0984230 2.1535 68.29 0.1018035 2.2435 69.17 0.1056140 2.3450 70.10 0.1098645 2.4589 71.09 0.1145950 2.5863 72.12 0.1198155 2.7282 73.18 0.1255660 2.8857 74.26 0.1318565 3.0598 75.37 0.1387170 3.2515 76.48 0.1461375 3.4616 77.59 0.1541180 3.6901 78.68 0.1626385 3.9368 79.74 0.1716490 4.2003 80.77 0.1810495 4.4775 81.74 0.19071100 4.7637 82.65 0.20046105 5.0335 83.43 0.20944110 5.4499 84.50 0.22290115 5.9347 85.58 0.23801120 6.5062 86.68 0.25508125 7.1895 87.79 0.27452130 8.0211 88.91 0.29685

5.3.2 Phase equilibrium diagram for the system sucrose-water

The sucrose solubility in water is graphically expressed in Figure 5.1 in theform of a simplified phase diagram of sucrose-water. The curves of sucrosesolubility and ice solidification which theoretically intersect in the eutecticpoint are shown in Figure 5.1. Above these curves there exist undersaturated solutions. The solubility curve begins in the point of sucrose melting(186°C), separates undersaturated sucrose solutions from the two-phasesystem saturated solution-solid sucrose and finishes in the eutectic point(-13.9°C). The solidification curve illustrates the equilibrium between thesaturated solution and solid phase of the solvent, i.e. the ice. Under thetemperature of the eutectic point ice continues to form whereas sucroseconcentrates in concentrated amorphous solutions.The solidification curve can be described by the following equation

108 SUCROSE

10090807040 50 80

SUCI'O" S (%)

302010

,! : : ~ i j

: -=r~~-~t::r-:~;:~·r.:_··20 '---", i--r-T-"-r'I~IlOSE+~E I j_.--t----40+--....' --+---t-'--+-!--t-!--+---t---+-'--+---lo

2001 :,'-T-l:-T---;:~;:::;~::::;-r--,, MELTIHG POINT

180 '----r--+;--+--+-. -+--i--+- r-, i

180 ~--~

140 -~ -+--+----+I ! i

-.L..-L_L_+--i-'==::::+===+=::::'::::~-I-,L-1

I 80 i

I 80 ----;-

~ j

Figure 5.1 Phase equilibrium diagram sucrose-water.

which was evaluated by Bubnik et al. (1994) on the basis of the data ofWeast (1976-77) and Washburn et ai. (1927):

tfpd = 0.05176 X DS + 1.327 X 10-3 X DS2 - 2.416 X 10-5 X

DS3 + 7.552 X 10-7 X DS4 (5.20)

where t fpd is the temperature of the freezing point depression of puresucrose solution (OC) and DS is the dry substance (%).The values calculated by means of equation (5.20) are shown inTable 5.4. The values for other saccharides - glucose, fructose, lactoseand maltose (Washburn et al., 1927; Weast, 1876/77) - are presented forcomparison in chapter 8.

5.3.3 Supersaturateq. solutions

If we evaporate or cool a saturated solution, a supersaturated solutioneasily appears where the sucrose concentration is higher than thatcorresponding to the saturation state. This state keeps for quite a long timein the absence of a solid phase although it is a thermodynamically instablestate. It is caused by the increase in viscosity and by limitation ofmovement of the molecules. This worsens the conditions for formation ofstable crystal nuclei. Creation of supersaturated solution is a condition forseparating the solid phase from the solution.

SUCROSE SOLUBILITY

Table 5.4 Freezing point depression of sucrose solutions

109

g sucrose per 100 g solution

1.002.003.004.005.00

6.007.008.009.0010.00

11.0012.0013.0014.0015.00

16.0017.0018.0019.0020.00

22.0024.0026.0028.0030.0032.0034.0036.0038.0040.00

42.0044.0046.0048.0050.00

52.0054.0056.0058.0060.00

62.0064.0066.0068.0070.00

Freezing point depression Cc)

0.050.110.170.230.29

0.350.420.490.560.63

0.710.790.870.951.03

1.121.211.301.401.49

1.701.922.162.422.713.023.353.724.134.58

5.075.626.226.887.6\

8.409.2810.2411.3012.45

13.7115.0916.5818.2119.97

110 SUCROSE

The amount of supersaturation can be expressed by means of thecoefficient of supersaturation SS(1) which expresses the ratio between thegiven sucrose concentration in the solution, SW (g sucrose per g water),and concentration of the saturated solution, SWsat , at the same temperature.

SSSW

SWsat,pure(5.21)

Table 5.5 shows dry substance contents of supersaturated solutions forthe values of supersaturation usual 10 the technical practice(SS = 1.0-1.30).

5.4 Sucrose solubility in impure sugar solution and other solvents

5.4.1 Three-component triangle diagram

Adding a third component into the binary system sucrose-water is betterillustrated graphically by use of a diagram in the shape of an isoscelestriangle. Each of the three corners of the triangle represents a purecomponent (i.e. component concentration equal to 100%) while theconcentration of this component equals zero at the opposite side. The tieline of the two vertices represents a mixture of the two given componentsand every point inside the triangle gives the composition of the mixtures ofall three components. The diagram can be expressed for one temperature(isothermal diagram) or for a certain range of temperatures (polythermaldiagram). The relative proportions among the components used to beexpressed in weight percent or in molar fractions.The system sucrose-fructose-water shown in Figure 5.2 can be given asan example. The grid lines are also constructed in the figure to make thereading of the values from the diagram easier. The figure comprises a scaleof sucrose concentration as well. The tie-line of the points AC represents asolution saturated with sucrose while the curve BC shows a solutionsaturated with fructose. The point C is called a triple point and the solutionis saturated with both components. The vertices of the triangle and thepoints A, Band C divide the triangle surface into four areas. The surfaceWACB (W is the vertex of the triangle with water concentration 100%)represents a solution non-saturated by none of the compounds. In thespace limited by the points SAC (S is the vertex with sucrose concentration100%) the mixture of crystalline sucrose with the solution saturated bysucrose is found while among the points FBC (F is the vertex with afructose concentration of 100%) there is the mixture of crystalline fructoseand a solution saturated by fructose. The mixture of saturated solutionwith crystalline sucrose as well as fructose is found in the area among thepoints SCF.

Tab

le5.5Supersaturatedsolutionsofsucroseinwater:drysubstance,DS(%)ofsupersaturatedsolutions

SupersaturationcoefficientSS(1)

Temperature

CC

)1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

1.16

1.18

1.20

1.22

1.24

1.26

1.28

1.30

- -10

63.8

64.2

64.7

65.1

65.5

66.0

66.4

66.8

67.1

67.5

67.9

68.2

68.6

68.9

69.3

69.6

-564.1

64.5

65.0

65.4

65.8

66.2

66.6

67.0

67.4

67.8

68.2

68.5

68.9

69.2

69.5

69.9

064.4

64.9

65.3

65.8

66.2

66.6

67.0

67.4

67.8

68.1

68.5

68.9

69.2

69.5

69.9

70.2

564.9

65.3

65.8

66.2

66.6

67.0

67.4

67.8

68.2

68.6

68.9

69.3

69.6

70.0

70.3

70.6

1065.4

65.9

66.3

66.7

67.1

67.6

67.9

68.3

68.7

69.1

69.4

69.8

70.1

70.5

70.8

71.1

r.n15

66.0

66.5

66.9

67.3

67.7

68.1

68.5

68.9

69.3

69.6

70.0

70.3

70.7

71.0

71.3

71.7

c:: (")

2066.7

67.2

67.6

68.0

68.4

68.8

69.2

69.6

69.9

70.3

70.6

71.0

71.3

71.6

72.0

72.3

'"025

67.5

67.9

68.3

68.7

69.1

69.5

69.9

70.3

70.6

71.0

71.3

71.7

72.0

72.3

72.6

72.9

r.n tTl

3068.3

68.7

69.1

69.5

69.9

70.3

70.7

71.1

71.4

71.8

72.1

72.4

72.8

73.1

73.4

73.7

r.n35

69.2

69.6

70.0

70.4

70.8

71.2

71.5

71.9

72.2

72.6

72.9

73.2

73.6

73.9

74.2

74.5

040

70.1

70.5

70.9

71.3

71.7

72.1

72.4

72.8

73.1

73.5

73.8

74.1

74.4

74.7

75.0

75.3

r' c::45

71.1

71.5

71.9

72.3

72.6

73.0

73.4

73.7

74.0

74.4

74.7

75.0

75.3

75.6

75.9

76.2

c::I t=

5072.1

72.5

72.9

73.3

73.6

74.0

74.3

74.7

75.0

75.3

75.6

75.9

76.2

76.5

76.8

77.1

~55

73.2

73.6

73.9

74.3

74.7

75.0

75.3

75.7

76.0

76.3

76.6

76.9

77.2

77.5

77.7

78.0

6074.3

74.6

75.0

75.4

75.7

76.0

76.4

76.7

77.0

77.3

77.6

77.9

78.2

78.4

78.7

79.0

6575.4

75.7

76.1

76.4

76.8

77.1

77.4

77.7

78.0

78.3

78.6

78.9

79.1

79.4

79.7

79.9

7076.5

76.8

77.2

77.5

77.8

78.2

78.5

78.8

79.0

79.3

79.6

79.9

80.1

80.4

80.6

80.9

7577.6

77.9

78.3

78.6

78.9

79.2

79.5

79.8

80.1

80.3

80.6

80.9

81.1

81.3

81.6

81.8

8078.7

79.0

79.3

79.6

79.9

80.2

80.5

80.8

81.1

81.3

81.6

81.8

82.1

82.3

82.5

82.8

8579.7

80.1

80.4

80.7

81.0

81.2

81.5

81.8

82.0

82.3

82.5

82.8

83.0

83.2

83.4

83.7

9080.8

81.1

81.4

81.7

81.9

82.2

82.5

82.7

83.0

83.2

83.4

83.7

83.9

84.1

84.3

84.5

9581.7

82.0

82.3

82.6

82.9

83.1

83.4

83.6

83.9

84.1

84.3

84.5

84.7

84.9

85.1

85.3

100

82.7

82.9

83.2

83.5

83.7

84.0

84.2

84.4

84.7

84.9

85.1

85.3

85.5

85.7

85.9

86.1

......

......

......

112 SUCROSE

WATER

100

SUCROSE80 40 o

FRUCTOSE

Figure 5.2 Triangle diagram of the system sucrose-fructose-water at 30°C.

An example of reading of the composition of a three-component mixturefrom the triangle diagram using the cited system of the grid lines is shownin Figure 5.2 for a mixture characterized by the point P. We proceed asfollows: the lines parallel to the sides of the triangle are drawn startingfrom the point P. These lines express the constant value of componentconcentration whose crystalline state (i.e. concentration 100%) is described'by the opposite vertex, e.g. the parallel line X1-X2 expresses the constantsucrose concentration. In the point where the parallel lines intersect thesides of the triangle (i.e. the points Xl and X2; Y1 and Y2; Zl and Z2), weread the concentrations of all the three components. In this case thefollowing composition corresponds to the solution in the point P: sucrose15%, fructose 35% and water 50%.Further, we present similar three-component diagrams for the systemswhere the third component of the system sucrose-water are glucose (Figure5.3) and sodium chloride (Figure 5.4) according to Kelly (1954, 1959).Fructose, glucose and NaCI belong to the typical compounds accompanying sucrose in its impure solutions and characterize a certain group of nonsugars (see section 5.4.2).The selected data describing solubility of sucrose in water in thepresence of another compound are given for the system sucrose-invertsugar-water according to Lyle (1950) (Table 5.6) (equimolar mixture of

WATERo~------------,~------------,

oGLUCOSE

20406060

60

20 ..----.---------*

::., --"7:,.-.-+'~-jt-·~;~.~ >< --,/ -/!f':'\""""~:-i100~-+---'T... ,~.-...;..¥-._.-\.....\ ....;' -_...+--...:;....../-'_\"4-""_;'_\";;\'f-';'_/_··.:.;·v_·--lI

100

SUCROSE

Figure 5.3 Triangle diagram of the system sucrose-glucose-water at 30°C.

WATER0,------------.,.--------------,

-*------------

20+-----

40 --.--..----I:---+-----j~-:.::--~-----_l

60 - - - -I: - }:..,,r---.+.-..- +----';!,..- --i; -.-._\:

oN.CI

20408080100 r--+--'T--'i---+--:.t---';'---+--+-....:.r~:::::..lI

100

SUCROSE

Figure 5.4 Triangle diagram of the system sucrose-NaCi-water at 30°C.

114 SUCROSE

Table 5.6 Solubility of sucrose in the presence of invert sugar

Temperature Solubility of sucrose (g per g water) at concentration of invert sugar(0C) (g per g water)

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8

40 2.05 2.00 1.95 1.91 1.87 1.83 1.80 1.77 1.75 1.7245 2.16 2.11 2.06 2.02 1.98 1.95 1.91 1.88 1.86 1.8450 2.27 2.22 2.17 2.13 2.09 2.06 2.03 2.00 1.98 1.9655 2.38 2.33 2.28 2.24 2.20 2.18 2.14 2.12 2.10 2.0860 2.50 2.45 2.40 2.36 2.32 2.29 2.26 2.23 2.22 2.2065 2.61 2.56 2.51 2.47 2.43 2.41 2.37 2.35 2.33 2.32

Table 5.7 Solubility of sucrose in the presence of raffinose

Temperature Dry substance of sugar solution DS(%)CC) at concentration of raffinose (g per 100 g solution)

0 2 3 4 5 6

30 68.3 68.1 67.8 67.5 67.1 66.7 66.235 69.2 69.0 68.7 68.4 68.1 67.7 67.340 70.1 69.9 69.7 69.4 69.1 68.8 68.445 71.1 70.9 70.7 70.5 70.2 69.9 69.550 72.1 72.0 71.8 71.6 71.3 71.0 70.755 73.2 73.1 72.9 72.7 72.5 72.2 71.960 74.3 74.2 74.1 73.9 73.7 73.4 73.165 75.4 75.3 75.2 75.1 74.9 74.7 74.470 76.5 76.5 76.4 76.3 76.1 75.9 75.7

glucose and fructose is understood as invert sugar), and sucrose-raffinosewater according to Liang et al. (1988) (Table 5.7).

5.4.2 Influence of beet and cane non-sugars on the solubility of sucrosein technical sugar solutions

The complicated situation found for sucrose solubility in impure sugarsolutions becomes particularly important in the lower purity of syrups andmolasses, where the influences of the quality and amount of non-sucrose.Sucrose solubility in impure solutions has already been studied by manyinvestigators but no equation providing a general application for all kindsof molasses and impure solutions was found.According to Vavrinecz (1978-79) the compounds present in impure

aqueous sucrose solutions can be divided into four groups.

(a) Substances which are influenced neither by water nor by sucrose insoluble materials and carbohydrates (e.g. fructose) which do notimmobilize water.


(b) Substances which bind water - glucose, glucose-containing mixtures(starch syrups), maltose, raffinose and other carbohydrates whichhave water of crystallization, inorganic salts with high amounts ofwater of crystallization and no direct combination with sucrose (e.g.MgS04)·

(c) Substances which bind sucrose - alkali hydroxides and alkali salts ofweaker acids (e.g. potassium carbonate and potassium acetate).Most of the substances in this group also bind a certain amount ofwater.

(d) Substances which bind water as well as sucrose - e.g. KCI and NaCI(group of the most positive melassigenic compounds).

5.4.3 Equation for solubility of sucrose in impure solutions

The influence of impurities on sucrose solubility can be expressed as thesaturation coefficient, SC(l):

SW .SC = sat,lmpSWsat,pure

(5.22)

where SWsat,imp is the sucrose to water ratio in saturated impure solution (gsucrose per g water); and SWsat,pure is the sucrose to water ratio insaturated pure solution (g sucrose per g water).The saturation coefficient, SC, was studied by a number of researchers.Through the combined efforts of Wiklund (1955), Wagnerowski et al.(1961, 1962) and Vavrinecz (1978-79) a correlation was developedbetween the saturation coefficient SC(l) and the non-sucrose to waterratio, NSW (g non-sucrose per g water). This relationship is expressed bythe following equation:

SC = a X NSW + b + (l-b) X e-exNSW (5.23)

where a, b, c are coefficients depending on the quality of non-sucrose.Vavrinecz (1978-79) summarized in his review the results of the

investigation of sucrose solubility in impure solutions and computed valuesof the coefficients a, band c for each of the groups of examined data. Thevalues of the coefficients varied in a very large range: a = 0.20-0.43, b =0.43--0.83 and c = 1.36-2.85, the average values are a = 0.292, b = 0.691and c = 1.80.The computed curve according to the average values a,b,c and to someresults of various authors is shown in Figure 5.5. The differences betweencurves do not allow an unambiguous application of the solubility values. Inaddition, the results of some authors show that the course of curveaccording to equation (5.23) is not independent on the temperature andconsequently the choice of the solubility data is further made difficult.

116 SUCROSE

-,..ufI)-CGlUlEGloU

S· 1.1+---+----H7L-?'-7L¥-+-+---+----I

~~

1iifI)

v3.00.5 1.0 1.5 2.0 2.5

Nonsucrose/water ratio NSW (1)

0.9+---+---+---+---+---+----10.0

Figure 5.5 Dependence of saturation coefficient on the non-sucrose to water ratio. Datataken from O. measured values by Orazi (1938); W, Wagnerowski (1961,1962); B, Bubnfkand Kadlec (1982, 1988); G, Grut (1937-38); R, Reinefeld (1979); Y, average values by

Yavrinecz (1978-79).

The composItIOn of different products and intermediate products insugar technology can be schematically and simply illustrated by means of atriangle diagram which describes the system sucrose-water-non-sugars(see Figure 5.6.). The straight lines going through the vertex of a trianglewith water concentration 100% express the constant ratio between sucrosecontent S (%) and non-sugars NS (%) and it is possible to calculate thepurity Q (%) from these ratios.Bubnik and Kadlec (1992) carried out the evaluation of the status quowith the following results.

(a) If the exact solubility data are required, it is necessary to determinethe solubility by means of experimentation (crystallization ordissolution), especially for the low purity solutions.

(b) If the analytical composition is known (content of K, Na, Ca, Mgand others), it is possible to apply some of the more complicatedequations, which include the influence of some main non-sugars onthe solubility (e.g. Schneider et al., 1961; Reinefeld et al., 1979).

(c) If only the rough values for the technical use are required, it is ofpractical interest to choose some large groups of data, which weredetermined with sufficient accuracy, the sugar-beet being cultivatedin similar conditions and the examined impure solutions produced

S

90\ syrupC85 Q=65% J ~80 Purity " I '\! 'X syrup A75 Q=56 % "'" '\ jV ':j(70 "tltolasse~/ J 'V\... 'X/\65 """ ~" ..JV\ ~_~thickJulce

:~ "--' , ...]~ ~0)E-JII!t.~<-- ---1

:~ I ~~~ . '\ rawiuice1\ j '\ ~ j j\ l

100,----------...=.=----:::--:-:=..,..-:-::-::-::-::----,95 ~ --.white sugar

raw sugar.-·--;'

40 1\/ \ J j\iW. '\ '\ '\ '\ \35 '\1\ '\/ '\ J ~ J'Y:. .1\1 I \thin Juic e30 I I\! 1\ J '\ I\~ /\ '\ j

25 1\ '\/\ 'V '\ j '\ '\IV "'0:~J ,/\ i20 1\ I j\ "IV j\ '\ j\1 j\ J"'ci- ~i\", i

15 /\/ ./\ I 1\/\ 'V\/ '\1\/\/ 'V\~ ~10 IV\\!\ \!V IV\ ./\/\/\ .IV'j\~~ -f-/....l\Jf-.'...,\.1'--'(-/_\+.1.....:/>(-'...,\.1'--'(-\/_'\¥-./""':\>(--1/'-I'-Of\'/'_'\+./...,\f---OT/''-\-Of./_!\+(-'\./>(--1'--'(-~....3!

NS 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100yJ

Figure 5.6 Triangle diagram of the system sucrose-water-non-sucrose for impure sugarsolutions.

by classical technology (no Quentin process, etc.). As a typical caseit is possible to take the work of Gmt (1937, 1937-38).

The dependence of the saturation coefficient SC (Gmt, 1937-38) on thenon-sucrose to water ratio NSW can be described very precisely by thefollowing equations.

(5.24)

(5.25)

(a) According to Vavrinecz (1978-79) equation (5.23) with coefficients:

a = 0.178, b = 0.820 and c = 2.1According to Bubnfk and Kadlec (1992):

for NSW < 0.864 SC = 1 - 0.1387 X NSW+ 0.1606 X (NSW)2for NSW > 0.864 SC = 1+ 0.1659 X (NSW - 0.864)

(b)

If we substitute the value of the saturation coefficient SC from theequations (5.23) or (5.24) and (5.25) into the equation (5.22), we cancalculate the solubility of sucrose in the form of the sucrose to water ratio,SWsat,imp (g sucrose per g water). We obtain the value of the sucrosesolubility in water SWsat,pure (g sucrose per g water) from equations (5.14)or (5.15).

118 SUCROSE

Using the equations cited, the tables of sucrose solubility in impure sugarsolutions were calculated (Bubnfk and Kadlec, 1992; Bubnfk et al., 1994)with the solubility expressed (see Table 5.8) by the sucrose to water ratio,SWsat,imp (g sucrose per g water) and also by the dry substance content DS(%) (see Table 5.9). The tables are convenient for technical use, especiallyin the sugar industry, where it is of practical interest to have simpletables with a limited validity. Table 5.9 is presented for the temperaturerange 4G-90°C and purity 55-100%. The values under 30°C and over 80°Cand purity under 60% were obtained by means of approximation and haveonly a partial validity.

5.5 Solubility of sucrose in other solvents

The sucrose solubility in nonaqueous solvents is generally significantlylower than in water and sucrose does not dissolve in non-polar solvents.Some more important values of solubility are shown by sucrose (Dobrzycki,1984) in condensed ammonia (72% sucrose in solution), dimethylsulphoxide (42%) and methylamine (more than 25%). Liquid sulphurdioxide shows lower values, also formic and acetic acid, dimethylformamide, pyridine (approx. 6%), propyleneglycol, glycerol (7%),methanol, ethanol, acetone and dioxane.

5.5.1 Ternary systems: sucrose-water-organic liquid solvent

Similarly to the three-component diagram sucrose-water-non-sucrose wecan graphically illustrate the ternary system sucrose-water-solvent by atriangle diagram. We present as an example the diagram for the systemsucrose-water-acetone at 30°C according to Verhaar (1940) (see Figure5.7). This figure shows that two liquid phases (Ll and L2), at a giventemperature, exist at equilibrium.Other important ternary systems are described in Table 5.10 for thesystem sucrose-ethanol-water (Schiwek and Kolber, 1985), Table 5.11 forthe system sucrose-propyleneglycol-water (Fey et al., 1951) and Table 5.12for the system sucrose-glycerol-water (Segur and Miner, 1953).

5.6 Solubility of some saccharides

Solubility of sucrose was compared with other usual saccharides. Thiscomparison is summarized and the basic data of solubility of glucose,fructose, invert sugar, maltose, lactose and raffinose in water are reportedin chapter 8.For the calculation of the dependence of solubility of the cited

Tab

le5.

8S

ucro

seto

wat

erra

tio

insa

tura

ted

impu

resu

gar

solu

tion

s,S

W(g

sucr

ose

per

gw

ater

)

Pur

ity,

Suc

rose

tow

ater

rati

o,S

W(g

sucr

ose

per

gw

ater

)fo

rte

mpe

ratu

re,

t(O

C)

Q(%

)20

2530

3540

4550

5560

6570

7580

8590

100

2.01

2.07

2.15

2.24

2.35

2.46

2.59

2.73

2.89

3.06

3.25

3.46

3.69

3.94

4.20

951.

972.

042.

122.

202.

302.

412.

532.

672.

822.

993.

183.

383.

603.

834.

0990

1.95

2.02

2.09

2.18

2.28

2.39

2.51

2.64

2.80

2.96

3.15

3.35

3.58

3.82

4.07

851.

942.

012.

092.

172.

272.

382.

512.

652.

802.

983.

173.

383.

623.

884.

1575

1.97

2.04

2.12

2.22

2.33

2.45

2.59

2.75

2.94

3.14

3.38

3.64

3.94

4.27

4.64

702.

012.

092.

182.

282.

402.

542.

702.

883.

083.

323.

593.

904.

254.

655.

0965

2.08

2.17

2.27

2.38

2.52

2.67

2.85

3.06

3.30

3.58

3.90

4.27

4.70

5.19

5.76

602.

192.

282.

402.

532.

692.

873.

083.

323.

613.

954.

354.

815.

366.

026.

79rJ'

J c:::55

2.34

2.45

2.59

2.75

2.93

3.15

3.41

3.71

4.08

4.51

5.03

5.66

6.43

7.38

8.56

(")

:>:l

0 rJ'J tl'1 rJ'J

Tab

le5.

9D

rysu

bsta

nce

of

satu

rate

dim

pure

suga

rso

luti

ons

DS

(%)

0 t"'" c::: a:I

Pur

ity,

Dry

subs

tanc

eo

fsa

tura

ted

impu

reso

luti

ons

atte

mpe

ratu

re,

tCC

)r:

Q(%

)~

2025

3035

4045

5055

6065

7075

8085

90

100

66.7

67.5

68.3

69.2

70.1

71.1

72.1

73.2

74.3

75.4

76.5

77.6

78.7

79.7

80.8

9567

.568

.269

.069

.970

.871

.772

.773

.874

.875

.977

.078

.179

.180

.181

.190

68.4

69.1

69.9

70.8

71.7

72.6

73.6

74.6

75.7

76.7

77.8

78.8

79.9

80.9

81.9

8569

.670

.371

.171

.972

.873

.774

.775

.776

.777

.878

.979

.981

.082

.083

.080

70.9

71.6

72.4

73.2

74.1

75.0

76.0

77.0

78.1

79.1

80.2

81.3

82.4

83.4

84.4

7572

.473

.173

.974

.875

.676

.677

.678

.679

.780

.781

.882

.984

.085

.186

.170

74.2

74.9

75.7

76.5

77.4

78.4

79.4

80.4

81.5

82.6

83.7

84.8

85.9

86.9

87.9

6576

.276

.977

.778

.679

.580

.481

.482

.583

.584

.685

.786

.887

.888

.989

.960

78.5

79.2

80.0

80.8

81.8

82.7

83.7

84.7

85.8

86.8

87.9

88.9

89.9

90.9

91.9

5581

.081

.782

.583

.384

.285

.186

.187

.188

.189

.190

.191

.192

.193

.194

.0.....

......

.\0

120 SUCROSE

Table 5.10 Solubility of sucrose in ethanol-water solutions (g sucrose per 100 g solution)

Temperature, Ethanol concentration (g ethanol per 100 g solution)t eq

0 4 8 12 16 20

15 66.04 62.07 58.10 54.15 50.21 46.2820 66.72 63.00 59.09 55.20 51.31 47.4325 67.47 63.96 60.12 56.27 52.43 48.5830 68.29 64.96 61.18 57.37 53.56 49.7435 69.17 65.99 62.26 58.50 54.71 50.9040 70.10 67.04 63.38 59.65 55.88 52.0645 71.09 68.13 64.52 60.83 57.06 53.2350 72.12 69.25 65.70 62.03 58.27 54.4155 73.18 70.41 66.90 63.26 59.49 55.9960 74.26 71.59 68.14 64.52 60.73 56.7765 75.37 72.80 69.41 65.80 61.98 57.9670 76.48 74.05 70.70 67.11 63.25 59.15

SUCROSE100,-----------......,...------------,

80

eo

40 +-.-..--.-...-..--.-~- .....-..-...;;:...--.--+(....-\_

20

80eo4020o*-~~-,;~....:.r'---+--+-+-+-+"";::""-lfo

WATER

Figure 5.7 Triangle diagram of the system sucrose-water-acetone.

saccharides on the temperature, Bubnik et ai. (1994) evaluated theequations as follows:

(5.26)

where DS is dry substance of saturated solution (%), t is temperature CC),and ao ... a4 are coefficients.


Table 5.11 Solubility of sucrose in propyleneglycol-water solution (at temperature 25°C)

Propyleneglycolin water(% w/w)

Sucrose

g per 100 ml solution g per 100 g solution

o(water)255062759599

84.475.752.239.424.54.82.0

64.458.842.733.521.84.51.9

Table 5.12 Solubility of sucrose in glycerol-water solution

Glycerol Sucrose solubility (g per 100 ml solution) Sucrose solubility (g per 100 g solution)in water at different temperatures (OC) at different temperatures (OC)(% w/w)

15 25 35 15 25 35

o(water) 87.4 84.4 92.3 66.0 64.4 69.225 73.2 78.2 82.2 55.9 59.4 61.950 54.6 38.0 63.7 42.4 44.8 48.775 28.6 32.4 38.1 22.7 25.4 29.582 20.1 25.5 29.0 15.9 20.0 23.395 5.8 12.6 16.6 4.6 9.9 13.099.95 7.2 5.7

The difference between original data and that calculated according toequation (5.26) is lower than 0.1% for glucose and fructose, 0.1% forinvert sugar (average value), 0.2% for lactose and raffinose and 0.3% formaltose (average value).The values of the coefficients for various saccharides are given in thefollowing survey together with the reference and the range of validity (seeTable 5.13).

5.7 Conclusion

Solubility of sucrose in water and non-aqueous solvents belongs to its basicproperties and influences significantly technological processes. Solubilityof sucrose in impure aqueous sugar solutions is especially important. Theseare treated during industrial sugar production in the sugar factories. Thecontent as well as composition of the admixtures (i.e. non-sugars) issuingfrom the natural raw material (sugar-beet or sugar-cane) influencesignificantly solubility of sucrose and therefore white sugar extraction. Theinfluence of non-sugars on the solubility is in a tight bond with molasses

122 SUCROSE

Table 5.13 Coefficients of equation (5.26) based on previously published data

au al a2 a3 a4

Glucose 32.2725 0.73964 -1.2121 X 10-5 0 0(Honig, 1953) for o-50°CGlucose 33.6227 1.20103 -1.1655 X 10-2 5.11882 X 10-5 0(Corn Refiners Association, 1975), for 50-91°CFructose 77.1708 -0.122121 1.42655 x 10-2 -2.0944 X 10-4 0.01515 X 10-6(Bates et ai., 1942) for 2o-55°CInvert 50.846 0.40579 2.0930 X 10-2 --6.6613 X 10-4 6.660 x 10-<>(Honig, 1953) o-50°CMaltose 35.911 0.32528 3.1532 X 10-3 2.74441 X 10-8 1.5575 X 10-10

(Washburn et ai., 1927) o-97°CLactose 10.8294 0.135663 6.34667 x 10-3 -2.2804 X 10-5 0(Washburn et ai., 1927) o-80°CLactose -32.597 1.58966 -8.2194 x 10-3 1.7030 X 10-5 0(Roetman and Burna, 1974) 8o-200°CRaffinose 6.6676 0.089312 0.011045 9.5174 X 1~ 0(Washburn et ai., 1927) o-24°C

formation and is quantitatively expressed by means of melassigeniccoefficients. Many authors have been therefore dealing with studies of therelations between non-sugars composition in beet and extraction (orsolubility). Even nowadays these questions are being intensively studied.Solubility is also a very important value for crystallization processeswhere it determines the rate of supersaturation of the solutions and so alsothe value of crystallization rate. The solubility increases significantly withthe temperature which is manifested at higher temperatures, commonlyused in industrial crystallization, by high content of dry solids and highviscosity of mother syrups.The values of sucrose solubility in water are known with a sufficientaccuracy. For impure solutions it is possible to use either some of thepublished figures or, when accurate data are needed, to carry outexperimental determination. The sucrose solubility is experimentallydetermined through reaching the saturation state. The most frequent arethe crystallization experiment with a supersaturated solution, sucrosebeing dissolved in an undersaturated solution at the given temperature(e.g. the so-called Polish test) or an increasing temperature of asupersaturated solution (saturoscopic method).Sucrose solubility in non-aqueous solvents is much lower. It is used, forexample, in pharmaceutical production, in analytics (e.g. moisturedetermination of crystalline sucrose - pyridine and ethanolamine,dispersion medium for measurement of crystals - glycerol, etc.).Solubility in ternary mixtures sucrose-water-solvent (e.g. ethanol oracetone) is also important.

List of symbols


a

a,b,caO,al,aZ,a3,a4A,BC

M sMw

NSNSWQSSCSSSWSWsat

SWsat,pUTe

SWsat,imp

WSWSsat,pUTe

Activity (mol sucrose per kg water)CoefficientsCoefficientsCoefficientsConcentration (g sucrose per litre solution), partialspecific gravityMolality (mol sucrose per kg water)Molar concentration (mol sucrose per litre solution)Dry substance (%)Activity coefficient (1)Non-sucrose molecular weight (sugar-beet non-sucrose114 g/mol, sugar-cane non-sucrose 205 g/mol)Sucrose molecular weight (342.30 g/mol)Water molecular weight (18.016 g/mol)Non-sucrose (non-sugars, impurity) (%)Non-sucrose/water ratio (g non-sucrose/g water)Purity (%)Sucrose content (%)Saturation coefficient (1)Coefficient of supersaturation (1)Sucrose to water ratio (g sucrose per g water)Sucrose to water ratio (g sucrose per g water) of thesaturated sucrose solutionSucrose to water ratio (g sucrose per g water) of thesaturated pure sucrose solutionSucrose to water ratio (g sucrose per g water) of thesaturated impure sucrose solutionTemperature (0C)Temperature of freezing point depression of sucrosesolution CC)Water to sucrose ratio (g water per g sucrose)Water to sucrose ratio (g water per g sucrose) of thesaturated pure sucrose solutionMolar fraction (mol sucrose per molsucrose + non-sucrose + water)Mass fraction (g sucrose per g solution)Specific gravity (density) (kg/m3) of sucrose solution

124

References

SUCROSE

Bates, F.J. et al. (1942) Polarimetry, Saccharimetry and Sugars, Circular NBS No C440,Washington, DC, USA.Bubnik, Z. and Kadlec, P. (1988) Krystalizacni rychlost sacharosy, Dil II [Growth rate ofsucrose crystals, Part II]. Listy Cukrov., 104, 201.Bubnik, Z. and Kadlec, P. (1992) Solubility of sucrose in impure sugar solutions. Zuckerind.,117,619--625.Bubnik, Z., Kadlec, P., Bruhns, M. and Urban, D. (1994) Handbook of Sugars. Chemical

and Physical Data of Sugar Manufacture and Processing. A Bartens, Berlin, Germany.Charles, D.E. (1960) Solubility of pure sucrose in water. Int. Sugar J., 62, 126.Ciz, K. and Valter, V. (1967) Roztoky sacharosy [Solutions of sucrose]. In Zdklady

cukrovarnictvi (dil IV) [Sugar Technology, Vol. IV] (ed. Bretschneider, R.) SNTL,Prague, Czech Republic, p. 53.Corn Refiners Association (1975) Critical Data Tables (3rd edn).Dobrzycki, J. (1984) Chemiczne Podstavy Technologii Cukru [Chemical Principles of SugarTechnology]. Wyd. Naukowo-Techniczne, Warszawa, Poland, p. 254.Fey, M.W., Weil, C.M. and Segur, J.B. (1951) Solubility of sucrose in aqueous glycerol andpropyleneglycol. Ind. Eng. Chern. Ind., 43, 1435-1436.Grut, E.W. (1947) Untersuchung von Ubersattigung und Viskositat in Kuhlmaischen fUrErst-Product. Cbl. Zuckerind., 45, 345.Grut, E.W. (1937-38) Setfeni 0 pfesyceni a viskozite v refrizerantech pro prvni produkt[Investigation of saturation and viscosity by cooling crystallization of A-product]. Dil II.Rozpustnost sacharosy v Ciste vode [Part II. Solubility of sucrose in pure water]. ListyCukrov., 56, 53-55. Dil III. Rozpustnost sacharosy v neCistych roztocich [Part III.Solubility of sucrose in impure sucrose solutions]. Listy Cukrov., 56, 62--64.Heitz, F. (1974) Crystallizing qualities of sugar solutions. Proceedings of the 16th Session

ICUMSA, Subject 25, Ankara, Turkey, p. 353.Herzfeld, A. (1892a) Uber die L6slichkeit des Zuckers in Wasser, Wessen und Ursache derMelassebildung im Algemeinen Einfluss der Raffinose darauf im besonderen. Z. Ver.Rubenzucker-Industrie, 42, 174.Herzfeld, A. (1892b) Die Loslichkeit des Zuckers in Wasser. Z. Ver. Rubenzucker-Industrie,42,232.Honig, P. (ed.) (1953) Principles of Sugar Technology (Vol. 1). Elsevier, Amsterdam, TheNetherlands, p. 84.Kadlec, P. and Sarka, E. (1977) Molami hmotnost necuknY v melase [Molar weight ofnonsucroses in molasses]. Listy Cukrov., 93, 131.Kadlec, P. and Hyndrak, P. (1979) Ebuliometricke stanoveni molami hmotnosti necukn.l'[Ebuliometric method for determination of molar weight of nonsucroses]. Listy Cukrov.,95, 13.Kelly, F.H.e. (1954) Principles of Sugar Technology (Vol. II). Elsevier, Amsterdam, TheNetherlands, p. 103.Kelly, F.H.e. (1959) The solubility of sucrose in impure solutions. In Principles of Sugar

Technology (Vol. 11) (ed. Honig, P.). Elsevier, Amsterdam, The Netherlands, p. 96.Liang, B., Hartel, R.W. and Berglund, K.A. (1988) Solubility of sucrose in water in thepresence of raffinose. Int. Sugar 1., 90, 25-28.Lyle, O. (1957) Technology for Sugar Refinery Workers. London, UK.Orazi, G. (1938) La solubilita dello zucchero nelle soluzioni impure (Coefficiente disaturazione). Ind. Sacch. Ital., 31, 401.Reinefeld, E., Emmerich, A., Fantar, N. and Gerlach, M. (1979) Grundlagen zurVerbesserung der Melasseerschopfung. Zuckerind., 104, 599.Roetman, K. and Burna, T.J. (1974) Neth. Milk Dairy J., 28, 155.Schiweck, H. and Kolber, A. (1985) Zur Frage der Optimalen Ausbildung der Zuckerkrustebei Krustenpralinen. Susswaren Technik Wirtschaft, 19, 273.

Schneider, F., Emmerich, A., Reinefeld, E., Walter, E. and Keirn, W. (1961) Auswirkungder Nichtzuckerstoffe der Rube, insbesondere auf die Melassebildung. 1. Beziehung

SUCROSE SOLUBILITY 125.

zwischen Nichtzuckerzusammensetzung und Entzuckerungsgrad von Melassen. Zucker, 14,208.Segur, J.B. and Miner, C.S. (1953) J. Agric. Food Chern., 1,567-568.Smelik, A., Vasatko, J., Dandar, A. and Matejova, J. (1970--72) Die Loslichkeit derSaccharose [Solubility of Sucrose). I. Mitteilung (1970a) Bewertung der Untersuchungsergebnisse tiber die LOslichkeit der Saccharose in Wasser, Zucker, 23, 133. II. Mitteilung(1970b) Ouadratische Gleichungen fUr die Loslichkeit der Saccharose, Zucker, 23, 595. Ill.Mitteilung (1971) Die Bewertung der Messungen der Saccharoseloslichkeit in Wasser mitHilfe der Ausgeglichenen Ouadratischen Modelfunktion, Zucker, 24, 138. IV. Mitteilung(1972) Loslichkeit der wasserfreien Saccharose und theoretische Approximation derLoslichkeit des Hemipentahydrates der Saccharose in Wasser, Zucker, 25, 89.

Taylor, M. (1948) The solubility of pure sucrose in water at high temperatures. Int. Sugar J.,50,292.Vavrinecz, G. (1962) Neue Tabelle tiber die Loslichkeit reiner Saccharose in Wasser. Z.

Zuckerind., 12, 481.Vavrinecz, G. (1978-79) The formation and composition of beet molasses. Sugar Technol.

Rev., 6,117.Verhaar, G. (1940) Arch. Suikerind. Ned. en Ned.-Indie. In Principles of Sugar Technology(Vol. II) (ed. Honig, P.). Elsevier, Amsterdam, The Netherlands, p. 110.Wagnerowski, K., Dabrowska, D. and Dabrowski, C. (1961) 0 ilosciowych stosunkachskladnik6w melasu rzeczywistego, Gaz. Cukrown., 63, 97.Wagnerowski, K., Dabrowska, D. and Dabrowski, C. (1962) Probleme der Melasseerschopfung, Z. Zuckerind., 12, 664.

Washburn, E.W., West, c.J., Dorsey, N.E., Bichowsky, F.R. and Klemenc, A. (1927)International Critical Tables (Vol. II). McGraw-Hili, New York, USA.

Weast, R.C. (ed.) (197~77) CRC Handbook of Chemistry and Physics (57th edn). ChemicalRubber Co., Cleveland.

Wiklund, O. (1955) Molektilverbindungen zwischen Saccharose und Salzen. Ein Beitrag zurBeleuchtung des Mechanismus der Melassebildung. Zucker, 8, 266.

6 Rheological properties of sucrose solutionsand suspensIOnsM. MATHLOUTHI and J. GENOTELLE

6.1 Introduction

Viscosity of sugar solutions is informative on the transport properties ofthe sucrose molecule in the aqueous medium. For this reason, it is aphysical property particularly important in crystallization studies. Viscosityis affected by temperature, concentration of solute and of suspendedmatter. Slight amounts of impurities, especially macromolecules, provokean appreciable increase in viscosity and a change in the rheologicalbehaviour of concentrated sucrose solutions. As stressed in an earlierreview (Mathlouthi and Kasprzyk, 1984) theory is a good guide to practiceand the solving of the day-to-day problems posed in a sugar factory such asthe flow in pipes, cooling of crystallizers, centrifugation or molassesexhaustion, require that rheological properties of syrups, molasses andmassecuites be taken into account.The rheological behaviour of sugar solutions and suspensions is derivedfrom the rate of deformation or rate of flow obtained when a force isapplied to the sample. When the relation between the shear stress (forceapplied per unit area) and the shear rate (velocity gradient resulting fromthe application of shear stress) is linear, the fluid is said to be Newtonian.This is the case for pure sucrose solutions at all temperatures andconcentrations experimented. When the increase in shear stress gives morethan a proportional increase in shear rate, with the curve beginning at theorigin, the flow behaviour is pseudo-plastic. It is also described as a shearthinning behaviour. The apparent viscosity decreases in this type of flowwith increasing shear rates, eventually reaching a constant value at highshear rates. At low shear rates, a pseudo-plastic fluid may exhibit a nearlylinear variation of shear stress in function of shear rate. This is called a'Newtonian regime' (Bourne, 1982). It illustrates the necessity of avoidingthe single-point measurements and the danger of restriction of measurements to low shear rates. Pseudo-plasticity is found for low-grademassecuites (Kaga, 1961) while the rheological behaviour of molasses maybe Newtonian or pseudo-plastic depending on their composition. Canesugar molasses, because of the concentration of soluble colloids, exhibits a


RHEOLOGICAL PROPERTIES OF SUCROSE 127

(6.1)

pseudo-plastic behaviour (Smolnik and Delavier, 1972). If no highmolecular-weight macromolecules or gas blisters are found in molasses, itsbehaviour remains Newtonian (Schneider et ai., 1967).Another non-Newtonian flow behaviour less common in sugar solutionsand suspensions is the dilatant or shear-thickening phenomenon. Itcorresponds to an increase in apparent viscosity with increasing shear rate.Viscosity of the two-phase system constituted by massecuites is not easy tomeasure. It is dependent on different factors such as the crystal content,size and shape of crystals, concentration and composition of mother liquoras well as the type of viscometer (rotating cylinder or pipeline viscometer).However, massecuites at the usual conditions of temperature, concentrationand crystal content keep their fluidity. They do not behave like plasticswhich do not flow until a minimum stress is reached.We now present the theoretical basis of viscosity relations used for puresucrose solutions as well as the practical approach of prediction of viscosityand flow behaviour for industrial sugar solutions and suspensions. Methodsof measurements used in the laboratory and the factory are reviewed andtheir validity for the different types ofsamples commented on. Interpretationof viscosity data is made at a molecular level to show the role of solventassociation to sucrose as well as solute-solute interactions or the role ofimpurities on the rheological behaviour of impure solutions. This information is applied to the understanding of the role of viscosity in sugarfactory processes.

6.2 Theoretical basis of viscosity relations

6.2.1 Einstein's equation

One of the earliest attempts to use hydrodynamics equations to calculatethe molecular dimensions of a spherical solute in dilute solution is due toEinstein (1906). This was applied to sucrose solutions. At very highdilution in a fluid strictly Newtonian, Einstein showed that the relativeviscosity of a very dilute suspension of rigid spherical uncharged particlesin a continuous liquid medium is given by a relation which, in the firstorder, gives

IIllrel = --= 1 + 2.5<1>

llo

where II and llo are, respectively, the absolute viscosities of solution andsolvent and the volume fraction occupied by the solute.Application of this relation requires that the solute molecules are largecompared with a molecule of the solvent and that they are far enough

128 SUCROSE

apart. Using Landolt and Bornstein tables, Einstein calculated ll/llo =1.0245 (at 20°e) for a 1% aqueous sucrose solution and found that 1 g ofsucrose dissolved in water has the same effect on the viscosity as smallsuspended rigid spheres of total volume 0.98 cm3 ( = 0.024512.5). As thespecific volume of 1% sucrose solution is 0.61 cm3 g-l, the difference isascribed to the fact that sugar molecules limit the mobility of water andthat the dissolved sugar molecule behaves in hydrodynamics relations as ahydrated molecule having a volume of 0.98 X 342 N cm-3 with 342 beingthe molecular weight of sucrose and N the Avogadro number.The calculation of hydrodynamic radius (P) of the sucrose molecule byuse of equation (6.1) and the Kirchhoff-Stokes-Einstein relation giving thediffusion coefficient (D):

D = RT/6rtllNP

was made. It results in the following data:

P = 0.49 X 10--6 mmN = 6.56 X 1023

(6.2)

The Avogadro number is given with a relatively low error (5%) while thehydrodynamic radius of sucrose is 20% higher than the value obtainedfrom crystallographic data (Pc = 4.04 A). The difference betweenhydrodynamic radius (4.9 A) and crystallographic radius (4.04 A) permitscalculation of a hydration number of 5.3 water per sucrose molecule whichis in good agreement with the value generally admitted (Akhumov, 1981;Culp, 1982). Most of equations used for polymer solutions in infinitelydilute state are comparable to Einstein's relation (equation (6.1». Theyare at the basis of the empirical relations used in the sugar industry.

6.2.2 Viscosity-concentration relations

The calculations of Einstein were made for a spherical particle. In fact, theshape of the sucrose molecule is far from spherical. To account for thisparameter and for the deformation of the solvent by the solute, Simha(1940) introduced a shape factor in Einstein's equation:

llrel = 1 + v<l> (6.3)

(6.4)

where v is the axial ratio for ellipsoids. It was calculated for discs and rods.Extension of Einstein's relation to the second order (Simha, 1952) gives

_ll__ 1 = 2.5<1> + 12.6<1>~llo

were <1>2 is the mole fraction of solute (<1>2 = (v2c2)/m2, where V2 is themolar volume of solute, m2 its molecular weight and C2 the concentration).

Replacing <1>2 by its value and dividing by C2 gives

11 - 110 = 2.5(~) + 12.6 ( V2 )2 C211oC2 m2 m2

(6.5)

Equation (6.5) is comparable to the Huggins (1942) equation:

11 - 110--- = [11] + k'[11Fc+ ... (6.6)

where

lim 11 - 110

C ~ 0 110c

is the intrinsic viscosity. The parameters [11] and k' (Huggins constant) aregenerally used to characterize the size and solute-solvent interactions inpolymer solutions. The application of a triple extrapolation procedurepermitted an increased accuracy in the determination of intrinsic viscosity[11] of sugars and sweeteners (Mathlouthi et ai., 1993). Beside extrapolationof equation (6.6) two other functions are extrapolated towards zero (seeFigure 6.1). These functions are

2.8

2.6

2.4

765432

2~_~_~_"'--_"-_~_~----' Co

Figure 6.1 Triple extrapolation of (0) reduced specific viscosity, (+) inherent viscosity and(*) reduced differential viscosity in function of concentrations c (g % mt) to obtain intrinsic

viscosity [TIl.

130 SUCROSE

(6.7)

• the inherent viscosity given by the relation of Kraemer (1938):

1 T] 2T]inh = -In (-) = [T]] - f3[T]] C+ ...

C T]o

• the reduced differential viscosity (Meffroy-Biget and Unanue, 1977):

T]diff

CT] - T]o = [T]] + (m + b - 1)[T]Fc+ ...T] C

(6.8)

Another equation frequently used in the literature for sugar solutions isthat of Jones and Dole (1929):

T]T]rcl = -- = 1 + Bc + Dc2+ ..

T]o

which may be written

T]sp T] - T]o-------= B + Dc+ . ..

(6.9)

(6.10)c T]oc

where B is comparable to [T]] in the Huggins equation (6.6). B is expressedin litres mol-I and the concentration c in mol litre-I, T] and T]o being,respectively, the viscosity of solution and solvent and c the concentration.The Jones and Dole equation is comparable to the Einstein-Simharelation:

T]r = 1 + v</> + C</>2 + . . . (6.11)

where </> is the volume fraction occupied by the hydrated solute (</> =(cM2 ~)/1000) and v a coefficient accounting for the shape of solute (v =2.5 for spherical particles). Comparison of equations (6.10) and (6.11)permits likening of B to a parameter accounting for the size of hydratedsolute called Bsizc ' For spherical particles:

Bsizc2.5 M2V~

1000(6.12)

M2 being the molecular weight of solute and ~ its apparent specificvolume.The experimental coefficient B is an overall hydrodynamic volume. It isexpressed as the sum of two components one due to the size and shape ofsolute (B sizc ) and the other originating from its effect on solvent structure(Bs,ruc,urc) :

B = Bsizc + Bs,ruc,urc (6.13)

Another relation between relative viscosity and concentration is due toVand (1948):

(6.14)k\c + '2 (k2 - k l )C

2

In lJr =1 - Qc

where k l is a shape factor for single spheres and k2 a shape factor forcollision doublets, '2 a collision time constant, Q a hydrodynamicinteraction constant and c the concentration.The two first terms of equation (6.14) multiplied by (1 - Qc) andneglecting Q2C2 gives

In lJr = klc + k\Q + '2 (k2 - k l ) c2+ . . . (6.15)

(6.16)lJr =

This relation is comparable to equation (6.7). Such a comparison permitsone to deduce that k\ = [lJ] and

Q '2(k2 - k j )

f3 = (~- k1 )

For a sucrose molecule taken as a model of solute with an ellipsoidal shapehaving an axial ratio alb = 1.84, Vand (1948) obtained the values of k l =2.75, k2 = 3.175, Q = 0.60 and '2 ranging from 2.089 at ooe to 1.489 at100°C. He could also deduce hydration numbers of 10.54 at ooe and 2.45 at100°C.The model of Vand (1948) was adapted by Robinson and Stokes (1959)for the study of dilute solutions of small molecules. They found a hydrationnumber of 7.7 water per sucrose molecule at 20°C which is in goodagreement with previous results. Furuse (1969) applied a relation inspiredfrom the work of Vand (1948) to describe the viscometric behaviour ofwater-sugar and water-glycerine solutions:

1 + 0.5<1>2

1 - <1>2

where has the same meaning as in equation (6.1).Reviewing the equations expressing the viscosity of dilute sucrosesolution as a function of concentration reveals their similarity with therelations applied to dilute solutions of polymers. This is the case for theequation used by Inhat et ai. (1968) to calculate the hydration numbers ofdifferent saccharides from their hydrodynamic volume, which is comparableto equation (6.6) of Huggins (1942). Among the other viscosity-eoncentration relations, that of Kaganov (1949) seems particularly adapted to sugarsolutions:

(6.17a)

where N is molar concentration, al and b l constants; equation (6.rt7a) maybe written as follows:

(6.17b)

132 SUCROSE

In this form, Kaganov's relation is comparable to that of Arrhenius (1916):

(6.17c)

in which Staudinger (1932) showed that k is comparable to intnnslcviscosity [TJ]' c being the concentration. Equation (6.17a) was applied(Genotelle, 1978) to the calculation of the viscosity of sugar solutions in therange of concentrations (0-85% (w/w)) and temperatures between 10 and80°C. This relation is also suitable for molasses and run-offs providing thata calibration taking into account the effect of non-sucrose is made.Wagnerowski (1976) has published a relation taking into account boththe concentration and the temperature of sucrose solutions:

log TJ = N4120

( -- - 11.75) - 2.10t + 91

(6.18)

(6.19)

when TJ is viscosity in poises; t is temperature in °C and N is the molefraction of sucrose (N= B/(1900-18B) ); B being the concentration in °Brix.For pure sucrose solutions, most ofthe equations found in the literature arederived from Einstein, Huggins or Kraemer's relations. Hence, Moulik(1968) adapted Einstein's equation (equation (6.1)) to the calculation ofthe viscosity of concentrated solutions. To take into account the shapes ofsugar molecules which are not spherical, Moulik and Khan (1977) applied amodified Vand's equation (equation (6.14)) and declared it to be auniversal equation valid for electrolytes and non-electrolytes in concentrated solutions. Burianek (1956) proposed an equation for the viscosity ofindustrial sugar syrups which is comparable to Kraemer (1938), seeequation (6.7).

6.2.3 Viscosity-temperature relations

The viscosity of a liquid was considered as a rate process and treated by themolecular kinetic theory (Glasstone et al., 1941). Indeed, viscosity involvesa velocity gradient in its definition. If two layers of molecules in a liquid ata distance l from each other are submitted to a force F, then the viscositycoefficient TJ describing the resistance to the displacement of one layerpast the other is defined as

FxlTJ=

~v

where ~v is the difference in velocity of the two layers. The viscous flow ofthe liquid implies that molecules jump from one equilibrium position toanother. For this movement to occur it is needed that the system passesover a potential energy barrier (Glasstone et al., 1941). Considering anactivation energy for the viscous flow Evisc> viscosity was expressed infunction of temperature using an Arrhenius relation:

RHEOLOGICAL PROPERTIES OF SUCROSE

II = BeEvisdRT

133

(6.20)

(6.22)

where B is a constant, R the gas constant and T absolute temperature.An equation comparable to equation (6.20) was used by Andrade (1930)and found valid for different liquids:

II = aV1/3eb/T (6.21)

where a and b are constants and V the molar volume of liquid. Theoccurrence of molar volume in viscosity relations originates from theimportance in the fluidity of liquids of the free volume theory of Eyring. IfV is the molar volume of liquid and Vs that of solid, the difference (V - Vs)is due to holes in the structure of the liquid. It was found proportional tofluidity (lIll) and viscosity expressed as

Cll= -

V- Vs

where C is a constant.An equation comparable to equation (6.22) was proposed by Hildebrand(1971): II = c/(V - Vo) where c and II have the same significance as inequation (6.22), V being the molal volume of liquid and Vo the molalvolume at which fluidity is zero. Hildebrand and Lamoreaux (1973)expressed the fluidity <p as a linear function of the relative molarexpansion «V - Vo)/Vo):

1<p = -= B

II(6.23)

The parameters B = Vde and V0 are obtained from the extrapolation of theplots of <p against V, which are linear over a wide range of temperatures.Equation (6.23) implies that no 'solid-like' clustering takes place in theliquid (Hildebrand and Lamoreaux, 1973; Hildebrand, 1978).Although the 'hole' or free volume concept was established for pure

simple liquids, it was applied to aqueous sugar solution (Soesanto andWilliams, 1981) in the region of temperatures and concentrations whereglassy behaviour is approached. The treatment of amorphous sugarsystems was inspired from that of amorphous synthetic polymers using thewell-known WLF equation (Williams et ai., 1955):

logCt (T - Tg)

Cz + (T - Tg)(6.24)

where II is viscosity of amorphous system, Q its density at temperature Tabove glass transition, llg and Qg being the viscosity and density at Tg theglass transition temperature taken as reference. Cl and Cz are coefficientsthat describe the relaxation process (here viscosity) at T > Tg ; CI is

134 SUCROSE

proportional to inverse of free volume of the system at Tg and C2 is the ratioof free volume at Tg over the expansion coefficient of the free volume.From experimental data on synthetic amorphous polymers, Williams et

al. (1955) extracted the values of universal constants CI and C2 (Cl = 17.44;C2 = 51.6). Thus, the WLF equation is often used in the following form:

log (1]/1]g) = - 17.44 (T-Tg)/[51.6 + (T-Tg)] (6.25)

The application of equation (6.25) to amorphous food polymers,oligomers and monomers, especially carbohydrates was recently reviewed(Slade and Levine, 1994). The good fitting between experimental data forsucrose-fructose mixtures at temperatures more than 20°C above Tg andconcentrations above 90% (w/w) with the calculated data using the WLFequation (6.25) are among the earliest applications of the WLF relation tosugar solutions (Soesanto and Williams, 1981). Prediction of the viscosityof undercooled melts of glucose and fructose was also found to fit withexperimental results (Ollett and Parker, 1990). As the amorphous state ofsugar has a practical importance (see chapter 4), it is of relevance to find atool for prediction of its mobility transformation (viscosity) from glasstransition temperature easily determined by differential calorimetry.Plotting log 1] against liT by analogy with Arrhenius equation was

criticized by Hildebrand (1978). Interpretation of log 1] as an 'activationenergy', implies the presence of quasi-lattice structures opposed to the flowin the liquid. This is a major difference between Eyring (Glasstone et al.,1941) and Hildebrand's theories. Although Arrhenius type equationsexpressing viscosity as a function of temperature are less accurate foramorphous concentrated solutions than the WLF model, many empiricalequations used to describe experimental viscosity-temperature data bothfor aqueous solutions and supercooled sugar melts are found in theliterature. Among these are the Vogel-Tammann-Fulcher (VTF) equation(Angell et al., 1982):

1] = A exp (B(T-To ))

and the power-low equation (Hill and Dissado, 1982):

(6.26)

(6.27)

where To and T' g are reference temperatures, A, B, A f and r constants.The sucrose literature involves some of these relations of the Arrheniustype like Kaganov's equation (1949) (1] = AebN1I) where A and bareconstants, N the concentration and T the temperature, and the relationsproposed by Pidoux (1961) and Barber (1966).Doolittle (1954) expressed viscosity as a function of free volume using anArrhenius equation:

1] = A exp (B VjVr) (6.28)


where A and B are constants V f = V - Va, V f being the free volume, V thespecific volume and Va the volume occupied at OK (absolute zero)obtained by extrapolation of a V - T plot. Using a modified Arrheniusequation and Doolittle's relation (6.28), Miller (1963) found that such amodel is applicable even to associated liquids like water if non-boundwater is taken as the free volume. The Eyring theory was adopted byThomas (1965) who found an empirical relation in which the ratio of theactivation energy of viscous flow to the energy of vaporization at the sametemperature is in good correlation with the size, shape and polarity ofthe solute molecule. The viscous flow does not seem to rupture hydrogenbonds in associated liquids (Thomas, 1965). The absolute reaction-ratetheory of Eyring was found to fit for both Newtonian and non-Newtonianflow and to agree with Einstein's equation (Utsugi and Ree, 1971).Dynamic viscosity of pure sucrose solutions as well as diffusion andmolecular structure of these solutions were interpreted using the freevolume theory (Erszterle, 1990). Free volume was found to decreaselinearly with sucrose concentration in undersaturated solution and todiminish with increasing supersaturation in supersaturated solutions.Equations were proposed for viscosity in both regions of saturation and thecomputed values compared to experimental results (Erszterle, 1990).

6.2.4 Results and interpretation

6.2.4.1 Dilute solutions. The values of intrinsic viscosity [ll] Hugginsconstant (k') and the B coefficients of sucrose and the monosaccharides (Dglucose and D-fructose) constituting it are listed in Table 6.1. Intrinsicviscosity accounts for the hydrodynamic volume of solute and k' for theinteractions between solute and solvent. To interpret these viscometricconstants two other characteristics of sugars in dilute aqueous solutions,the apparent specific volume and h, the hydration number derived fromthe apparent molar volume B coefficient (Mathlouthi et al., 1993), are givenin Table 6.2.

Comparison of intrinsic viscosity [ll] to apparent specific volume V~

comes to compare a hydrodynamic to a static volume. Likewise, k'

Table 6.1 Intrinsic viscosity [ll] and Huggins constant k' for o-glucose, o-fructose andsucrose (25°C)

Sugar [ll] k'(em) g-I)

o-Glucose 2.38 0.89o-Fructose 2.28 0.84Sucrose 2.41 0.95

136 SUCROSE

Table 6.2 Apparent specific volume (ASY) and hydration number for o-glucose, o-fructoseand sucrose (25°C)

Sugar

o-Glucoseo-FructoseSucrose

0.620.610.62

h(water per mole of sugar)

3.452.876.63

-----~----

-~-

-u------~-----

Scheme 6.1 Schematic representation of the flow of a hydrated solute showing thedeformation of force lines around the solute «, hydration water).

describes a dynamic hydration whereas h measures a static hydration. k'accounts for the effect of the flowing solute molecule on the bulk water at acertain distance beyond the primary hydration shell given by h the numberof molecules of water bound to the sugar and affecting its kineticsproperties (viscosity, diffusion ... ) (see Scheme 6.1).The solution properties listed in Tables 6.1 and 6.2 were found helpful ininterpreting the ease of accession of sugars to the receptor site of the tastebud (Mathlouthi et al., 1993). Pauling (1946) has stressed the fact that thesizes and shapes of molecules are of great significance in determining theirphysiological behaviour.

6.2.4.2 Concentrated solutions. The standard values of viscosity ofsucrose solutions adopted by ICUMSA are the data tabulated at theNational Bureau of Standards (NBS) by Swindells et al. (1958) supplemented by the viscosity table for concentrated pure sucrose solutions(75-85°Brix and 5-80°C) published by Schneider et al. (1963). These dataare reported in chapter 8.

Analysis of the NBS-Schneider results was performed by Genotelle(1978). An empirical relation was found to fit with a good accuracy (± 1 %)with these results. A temperature function (<p = (30-t)/(91 + t) ) valid forwater was corrected as a function of concentration. This relation is

log II = 22.46 N - 0.114 + <p (1.1 + 43.1 N1.2S) (6.29)

where N = (BI1900 -18 B) is the mole fraction of sucrose B being °Brix and<p = (30 - t)/(91 + t), t being the temperature. The good fitting betweenviscosities obtained from equation (6.29) and NBS-Schneider results areillustrated in Table 6.3.

Table 6.3 Comparison of experimental and calculated values of viscosity of pure sucrosesolution (mPa s)

Temperature (0C)

% MS 10 20 30 40 50 60 70 80 Ref.'

B=O 1.30 1.00 0.80 0.65 0.55 0.47 0.41 0.36 I1.27 0.97 0.77 0.63 0.54 0.47 0.41 0.37 3

20 2.64 1.95 1.49 1.18 0.97 0.81 0.68 0.68 I2.71 1.97 1.51 1.20 0.99 0.83 0.72 0.72 3

30 4.49 3.19 2.37 1.83 1.47 1.20 1.00 0.85 14.70 3.26 2.40 1.86 1.49 1.23 1.04 0.90 3

40 9.17 6.17 4.37 3.24 2.49 1.97 1.60 1.32 I9.76 6.35 4.44 3.28 2.53 2.01 1.65 1.39 3

50 25.2 15.4 10.1 6.99 5.07 3.81 2.94 2.34 I26.8 15.8 10.2 7.03 5.11 3.87 3.04 2.45 3

60 III 58.5 33.8 21.0 14.0 9.66 6.98 5.20 I118 59.7 33.8 20.9 13.8 9.66 7.06 5.35 3

65 313 147 77.3 44.4 27.5 17.9 12.4 8.81 I330 149 76.9 43.8 27.1 17.8 12.4 8.94 3

70 1206 482 222 114 64.4 39.0 25.0 16.8 I1250 485 220 113 63.4 38.5 24.9 16.9 3

75 7402 2328 885 389 193 105 61.4 38.3 I and 27478 2340 889 391 193 105 61.4 38.3 3

80 93300 20700 6280 2250 855 394 203 114 293000 21250 6195 2180 890 409 207 114 3

85 4.21.106 541000 111000 30000 7000 2740 1170 598 24.18.106 579000 111100 27450 8270 2920 1174 525 3

'Experimental values taken from: 1, Swindells et al. (1958); 2, Schneider et al. (1963).Calculated values from: 3, Genotelle (1978).

138 SUCROSE

(6.30)

6.3 Viscosity of impure solutions

Most of results reported in the literature show that pure sucrose solutionsas well as industrial run-offs, syrups and molasses have a Newtonianbehaviour in the whole range of concentrations and temperatures practisedin sugar factories. However, it happens that, in presence of macromolecularimpurities (dextrans, pectins ... ), the rheological behaviour becomespseudo-plastic or viscoelastic. Each of the constitutents of the non-sucrosein technical sugar solutions, because of its specific association with water,may exert a specific influence on the viscosity of sucrose solutions. Most ofthe equations proposed to predict the viscosity of impure solutions areempirical adaptations of the relations described above for pure sucrosesolutions.

6.3.1 Relations applicable to homogeneous phases

One of the earliest relations used in sugar factories is that of Kaganov(1949). This relation was adapted by Silina (1953) to draw a nomogrampermitting estimation of the viscosity of molasses at different temperaturesand concentrations providing that the viscosity is known at a certainconcentration and temperature.The availability of mathematical relations for the calculation of viscosityis needed in the sugar factories, especially as the means of rapidcomputation are widespread and the usefulness of rheology is recognizedboth for simulation research programs and for process control.Wagnerowski (1976) proposed, for the Polish molasses, an equationcomparable to equation (6.18) where only the coefficients are modified:

2440log II = N ( -- - 6.66) - 1.70

t + 63where N = B/(1900-18 B) is the mole fraction of sucrose, B being theapparent % of dry matter (refractometric 1/1°Brix). For intermediateproducts between molasses and pure sucrose solutions, Wagnerowski useda relation derived from equations (6.18) and (6.30) taking into account thepurity of product. It may be noticed that N is calculated supposing thatmolecular weight of apparent dry matter is equivalent to that of sucrose.Although this is far from being true, equation (6.30) proves to be useful forthe Polish molasses.The application of equation (6.29) was extended to all industrial beetsugar solutions (Genotelle, 1978). The dry matter content expressed by'Brix' was corrected considering the effect of non-sucrose on its measuredvalue and the effect of the composition of impure solutions on the specificviscosity. This yielded the following formula:

10g'YJ = 22.46 N-2.114 + (1.1 + 43.1 A X N1.2s) (6.31)

with N = (BJ(1900-18 Bo )); Bo = B a (k + (1 - k) X PllOO), = (30 - t)1(91 + t) and A = 0.85 + 0.15PllOO, Band P are, respectively, the % of drysubstance and the purity of product; k being a coefficient of calibrationobtained from an experimental value of the viscosity of molasses.The flow behaviour of cane molasses was found non-Newtonian (Kangaand Raja Rao, 1978) fitting with a power law model of equation expressingthe wall shear stress Cw as a function of the dimensions of tube and thepressure drop:

C =w

D!!.P---=k'

4L

8V(_) nt

D(6.32)

where D and L are diameter and length of the tube, !!.P the pressure dropalong the tube, V the velocity of fluid, k' is the flow consistency and n', abehaviour index (k' = ke E1RT

) where E is the activation energy of flow andT the absolute temperature. k' and n' were calculated from the applicationof equation (6.32) to molasses and the results found useful for the design ofpipes.

(6.33)log

6.3.2 Relations applicable to heterogeneous phases

The fluids flowing in the crystallization station of a sugar factory areheterogeneous in nature (mixtures of syrups and crystals) called massecuites.The rheological behaviour of the massecuites is different from that ofconcentrated homogeneous solutions. The mathematical treatment of suchmixtures is complex and most of the relations given in order to predict theviscosity (or consistency) of massecuites are empirical. They tend to find acorrelation between the viscosity of molasses and that of massecuites. Silina(1953) studied the viscosity of beet low-grade massecuites. She found thatthis parameter obey's Poiscuille's law and is proportional to the mothermolasses viscosity. She also found a relationship between the percentage ofcrystals in the massecuites and the ratio of viscosities of massescuites andmolasses. These results are summarized in Table 6.4.Viscosity of beet molasses and massecuites were extensively studied byWagnerowski (1983). He proposed a modified Silin equation to express therelation between the viscosity of massecuites and that of mother liquor:

'YJMC 85--= 0.01326 X BMC ( - 1)'YJEM 85 - Cr

where 'YJMC and 'YJEM are the viscosities of a massecuite and its motherliquor; BMC being the % dry substance of massecuite and Cr the % ofcrystals. A relation was established for Polish beet massecuites with 42% of

140 SUCROSE

Table 6.4 Ratio of massecuites to molasses viscosities T]MClT]molas a function of crystal content

Crystals % massecuite

30354042444648

4.356.5811.2414.5019.2526.335.7

130log-8 + 50

crystals at a given temperature (8); the optimal cooling temperature being(8op!) (Wagenerowski, 1983):

f8op! - 40 ~log T]MC(e) = + 10

12.5

+ 0.3158 - 0.267 (6.34)

This empirical formula is important for mastering the process of lowgrade crystallization.More recently (Broadfoot and Miller, 1990), found that the nonNewtonian behaviour of massecuites and molasses can be adequatelydescribed using a 'Power law' model:

(6.35)

where 't is shear stress at the interface of fluid and the shear producingelement, K is the consistency, and Yis the shear rate and n the flow index.The viscosity, fl, of the massecuites and molasses which generally show apseudo-plastic behaviour is derived from equation (35) and expressed by

(6.36)

where Yeorr is the corrected shear rate at the interface. For pseudo-plasticmaterial n varies between 0 and 1 and shows an increase in pseudoplasticity as it goes away from unity. The range of shear rates usually foundin sugar factories lies between 0.1 and 10 S-1 (Broadfoot and Miller, 1990).The influence of temperature T on the consistency K of massecuites wasexpressed by an exponential relation (K =Ae-mt

) (Diaz Garcia, 1977). Thesame author established from this relation and other empirical formula ofconsistency taking into account the crystal content, nomograms predictingthe influence of temperature and crystal content on the viscosity ofmassecuites.

6.3.3 Results and interpretation

The rheological behaviour of molasses and massecuites depends on theircomposition. Cane sugar molasses were found to exhibit pseudo-plasticitymainly due to soluble colloids (gums) (Smolnik and Delavier, 1972)whereas beet molasses have shown a viscoelastic behaviour (Devillers andPhelizot, 1971).

6.3.3.1 Homogenous phases. Different factors affect the viscosity ofmolasses. Among these are the increase in concentration and thecomposition of inorganic and organic non-sugars.

6.3.3.1.1 Effect of inorganic non-sugars. The concentration of cationsand anions in low-grade products greatly affects the solubility of sucrose.The influence of non-sugars on sucrose solubility is estimated by use of anempirical relation and a laboratory test called the 'Polish test' very helpfulin beet sugar factories (Wagnerowski, 1983). Another parameter influencedby non-sugars is the concentration of molasses and run-offs. Themeasured value of % DS is far more from the true value (obtained bydesiccation in an oven or by Karl Fischer titration) the more the non-sugaramount is high. These discrepancies are of major importance in comparingthe viscosities of pure and impure solutions. The proportion and nature ofcations determines the reasons for viscosity increase in impuresolutions. Their effect augments as follows: K+ < Na+ < Ca2+< Mg2+. It iscombined to that of anions which show increasing influence: N03-<CI-<glutamate < lactate. The global effect is linked to the importance ofhydration of sucrose and non-sugars as well as the possible formation ofsucrose-eations adducts. The effect of cations added to pure sucrosesolutions was investigated both for their basic molecular associationinterest (Misra and Misra, 1977; Mohanty et al., 1981) and for the factoryconcern (Breitung, 1956; Khalikovskii, 1965). The effect of some cationsand anions on the viscosity of saturated sucrose solutions at 40°C isillustrated in Figure 6.2. It should be noticed that Mg2+associated with Ciprovokes an increase in the viscosity of saturated sucrose solution morethan ll-fold. This is especially interesting as Mg2+ is exchanged againstNa+ and K+ in an ion exchange process (Quentin process) to improvemolasses exhaustion.

6.3.3.1.2 Effect of organic non-sugars. The non-Newtonian flow ofmolasses is mainly due to the presence of organic macromolecules(dextrans, gums). Macromolecular components of cane molasses wereobtained after dialysis of the dilute syrup (Cortis-Jones et al., 1963). Thefraction having the highest effect on molasses viscosity is composed ofpolysaccharides. The dextran of bacterial (Leuconostoc mesenteroides)origin has the major effect. The increase in viscosity under the effect of

142

10

8

6

4

2

o

SUCROSE

o

K+ Na+ Ca++ Mg+ + Betaine Invert

Figure 6.2 Effect of some anions and cations as well as of betaine and invert on the viscosityof sucrose solutions at 40°C. (Tl,lTls) is the ratio of impure to pure saturated solution.

(Adapted from Breitung (1956).)

dextrans (Geronimos and Greenfield, 1978) was investigated and amathematical relation of viscosity as a function of dextran concentrationproposed. The pseudo-plasticity induced by dextrans in pure sugar solutionwas less important than in molasses. In cane sugar molasses, the pseudoplastic behaviour was much more correlated with temperature than withconcentration (Smolnik and Delavier, 1972).Another class of organic additives used to decrease molasses' viscosity isthat of surface active agents. This topic has been reviewed (Berger, 1976).It was reported that several surfactants are efficient in reducing viscosityand enhancing crystal yields. However, the dosage of surfactants seems tobe critical in obtaining the optimum viscosity.

6.3.3.2 Heterogeneous phases. The rheological behaviour of massecuiteswas found pseudo-plastic at low shear rates and approaching the


Newtonian flow at higher shear rates (Ness, 1979). Results of determinationof massecuites viscosity depend on the method of measurement. Thepipeflow method was offered as an alternative to the rotating cylindermethod (ICUMSA, 1990) for determining the rheological properties ofmolasses and massecuites was found to depend on the tube dimensions.Wall effects which are at the origin of discrepancies of the results areminimized with a large LID ratio (Ness, 1980).The viscosity of massecuites depends on the crystal content, crystal sizeand shape as well as on the size distribution of crystals and the viscosity ofmolasses (Rouillard and Koenig, 1980). The effect of crystal content wasdetermined by Silina (1953) (see Table 6.3). They were found comparableto more recent data (Rouillard and Koenig, 1980). Another approachconsisting in calculating the velocity of flow of crystals in a massecuiteconsidered as a fluidized bed permitted Kelly (1957) to calculate the crystalcontent in which massecuite loses fluidity. It was even established thatcrystals are more compressed in massecuite than in granulated dry sugar(Kot et ai., 1968).Another source of heterogeneity is the entrapped gas in molasses andmassecuites. Wagnerowski and Dabrowski (1965) evaluated that theincorporation of 20 ml of gas per g of molasses provokes the doubling of itsviscosity. The effect of entrapped gas was also found to increase thepseudo-plastic behaviour (lower value of n in equation (6.35)) and theconsistency of molasses (higher value of K). The entrapped gas may haveas its origin in beet molasses a fermentation which occurs in immature orfrozen beets (Wagnerowski et ai., 1961).

6.4 Methods for determining viscosity and flow properties

Various types of viscometers and empirical devices for determining therheological properties of sucrose solutions, molasses and massecuites wereused in laboratories and in the factories. The principles of measurementencountered may be classified as follows:

• flow time in a capillary tube;• rate of fall of a calibrated sphere in the viscous medium;• torque required to rotate a solid body at a definite angular velocity in aviscous sample;

• damping effect of a medium on a thin plate vibrating in the medium;and

• rate of flow under pressure through orifices and pipelines.

The different methods are grouped, depending on the interest in theiruse, as laboratory or factory methods.

144 SUCROSE

6.4. J Laboratory methods

6.4.1.1 Capillary viscometers. These viscometers measure the timenecessary for a given volume to flow through a capillary. If l is the length ofcapillary, r its radius and P the pressure difference under which aNewtonian liquid flows, the friction force per unit surface is equal to theproduct of viscosity, lJ, by the gradient of velocity dv/dr (Champetier andMonnerie, 1973) for a laminar flow, we have

dv Pr

(6.38)

(6.37)-=-dr 2lJl

Intergration of equation (6.37) yields a parabolic expression:

P 22V =-(R -,)

4lJl

From equation (6.38) and the volume Q of liquid flowing in the time t,an expression of viscosity is derived:

rrR4lJ = -Pt (6.39)

8Ql

This value of viscosity is only valid for laminar flow. Absolute viscosity isderived from equation (6.39). What is measured is the time t necessary fora volume Q to flow in a vertical capillary. Writing equation (6.39) as t = f(lJ) is known as the Poiseuille law:

(6.40)8Ql

t = :rtr4 p lJ

Corrections must be made to account for the convergence anddivergence of streamlines respectively at the inlet and outlet of capillary(Couette correction) and considering the fact that a part of the pressure Pis used to communicate the kinetic energy to the liquid instead ofvanquishing the friction forces. Corrections were discussed in detail forcapillary viscometers used for polymers (Vanwazer et al., 1963; Champetierand Monnerie, 1973). Capillary viscometers are generally used for dilutesolutions to determine intrinsic viscosity. Their dimensions and conditionsof utilization were previously (Mathlouthi and Kasprzyk, 1984) commented.Results obtained with an Ubbelhode capillary viscometer are listed inTable 6.1 for sucrose, glucose and fructose.

6.4.1.2 Rotating viscometers (Couette type). Viscosity depends on shearrate. It is necessary to vary this rate to measure viscosity of sugar solutionsand suspensions with rotating viscometers and coaxial cylinders. Thestudied sample is placed in the space between the two coaxial cylinders.


One of the cylinders rotates at an angular velocity (w), the other is fixed.The torque (C) resisting to shear stress is measured on the axis of one ofthe cylinders (the rotating one). To each velocity (w) corresponds a shearrate (y). Variation of shear rate may be obtained continuously ordiscontinuously. Shear stress values (0) are proportional to torque values.Viscosity derives from the ratio of shear stress to shear rate (TJ = 0/ y);TJ is expressed in Pa s; 0 in Pa and y in S-I.

(6.41)

6.4.1.3 Falling-ball viscometers. This measurement is based on Stokes law.When the movement of a falling sphere takes place under gravity with avelocity v, the friction force F exerted on the sphere with a radius R in aliquid of viscosity TJ is

4F =-Jt R3 (Q - Qo)g

3

(6.42)'TJ=

where Q and Qo are specific gravities of the sphere and the liquid,respectively. In steady state conditions viscosity 'TJ is expressed by

2gR2(Q - Qo)

9v

However, corrections should be made to take into account the influenceof the proximity of the walls. Moreover, the falling ball is not suitable tomeasure the viscosity of very viscous and dark liquids like molasses. This isthe reason why it was recommended by ICUMSA (1990) to delete it as asuitable alternative method for measuring the viscosity of molasses.

6.4.2 Factory methods

Industrial viscosity measurements are generally derived from laboratorymethods and adapted to the specific problems posed by the flow of theproducts. The use of viscometers to control crystallization was reported ina review of automatic pan boiling (Knovl and Moller, 1976).

6.4.2.1 Rotating viscometers. The viscometers most used in the sugarindustry are the Couette type viscometers. The theory is the same asdescribed above. However, it is recommended that the geometry of themeasuring system, particularly the ratio of the diameter of the spindle tothe diameter of the coaxial cylinder reservoir, should be defined and theextent of its influence on the parameters of the Power law (equation (6.35))determined (ICUMSA, 1990). The rotating cylinder viscometers werefound satisfactory and adopted as the official method by ICUMSA (1978).The range ofviscosities covered by these viscometers iswide (10-3 to 105 Pa s).Their accuracy may be tested using silicone standards with known absoluteviscosities (Kitterman, 1974).

146 SUCROSE

Beside the analysis of viscosity of molasses, run-offs and syrups, therotating body viscometers were also applied to control low-grade cyrstallization stations. The cooling of massecuite and dilution before centrifugation may be monitored by use of a rotating viscometer. It is needed thatset values be regularly checked with a laboratory measurement and thatthe rotating body be rinsed from time to time to avoid the formation ofcrystal crusts.

6.4.2.2 Pipeflow viscometers. Instruments employing the principle for therate of flow of a fluid under pressure through a pipeline can measure a widerange of viscosities (10-2 to 1010 Pa s). This principle is comparable to thatof the capillary viscometer. Comparison of pipeflow to rotating viscometerin determining low-grade cane massecuites viscosity was made (Ness,1983). The shear stress Tw and shear rate Yw at the wall of pipeflowviscometer were derived from experimental data. The viscosity of themassecuite is given by

and

Twl) = --with Tw =

Yw

DI1P

4L

where D and L are, respectively, the diameter and length of the tube, 11 Pthe air pressure applied, V the average flow velocity, n1 the slope of theloglog plot of Tw versus 8V/D. Applying the power law, this plot Tw islinear and n J is a constant written as n (Ness, 1983). The flow index n wasfound independent of the tube dimensions and the consistency dependenton D and L.

6.4.2.3 Other viscometers. Viscosity measurements for factory, laboratory or process control do not need to be as accurate as research results.However, one should be aware of the importance of performing themeasurements at the good shear rate. For pseudo-plastic products, a singleviscosity value is useless. A more complete information is provided by therheological behaviour obtained in a wide shear range or from consistencyand behaviour index derived from the Power law. Among the instrumentsutilizable for massecuites are the orifice and vibrating viscometers. Theflow properties were investigated for massecuites by measuring the headloss across an orifice at a given flow rate (Maudarbocus, 1980). Theobtained results were comparable to that of the pipeflow viscometer. In thevibrating instruments, the rate of damping of vibration of a probe ismeasured. The probe may be a metallic strip or a sphere immersed in the


liquid or the massecuite. The viscosity of massecuites measured by anultrasound viscometer was found to be less influenced by the crystalscontent than that obtained with a rotating viscosity (Ahari et al., 1967).The use of viscosity-eonsistency instruments was analysed and theproblems posed by automatic control of crystallization especially for theeffect of crystal content on rotating viscometers discussed (Schliephakeand Austmeyer, 1975).

6.5 Applications

The viscosity of sucrose solutions is important for more than one purpose.Viscosity ofpure solutions may be used to calibrate viscometers. In the sugarfactory, the rheological characteristics of syrups and masscuites play amajor role during the boiling and the crystallizing processes. It is a limitingfactor for the workability of low-grade masscuites and hence for the yieldof sugar extracted.

6.5.1 Effect of viscosity on crystallization

Crystal growth in a supersaturated solution may be roughly divided in twosteps:

(1) a diffusion step consisting in the transfer of more or less hydratedsucrose molecules from the bulk solution to the vicinity of thecrystal; and

(2) an integration step consisting in incorporating the vicinal moleculesin the crystal lattice after disassociation of hydration water.

The rates of the two steps are different depending on temperature. Fortemperatures above 40°C commonly applied in sugar factories, diffusion isthe limiting step. As diffusion coefficient is inversely proportional toviscosity, the rate of growth (Rc) may be expressed by

Rc = K (0 - 1)1'Yl (6.43)

where 0 is supersaturation and 'Yl viscosity, K being a constant. It appearsfrom equation (6.43) that at a constant supersaturation an increase inviscosity provokes a decrease in growth rate. However, the role of viscosityin crystal growth seems controversial. For Van Hook (1981) the limitingparameter is the orientation and incorporation of molecules to the faces ofthe crystal rather than viscosity. An other argument is that certain colloids(acacia gums, pectins and starch) increase tremendously the viscosity ofsugar solutions but have no effect on crystal growth (Van Hook, 1952).This argument was criticized by Silin (1958) who considers that colloids act

148 SUCROSE

as screens for sugar molecules. They increase viscosity but do not limitdiffusion and crystallization rate.

It is to be noted that for the same concentration in dry matter, viscosityincreases rapidly when the purity is lowered for saturated solutions(Maurandi, 1971). On the other hand, viscosity dependence on temperaturedoes not follow the same behaviour for high and low purity products. Thisis observed in Figure 6.3. For purities close to 100%, viscosity decreases astemperature is increased above 80-90°C. However, for lower purities, itmay be observed (see Figure 6.3) that viscosity reaches a minimum, andthis minimum takes place at lower temperatures for lower purities. Theoptimum is situated at 40°C for molasses, which is the usual end of coolingof low-grade massecuites. Therefore, it seems wrong to work at very hightemperatures in low-grade boiling. This corresponds to lower crystallizationrates and lower crystal yields. For first strike boilers, the temperature is a

----

Log 111

0.8

0.6

0.4

0.2

40 60

/,/

"

80

P:90'C

100

Figure 6.3 Variation of viscosity as a function of purity and temperature. (- - -) b = 1.05;(--) 0 = 1.0.


compromise between the crystalIization rate (favoured at high temperatures) and the crystals yield (higher at low temperature). In general, theoptimum is situated at 80°C.

6.5.2 Effect of viscosity on molasses formation and exhaustion

The growth of sucrose crystals in low purity media such as molasses isconsiderably reduced under the effect of an increasing viscosity whereasthe limit of exhaustion only depends on temperature, concentration in nonsugars and the specific effect of the non-sugars on solubility. This effect isexpressed by the equation relating the solubility coefficient (Ksat ) to nonsugars expressed as the ratio of non-sugars to water content (NS/W): (Ksat

= a (NS/W) + b). This expression is at the basis of the 'Polish test' ofcontrol of molasses exhaustion. Although the Mg2+ ion is known toenhance viscosity as compared to (Na+ and K+) it proves to be a goodmeans of molasses exhaustion because of a favourable effect at Mg2+ onthe saturation coefficient Ksat . Apart from increasing the viscosity, the ionexchange yields a higher apparent refractometric Brix although theexchange of cations decreases the weight of dry matter. PracticalIy theratio of actual to apparent non-sugars is 0.89 for Quentin products against0.93 for usual molasses. Sucrose solubility is decreased because of theimportant hydration of sucrose and Mg2+, which leads to a decrease in theslope of the saturation coefficient (Ksat = a (NS/W) + b). As an example,the characteristics, especially viscosity, of molasses brought to saturation at50°C are compared with the same molasses after 60% of ion exchange(Quentin), without any change in water content (Table 6.5).From these data, it is observed that the ion exchange provokes an

important increase (X2.5) of viscosity. After crystallization of sucrose inthe Quentin molasses to reach saturation, at constant NS/w ratio, itscharacteristics become: apparent Brix = 85.7%; a = 1.0, purity = 52.3%and 'Y) = 3500 mPa s. It is noticeable that the increase in viscosity due toMg2+ is practicalIy compensated by a better exhaustion of molasses, which

Table 6.5 Comparison of molasses before and after ion exchange (Quentin)

Parameters

Polish test coefficientsabViscosity of a test sample (8 = 85, T = 45°C)Apparent refractometric BrixApparent purityNSIWSupersaturationViscosity

Initial

0.280.68

3000 mPa s86.5%58.0%2.701.0

4000 mPa s

Quentin

0.170.72

5000 mPa S87.4%59.0%2.851.31

10200 mPa s

150 SUCROSE

occurs after bringing the molasses at saturation to a lower viscosity.However, it is difficult to estimate the real gain in yield because the routineanalysis gives apparent values which are not comparable for treated andnon-treated molasses.Wagnerowski (1983) thinks that viscosity is not a fundamental parameterin optimizing molasses exhaustion. He recommends to reduce the viscosityof massecuites by reducing the crystal contents with a preliminarycentrifugation of low-grade massecuites. It is theoretically possible toreduce the purity of molasses by lowering the temperature. However, thisneeds a very long time and the viscosity reaches very high values beyondthe limits of the equipment. Practically, a 'standard' viscosity of molasseswas defined by Silin (1958) with the following characteristics (Brix = 82; T= 40°C, 'YJ = 4400 mPa s), which corresponds to a limit of mother liquorviscosity in the centrifuges = 7000 mPa s. These values are now pushedbeyond these limits because of improvement of materials power. Takinginto account both the results of Polish test and reference viscosity a graphwas drawn (see Figure 6.4) to give an overall view of the effect of theconditions of work on molasses exhaustion. For example, the purity of

0"01.05

:!>

''so\Q\0

\\\\\\\,,

\

:!>

'~\~,,,

\

85

80

55 L.r------.-----r--~-r---~~ lSI.2 2.5 3 3.5

Figure 6.4 Effect of work conditions on molasses exhaustion. For a standard viscosity at50°C,1] = 300 Pa.s, the Polish coefficients being a = 0.30 and b = 0.70. From the NSIW ratio

it is possible to deduce saturation, viscosity and purity of molasses.


molasses is about 58% for T = 50°C, 0 = 1.05 and T] = 10 000 mPa s (1000Pa s); (see Figure 6.4).

6.5.3 Effect of viscosity on machines running

Sizing of pumps, pipes and flow orifices is calculated in function of the flowrate and the viscosity of product so that head losses are limited toacceptable values. For concentrated sugar solutions like syrups, run-offs,molasses and massecuites an important margin of security is neededbecause the fluctuations of viscosity may limit the flow. For low-gradeproducts the compacity ofmassecuites may reach values as high as 1000 Pa s.An on-line control of viscosity limits this value to 300 Pa s at the inlet ofpumps. The efficiency of separation centrifugals which is at the basis of rawsugar quality needs that mother liquor viscosity is limited to about 10 Pa sfor discontinuous machines and 30 Pa s for the continuous ones.The rheological properties of technical sugar solutions have been takeninto account for the calculations of the diameters of pipes and orifices andfor the conditions of work in crystallizers and centrifuges (Blanc, 1970;Gebler and Ciz, 1979). It was shown that the factors governing the laminarflow of massecuites across circular or rectangular apertures are derivedfrom viscosity measurements and equations.

6.6 Conclusion

Viscosity of sugar solutions is important both from the basic and practicalpoint of views. The mathematical treatment of viscosity shows thecomplexity of the problem. This is due to the solvation and molecularassociation in pure sucrose solutions. In presence of impurities or crystalsin suspension it is necessary to apply empirical relations. Most of theequations applied either to pure or impure solutions are derived frompolymer science studies. The values of the viscosity of pure sucrose solutionshave been measured for a long time and adopted as a means of calibration ofviscometers. However, the rheological properties of run-offs, molasses andmassecuites need to be known accurately so that a good method (rotating orpipeflow viscometer) be adopted and that the measurements be performedin the good shear rate range. Although viscosity is a limiting factor todiffusion, the real effect in low-grade products depends on the nature ofeach non-sugar component and needs to be elucidated at the molecularlevel. Hydrodynamic problems posed by friction in pipes, pumps andcentrifugals as well as crystallization rates can be better investigated by useof rheological characteristics of technical sugar solutions.

152

References

SUCROSE

Ahari, D., Genotelle, J., Heitz, F. and Vicaigne, J.M. (1967) Utilisation d'un viscosimetre 11ultra-sons pour Ie contr61e de la cristallisation: Application industrielle. Proceedings CITSFalsterbo, Tienen, Belgium, pp. 2257-2263.Akhumov, E.!. (1981) Hydration in two-component water-non-electrolyte solutions.

Russian 1. Phys. Chern., 55, 837-839.Andrade, E.N. da C.(1930) Viscosity of liquids. Nature, 125, 309-310.Angell, C.A., Stell, RC. and Sichina, W. (1982) Viscosity-temperature function forsorbitol from combined viscosity and differential scanning calorimetry studies. J. Phys.Chern., 86, 1540---1542.Arrhenius, S. (1916) Medde/. Vetenskepsakad. Nobe/Inst., 4, 13.Barber, E.J. (1966) Calculation of density and viscosity of sucrose solutions as a function ofconcentration and temperature. Nat. Cancer Monograph, 21,219-239.Berger, P.O. (1976) Surfactants and surface activity in sugar manufacturing. Sugar Techno/.

Rev., 3, 241-273.Blanc, J. (1970) Note sur Ie calcul des tuyauteries pour liquides visqueux. Indust. A/irn.

Agric., 87, 809-813.Bourne, M.C. (1982) Food Texture and Viscosity. Academic Press, New York, USA, pp.199-246.Breitung, H. (1956) Viscosity of technical sugar solutions. Z. Zuckerind., 6, 254-260.Broadfoot, R. and Miller, K.F. (1990) Rheological studies of massecuites and molasses, Int.

Sugar 1.,92, 107-115, 143-146.Burianek, J. (1956) Theory of sugar solutions. Listy Cukr., 72, 16-18, 31-33, 57-59.Champetier, G. and Monnerie, L. (1973) Introduction a /a Chirnie Macrornolecu/aire,Masson, Paris, France, pp. 336-350.Cortis-Jones, B., Wickam, R. and Goddard, J. (1963) The viscosity of mill syrups. Int. Sugar

J., 65, 231-234.Culp, E.J. (1982) Science and the art of low purity cane sugar crystallization. Proc. Cane

Sugar Ref. Res., Atlanta, pp. 1-15.Devillers, P. and Phelizot, R (1971) Pseudoplasticite et visco-elasticite des melasses, Sucrerie

Fram;aise, 112,37-44.Diaz Garcia, A. (1977) Estudio reologico para la prediccion del effecto de la temperatura y elcontenido de cristales sobre la viscosidad de mieles y massas cocidas de tercera. CubaAzucar, July-Sept, pp. 45-51.Doolittle, A.K. (1954) The Technology of Solvents and Plasticizers, John Wiley and Sons,New York, USA.Einstein, A. (1906) A new determination of molecular dimensions. Ann. der Physik, 19,289306 (and corrections, 34 (1911) 591-592).

Erszterle, M. (1990) Viscosity and molecular structure of pure sucrose solutions, Zuckerind. ,115,263-267.Furuse, H. (1969) Temperature dependence of equivalent rigid spheres of solute suspended inliquid. 1. Phys. Chern. Soc. Japan, 26, 583.Gebler, J. and Ciz, K. (1979) Contribution 11 l'etude des proprietes rheologiques des massescuites. Indust. A/irn. Agric., 96, 799-805.Genotelle, J. (1978) Expression de la viscosite des solutions sucrees. Indust. A/irn. Agric., 95,747-755.Geronimos, G.L. and Greenfield, P.F. (1978) Viscosity increases in concentrated sugarsolutions and molasses due to dextrans. Proc. Queens/. Soc. Sugar Cane Techno/., Mackay,Australia, pp. 119-126.Glasstone, S., Laidler, K.J. and Eyring H. (1941)The Theory of Rate Processes. McGrawHill, New York, USA, pp. 477-551.Hildebrand, J. (1971) Motions of macromolecules in liquids: viscosity and diffusivity. Science,84,490---493.Hildebrand, J. (1978) Theories and facts about liquids. Discuss. Faraday Soc., 66, 151-159.Hildebrand, J. and Lamoreaux, R.H. (1973) Fluidity and liquid structure. J. Phys. Chern.,77, 147-1473.


Hill, R.M. and Dissado, L.A. (1982) The temperature dependence of relaxation processes.J. Phys., CIS, 5171-5193.Huggins, M.L. (1942) The viscosity of dilute solutions of long chain molecules IV.Dependence on concentration. 1. Am. Chem. Soc., 64, 2716--2718.ICUMSA (1978) Referee's report, Subject 23. Rheological Properties, 1-13.ICUMSA (1990) Referee's report, Subject 12. Rheology, 271-282.Inhat, M., Szabo, A. and Goring, D.A.I. (1968) A comparison of the viscometricallydetermined hydrations of glucose and tetrahydropyran-2-ylmethanol between 10 and 65°C.1. Chem. Soc. (A), 1500--1503.Jones, G. and Dole, M. (1929) Viscosity of aqueous solutions of strong electrolytes withspecial reference to barium chloride. 1. Am. Chem. Soc., 51, 2950-2964.Kaga, T. (1961) On the viscosity of low-grade massecuites. Proc. Res. Soc. Japan Sugar

Refineries Technol., 10, 27-38.Kaganov, M. (1949) Chemical theory of molasses formation. Sakharnaia Prom., 22(3), 21-23.Kanga, M.R. and Raja Rao, M. (1978) Rheological characteristics of some industriallyimportant non-Newtonian fluids. Chem. Engng World, 13, 57-{jl.Kelly, F.H.C. (1957) The maximum viscosity of massecuites. Int. Sugar J., 59, 92-93.Khalikovskii, T.P. (1965) Influence of the principal non-sugars of molasses on its viscosity.

lzvst. Vysshik Ucheb. Zaved., Pishch. Technol., 3, 51; cited in Sugar Ind. Abstr., 27, 240.Kitterman, J.S. (1974) An improved Brookfield viscosity test. Cereal Sci. Today, 19, 285286.Knovl, E.A. and Moller G.R. (1976) Progress in automatic pan boiling. Sugar Technol.

Rev., 3, 275-309.Kot, Y.D., Yassinshava, T.V. and Suschenko, A.K. (1968) Viscous properties ofmassecuites. Sakharnaia Prom., 22, 106-125.Kraemer, E.O. (1938) Molecular weights of cellulose and cellulose derivatives. Ind. Engng

Chem., 30, 1200--1203.Mathlouthi, M. and Kasprzyk, P. (1984) Viscosity of sugar solutions. Sugar Technol. Rev.,11,209-257.

Mathlouthi, M., Bressan, C., Portmann, M.O. and Serghat, S. (1993) Role of waterstructure in sweet taste chemoreception. In Sweet Taste Chemoreception, (eds Mathlouthi,M., Kanters, J.A. and Birch, G.G.). Elsevier Applied Science, London, UK, pp. 141174.

Maudarbocus, S.M.R. (1980) The orifice viscometer: A new technique for measuringrheological properties of massecuites and molasses. Proc. Int. Soc. Sug. Cane Technol. 17thCongress, Manilla, pp. 2257-2263.

Maurandi, V. (1971) Theory and practice of syrup cooking. Ind. Sacch. Italiana, 3, 77-93.Meffroy-Biget, A.M. and Unanue, A. (1977) Equation transcendentale adaptee a I'etuderheologique des solutions d'oligomeres. CR Acad. Sci. Paris, 284, 57-59.Miller, A.A. (1963) 'Free volume' and the viscosity of liquid water. J. Chem. Phys., 38,1568-1571.

Misra, V.N., and Misra, V.P. (1977) Studies on electrolytes-nonelectrolytes interactions:viscosity behaviour of alkali halides in aqueous sucrose solutions. Carbohydr. Res., 59, 3546.Mohanty, S., Das, B.K. and Das, P.B. (1981) The thermodynamics of the Potassiumchloride-water-sucrose system from viscosity studies. Thermochimica Acta, 43, 385-391.Moulik, S.P. (1968) A proposed viscosity--eoncentration equation beyond Einstein's region.

J. Phys. Chem., 78, 4682-4684.Moulik, S.P. and Khan, D.P. (1977) Viscosities of concentrated solutions of polyhydroxynonelectrolytes, glucose, sucrose, mannitol and sorbitol in relation to solute-solventinteractions and a universal viscosity equation. Ind. J. Chem., 15A, 267-272.Ness, J.N. (1979) The rheology of massecuite by pipeline viscometry, Proc. First Nat. Conf.

Rheology, Melbourne, pp. 47-50.Ness, J.N. (1980) Massecuite viscosity, some observations with a pipeline viscometer, Proc.

Aust. Soc. Sugar Cane Technol., Mackay, Australia, pp. 195-200.Ness, J.N. (1983) On the measurement of massecuite flow properties. Proc. ISSCT 18th.

Congress V,3 Factory Commission, pp. 699-721.Ollett, A.L. and Parker, R. (1990) The viscosity of supercooled fructose and its glasstransition temperature. J. Text. Stud., 21, 344-362.

154 SUCROSE

Pauling, L. (1946) Analogies between antibodies and simpler chemical substances, Chern.Engng News, 30, 1200-1203.Pidoux, G. (1961) Expression de la viscosite entre 0 et 100°e. fndust. Alim. Agric., 78, 729741.Robinson, R.A. and Stokes, R.H. (1959) Electrolyte Solutions. Butterworths, London, UK,pp.302-313.Rouillard, E.E.A. and Koenig, M.F.S. (1980) The viscosity of molasses and massecuites,

Proc. South Afr. Sugar Technol., Durban, pp. 1-4.Schliephake, D. and Austmeyer, K. (1975) Analyse Rheologischer Mebsysteme zur Reglungder saccharosekristallisation. Zucker, 28, 546-554.Schneider, F. Schliephake, D. and Klimmek, A. (1963) Uber die Viskositat von ReinenSaccharoselossungen. Zucker, 16,465-473.Schneider, F., Emmerich, A. and Finke, D. (1967) Zur Viskositat HochkonzentrierterZuckersirupe. Proc. CfTS, Falsterbo, Tienen, Belgium, pp. 333-345.Silina N.P. (1953) Kristallizacja poslednewo utfiels. Sakharn. Promyszl., 9,14.Silin P.M. (1958) Technology of Beet Sugar Production and Refining. Piszczepromizdat,Moscou, pp. 449-450.Simha, R. (1940) The influence of Brownian movement on the viscosity of silutions. J. Phys.

Chern., 44, 25-34.Simha, R. (1952) A treatment of the viscosity of concentrated suspensions. J. Appl. Phys., 23,1020.

Slade, L. and Levine, H. (1994) Glass transitions and water-food structure interactions. InAdvances in Food and Nutrition Research (ed Kinsella, J.E.). Academic Press, San Diego,CA, USA, (in press).Smolnik, H.D. and Delavier, H.J. (1972) Fliebanomalien von Rohrzuckerfabriksmelassen.Z. Zuckerind., 22 (11), 498-506.Soesanto, T. and Williams, M.C. (1981) Volumetric interpretation of viscosity forconcentrated and dilute sugar solutions. J. Phys. Chern., 85, 3338-3341.Staudinger, H. (1932) Die Makromolekularen Organischen Verbidungen. Springer Verlag,Berlin, Germany.Swindells, J.F., Snyder, e.F., Hardy, R.e. and Golden, P.E. (1958) Viscosities of sucrosesolutions at various temperatures; tables of recalculated values. Nat. BuT. Stand. Suppl. toCiT. n° C440.Thomas, L.H. (1965) Temperature variation of viscosity and the structure of liquid. 1. Chern.

Soc., 328-335.Utsugi, H. and Ree, T. (1971) Application of the absolute reaction-rate theory to nonNewtonian flow. In Advances in Chemical Physics (ed. Progogine, I.) Wiley, New York,USA, pp. 273-287.Vand, V. (1948) Viscosity of solutions and suspensions. I. J. Phys. Chern., 52, 277-299.Van Hook, A. (1981) Growth of sucrose crystals. Sugar Technol. Rev., 8, 41-79.Van Hook, A. (1952) The place of viscosity in sugar boiling and crystallizing. The Sugar J.,

14,9-10,32.Vanwazer, J.R., Lyons, J.W., Kim, K.Y. and Cowell, R.E. (1963) Viscosity and Flow

Measurements. Interscience Publ, New York, USA.Wagnerowski, K. (1976) Equation for the viscosity of concentrated pure and impuresolutions. Gaz. Cukrow., 84, 11, 241-246, 256.Wagnerowski, K. (1983) Rationalisation du Processus d'Epuisement de la Melasse, Frenchtransl. D. Mottard-Dabrowska, APRIA, Paris, France, pp. 119-146.Wagnerowski, K. and Dabrowski, C. (1965) Uproszczona Metoda Oznaaczania Czystoaci

Normalnej Melasu, 73, 230-235, 257-263.Wagnerowski, K. Dabrowska, D. and Dabrowski, C. (1961) Szybka metoda oznaczaniarozpuszczslnoaci sacharozy w melazach. Gaz. Cukrow., 63, 262-269.Williams, M.L., Landel, R.F. and Ferry, J.D. (1955) The temperature dependence ofrelaxation mechanisms in amorphous polymers and other glass-forming liquids. J. Am.Chern. Soc., 77, 3701-3706.

7 Analysis of sucrose solutionsJ.P. LESCURE

7.1 Introduction

The role of the analyst is to give the possessor or the user of a product or asubstance quick and precise answers to any question related to itscharacterization, its dosage and its technological value. In this chapter, weintend to gather elements which may serve this purpose for all types ofhigh-sucrose solutions, from technical sugar solutions to liquid sugars orprocessed products. This paper will also deal with issues of quality, fromthe point of view of both norms and regulations.Sucrose and other carbohydrates can be easily distinguished, either bytaste or by means of easily developed physical and chemical reactions. Thetaste of sucrose is always taken as the reference taste. Its quality of tasteprovides sweetness, smoothness and mouthfeel that are difficult toduplicate.Chemical characterization of sucrose may use colorimetric reactions,oxidation-reduction properties, or chromatographic separations. Theprocess can be performed on sucrose solutions directly, on a product driedin an oven at atmospheric pressure (130 ± 2°C), in the case of sucrosesolutions, or under a reduced pressure of 3.4 kPa (34 mbar), in the case ofa reducing-sugar-enriched mixture.For colorimetric reactions, the most common reactions are performed inthe presence of sulphuric acid. Dehydration reactions occur after hydrolysisby warm drying, leading to the formation of deoxyulose, which yield to 5hydroxymethyl-2-furaldehyde (HMF), as well as further degradations ofno interest here.Colorants in the visible wavelength can be easily obtained fromchromogenic HMF, whose absorption band in the ultraviolet range is quitestrong at about 280 nm. The so-called Molisch reaction is the most usualone: the addition of a newly prepared solution of a-naphthol andconcentrated sulphuric acid leads to the formation of a highly characteristicviolet-red ring. In the presence of sulphuric acid, it is also possible to addother chromophoric radicals that shift the absorption towards visiblewavelengths, for instance, orcinol at 420 nm, anthrone at 585 nm andphenol at 480 nm. These reactions, which are not highly selective, arecommon to all sugars. They are widely used in biology for the titration of


156 SUCROSE

total sugars. They are still commonly used to reveal stains after sugarseparation by thin-layer chromatography.Sucrose does not have a free carbonyl group in its molecule and does not

give any of the reactions specific to reducing sugars. More particularly, itdoes not react with a hot copper solution in an alkaline medium. After acidhydrolysis, the glucose-fructose mixture easily reacts with a boiling coppersolution and gives a red-orange cuprous oxide precipitate.This behaviour is quite typical of a non-reducing oligoside. It is aneffective indicator of the presence of sucrose, that can be easily confirmedthrough thin-layer chromatography or spectral analysis.Thin-layer separation of sugars can be carried out on various supports:cellulose, silica gel, magnesium silicate, kieselguhr, with the use ofadequate eluent. Many operation methods can be found in the literaturepublished sometime ago, such as the bibliographical review by Berger andBorodkin (1967), or the monograph on thin-layer chromatographypublished by Stahl (1969).As already mentioned, sugar characterization involves colorimetricreactions. Several specific colorants have been specially developed,particularly a mixture of aniline and diphenylamine in a phosphoric acidmedium recommended by Berger and Borodkin (1967) in order to avoidthe interference of heterolevulosane I and II found in non-refined sugarremelts. The European Pharmacopoeia (Anon., 1992) is still using a thinlayer chromatography technique on a silica gel support. The eluent is amixture of water/methanol/acetic acid/ethylene chloride (10:15:25:50).Colour is developed with thymol in sulphuric acid at BO°C.The current development of gas chromatography and of high-performance liquid chromatography (HPLC) , which allow precise quantitativedosage, has resulted in a more restricted use of thin-layer chromatography.The latter serves mainly for identification purposes. It should be noted,however, that Poole and Poole (1994) have reviewed the most recentdevelopments of this technique.

7.2 Sucrose identification by vibrational spectroscopy

Spectroscopic techniques basically result from the interaction of a radiation,generally an electromagnetic radiation with matter. Although vibrationalspectroscopy requires specific laboratory equipment rather than mereroutine equipment, it is highly valued for its fast and accurate identificationof unknown substances. It is capable of rapidly generating highlycharacteristic data which constitute a real fingerprint of the product.

ANALYSIS OF SUCROSE SOLUTIONS 157

7.2. I Infrared

Infrared spectroscopy has been greatly modified by the advent of powerfulcomputers and of Michelson's interferometer. It has been possible for thelast 30 years to record in a few minutes Fourier-transform infrared (Ff-IR)spectra, which give a far better resolution than traditional grating andprism devices. As an example Figure 7.1 shows an Ff-IR spectrum ofcrystallized sucrose in the 4000-400 cm-1 frequency range.Information provided by this spectrum is essentially the same as thatprovided by traditional infrared. Early work by Barker et al. (1954)focused on a- and j3-anomers' characteristic bands and in pyranoidicmonosaccharides observed at 844 cm- l and 890 em-I. Shallenberger andBirch (1975) have shown the narrow bands due to free OH groups at3600 cm-1 and that due to OH group linked by hydrogen bonds around3400 em-I.In spite of a wide range of water absorption in this area, multiple

correlations allow quick titration of sucrose in the near-infrared (NIR).Recent developments of this technique are examined in the sectionconcerning sucrose titration. Comparative studies of vitreous state andconcentrated sucrose solutions were made by Mathlouthi et al. (1986) inthe range 1200-800 em-I, where water absorption is far more reduced

4000 2000 400Figure 7.1 Ff-IR spectrum of crystallized sucrose in the (4000-400 em-I) wavenumber range.

(By courtesy of M. Mathlouthi, University of Reims, France.)

158 SUCROSE

(Figure 7.2). Dupuy et al. (1993) have shown how, in the middle infrared,sample preparation may alter the actual performance.

(3)

1111

(b)

4111 2010

III

181Figure 7.2 FT-IR spectra of aqueous solution (65% w/w) of sucrose (a) in the wavenumberrange (18Q0--800 em-I) and of maximally concentrated (85% w/w) amorphous solution; (b) inthe wavenumber range (400-800 em-I). (By courtesy of M. Mathlouthi, University of Reims,

France.)


As concerns identification, it should be noted to what extent operationconditions can modify the infrared spectrum aspect. It is advisable toconduct a comparison with a standard sample of pure product in the sameconditions. Maximum absorption levels should be observed at the samefrequencies.

7.2.2 Raman spectroscopy

The use of high energy permitted accurate data lasers with sucrose solutionsto be obtained. Mathlouthi and Luu (1980) have studied the structure ofa concentrated sucrose solution (W = 60%), and have constructed a tableof characteristic frequency bands with corresponding intensities (Table7.1). This table could be extremely useful for identification purposes.Nguyen Quy Dao et al. (1992) have mentioned applications of the laserRaman technique, coupled with optical fibres, tested for the on-linecontrol of sugar solutions in the foodstuffs industry (fruit juices).

7.2.3 NMR spectroscopy

Proton spectroscopy was applied to sucrose by Lemieux and Stevens(1966). The spectrum reveals the anomer proton, bound to the carbon

Table 7.1 Raman spectroscopy: Assignment of frequencies observed in aqueous solution(according to Mathlouthi and Dang Vinh Luu (1980»*

v (em-I) Q Attributions

1628 16.4 0.56 b(HOH)1456 33.6 0.93 b(CH2)1366 45.7 0.67 w(CH2)1340 37.8 0.72 r(CH2)1266 25 0.71 1:(CH2)1130 68 0.53 b(COH)1110 60 0.20 v(C-O)endo1064 89 0.31 v(C-O)exo920 25 0.34 b(C-H)836 100b 0.10 v(C-C)746 21 0.38 b(CCO )endo(Fru)640 35.7 0.24 b(CCO )exo(Fru)600 31.4 0.45 b(OC0 1)

548 60.7 0.25 b(CCO)endo(GIc)528 69 0.17 b(CCO)exo(GIc)470 38 0.10 b(CCC)(Fru)456 36.4 0.23 b(CCC)(GIc)416 17.8 0.84 b(O-H-O)374 40 0.25 b(COC)

*Key: /, = relative intensity; Q, depolarization ratio; endo, = endocydic; exo, =exocydic; Fru, = D-fructosyl moiety; b, = taken as reference; GIc, = D-glucosyl moiety; b, =bending; w, = wagging; r, = rocking; 1:, = twisting; and v, = stretching.

160 SUCROSE

located between two oxygen atoms, the resonance doublet of which clearlydecoupled as compared with the other protons bound to other carbons ofthe rings or to the hydroxymethyl groups of sucrose.

It should be noted that Lowman and Maciel (1979) have used Fouriertransform H NMR to determine sucrose content in beet press juice, basedon time decay of water resonance in presence of Cr(III). Different authors(Mathlouthi et al., 1986; Grabka, 1993) have investigated the 13C NMRspectrum of sucrose.

7.3 Methods of titration of sucrose

The following section gives a survey of the various techniques used to dosesucrose or sucrose equivalent in technical or purified sugar solutions,especially those which are marketed under the name of 'liquid sugars', ormore or less highly inverted sugar solutions. These techniques will mainlybe used for high-sucrose processed products. They involve either physicalor chemical properties of sucrose and reducing sugars, or biologicalreactions.

7.3. I Physical methods

7.3. 1. I Polarimetry

7.3.1.1.1 Saccharimetric scale. The rotatory power due to moleculeasymmetry is high for sugars, especially for sucrose. Considering Biot'slaws obeyed by a substance for a given radiation, rotation is proportionalto the path length and to the concentration of this solution.Hence

a[alA =

c X I

defines the substance's specific rotation for the radiation considered. [a] iscommonly expressed in degrees and tenths of degrees, the length I indecimeters and the concentration c in g/cm3.However, sucrose polarimetric titration is of such importance in thesugar industry, and in trade and customs controls, that it has becomenecessary to set up an operation method and a measuring instrument scalein order to directly obtain the strength w for 100 g of analysed product.The principle is based on the use of a strictly defined mass mo of thesubstance to be analysed, the so-called 'normal weight' (or previously'typical head'). This mass is always dissolved in the same volume of waterYo, and rotation measurements are performed using a tube of constant


length, 10 , It is agreed that the rotatory power of sugar has a constant valuewhich does not depend on concentration. The measuring instrument isscaled so as to directly give the strength w for 100 g of the analysedsolution. This is the basis of the International Sugar Scale. It is the result ofparallel efforts in different countries that led to the introduction of a singlescale. The various stages can be summarized as follows:

• In Germany, Wentzke (1842-1843) attempted to determine a scaleusing a solution with a relative density of 1.100 with respect to water at17SC. This scale, based on old units, let to a 'normal weight' close to26.003 g in vacuo and to a specific rotation for sucrose:

[aHJ·5 = 66.685°

• In France, an attempt was made to define the value of 100 on the scaleusing the rotation performed by the yellow doublet of sodium,through a I-mm-thick quartz sheet at 20°C. The corresponding'normal weight' was determined after numerous investigations andwas finally fixed by the administration at 16.29 g weighed in air againstbrass counterweights, and at 16.30 g weighted under vacuum. Thecorresponding specific rotation of sucrose was:

[alba = 66.41°The lack of accuracy of these purely physical definitions led ICUMSA toabandon them, during its first meeting in 1897, and to define a commoninternational scale based on an apparent 'normal weight' of 26 g, weighedin air against brass counterweights, dissolved in 100.00 cm3 of water, thesolution being observed at 20°C using a 2 dm tube.Pure sucrose sets the value 100 on the scale. The difficulty lies inpreparing a solution from a sufficiently purified sugar in order to determinethe corresponding angle with precision. To take into account advances inthe chemical and metrologic fields, ICUMSA redefined the angle atthe value of 100 on the scale in 1904, 1932, and finally reached the 1986agreement resulting from extremely accurate and concording measurementsperformed by the Physikalisch-Technische Bundesanstalt (PTB) ,Brunswick, Germany, and by the Bureau of Standards, USA. Moreover,in order to avoid any sort of confusion with the former values, it wasdecided to change the notation of the international unit from oS to oZ.The definition of the value 100 on the scale is currently as follows.'Normal sugar solution' is defined as 26.016 g of pure sugar, weighed invacuum and dissolved in 100.00 cm3 of pure water at 20.00°C. Thiscorresponds to a solution of 26.0000 g of sugar weighed in air in normalconditions (101.3 kPA pressure, 20°C, and 50% relative humidity), in100.000 cm3 at 20.00°C.The value 1000Z on the international sugar scale corresponds to the

162 SUCROSE

optical rotation of the normal sugar solution as defined, at the wavelengthof the green rays of the isotope 198Hg (A. = 546.2271 nm in vacuum, at20.00°C through a 200-mm-long tube).In these standard conditions, the rotation angle is

[a]20.OO°C

546.2271= 40.777 ± 0.001°

The following relation is used to determine the value 1000Z at wavelengthsother than 546.2271 nm:

aO.S462271=a+ +

d+-:;:s

where A. is the wavelength and a = -0.001 7982; b = +0.276531 8; c =+0.006 557 36; and d = + 0.000010 382 5.Hence, for filtered sodium D-line:

[a]20.oo°C

589.4400= 34.626 ± 0.001°

In such conditions specific rotation for sodium D-line is expressed asfollows

=100.34.426

2.26.000= 66.59

if weighing is performed in air, or

=100.34.426

2.26.016= 66.55

if weighing is performed in vacuo.Quartz-standards are defined according to this scaleIt should be noted, however, that in current laboratory practice, theaccuracy of the saccharimeters cannot generally reach internationalstandards. In current practice, it is possible to use instruments withstandards established prior to 1986; the shift from the former S scale to theZ scale can be easily made by cutting down the result by 0.029%.Polarization is one of the most accurate measurements performed in thelaboratory. In order to preserve this accuracy, it is necessary to proceed ascautiously as possible following the adequate laboratory practice as definedby Schneider (1979). Here are some of the main principles:

• Equipment must be carefully checked. Tubes need to fulfil theICUMSA requirements: they must be closed by strictly parallel coverglasses, and should not produce any polarization when filled withwater and submitted to a gradual rotation of 180° around their axis.


• Gauged glassware should be strictly standardized with distilled water.

In most cases, defecation is unnecessary as sugar solutions are clearenough. It would otherwise be necessary to perform defecation with basiclead acetate, in the proportion of 1 cm3 of basic lead acetate of density to1.24 ± 0.01 of water, for 100 cm3 . For more detailed information, readersare referred to the ICUMSA (Anon. 1994a) methods.Hygiene and environment concerns have prompted the study of otherdefecants, but basic lead acetate is the only one to be currently recognizedworldwide.Measurements are only valid if no other polarizing substance is present.Otherwise double polarization may be necessary.

7.3.1.1.2 Double polarization. The method is based on the followingprinciple: when a sugar solution contains active substances other thansucrose, the result given by the saccharimeter is faulty.

(7.1)

where W is the solution strength and no the variation due to the presence ofother polarizing substances.In theory, it is possible to perform a selective hydrolysis of sucrose inorder to transform into an equimolar mixture of glucose and fructose. Thedirection of polarization changes because the negative rotation of fructoseis higher than the glucose one (this operation is called accordingly'inversion').After inversion, the result read on the saccharimeter can be written as

follows:

Rnz = --x W+ no

100(7.2)

where R denotes the result read after complete inversion of the normalweight of pure sucrose in 100 cm3 .Interference is suppressed by difference and the strength is given by thefollowing relation:

100 x (nJ nz)W=

100 - R(7.3)

(100 - R) is called Clerget's divisor, after the name of the first man whodeveloped this technique in 1849.

7.3.1.1.3 Techniques of double polarization. In order to be universal,relation should meet several conditions that are unfortunately not alwaysfilled:

164 SUCROSE

(1) It is based on the assumption that the specific rotation [a] does notdepend on temperature and concentration. Now, the saccharimetricscale is an approximation close enough to titrate pure sucrosesolutions, variations due to concentration are much higher for invertsugars, which are also much more sensitive to temperature.

(2) Selective hydrolysis of sucrose is extremely difficult to carry out: inan acid medium, the other oligosides can be more or less hydrolysed;through enzymatic processes it is possible to obtain a more or lesscomplete hydrolysis of raffinose and kestoses.

The following methods are noteworthy:

(a) Dutton's (1979) method of double polarization, and enzymaticprocess corrected by a separated chromatography titration ofraffinose and kestoses.

(b) The Clerget (1849) method is broadly used in France for conventional dosage of sugar in molasses. This method uses the formerFrench normal weight, namely, 16.27 g. When polarization isperformed with a saccharimenter at 26 g, recorded results have tobe corrected accordingly, that is, multiplied by the coefficient26/16.27. At 20°C, the value of Clerget's divisor is then 134 or, moregenerally, at a different temperature t (in 0C):

100 X (n ! - nz)w= X144 - 1/2 X I

26

16.27(7.4)

(7.5)

(c) The Clerget-Herzfeld (Herzfeld, 1888) method uses the international half-weight. Hence the formula becomes

200 X (n! - nz)W=------

142.66 - 1/2 X t

(d) The American Custom Laboratory Methods (Anon., 1942),provide more elaborate temperature corrections and makes itpossible, in theory, to measure sucrose and raffinose.

These all are conventional methods. Hence, operation requirements haveto be strictly applied and calculations must be done using the Clergetdivisor specific to each method.

7.3.2 Refractometry

The biunivoque relation between the refractive index of pure sucrosesolution and its concentration can be used to titrate sucrose in aqueoussolutions.Many instruments are graduated as saccharimetric scales and givedirectly the strength W of the solution for 100 g (in the sugar industry, this


magnitude is usually expressed in degrees Brix (OBx) on the Brix scale l°Bx= 1% of dry substances (DS). (This denotation is for internal use only, itdoes not have a legal value in the European Union.) For practical use ofrefractometry, readers are referred to chapter 8. Tables are to be read at20°C. Potential corrections depend on type of device used.

In 1936, Landt (1936) published correction tables for the lo-30°C rangewhich are still valid for measurements made in the air.With a dipping refractometer, air is not taken into account, andRosenbruch (1978) published specific tables for temperatures ranging from10 to 40°C. These corrections can be applied at 546 nm (green ray ofmercury) as well as at 589 nm (doublet of sodium).Development of computers has increased calculation speed, and the useof tables is gradually declining. Rosenhauer's (1966) equations can be usedto determine the strength of solutions with up to 85% sucrose for solutionsprepared in air in standard conditions (101.325 kPa, 20°C and 50% relativehumidity).

Pair = ao + aj X n + az X nZ + a3 X n3 + a4 X n4

T~e c~efficients values are a function of the wavelength used to determinen In au:

Mercury greenline,546 nm

ao - 0.337 500 359 X 105

al + 0.861 909 872 X 105az - 0.834 048 354 X 105

a3 + 0.362 171 461 X 105a4 - 0.593 099 058 X 104

Doublet of sodium,589 nm

- 0.355 515 999 X lOS+ 0.914 330 824 X 105- 0.891 061 027 X lOS- 0.389 665 844 X lOS- 0.642 726 227 X 104

Several instruments are available today which can automatically determinethe strength and display it on a digital dial. Temperature correction isusually integrated.Many marketed syrups are blends which contain varying proportions ofinvert sugars. De Whalley (1935) proposed the following formula of linearcorrection, taking into account the level of invert sugar present in asolution which concentration W is expressed for 100 g:

Wp = Wr + 0.022 X Wi

where Wp denotes the corrected concentration in % DS and Wr denotesthe apparent refractometric concentration similating the product tosucrose. This formula was recorded in the EEC regulation 79/797/ (EEC,1979).Rosenbruch (1986) wrote a more accurate third degree polynomial

correction formula, which allows good accuracy even with highly invertedsolutions:

166 SUCROSE

t!.W = (A + B X Wr + C X W/ + D X W,3) X Wi + E X W?

where A = 6.222 X 103 ; B = 2.372 5 X 10-4; C = -1.8165 X 1O--{); D =1.8906 x1O-8 ; and E = 2.328 X 10-5 .This method of correction of the refractometric concentration wasofficially confirmed through a series of interlab tests organized by theBrunswick Institute (Rosenbruch, 1990).It can be simplified for poorly inverted solutions as follows:

t!.W = (A I + B1 X Wv ) X Wi

where Al = 5.830 X 103 and B1 = 2.251 X 10-4.Use of refractometry in measuring the apparent purity of solutions isfurther developed in this chapter. Refractometry is widely used, not onlyfor the control of marketed solutions, but increasingly for on-linemeasurements with sensors whose reliability is constantly improving(Burzawa et al., 1990).

7.3.3 Polarography

Polarography is a particular application of the oxidation-reduction studieswhich were carried out using intensity-potential curves. Originally, thetechnique was developed in relation to systems using mercury dropselectrode. Similar methods employing solid electrode use the samemethodology, and are often designated by more specific terms'voltamperometry', 'pulsed amperometry', etc. Various polarographicmethods were proposed. Amid these are the lowering of the maximumpotential due to oxygen for pure sugar titration using the surface tensionproperties of refined sugar solutions, or the oxidation of carbonylgroups.Polyachenko (1956) developed a polarographic method for the titrationof fructose in invert solutions. Shul'man (1958) has introduced a techniquefor dosing both sucrose and fructose. Glucose cannot be detected, as it isentirely dissimulated by the fructose wave.These techniques would not be of much interest if not for the recentdevelopment of a pulsed amperometric method for the polarographicanalysis of OH groups by absorption on a gold electrode. This highlysensitive technique is used in chromatographic detection, as will be seen inthe section on HPLC.

7.3.4 NIR spectrophotometry

This technique has considerably developed since the mid-1980s. Itsprinciple is quite simple and is gaining notoriety (McClure, 1994). Amonochromatic beam of light runs through a sample laid in a measurementcell. Part of the energy is absorbed, and the rest is reflected. The reflected


light is integrated in an Ulbricht sphere and undergoes analysis. Byselecting a few specific wavelengths, the instrument can be standardized onthe basis of similar products previously analysed and selected to cover theentire range of products components to be a multilinear relation. Theaccuracy of the analysis depends on the choice of wavelengths, and on theextent to which the data collected for sampling is representative.In current laboratory practice, the result is rapidly achieved from theexperimentally established regression. Although, as previously mentioned,the infrared spectrum is not very selective in such conditions, thistechnique yields good results, e.g. the correlation coefficient r between thelaboratory and the automatic measurement ranges from 0.994 to 0.999 forthe dry substance content, expressed in percentage (Brix scale), of themore or less pure processed products, from syrup to molasses. Regardingsucrose analysis (from polarization), r is suitable for high purity products (r= 0.996), although it is affected by the nature of impurities.Using molasses collected from several French sugar refineries, the IRISlaboratory obtained a correlation coefficient of 0.92 using three wavelengths. In better targeted collections, taken from a single factory,Burzawa et ai. (1991) and Vaccari et ai. (1987) obtained far bettercorrelations, around or higher than 0.990. Vaccari et ai. (1990) also showedthat performance deteriorates rapidly when molasses are collected in twodifferent sites and cross-tested. Specialized literature gives severalexamples of on-line control using this technique (Vaccari et al., 1987, 1990;Burzawa et al., 1990). Readers are reminded that NIR spectroscopy wasselected for the automatization of the crystallization plant of the sugarrefinery in Bucy-le-long, France.The operation has been quite successful for the last few years. Thistechnique could be easily adjusted to other applications, especiallydownstream in the fabrication process, and in sucrose solutions development.

7.3.5 Isotope dilution

This technique consists in adding a known quantity of radioactive 14C_sucrose to the solution to be analysed. Sucrose is then extracted from thesolution and purified regardless of its origin. It is an internal standardization method involving the comparison of the radioactivity of the finalproduct with that of the radioactive sucrose added to the sample.Various purification methods have been developed by Horning andHirschmi.iller (1959) in Germany, and by Sibley et al. (1965) in the US.These methods are considered as reference methods, and are not suitablefor routine laboratory control.

168 SUCROSE

7.4 Chemical methods of analysis

In spite of their high degree of accuracy, physical methods such aspolarimetry, refractometry, and densimetry can only be used for thetitration of pure sucrose solutions. If solutions are more complex, with thepresence of reducing sugars, colorants or flavour enhancers, chemicalmethods can be employed to determine sugar content, and especiallysucrose. These methods involve reducing-sugar titration.

7.4.1 Reducing sugars

These methods are based on the reducing properties of free carbonyl groupin reducing sugars reacted with a copper solution in alkaline medium. Astotal alkaline degradation of the reducing sugars cannot be easily avoidedin the course of titration, it is important to respect all operation details inorder to achieve good reproducibility. Sucrose interferes slightly in thesetitrations and they must be corrected accordingly.

It should be noted that the simplest methodology, gravimetric separationand weighing of cuprous oxide, as described by Saillard (1923), is no longerin use.The methods officially recognized by ICUMSA and quoted in regulations or standards documents together with the Luff-Schoorl method arereviewed in the following survey:

(a) Knight and Allen (Anon., 1994b): to be applied for very lowconcentrations (0.002-0.017%) only. It uses back-titration of theexcess of copper solution after hot precipitation of the cuprousoxide by means of an EDTA solution.

(b) The Berlin Institute's Method (Anon., 1994c): to be applied tosolutions or products whose reducing sugars-content does notexceed 10%; the characteristic of this method is the use of a Mullersolution prepared with carbonate instead of sodium hydroxide inFehling's solution. It is based on the following principle: thecuprous oxide formed by the reduction of copper salts is oxidized byan excessive addition of iodine, the latter being subsequently backtitrated by sodium thiosulfate. The result has to be corrected inorder to account for water blank, blank on a cold solution andsucrose interference. Gfner (Schneider, 1979) has introducedanother method based on the same principle that only differs interms of operation instructions.

(c) Lane and Eynon (Anon., 1994d): this method has the widestapplication. It is suitable from 0.1 g of reducing sugar for 100 cm3 ofsolution. Beyond 0.8 g for 100 cm3 , work should be performed after


dilution. The method consists of performing the titration of a knownquantity of Fehling's solution, at boiling temperature, using thesugar solution in the burette. The final stage of titration is revealedby total decoloration. The end-point can be improved by theaddition of methylene blue, which then turns from the normal form(blue) into the reduced one (decolorized). The ICUMSA currentlyrecommends a constant volume method. In the presence of sucrose,correction is provided by the tables.

(d) Luff-Schoorl (Anon., 1994e): this method was introduced in variousEuropean regulations (EEC, 1971, 1979), expecially as regardsfeed. Instead of the Fehling's solution, it uses an alkaline solution ofcopper sulfate and hot sodium citrate in standardized conditions.Back-titration is carried out through reduction, in an alkalinesolution, of the excess of copper salts by potassium iodide. Iodine istitrated by sodium thiosulphate after the medium is acidified.

7.4.2 Chromatography

In spite of the development and the economic significance of the analyticaltechniques mentioned so far, we cannot but acknowledge the hugedifficulties faced by analysts in the past for mixtures of sugars with similarphysical and chemical properties.Chromatographic techniques are still greatly praised by specializedlaboratories. They have made it possible to finely separate sugars withsimilar characteristics by bringing into play partition coefficients betweenthe mobile phase that drives the mixture to be analysed, and the stationaryphase. These techniques are constantly improving. Theory of chromatography is beyond the scope of this chapter but it will only be dealt withapplication to the sugar analysis.Gas-liquid chromatography was the first technique that yielded a quickand specific dosage of sucrose in sugar compounds using another methodthan polarization. It was mostly developed in the 1970s.

7.4.2.1 Gas-liquid chromatography. The most widely used form of thistechnique is based on a partition coefficient between the mobile phase andan immobilized liquid which soaks a fixed support. This is the reason whyit is denoted by the abbreviation GLC (gas-liquid chromatography).

In spite of their light molecular weight, sugars are not sublimable mainlybecause of their instability at high temperature and of hydrogen bonds withhydroxyl groups. However, it is easy to obtain silylated derivatives whichare, despite a heavier molecular weight, easily volatile and far more stablethan sugars at high working temperatures.Devillers et al. (1974) have published a technique which was developedand used at the time to establish a mass balance in sugar factories based on

170 SUCROSE

sucrose. The technique was improved through the use of capillary columnsand it is still used for the dosage of raffinose.All other described techniques (Karr and Norman, 1974; Wong Sak Hoi,1980) present various drawbacks:

(1) long preparation time of samples;(2) preparation of the silylated derivate requires complete drying ofsample in an oven where degradations may occur; and

(3) reducing sugars polymorphism often leads to three different peaks.

Schiiffler and Morel du Boil (1984) developed a silylation technique in ahumid environment using a reagent in excess (hexamethylenedisilazane orHMDS) which reacts on water (Figure 7.3). The reaction takes place withtrifluoracetic acid. Silylation occurs very quickly (10 mn in a water-baththermostated at 80aC ultrasonically homogenized). International interlabtests showed that the technique was highly reproducible. It was awardedofficial status (ICUMSA, 1986). In the Republic of South Africa, it isused in the official analysis of sugar cane juice, but it has proved to be lessaccurate in the case of non-purified beet juice. According to Oikawa(1990), this lack of precision is due to protein interference.As regards the analysis of reducing sugars, Schiiffler and Morel du Boil(1984) have proposed reducing the number of one-doublet peaks, byperforming an oximation just before silylation. This method is elegant butit is hardly used, especially in situations where the sugar mixture is acomplex one.

x sx s

G

F

Figure 7.3 GC chromatograms according to the Schaffler method: left, sucrose syrup; right,molasses; G, glucose; F, fructose; S, sucrose; X, internal standard xylose. (By courtesy of G.

Deruy, IRIS, France.)


The use of HPLC is therefore strongly recommended for the analysis ofsugar mixtures.

7.4.2.2 HPLC. HPLC is a method widely used in many areas. As it hasbeen thoroughly described in the specialized literature, this paper will onlyoutline it briefly. An eluent liquid is injected into a separative columnusing a pump with constant flow (the regularity of the flow, especially theabsence of pulsations, is the touchstone of the technique; excellent suckerpumps, generally with double barrels, can be found in the market). At theend of the column, the liquid goes through a detection device, which isconnected to an electronic appliance designed to amplify, record andintegrate the signals. The column is generally fitted with small size particles(from 10 to several dozen !-lm), made up of a support, often chemicallybonded, which constitutes the stationary phase; the best separations areperformed with perfectly spherical particles of very small diameter.

It is most important that the flow inside the device is not disturbed inorder to avoid an anomaly in the detected signals. The analysed sample istherefore introduced through a specific valve which, in a normal position,allows for the direct flow of the eluent and an 'injection' position allowsfor the integration in the circuit of a sampling ring of constant volume(10-20 !-lm) filled with the product to be analysed. In order to avoidanomalies, the gas must be removed from the eluent before use.

It should be noted that different possibilities may be added to the basicdevice as described below:

• The column can be thermostated at temperatures different from theambient temperature, especially in order to reduce the viscosity of theeluant liquid.

• The eluant can be modified in a pre-programmed way, it is thenreferred to as the elution gradient.• Lastly, a chromophore group can be injected so as to enhance thedetection at the end of the separative column, the so-called postcolumn coloration.

By combining the three factors involved in both the separation and theselective measurement of the analyte under study, i.e. nature of thestationary phase, composition of the eluant and detection technique,various analytic techniques have been developed. They are summarized inTable 7.2.The accuracy of the method depends on the technical possibilities ofpeaks separation, on reproducibility and on the ageing of the separativecolumn, and above all on the complexity of the product.Thus, in 1990, in the course of assessing intercomparison-basedmethods, Lescure (1990) had the following results (Table 7.3) on thevarious products processed from beet sugar.

172 SUCROSE

Table 7.2 High-performance liquid chromatography: Review of the various combinationsusable for the HPLC analysis

Nature of thestationary phase

Principle ofseparation Eluent Particularities

Silica Water or water Bad separation, improved with+ amine amine

CI8 bonded silica Decreasing(or reverse phase) polarity

Bonded silica: NH2 By difference in(amine) polarity

Water

Acetonitrile/water: 80/20

Separationby increasing molecular weightsunseparated glucose and fructose

Separationby increasing molecular weightsoverlapping glucose and fructose

ProblemSchiff's basis formation

Cation exchangeresin form: Na,Ca, Pb

Anion exchangeresin

Ligand exchange, Water + metalabsorption/ acetatesdesorption

pK. of the 0.1M NaOH'alcohol' groups solution

Separationin the reverse order of molecularweightbad separation of osides

Very good separation and very highsensitivity

Table 7.3 High-performance liquid chromatography: Assessment ofrelative accuracy (CV%) for HPLC and polarization methods ofanalysis of sucrose in different sugar products (Lescure, 1990)

Products

BreiSyrupMolasses

Polarization'

0.70.41.1

Sucrose analysis byHPLC', separation of Ca2+cation exchange column

1.60.81.3

'Methods: polarization, ICUMSA method GS1/2/3, HPLC,ICUMSA method GS8--1. (Anon., 1994).

Using the same method applied to the collection of cane and beetmolasses, Schaffler (1990) obtained a repeatability variation coefficientCVr = 0.58 and a reproducibility coefficient of CVr = 1.82. With a similarmethod, the ISO (1993) obtained a repeatability variation coefficientranging from 0.65 to 3.1 for glucose titration (dextrose). It should be notedthat in this case, the technique at work is slightly more complex, as adeionization is needed to suppress the refractometric interference of thesalts present in these solutions. Deruy and Lescure (1988) published adetection technique through post-column coloration using para-aminobenzoic hydrazide (HAB) , which is extremely selective. Bugner and


Feinberg (1990) demonstrated that the reproducibility deteriorates whenproducts are dosed in complex systems such as finished products (biscuits,beverage, desserts, etc.): the reproducibility CV varies considerablyaccording to the difficulty of extraction and the analyte concentration.Recent studies have shown the great opportunities offered by pulsedamperometric detection, after sugar separation on anionic resin.Bichsel (1990) and later Peschet and Giacalone (1991) have dosed tracesof reducing sugars and higher glycosides (raffinose and kestose) in asucrose syrup. Figure 7.4 illustrates the separation achieved in the IRISlaboratory by Deruy using this technique.Before concluding this chapter, it seems appropriate to give a survey ofthe industrial applications of chromatography (see Guerain et al., 1986;Buttler et al., 1993). It is mainly applied to on-line control of variousdistilling procedures, but it might well be used in the control of other typesof processing.

7.5 Enzymatic methods

The principle of these methods is extensively described in the specializedliterature, especially in Bergmeyer's work (1974). Only specific reactionswill be described.

7.5.1 Methods description

As a general rule, dosage concerns reducing sugars, glucose and fructose,although sucrose can be easily hydrolysed through the action of /3fructofuranosidase at pH 4.5.

Glucose + FructoseSucrose~

~

/3-Fructofuranosidase

Its concentration is calculated through the titration of glucose which ispresent before and after hydrolysis. Several methods are possible:

(a) Kinetic method (Devillers et al., 1975)

~

~

Glucose oxidaseGluconic acid + H20 2

Hydrogen peroxide thus formed is used with peroxidase to oxidizethe 4-aminophenazone:

H20 2 + 4-aminophenazone ~ H20 + 4-phenylaminophenazone+ phenol

(reduced form) (oxidized form)

174

(a) A

!G

F

(b)

L

s

GF

SUCROSE

s

R

R

M

Figure 7.4 HPLC chromatograms with Carbopac ACI column (pulsed amperometrydetection). (a) Mixture of sugars: A, arabinose; G, glucose; F, fructose; L, lactose; S,sucrose; R. raffinose; and M, maltose. (b) Molasses: G, glucose; F, fructose; S, sucrose; R,

raffinose. (By courtesy of G. Deruy, IRIS, France.)


The reaction elicits a colorant which is measured at 505 nm. Thistechnique is suitable for automatic analysis using a continuous flowinstrument and yields excellent results with beet sugar.Some authors have expressed doubts regarding the use of thistechnique with cane sugar. In this case, the sucrose molecule isassociated with kestoses, trisaccharidic compounds resulting fromthe condensation of a furanosyl moiety on one of the threehydroxymethyl groups of sucrose. Kestoses, in trace amounts,interfere in sucrose titration through an enzymatic process and aremore frequent in cane than in beet.

(b) 'End point' manual titration of reducing sugars: it uses anotherreaction with hexokinase in presence of ATP (adenosine triphosphate), and glucose-6-phosphate dehydrogenase with NAPD(nicotinamide adenine dinucleotide phosphate).

GlucoseATP ~ Glucose-6-phosphate + ADPGlucose-6-phosphate ~ Gluconate-6-phosphate

+ APD + NADP + H+Glucose-6-phosphate dehydrogenase

The reaction is shifted to the right by use of a buffer of pH 7.6 whichrecombines the freed protons.Fructose also reacts with ATP and gives

Fructose + ATP ~ Fructose-6-phosphate + ADPHexokinase

When phosphoglucose-isomerase is added, the reaction can bewritten:

Fructose-6-phosphate ~ Glucose-6-phosphatePhosphoglucose isomerase

and is to be analysed as before.

NADPH is dosed by ultraviolet spectophotometry, either at 340 or at334 nm, according to the type of instrument used. Its concentration isproportional to the increase in absorbance induced by the reaction.These reactions use a costly reagent as the enzymes need to be preparedin an extremely pure form in order to retain their specificity. Generallyspeaking, products are provided by the specialized firms Boehringer orSigma. Other firms, such as Leeds & Northrup, Yellow Springs, andTacussel, sell fixed enzyme reactors. These reactors always combine areaction column or chamber with an amperometric device coupled up withan exploitation system designed to give a quantitative response afterstandardization. This equipment has been mainly designed for medicalapplications and is used mostly for sucrose dosage. Yellow Springs alsocombines invertase and fixed enzymes, and sells a special version of theequipment for the direct dosage of sucrose.

176 SUCROSE

Immobilized enzymes are also used in the so-called FIA technique (flowinjection analysis) as described by Bengtsson and Tjebbes (1991). In thiscontinuous flow technique, a pump injects a transport solution which maycontain one or several reagents. Samples are injected at regular intervalsthrough a by-pass valve similar to the injection device used in liquidchromatography. Secondary reagents can be injected at the end of thereaction.The scheme used by Bengtsson for sucrose titration is shown on Figure7.5.The first fixed enzymes reactor contains mutarotase, glucose oxidase andcatalase. It makes it possible to eliminate glucose before sucrosehydrolysis. The second one contains the same reagents as in the Devillersmethod, although in a fixed form. The other similarity with the Devillersmethod is the injection of a colorant which reacts with hydrogen peroxideand is detected by a colorimeter. The recorded height of the peaks isproportional to the concentration of sucrose.

7.5.2 Situation of enzymatic methods

The ICUMSA only investigated final stage enzymatic dosages of glucoseand fructose in 1982. At the time, reproducibility was not thought goodenough to lead to an official recommendation. It should be noted thatcontinuous flow methods have developed considerably, and have beencompared to traditional polarization methods. The methods are used tocharacterize the technological value of beets in various countries aroundthe world (France, Austria, Spain and Morocco with the Devillers method,and Sweden with the BengtssonlFlA method).The practical difficulties involved in organizing interlaboratory testingusing homogenous equipment across the world is the only factor standingin the way of their official recognition.

7.6 Determination of the syrups quality

7.6.1 Purity

Sugar syrup analysis also aims at characterizing extraneous elements whichmight be present, most often in trace amounts.

It should be recalled that percentage is frequently used for the ratio ofsucrose to dry substances. This also leads to the definition of the notion ofnon-sugar, namely, all dissolved substances other than sugar:

(non sugar) = (dry substances) - (sugar)

It is possible to dose dry substances with great precision (oven method orKarl Fischer method) but the methods are too time-consuming for current

Pum

p

gluc

ose

-f--

Suc

rose

reac

tor

~VV'-

Det

ecto

r"k

ille

r"

Inje

ctor

Com

pute

r

HP1000

Fig

ure7.5FlowdiagraminanFIA(flowinjectionanalysis)device.(AfterBengtssonandTjebbes(1991).)

er

> Z > r oo( VJ Vi o ..,., VJ C (") ~ VJ trl

VJ o r c: ::l o z VJ --....I -....I

178 SUCROSE

laboratory practice. It is therefore necessary to resort to refractometric ordensimetric determination. Dry substances are assimilated to sucrose. Themargin of error is narrower in refractometry than in densimetry. The usualdenotation is refractometric or densimetric apparent purity.

It is understood, however, that, whereas the notion of purity is veryimportant with regards to process control or to estimate a crystallizationyield in the prescribed manner, it is of little importance regardingmarketed solutions made of highly purified sugars, always purer than99.7%. Users should then try to quantify certain categories of impuritiesmore specifically inasmuch as their presence is not wanted, in particularmineral salts, which are globally dosed in the form of ash or colour.

7.6.2 Ash

As some ions are volatile, chlorides in particular, they are replaced by ionsulphate. Sulphates have a greater stability at high temperatures and fixfewer carbon particles. A double sulphatation is performed. Operationinstructions are given by the ICUMSA (Anon., 1994g). Use 15-35 g ofsyrup in a platinum dish. First, 8 cm3 of 1.84 g/cm3 sulphuric acid is added,followed by calcination at 550°C until the apparent disappearance ofcarbon. A few drops of concentrated sulphuric acid are added again,followed by a second calcination at 650°C. Resulting ash must be flaky andshould not adhere to the dish.In fact, this method remains delicate and lacks precision for white sugarsor syrups prepared with white sugar. The indirect conductimetric method isfar more reproducible and is therefore preferable. It works on the principleof the proportionality between conductivity and the concentration ofdissolved salts. Solutions with 28% (m/mH) concentration are used asconductivity then reaches its maximum and is less affected by concentration.C28 conductivity at 20°C is expressed in r-tS/cm. Conductrimetric ash isconventionally expressed for 100 g by

conductimetric ash = 6 and 10-4 X Czs

(C28 denotes measured conductivity, the index 28 indicates that work isperformed on a sugar solution at 28% m/mH).Diluting water used should be of very low conductivity «2 r-tS/cm) and

the value of C28 has to be corrected in order to eliminate waterconductivity:

e28 = C read - 0.35 X Cwater

For more details, readers are referred to the ICUMSA methods (Anon.,1994g).The colour point used by the European Union is written as follows:

1 point = 3.13 X C28

7.6.3 Colour


Measurement is made by absorption spectrophometry at 420 nm on a sugarsolution at 50 g DS for 100 g of solution.The measure is expressed in ICVMSA units (IV) = (100 X E42J(L X w

X d) where l is the length of cell, E420 is the solution absorbancy, w is theconcentration in grams for 100 g (OBx), and d is the apparent density.The solution is filtered beforehand to avoid the interference ofsuspended matter.

It should be noted that a sucrose solution can never be a true solution, asunderstood in chemistry. There is always a diffraction of light bymicroparticles (seed of crystallization). Results vary slightly according tothe device used, in function of their respective configuration. This methodhas been published by the ICVMSA (1994a).

In the European Vnion, the colour point is often written as follows:

1 point = 7.5 IV

Other impurities are taken into account in the regulations. The CODEXhas set maximum concentration in the products delivered for humanconsumption. The problem is to dose the traces in the sugar solution.

7.6.4 S02

The rosaniline method is the ICVMSA (1994b) official method. Spectrophometry at 560 nm is used to measure the colour of the suphite/rosalininecompound formed after reaction with a formaldehyde solution. Theaddition order of reagents has to be strictly followed to ensure the suitableand reproducible development of colour. Standardization starts with afreshly prepared range.Rosaniline (or decolorated fuschine) is considered dangerous for thehealth. This is the reason why it is currently replaced by the ICVMSA(Anon., 1994c) method developed by the Boehringer firm. Sulphiteoxidase forms hydrogen peroxide which is reduced by NADH peroxidaseand gives colourless NAD+ at 340 nm in presence of sodium hydroxide.The result is obtained by measuring the difference in absorbancy.

7.6.5 Heavy metals

Atomic absorption spectrophotometric methods are the most widely usedtoday.The ICVMSA has adapted some methods for use with arsenic and leadalthough it continues to use colorimetric methods for lead, copper, arsenic,and iron.

180 SUCROSE

The AOAC (1990) has proposed atomic absorption spectrophotometricmethods.

7.7 Microbiology

Contamination by pathogen strains is hardly a major risk, as shown by theinvestigation led by Gireaudot-Liepmann and Catteau (1992). Whenpathogen strains (e.g. Proteus mirabilis, Salmonella typhimurium,Escherichia coli pathogen strain, Escherichia coli ATCC 10537, Salmonellamontevideo, Pseudomonas aeruginosa) are introduced into a liquid sugar(marketed product taken during processing), whatever regenerationprocess in used to avoid osmotic shocks, a decrease in the number of viablebacteria can be observed with time, and they completely disappear after an18 h stabilization period. Bacteria which develop or resist in a concentratedsugar solution are thus more osmophilic, and noxious for applications orthe preservation of products containing insufficiently sterilized syrups,although they are far less dangerous than the pathogen strains mentionedabove. In particular moulds and yeasts may develop on the surface andgradually contaminate the whole mass through convection movements. Itis also necessary to count total mesophilic germs (incubation at 30°C), totalthermophilic aerobic germs, flat-sour germs, and HzS producing anaerobicthermophilic germs.

It should be recalled that 'flat-sour' germs can turn glucose into acid, andthat they are characterized by a lowering of pH level, as shown by thepurple bromocresol (PBC) indicator which turns from violet to yellow inthe form of a halo around the colonies. HzS producers are easily identifiedby adding iron citrate in the solution which turns black as a result ofsulphide formation under the action of microorganisms.Besides general recommendations on foodstuffs hygiene, there are noregulatory norms in the field of microbiology. Guarantees are determinedby mutual agreement between supplier and the buyer, though it is ofcommon practice to adhere to norms set by American traders.

(a) National Canners' Association (1970) standards: They mainlyconcern crystallized sugar and are to be performed on five samples.They set the following limits:

• less than 150 total thermophilic germs for 10 g in a sample, andon average fewer than 125 germs for 10 g in the 5 samples,

• less than 75 'flat-sour' germs for 10 g in a sample, and onaverage fewer than 50 germs for 10 g in the 5 samples.

(b) National Soft Drink Association Norms (American Bottlers ofCarbonated Beverages, 1962): They are more specific to liquid


sugars and are to be performed on 20 samples. They set thefollowing limits:

• on average fewer than 100 germs for 10 g of dry equivalent,only one sample can exceed 200 germs and it is not to beincluded in the overall average if all the other samplingsanalysed on the same day fulfil the previous requirement,

• less than 10 yeasts for 10 g of dry equivalent, only one samplecan exceed 18 and it is not to be included in the overall averageif all the other samplings analysed on the same day fulfil theprevious requirement,

• less than 10 moulds for 10 g of dry equivalent, on average, onlyone sample can exceed 18 and it is not to be included in theoverall average if all the other samplings analysed on the sameday fulfil the previous requirement.

Most of the bacteriological methods of analysis which may be applied tosugars are described in the specialized literature, particularly in IRIS(Anon., 1984) and ICUMSA (Anon., 1994).

7.8 Standards and regulations

Unlike products to be processed that are traded by mutual agreementunder the responsibility of contracting parties, foodstuffs ready forconsumption have to conform to a number of rules established byInternational Organizations in view of protecting consumer's health.Quality standards for sugars in the CODEX Alimentarius are set by theFAO (Food and Agriculture Organization), which is the United Nations'Special Commission on Food and Agriculture, and the World HealthOrganization. The European Regulation 79/796 (EEC, 1979) is enforceableafter transposition in the national legislations of all European states.Sugar is also used in pharmacology for preparations and specialties.Pharmacology is concerned with and has the duty of averting all sorts ofrisks. It established regulatory rules for its own use that are graduallybecoming international through the efforts towards harmonization of themain pharmacopoeia, that is, European, American and Japanese.In all market organizations, decision-making authorities set a number ofrules in order to determine qualities and quantities that are to be taken intoaccount, e.g. such as the disposition of the sugar regulation which isenforceable in all member-states of the European Union.Besides what they have primarily been designated for, these documentsare often used as reference tests in many ordinary transactions. Althoughthe survey cannot be exhaustive owing to frequent updatings in this area,

182 SUCROSE

the following section is an attempt to underline the main qualitative normsand the analysis methods recommended by decision-making authorities.

7.8.1 Codex Alimentarius

It does not include industrial norms specific to liquid sugar. By analogywith the dispositions concerning crystallized sugars, it should be noted thatliquid sugars have to respect the limits set for traces of contaminatingagents, namely:

• conductimetric ash: less than 0.1 g per 100 g of dry substances,• colour: less than 150 ICUMSA units,• S02: less than 20 mg/kg of dry substance,• arsenic: less than 1 mg/kg of dry substance,• copper: less than 2 mg/kg of dry substance, and• lead: less than 0.5 mg/kg of dry substance.

The Codex also draws attention to the fact that sugars must comply withthe general rules on foodstuffs regarding:

• pesticide residues,• hygienic rules, particularly as regards microbiology and parasites, aswell as products proceeding from microorganisms and liable to bedamaging to health.

As the Codex does not have a specific analysis method for sugar, itrecommends the ICUMSA methods for invert sugars, conductimetric ash,colour, and S02, and the AOAC methods for arsenic and copper titration.

7.8.2 The European Regulation 79/796

Unlike the Codex, the European Regulation describes a number ofcategories: liquid sugar, white liquid sugar, inverted white liquid sugar,invert sugar syrup, invert white sugar syrup. Dry substance content shouldbe determined by refractometry on the basis of Rosenhauer's equation andwith de Whalley's correction in the case of reducing sugars. Invert sugarsshould be determined according to the Luff-Schoorl method, however adispensation is allowed for if states opt for the Lane and Eynon methodwith constant volume.

7.8.3 Pharmacopoeia

According to the latest trends, sulfated ash is to be replaced withconductimetric ash following the pharmacopoeia's own procedure, that isto say, at 25°C instead of 20°C for the ICUMSA. Levels of invert sugarsshould not exceed 0.04 g for 100 g and sulphites 15 ppm. For the


pharmacopoeia's own methods, users are referred to reference works inthe countries concerned.

7.8.4 Sugar regulation

Regulating the total sugar in a sugar syrup, the regulations in force in themember states of the European Union provide for the application of thechemical method according to Lane and Eynon on an inverted solutionaccording to Clerget-Herzfeld. The total content is converted into sucroseafter multiplication by 0.95.

References

American Bottlers of Carbonated Beverages (1962) Standards and Tests Procedures for'Bottlers' Granulated and Liquid SURar.Anon (1942) Custom Laboratory Methods or Acid Methods of the Association of OfficialAgricultural Chemists. In Polarimetry, Saccharimetry and the Sugars (Circular of the NBSC440). Washington DC, USA, pp. 155-157.Anon (1992) Saccharose. Pharmeuropa, 4(4), 306-308.Anon (1984) IRIS Methodes d'Analyse. IRIS, Villeneuve d'Ascq, France.Anon (1994) ICUMSA Methods Book. (a) GS 2/3-1; (b) GS 2-5; (c) GS 1/3/4-5; (d) GS 1173; (e) GS 4-9; (f) GS 1/3/4/7/8-11; (g) GS 2/3-17. British Sugar Technical Centre, Colney,Norwich, UK.AOAC (1990) Official Methods of Analysis 952-12 and 971-20. Arlington, Virginia.Barker, S.A., Bourne, E.J., Stacey, M. and Whiffen, D.H. (1954) J. Chem. Soc., 171-176.Bengtsson, M. and Tjebbes, J. (1991) Automatic enzymatic determination of true sucrose inbeet and molasses. Int. Sugar. 1., 93 (1110), 121-125.Berger, P.D. and Borodkin, S.E. (1967) The application of thin-layer chromatography tothe separation of carbohydrates in sugar refining. Int. Sugar J., 69 (1), 3-7.Bergmeyer, H.U. (1974) Methods of Enzymatic Analysis (Vol. 1). Academic Press, NewYork, USA.Bichsel, S. (1990) ICUMSA Proceedings (20th Session). ICUMSA Publication,Peterborough, UK, pp. 352-357.Bugner, E. and Feinberg, M. (1990) Protocole de validation d'une methode de dosage dessucres simples par analyse interlaboratoire en milieu industriel. Analusis, 18 (10), 60D-607.Bugner, E. and Feinberg, M. (1992) Determination of mono and disaccharides in foods byinterlaboratory study: quantitation of bias components for liquid chromatography. J.AOAC Int., 75 (3).Burzawa, E., Groult, M., Melle, M., Philament, G. and de Pellegars (1990) Quelquescapteurs et analyseurs en ligne au banc d'essai. Ind. Alim. Agric., 107 (7-8), 659--{j64.Burzawa, E., Melle, M. and Groult, M. (1991) Application de I'analyse dans Ie procheinfrarouge au contr6le en ligne de la purete. CITS Proceedings (19th General Assembly).General Secretariat of CITS, Rain am Leck, Germany, pp. 457-476.Buttler, J.B., Johanson, K.A.J., Gorton, L.G.O. and Marko-Varga, G.A. (1993) On linefermentation process monitoring of carbohydrates and ethanol using tangential flowfiltration and column liquid chromatography. Anal. Chem., 65 (19),2629-2636.Clerget, T. (1849) Ann. Chim. Phys., 26, 175.Deruy, G. and Lescure, J. P. (1988) Dosage des sucres reducteurs separes par chromatographie11 haute performance avec coloration post-colonne. Sucrerie Fran!;., 129 (123), 117-124.Devillers, P., Cornet, C. and Detavernier, R. (1974) Dosage du saccharose et bilan du sucre.

Ind. A lim. Agric. 91 (7-8), 833-839.

184 SUCROSE

Devillers, P., Detavernier, R. and Roger, J. (1975) Application du dosage enzymatique dusaccharose en sucrerie pendant la campagne 1974-75. Sucrerie Fram;., 116 (7), 299-307.De Whalley, H.C.S. (1935) Int. SugarJ., 27 (441), 353-355.Dupuy, Nathalie, Huvenne, J.P. and Legrande, P. (1993) Etude critique de methodesd'echantillonnage pour I'analyse quantitative par spectrometrie moyen infrarouge dans Iedomaine de I'agro-alimentaire. Ind. Alim. Agric., 110 (1-2), 5-15.Dutton, J. (1979) Double polarimetric method. In Sugar Analysis (ed. Schneider, F.).ICUMSA Publication, Peterborough, UK, pp. 30-32.EEC (1971) Ottic. J. EC, 12/7/71, EEC 71/250.EEC (1979) Ottic. J. EC, 22/9179, EEC 791796.Gireaudot-Liepmann, M. F. and Catteau, M. (1992) Institut Pasteur de Lille, Rapportd'etude pour Ie SNFS Lille, France.Grabka, J. (1993) Etude de la structure chimique des saccharates et des sucre-carbonates decalcium. Ind. A lim. Agric., 110 (10), 714-719.Guerain, J., Cogat, P., Dumoulin, E., Azais, B., Duarte, A., Isambert, A. and Ghassemlou,B. (1986) Etude de capteurs en vue de I'automatisation d'un procede. Application a ladistillerie. Ind. Alim. Agric., 103 (1), 5-12.Herzfeld, A. (1888) Z. Zuckerind., 38, 699Horning, H. and Hirschmi.iller, H. (1959) Bestimmung des Saccharosegehalts in Zuckerri.ibendurch Isotopsen-Verdi.innungsmethode. Z. Zuckerind., 9 (10), 499-507.ISO (1993) Sirop de glucose et polyols - composition - methode par chromatographie liquideaHaute Performance. ISO/CO 10504. AFNOR, Paris.Karr, J. and Norman, L.W. (1974) The determination of sucrose in concentrated Steffenfiltrate by G.L.c. J. Am. Soc. Sugar Beet Technol., 18 (1), 53-59.Landt, E. (1936) ICUMSA Proceedings (9th Session). ICUMSA Publication, Peterborough,pp.22-25.Lemieux, R.U. and Stevens, J.D. (1966) The proton magnetic resonance spectra andtautomeric equilibria of aldoses in deuterium oxide. Can. J. Chern., 44 (3), 249-262.Lescure, J.P. (1990) Beet sugar processing, subject G8 Referee's report ICUMSA

Proceedings, (20th Session). ICUMSA Publication, Peterborough, UK, pp. 101-118.Lowman, D.W. and Maciel, G.E. (1979) Determination of sucrose in sugar beet juices bynuclear magnetic resonance spectrometry. Anal. Chem., 51 (1),85-90.Marchetti, G., (1990) Application of a NIR on-line automatic analyser system in a beet sugarfactory. Int. Sugar J., 92 (1l02), 210-215.Mathlouthi, M. and Dang Vinh Luu (1980) Laser-Raman spectra of D-Glucose and sucrosein aqueous solution, Carbohydr. Res., 81, 203-212.

Mathlouthi, M., Colli, A.L. and Koenig, J.L. (1986) Spectroscopic study of the structure ofsucrose in solid amorphous state and in aqueous solution. Carbohydr. Res., 147, 1-10.McClure, F.W. (1994) Near-infrared spectroscopy: the giant is running strong. Anal. Chem.,66 (1), 43A-52A.National Canners' Association (1970) Research Information 159. Washington, DC.Nguyen Quy Dao, Jouan, M. and Plaza, P. (1992) L'evolution de la technique despectrometrie Raman laser avec utilisation de fibres optiques. Spectra, 168, 8-16.Oikawa, S. (1990) ICUMSA Proceedings (20th Session) ICUMSA Publication,Peterborough, UK, p. 107.

Peschet, J.L., Giacalone, A. (1991) Un nouveau concept en analyse des sucres. Lachromatographie ionique couplee a I'amperometrie pulsee. Ind. Alim. Agric., 108 (7-8),583-586.Polyachenko, M.M. (1956) Polarographic determination of invert sugar. Sugar Ind. Abstr.,20 (2-3), 40.

Poole, C. F. and Poole, S. K. (1994) Instrumental thin-layer chromatography. Anal. Chem.66 (1), 27A-36A.Rosenbruch, K.J. (1978) ICUMSA Proceedings, (17th Session). ICUMSA Publication,Peterborough, UK, pp. 166-174.

Rosenbruch, K.J. (1986) Refractice index, subject 12 Referee's report. ICUMSA Proceedings(19th Session). ICUMSA Publication, Peterborough, UK, pp. 199-212.Rosenbruch, K.J. (1990) Refractive index, subject 13 Referee's report. ICUMSA Proceedings(20th Session). ICUMSA Publication, Peterborough, UK, pp. 283-289.


Rosenhauer, K. (1966) Refractive index, subject 12 Referee's report. ICUMSA Proceedings,(14th Session). ICUMSA Publication, Peterborough, UK, pp. 65-73.Saillard, E. (1923) Sucrerie de Betterave: 1- Les Methodes d'Analyses, 137-140.Schaffler (1990) Sucrose, glucose and fructose in cane molasses by cation exchangechromatography. ICUMSA Proceedings (20th Session). ICUMSA Publication,Peterborough, UK, pp. 248-253.

Schaffler, K.J. and Morel du Boil, P.G. (1984) A review of gas chromatography in the SouthAfrica Sugar Industry. Development and application of accurate methods for sugaranalysis. Sugar Techno!. Rev., 11 (2),95-185.Schneider, F. (1979) Sugar Analysis. ICUMSA Publication, Peterborough, UK, pp. 171-180.Schallenberger, R.S. and Birch, G.G. (1975) Sugar Chemistry. The AVI PublishingCompany, Inc., Westport, cr, USA.Shul'man, M.S. (1958) Sakhar. Prom., (1), 35-37 and abstr. Int. Sugar J., 60 (720), 372.Sibley, M.J., Eis, E.G. and McGinnis, R.A. (1965) Determination of the true sucrosecontent of sugar beets and refinery products by isotope dilution. Anal. Chern., 37 (13),1701-1703.

Stahl, E. (1969) Thin-Layer Chromatography. Springer-Verlag, Berlin, Germany.Tsang, W.S., Bengtsson, M., Tjebbes, J. and Clarke, M.A. (1991) Ion chromatogrpahy,flow injection analysis and other techniques for the future. Zuckerind., 116 (1), 42-47.Vaccari, G., Mantovani, G., Sgualdino, G. and Goberti, P. (1987) Near infrared spectroscopy utilization for sugar products analytical control. 24th General Meeting of the ASSBT.Vaccari, G., Mantovani, G. and Sgualdino, G. (1990) The development of near infrared(NIR) technique on-line in the sugar factory. Sugar J. 53 (5), 4-8.

Wentzke, K. (1842-1843) J. Prokt. Chern., 25, 101.Wong Sak Hoi, Y.L. (1980) ISSCT Proceedings, 17th Congress, (3), pp. 2152-2163 and(1982) Gas-liquid chromatographic determination of fructose, glucose and sucrose in canesugar products. Int. Sugar J., 84 (999), 68-72.

8 Physical propertiesP. REISER, G.G. BIRCH andM. MATHLOUTHI

8.1 Introduction

Although the physical properties of sucrose have been studied for nearlytwo centuries, there is a need to gather the numerous informationdispersed in different sources in the same book. The tables included in thischapter are a critical selection of data published in the sugar literature andare by no means an exhaustive compilation of all the knowledge in thefield. The three forms under which sucrose may be found in the laboratoryor the factory are the crystalline, amorphous and aqueous solution. This isalso the order of presentation of the physical properties selected andestimated of certain utility to the reader. Some of the properties(solubility, viscosity) reported here are more thoroughly developed inother chapters of the book. Sucrose is one of the purest chemicals availableat a low price and this is probably the reason why it has been always used asa standard for calibration of densimeters, viscosimeters, refractometers,polarimeters, etc. Although the modern computers offer huge possibilitiesof modeling and rapid computation, nothing can replace experimentalwork. That is why most of data listed in the following tables have as theirorigin experimental determinations.

8.2 Properties of the crystal

Sucrose crystallizes as an anhydrous monoclinic crystal. It belongs to spacegroup P2 1• Its structure was one of the earliest to be precisely determinedby neutron diffraction (Brown and Levy, 1963) and further refined usingX-ray (Hanson et al., 1973) and neutron diffraction (Brown and Levy,1973) data. Structural characteristics of crystalline sucrose (see chapter 2)and crystallization aspects (see chapter 3) including morphology and effectof impurities are given elsewhere in this book. The data reported here areintended to complement information given in the other chapters or tocover aspects of practical interest.


PHYSICAL PROPERTIES 187

8.2. J X-ray crystallinity

The crystal data are reported in chapter 2, the Miller indexes of thedifferent faces of the sucrose crystal in chapter 3 and X-ray diffractogramin chapter 4 of this book. However, from a practical point of view, it isrelevant, having a white powder looking like sugar and wishing to check ifit is crystalline sucrose to have a tool for verification of the X-raydiffractogram. This is achieved through ASTM (American Society forTesting Materials) card index reported in Table 8.1.

8.2.2 Melting point

Melting point is another means of ascertaining the nature of a substance.Unfortunately, most of the carbohydrates decompose or dehydrate attemperatures very close to their melting point. The presence of impurities,moisture or small amounts of non-crystalline sucrose may modify theresult. The generally admitted melting point is 186°C, although valuesranging from 182 to 192°C were reported even in recent literature (Roos,1993).

Table 8.1 ASTM index card for sucrose crystal'

dA 4.70 3.59 7.54 10.58 C1zH 220 11

//11 100 100 70 16 Sucrose

dA //11 hkl dA lilt

10.59 16 100 3.15 127.54 70 001 2.876 306.95 40 101 2.794 206.69 60 110 2.735 145.71 34 011 2.670 125.43 12 111 2.578 75.27 12 200 2.513 104.70 100 111 2.479 114.52 63 210 2.408 104.36 30 020 2.343 224.25 33 211 2.301 84.03 40 120 2.254 143.93 12 201 2.178 93.78 20 021 2.068 103.68 15 121 1.898 53.59 100 211 1.850 73.52 40 300 1.794 73.44 7 1.694 13.36 10 1.670 23.24 14 1.639 3

'dA, interplanar distance in A; 1Il» relative intensity; hkl, Miller indexes.

188 SUCROSE

8.2.3 Density

The density of sucrose crystal may be derived from crystallographic data(see chapter 2). It has been measured at 15°C by Plato since 1901 and thevalue found (Q = 1587.9 kg/m3) is still valid for practical purposes.Dependence of the density Q of crystallized sucrose on temperature (t)was calculated (Ciz and Valter, 1967) using the following equation:

Qzo

Q = 1 + 1.116 X 104 X (t - 20)

with reference value given by Helderman (1927): Qzo = 1588.4 kg/m3 .

8.2.4 Compressibility

The cubic compressibility coefficients (k) of sucrose crystals weredetermined (Bridgman, 1933) using the following relation:

k=a+bXP

The constants a and b are given in Table 8.2 in the intervals of pressure P(0 = 12 X 105 kPa) and temperature (3Q-75°C).

8.2.5 Granulometry

Crystallized sugar is commercially available with various grain sizesadapted to different food applications. Weight, surface area and volume ofsugar crystals of different mesh or screen openings are listed in Table 8.3.Bubnik and Kadlec (1993) obtained similar results when computing surfacearea and number of crystals as a function of linear dimension.

8.2.6 Electrical properties

Sucrose crystals were reported to be triboluminescent, i.e. they emit lightupon fracture (Hirschmiiller, 1953). Because of the polar character of thesucrose molecule, and the dipole-dipole interaction nature of the hydrogen

Table 8.2 Cubic compressibility coefficient (k) of sucrose crystals(From Bridgman, 1933)*

Temperature("C)

3075

Pressure(x 105 Pa)

0-120000-12000

a b(x 105 Pat! (x 105 Pat2

6.930 X 10-6 -161.1 X 10-12

7.486 X 10-6 -184.2 X 10-12

k = a + b x p. a and b are constants, p is pressure (105 Pa), k [(105

Pat!) may be calculated by means of the values in the table.


Table 8.3 Calculated weight, surface area, and volume of sugar crystals (from Pancoast andJunk, 1973)

Mesh Screen Weight of Number of Surface area Volume Surface arealopening crystals crystals of crystal of crystal mg of crystal(mm) (mg) per mg (mm2) (mm3) (mm2)

5 3.962 69.1 0.014 69.5 43.6 1.0046 3.327 40.9 0.024 49.0 20.7 1.2007 2.794 24.2 0.041 34.5 15.2 1.4268 2.362 14.6 0.068 24.7 9.25 1.6909 1.981 8.65 0.012 17.4 5.45 2.01010 1.651 5.00 0.200 12.1 3.16 2.42012 1.397 3.02 0.330 8.63 1.91 2.86014 1.168 1.77 0.560 6.03 1.12 3.41016 0.991 1.08 0.930 4.34 0.681 4.02020 0.833 0.642 1.560 3.08 0.405 4.79024 0.701 0.382 2.620 2.17 0.241 5.68028 0.589 0.227 4.400 1.54 0.143 6.79032 0.495 0.134 7.460 1.08 0.0845 8.06035 0.417 0.0805 12.400 0.768 0.0508 9.54042 0.351 0.0479 20.900 0.545 0.0302 11.4048 0.295 0.0285 35.100 0.385 0.0180 13.5060 0.246 0.0165 60.600 0.267 0.0104 16.2065 0.208 0.00999 100.00 0.191 0.0063 19.1080 0.175 0.00595 168.00 0.135 0.00375 22.70100 0.147 0.00352 284.00 0.0957 0.0222 27.20115 0.124 0.00212 472.00 0.0681 0.00134 32.10150 0.104 0.00125 800.00 0.0479 0.000788 38.30170 0.088 0.000755 1320.0 0.0342 0.000476 45.30200 0.074 0.000450 2220.0 0.0242 0.000284 53.70250 0.061 0.000252 3970.0 0.0165 0.000159 65.50270 0.053 0.000165 6060.0 0.0124 0.000104 75.10325 0.043 0.000088 11400.0 0.00817 0.0000555 93.00

bonding in the crystal, sucrose crystals possess a dipole moment anddielectric constant. Dipole moment was reported (Landt, 1934) to be equalto 3.1 X 10-18 C m and dielectric constant CT = 3:5-3.85 depending onorientation (Narayana, 1950). According to Pavlik (1939) a piezoelectriceffect (emission of electricity when pressure is exerted) has been observedon sucrose crystals. However, no electrical conductivity value was reportedfor sucrose as it is a non-conducting material.

8.2.7 Specific heat

The specific heats (Cp in kJ/kg K) of crystalline sucrose in a temperaturerange from 0 to 100°C are given in Table 8.13 with the values reported byLyle (1957) for aqueous solutions. Cp was also calculated for sucrosecrystals using equation of Anderson et al. (1950):

Cp = 1.1269 + 4.524 X 1O-3 .t + 6.24 X 1O--{) X .f

and the computed values found different from Lyle's results.

190 SUCROSE

8.3 Properties of amorphous sucrose

Occurrence and practical importance of amorphous sugar are described inchapter 4 as well as its structure and some of its thermal properties.

8.3.1 Density

Because of its structure, it is easily understandable that the density ofamorphous sucrose is lower than that of the crystal. Although differentstructures and packing of the molecules may be obtained from the differentmethods of amorphization of sucrose and consequently density may alsovary, the value of 1507.7 kg/m3 reported by Plato (1901) can be given as anexample.

8.3.2 Specific heat

The specific heat of amorphous sucrose was found to be about 1.432 kJ/kgK between 22 and 25°C.

8.3.3 Glass transition, recrystallization and melting

Differential scanning calorimetry (DSC) was used (Roos and Karel, 1990)to determine the glass transition (Tg), recrystallization (Tc) by moisturesorption and melting (Tm) of the rearranged amorphous sucrose aftersubmission to different equilibrium relative humidities. The values of Tg ,

Tc and Tm are listed Table 8.4.

8.3.4 /3C NMR spectra of amorphous sucrose

As already mentioned (see chapter 4) the structure of amorphous sucrosedepends on the method of preparation. Tentative assignments of CP-MAS

Table 8.4 Glass-transition temperatures (Tg ± standard deviation) for amorphous sucroseequilibrated to varying equilibrium relative humidities in the surrounding air (from Roos andKarel, 1990)

Water activity' H20 (g/100 g) Tg (0C) Tc (0C) Tm (0C)

0.00 0.0 56.6 ± 3.4 104.4 ± 2.3 183.5 ± 1.20.11 1.4 37.4 ± 8.0 83.7 ± 7.6 172.1 ± 4.20.23 3.8 27.9 ± 2.4 75.1 ± 4.1 165.0 ± 2.10.33 4.7 12.6 ± 0.9t 57.4 ± 1.2

'Water activity at 25°C, obtained with P205 (0.00) and saturated solutions of LiCi (0.11),CH3COOK (0.23) and MgCI 2(0.33).tMoisture content allows crystallization at room temperature.


Table 8.5 Tentative assignments DC chemical shifts (ppm downfield from Me4Si) for sucrosein various physical states (from Mathlouthi et al., 1986; Jones et al., 1979)

Freeze-dried Quenched melt Crystalline D 20 Solution Assignments

104.7 104.7 102.6 104.5 C-2'93.4 93.4 93.5 92.8 C-I

83.1 82.2 C-5'82.4 81.35 82.0 77.3 C-3'

73.9 74.8 C-4'73.6 73.7 73.2 73.4 C-3

73.3 C-572.1 71.9 C-268.2 70.0 C-4

65.3 65.0 66.3 63.2 C-6'61.3 61.8 61.5 62.2 C-I'

60.2 61.0 C-6

(cross-polarization magic angle spinning) BC NMR spectra chemical shiftsof amorphous, crystalline and aqueous sucrose solutions were proposed(Jones et al., 1979; Mathlouthi et al., 1986). They are reported in Table 8.5.

8.4 Aqueous solutions

The physical properties of sucrose solutions are important both forlaboratory studies (structure, nucleation and crystal growth), industrialpractice (beverage, confectionery, candying) and molecular biologyinvestigations (to produce the suitable gradient for separation of biologicalmaterials). Therefore, the information listed in the following tables mightmeet more than the interest of the sugar profession only.

8.4. J Concentration units

Although the subject is treated in detail in chapter 5 and Table 5.1 is givenfor sucrose concentrations expressed in different units with 5% dry matterincrement, it may be of relevance to complement chapter 5 with a tablegiving the different values of concentration with 1% dry matter increment.(see Table 8.6). Data reported in Table 8.6 are computed using equations(5.4), (5.6), (5.8), (5.9), (5.11), and (5.12).

8.4.2 Solubility

One of the most important properties of sucrose is its high solubility inwater. Solubility in pure water and impure solutions is essential indetermining the saturation conditions in boiling and crystallizing the sugarin cane or beet sugar factories. Tables of solubility exist for more than one

Table 8.6 Expression of various sucrose concentrations at 200e

Dry SW WS C Cm Cm Molarsubstance (g sucrose (g water (g sucrose (mol sucrose (mol sucrose fraction,DS(%) per g water) per g per litre per kg water) per kg X m (mol

sucrose) solution) solution) per mol)

0 0 0 0 0 01 0.0101 99.00 10.02 0.02951 0.02927 0.000532 0.0204 49.00 20.12 0.05963 0.05878 0.001073 0.0309 32.33 30.30 0.09036 0.08851 0.001624 0.0417 24.00 40.55 0.1217 0.1185 0.002195 0.0526 19.00 50.89 0.1538 0.1487 0.002766 0.0638 15.67 61.31 0.1865 0.1791 0.003357 0.0753 13.29 71.81 0.2199 0.2098 0.003948 0.0870 11.50 82.39 0.2540 0.2407 0.004559 0.0989 10.11 93.06 0.2889 0.2719 0.00517

10 0.1111 9.00 103.80 0.3246 0.3933 0.0058111 0.1236 8.09 114.60 0.3611 0.3349 0.0064612 0.1364 7.33 125.60 0.3984 0.3668 0.0071213 0.1494 6.69 136.60 0.4365 0.3990 0.0078014 0.1628 6.14 147.70 0.4756 0.4314 0.0084915 0.1765 5.67 158.90 0.5155 0.4641 0.0091916 0.1905 5.25 170.20 0.5565 0.4971 0.0099217 0.2048 4.88 181.50 0.5984 0.5303 0.0106618 0.2195 4.56 193.00 0.6413 0.5638 0.0114119 0.2346 4.26 204.50 0.6853 0.5976 0.01218

20 0.2500 4.00 216.20 0.7304 0.6316 0.0129821 0.2658 3.76 227.90 0.7766 0.6659 0.0137922 0.2821 3.55 239.80 0.8240 0.7005 0.0146123 0.2987 3.35 251.70 0.8726 0.7354 0.0154624 0.3158 3.17 263.80 0.9226 0.7706 0.0163325 0.3333 3.00 275.90 0.9738 0.8060 0.0172326 0.3514 2.85 288.10 1.026 0.8418 0.0181427 0.3699 2.70 300.50 1.081 0.8778 0.0190828 0.3889 2.57 312.90 1.136 0.9142 0.0200429 0.4085 2.45 325.50 1.193 0.9508 0.02103

30 0.4286 2.33 338.10 1.252 0.9878 0.0220431 0.4493 2.23 350.90 1.313 1.025 0.0230832 0.4706 2.13 363.70 1.375 1.063 0.0241533 0.4925 2.03 376.70 1.439 1.100 0.0252534 0.5152 1.94 389.80 1.505 1.139 0.0263735 0.5385 1.86 403.00 1.573 1.177 0.0275436 0.5625 1.78 416.30 1.643 1.216 0.0287337 0.5873 1.70 429.70 1.716 1.255 0.0299638 0.6129 1.63 443.20 1.791 1.295 0.0312239 0.6393 1.56 456.80 1.868 1.335 0.03253

40 0.6667 1.50 470.60 1.948 1.387 0.0338741 0.6949 1.44 484.50 2.030 1.415 0.0352542 0.7241 1.38 498.50 2.116 1.456 0.0366843 0.7544 1.33 512.60 2.204 1.497 0.0381644 0.7857 1.27 526.80 2.295 1.539 0.0396845 0.8182 1.22 541.20 2.390 1.581 0.0412546 0.8519 1.17 555.70 2.489 1.623 0.0428747 0.8868 1.13 570.30 2.591 1.666 0.0445548 0.9231 1.08 585.00 2.697 1.709 0.0462949 0.9608 1.04 599.80 2.807 1.752 0.04809

Table 8.6 Continued

Dry SW WS C Cm Cm Molarsubstance (g sucrose (g water (g sucrose (mol sucrose (mol sucrose fraction,DS(%) per g water) per g per litre per kg water) per kg X m (mol

sucrose) solution) solution) per mol)

50 1.0000 1.00 614.80 2.921 1.796 0.0499651 1.0410 0.96 629.90 3.041 1.840 0.0518952 1.0830 0.92 645.20 3.165 1.885 0.0539053 1.1280 0.89 660.60 3.294 1.930 0.0559854 1.1740 0.85 676.10 3.429 1.975 0.0581455 1.2220 0.82 691.70 3.571 2.021 0.0603956 1.2730 0.79 707.50 3.718 2.067 0.0627357 1.3260 0.75 723.40 3.873 2.113 0.0651658 1.3810 0.72 739.40 4.034 2.160 0.0677059 1.4390 0.69 755.60 4.204 2.208 0.07035

60 1.5000 0.67 772.00 4.382 2.255 0.0731161 1.5640 0.64 788.40 4.569 2.303 0.0760062 1.6320 0.61 805.10 4.767 2.352 0.0790263 1.7030 0.59 821.80 4.974 2.401 0.0821864 1.7780 0.56 838.70 5.194 2.450 0.0854965 1.8570 0.54 855.80 5.425 2.500 0.0889766 1.9410 0.52 873.00 5.671 2.550 0.0926267 2.0300 0.49 890.30 5.931 2.601 0.0964668 2.1250 0.47 907.80 6.208 2.652 0.100569 2.2260 0.45 925.40 6.503 2.704 0.1048

70 2.3330 0.43 943.20 6.817 2.756 0.109371 2.4480 0.41 961.20 7.152 2.808 0.114172 2.5710 0.39 979.30 7.512 2.861 0.119173 2.7040 0.37 997.60 7.899 2.914 0.124574 2.8460 0.35 1016.00 8.315 2.968 0.130275 3.0000 0.33 1034.50 8.764 3.022 0.136376 3.1270 0.32 1053.30 9.251 3.077 0.142877 3.3480 0.30 1072.20 9.780 3.132 0.149778 3.5450 0.28 1091.20 10.36 3.188 0.157179 3.7620 0.27 1110.40 10.99 3.244 0.1652

80 OOסס.4 0.25 1129.80 11.69 3.301 0.173881 4.2630 0.23 1149.30 12.45 3.358 0.183182 4.5560 0.22 1169.00 13.31 3.415 0.193383 4.8820 0.20 1188.80 14.26 3.473 0.204384 5.2500 0.19 1208.90 15.34 3.532 0.216385 5.6670 0.18 1229.10 16.55 3.591 0.229686 6.1430 0.16 1249.40 17.95 3.650 0.244287 6.6920 0.15 1269.90 19.55 3.710 0.260388 7.3330 0.14 1290.60 21.42 3.770 0.278389 8.0910 0.12 1311.50 23.64 3.831 0.2985

90 OOסס.9 0.11 1332.50 26.29 3.893 0.321291 10.1100 0.10 1353.70 29.54 3.955 0.347192 11.5000 0.Q9 1375.00 33.60 4.017 0.376893 13.2900 0.08 1396.50 38.81 4.080 0.411394 15.6700 0.06 1418.20 45.77 4.143 0.451795 OOסס.19 0.05 1440.10 55.51 4.207 0.499896 24.0000 0.04 1462.10 70.11 4.271 0.557997 32.3300 0.03 1484.30 94.46 4.336 0.629798 49.0000 0.02 1506.70 143.15 4.402 0.720499 99.0000 0.01 1529.20 289.22 4.467 0.8389100 0.00 1551.90 4.534 1.0000

194 SUCROSE

century (Herzfeld, 1892). The subject is thoroughly treated in chapter 5.To complement Table 5.2 which gives solubility of sucrose in water eachSoC, we report in Table 8.7 the solubility for unit increments oftemperature. The equations which fit with experimental data and whichwere used to calculate solubility in each of the ranges of temperature -13to 100°C (Charles, 1960; Vavrinecz, 1962); 100-125°C (Smelik et ai., 1972)and above 125°C (extrapolation by Bubnik and Kadlec) were discussed inchapter 5. The solubility of some mono- and disaccharides of common usein the food industry may be of relevance to include in a book on sucrosesolutions. This is given in Table 8.8.

Table 8.7 Solubility of sucrose in water

Temperature (0C) g sucrose per g water Dry substance (%) Molar fraction(mol per mol)

-13 1.7513 63.65 0.08439-12 1.7544 63.69 0.08453-11 1.7578 63.74 0.08468-10 1.7615 63.79 0.08484

-9 1.7654 63.84 0.08502-8 1.7696 63.89 0.08520-7 1.7740 63.95 0.08540-6 1.7787 64.01 0.08560-5 1.7837 64.08 0.08582

-4 1.7889 64.14 0.08805-3 1.7945 64.21 0.08630-2 1.8003 64.29 0.08655-1 1.8063 64.37 0.086820 1.8127 64.45 0.08710

1 1.8194 64.53 0.087392 1.8263 64.62 0.087693 1.8335 64.71 0.088014 1.8410 64.80 0.088345 1.8489 64.90 0.08868

6 1.8570 65.00 0.089037 1.8654 65.10 0.089408 1.8742 65.21 0.089789 1.8832 65.32 0.0901810 1.8926 65.43 0.09059

11 1.9023 65.54 0.0910112 1.9123 65.66 0.0914413 1.9226 65.78 0.0918914 1.9333 65.91 0.0923615 1.9443 66.04 0.09283


Table 8.7 Continued

Temperature Cc) g sucrose per g water Dry substance (%) Molar fraction(mol per mol)

16 1.9557 66.17 0.0933317 1.9674 66.30 0.0938318 1.9795 66.44 0.0943519 1.9919 66.58 0.0948920 2.0047 66.72 0.09544

21 2.0178 66.86 0.0960022 2.0313 67.01 0.0965923 2.0452 67.16 0.0971824 2.0595 67.31 0.0977925 2.0741 67.47 0.09842

26 2.0892 67.63 0.0990727 2.1047 67.79 0.0997328 2.1205 67.95 0.1004029 2.1368 68.12 0.1010930 2.1535 68.29 0.10180

31 2.1706 68.46 0.1025332 2.1882 68.63 0.1032733 2.2062 68.81 0.1040334 2.2246 68.99 0.1048135 2.2435 69.17 0.10561

36 2.2629 69.35 0.1064237 2.2827 69.54 0.1072638 2.3030 69.72 0.1081139 2.3237 69.91 0.1089740 2.3450 70.10 0.10986

41 2.3668 70.30 0.1107742 2.3890 70.49 0.1116943 2.4118 70.69 0.1126444 2.4351 70.89 0.1136045 2.4589 71.09 0.11459

46 2.4833 71.29 0.1155947 2.5082 71.50 0.1166248 2.5337 71.70 0.1176649 2.5597 71.91 0.1187350 2.5863 72.12 0.11981

51 2.6135 72.33 0.1209252 2.6413 72.54 0.1220553 2.6696 72.75 0.1232054 2.6686 72.96 0.1243755 2.7282 73.18 0.12556

56 2.7584 73.39 0.1267757 2.7893 73.61 0.1280158 2.8207 73.83 0.1292759 2.8529 74.05 0.1305560 2.8857 74.26 0.13185

196 SUCROSE

Table 8.7 Continued

Temperature (OC) g sucrose per g water Dry substance (%) Molar fraction(mol per mol)

61 2.9191 74.48 0.1331862 2.9533 74.70 0.1345363 2.9881 74.93 0.1359064 3.0236 65.15 0.1372965 3.0598 75.37 0.13871

66 3.0967 75.59 0.1401467 3.1344 75.81 0.1416168 3.1727 76.03 0.1430969 3.2118 76.26 0.1446070 3.2515 76.48 0.14613

71 3.2921 76.70 0.1476872 3.3333 76.92 0.1492573 3.3753 77.14 0.1508574 3.4181 77.37 0.1524775 3.4616 77.59 0.15411

76 3.5058 77.81 0.1557777 3.5508 78.03 0.1574678 3.5965 78.24 0.1591679 3.6429 78.46 0.1608980 3.6901 78.68 0.16263

81 3.736 78.89 0.1644082 3.7867 79.11 0.1661883 3.836 79.32 0.1679884 3.8861 79.53 0.1698085 3.9368 79.74 0.17164

86 3.9883 79.95 0.1734987 4.0403 80.16 0.1753688 4.093 80.37 0.1772489 4.1463 80.57 0.1791490 4.2003 80.77 0.18104

91 4.2547 80.97 0.1829692 4.3097 81.17 0.1848993 4.6652 81.36 0.1868394 4.4211 81.55 0.1887795 4.4775 81.74 0.19071

96 4.5342 81.93 0.1926797 4.5912 82.11 0.1946298 4.6485 82.30 0.1965799 4.7060 82.47 0.19852100 4.7637 82.65 0.20046

101 4.7405 82.58 0.19968102 4.8108 82.79 0.20204103 4.8830 83.00 0.20446104 4.9572 83.21 0.20692105 5.0335 83.43 0.20944


Table 8.7 Continued

Temperature CC) g sucrose per g water Dry substance (%) Molar fraction(mol per mol)

106 5.1120 83.64 0.21201107 5.1928 83.85 0.21464108 5.2760 84.07 0.21734109 5.3617 84.28 0.22009110 5.4499 84.50 0.22290

111 5.5409 84.71 0.22578112 5.6347 84.93 0.22873113 5.7316 85.14 0.23175114 5.8315 85.36 0.23485115 5.9347 85.58 0.23801

116 6.0414 85.80 0.24126117 6.1517 86.02 0.24459118 6.2658 86.24 0.24800119 6.3839 86.46 0.25150120 6.5062 86.68 0.25508

121 6.6329 86.90 0.25877122 6.7644 87.12 0.26255123 6.9007 87.34 0.26643124 7.0424 87.57 0.27042125 7.1895 87.79 0.27452

126 7.3426 88.01 0.27874127 7.5019 88.24 0.28307128 7.6678 88.46 0.28753129 7.8407 88.69 0.29212130 8.0211 88.91 0.29685

Table 8.8 Solubility of some saccharides in water

Dry substance DS (%) of saturated solutions

Temperature Glucose Fructose Invert Maltose Lactose Raffinose(0C) sugar

0 32.3 50.8 35.9 10.8 6.71 33.0 51.3 36.2 11.0 6.82 33.8 51.7 36.6 11.1 6.93 34.5 52.2 36.9 11.3 7.14 35.2 52.8 37.3 11.5 7.35 36.0 53.3 37.6 11.7 7.56 36.7 53.9 38.0 11.9 7.87 37.4 54.5 38.3 12.1 8.28 38.2 55.1 38.7 12.3 8.69 38.9 55.8 39.1 12.5 9.1

198 SUCROSE

Table 8.8 Continued



10 39.7 56.4 39.5 12.8 9.611 40.4 57.1 39.8 13.1 10.312 41.1 57.7 40.2 13.3 11.013 41.9 58.4 40.6 13.6 11.814 42.6 59.1 41.0 13.9 12.715 43.4 59.7 41.4 14.2 13.716 44.1 60.4 41.9 14.5 14.817 44.8 61.1 42.3 14.9 16.118 45.6 61.7 42.7 15.2 17.419 46.3 62.4 43.1 15.5 18.9

20 47.1 78.9 63.1 43.6 15.9 20.521 47.8 79.2 63.7 44.0 16.3 22.222 48.5 79.4 64.4 44.4 16.6 24.123 49.3 79.7 65.0 44.9 17.0 26.124 50.0 79.9 65.6 45.3 17.4 28.325 50.8 80.2 66.3 45.8 17.8 30.726 51.5 80.4 66.9 46.2 18.227 52.2 80.7 67.5 46.7 18.728 53.0 81.0 68.1 47.2 19.129 53.7 81.3 68.7 47.6 19.5

30 54.5 81.5 69.3 48.1 20.031 55.2 81.8 69.8 48.6 20.532 55.9 82.1 70.4 49.1 20.933 56.7 82.4 71.0 49.5 21.434 57.4 82.7 71.6 50.0 21.935 58.1 82.9 72.1 50.5 22.436 58.9 83.2 72.7 51.0 22.937 59.6 83.5 73.3 51.5 23.438 60.4 83.8 73.8 52.0 23.939 61.1 84.1 74.4 52.5 24.4

40 61.8 84.3 75.0 53.0 25.041 62.6 84.6 75.6 53.5 25.542 63.3 84.9 76.2 54.0 26.043 64.1 85.2 76.8 54.6 26.644 64.8 85.4 77.4 55.1 27.145 65.5 85.7 78.1 55.6 27.746 66.3 85.9 78.8 56.1 28.347 67.0 86.2 79.5 56.7 28.948 67.7 86.4 80.2 57.2 29.449 68.5 86.7 81.0 57.7 30.0

50 69.2 86.9 58.2 30.651 71.4 87.2 58.8 31.252 71.8 87.4 59.3 31.853 72.2 87.6 59.9 32.554 72.6 87.9 60.4 33.155 72.9 88.1 61.0 33.756 73.3 61.5 34.3


Table 8.8 Continued



57 73.7 62. I 35.058 74. I 62.6 35.659 74.4 63.2 36.2

60 74.8 63.7 36.961 75.1 64.3 37.562 75.5 64.8 38.263 75.8 65.4 38.964 76.2 66.0 39.565 76.5 66.5 40.266 76.8 67.1 40.967 77.2 67.7 41.668 77.5 68.3 42.269 77.8 68.8 42.9

70 78.1 69.4 43.671 78.5 70.0 44.372 78.8 70.6 45.073 79.1 71.1 45.774 79.4 71.7 46.475 79.7 72.3 47.176 80.1 72.9 47.877 80.4 73.5 48.578 80.7 74.1 49.279 81.0 74.6 49.9

80 81.3 75.2 50.681 71.6 75.8 51.382 82.0 76.4 51.983 82.3 77.0 52.584 82.6 77.6 53.085 82.9 78.2 53.686 83.3 78.8 54.287 83.6 79.4 54.788 83.9 80.0 55.289 84.3 80.6 55.8

90 84.6 81.2 56.391 85.0 81.8 56.892 82.4 57.393 83.0 57.894 83.6 58.395 84.2 58.896 84.8 59.397 85.4 59.898 86.0 60.399 86.6 60.7100 87.2 61.2

200 SUCROSE

8.4.3 Density of sucrose solutions

The density of sucrose solutions is generally used in sugar technology andin the sugar trade to measure the concentration of dissolved substances.Since the non-sucrose impurities present in technical sugar solutions affectthe density in the same way as sucrose, density can be used as anapproximate method for determination ofdry substance content. The densitytables for sucrose have existed for nearly one century (Plato, 1900) and arestill used as reference marks in brewery. ICUMSA recommended in its16th session (1978) that density should be determined at 20°C related towater at 4°C (weight corrected to vacuum) by means by hydrometers orpycnometers, the latter giving more accurate results.Following the recommendations adopted by ICUMSA in 1982, newmeasurements of density were performed by Wagenbreth et al. (1988)using Kell's (1975) relation of determination of water density Qw (kg/m3)when temperature t (0C) is varied:

Qw = (999.83952 + 16.952577 X t - 7.9905127 X 10-3 X (2- 46.241757 X 1O--{) X P + 105.84601 X 10-9 X t4 - 281.03006X lO-IZ X ~)/ (1 + 16.887236 X 10-3 X t)

A polynomial including Qw giving the density Q of sucrose solutions as afunction of mass concentration c (g sucrose % g of solution) at 20°C andtemperature t (0C) was adopted by ICUMSA (1990) to replace Plato'stable:

Q = Qw + atcZ + a3c3 + (blc + bzcz + b3c3) X (t - 20)

+ (CIC + czcz + C3C3) X (t - 20)Z + (d,c + dzcZ) X (t - 20?+ elc(t - 20)4

Coefficients .-G, = 385.850 74Gz = -13.03435G3 = -3.6663

d,= -5.110 X 10-5

dz = 1.580 X 10-5

hi = -0.459244bz = 7.5699 X lO-z

b3 = 6.2667 X lO-z

e, = 1.986 X 10-7

CI = 6.0198 X 10-3

Cz = -1.3008 X 10-3C3 = -4.907 X 10-4

The values of density derived from this polynomial are listed in Table 8.9with Q in kg/m3 , c in g sucrose % g of solution and t in °C.

8.4.4 Density and apparent specific volume

Theoretically the density (or mass per unit volume) represents the packingcharacteristics of solute molecules among water molecules, which dependson the molecular structure of both. Moreover, since water is 'structured',

Table 8.9 Density Q (kg m-3) of aqueous sucrose solution as a function of the mass fractionw (%) and temperature CC) (from ICUMSA, 1990)

Temperature CC)

w(%) 10 20 30 40 50 60 70 80

5 999.70 998.20 995.64 992.21 988.03 983.19 977.78 971.765 1019.56 1017.79 1015.03 1011.44 1007.14 1002.20 996.70 990.6510 1040.15 1038.10 1035.13 1031.38 1026.97 1021.93 1016.34 1010.2315 1061.48 1059.15 1055.97 1052.06 1047.51 1042.39 1036.72 1030.5620 1083.58 1080.97 1077.58 1073.50 1068.83 1063.60 1057.85 1061.6325 1106.47 1103.59 1099.98 1095.74 1090.94 1085.61 1079.78 1073.5030 1130.19 1127.03 1123.20 1118.80 1113.86 1108.44 1102.54 1096.2135 1154.76 1151.33 1147.28 1142.71 1137.65 1132.13 1126.16 1119.7940 1180.22 1176.51 1172.25 1167.52 1162.33 1158.71 1150.88 1144.2745 1206.58 1202.01 1198.16 1193.25 1187.94 1182.23 1176.14 1169.7050 1233.87 1220.64 1224.98 1219.93 1214.50 1208.70 1202.56 1196.1155 1262.11 1257.64 1252.79 1247.59 1242.05 1236.18 1220.99 1223.5360 1291.31 1286.61 1281.59 1276.25 1270.61 1264.67 1258.45 1251.9965 1321.46 1316.56 1311.38 1305.93 1300.21 1294.21 1287.96 1281.5270 1352.55 1347.49 1342.16 1336.63 1330.84 1324.80 1318.55 1312.1375 1384.58 1379.36 1373.98 1366.36 1362.52 1356.46 1350.21 1343.8380 1417.50 1412.20 1406.70 1401.10 1395.2 1389.2 1383.0 1376.6085 1451.30 1445.90 1440.50 1434.80 1429.00 1422.90 1416.80 1410.60

the interaction between solute and solvent is extremely complex. Electrostrictive forces counterbalance displacement of water molecules so thatwater molecules, in general, have good packing characteristics. In otherwords, sugars dissolved in water show higher densities (up to 1.5 g/cm3)than many other organic compounds. Sucrose is no exception to this ruleand, in fact, sucrose has a slightly higher density than many other sugarswhich emphasizes its good compatibility with water structure. Theinteraction between sucrose and water, giving rise to the solute-solventeffects, is largely due to hydrogen bonds. These have a lifetime of about10-9 s and occur at specific loci in the sucrose molecule. Generallyequatorial hydroxyl groups are more easily hydrated than axial hydroxylgroups but such effects are modified by the interplay between the hydroxylgroups of the sucrose molecule itself, i.e. intramolecular hydrogenbonding.Apparent molar volume <j>(V~) of the sugar can be calculated using the

density of solution and that of water:

<j>(V~) = M x(_l_ -~) /W2

QI Q2

with QI density of solution, Q2 density of water, WI mass fraction of water,W2 mass fraction of solute and M molecular weight of solute. The partialmolar volume is apparent molar volume at infinite dilution. Thecorresponding specific volumes are obtained by dividing molar volumes by

202 SUCROSE

molecular weight of the solute. These offer the advantage of comparing thecharacteristics of substances on a mass basis. The apparent molar volumesof dissaccharides are about twice those of monosaccharides and trisaccharides about thrice, etc.... However, the apparent specific volumes of mostsugars are all within 0.6O--D.63 cm3/g. Increase in temperature causes acorresponding increase in apparent specific volume which is presumablyrelated to a diminution in the overall number of hydrogen bonds.Apparent molar volumes (AMV in cm3/mol) are good experimentalindicators of the effective size of sugar molecules (see Table 8.10). Ofparticular importance in taste chemoreception is the apparent specificvolume (ASV in cm3/g) which is a measure of the packing ability of aparticular form of solute conformation in water (Shamil et ai., 1987). Thisparameter was useful to discriminate basic tastes. Sweet substancesgenerally fit into the range 0.51-0.71 cm3/g and sucrose, the optimumsweetener, is situated in the center of the sweet range (0.61 cm3/g).

8.4.5 Refractive index

Like densimetry, refractometry is a rapid method for determining totalsolids in aqueous sucrose solutions. The principle of the method is recalled inchapter 7 (in this book). The 20th Session of ICUMSA (1990) officiallyadopted formulae and tables recommended in the 17th Session (1978)which give the Refractive Index in vacuo for aqueous sucrose solutionswith concentrations ranging from 0 to 85% at temperatures between 15 and30°C, and wavelengths from 546.1 to 589.3 nm.For practical purposes, Table 8.11 reports the Refractive Index ofaqueous sucrose solutions prepared in standard air at 20°C and concentrations ranging from 0 to 85% for a wavelength of 589.3 nm (ICUMSA,1979).Temperature variations may seriously affect the reliability of refractometer readings and negative (1G-19°C) or positive (21-30°C) corrections torefractometer readings have been published (Pearson, 1976; ICUMSA,

Table 8.10 Apparent molar and specific volumes of sugars at 20°C and5% (w/v) (From ShamjI, 1988; Lopez-Chavez, 1993)

Sugar Apparent molar volume Apparent specific volume(cm3/mol) (cm3/g)

GlucoseFrucroseMannoseGalactoseSucroseTrehaloseMaltoseLactose

111.2108.1111.2109.8209.5208.6210.9211.5

0.61780.60060.61780.61000.61200.60800.61610.6179

Tab

le8.

11Refractiveindexofpuresucrosesolutionsat20°Cand589

nm(DatafromICUMSA,1979)

Sucroseg/100g

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

01.332986

333129

33272

333415

333558

333702

333845

333989

334132

334276

11.334420

334564

334708

334852

334996

335141

335285

335430

335574

335719

21.335864

336009

336154

336300

336445

336590

336736

336882

337028

337174

31.337320

337466

337612

337758

337905

330851

338198

338345

338492

338639

41.338786

338933

339081

339228

339376

339524

339671

339819

339967

340116

51.340264

340412

340561

340709

340858

341007

341156

341305

341454

341604

61.341753

341903

342052

342202

342352

342502

342652

342802

342952

343103

71.343253

343404

343555

343706

343857

344008

344159

344311

344462

344614

..., ::r::8

1.344765

34917

34509

345221

345373

345526

345678

345831

345983

346136

-< en9

1.34289

346442

346595

346748

346902

347055

347209

347362

347516

347670

n10

1.347824

347978

348133

348287

348442

348596

348751

348906

349061

349216

;I> t""" ...,

111.349371

349527

349682

349838

349993

350149

350305

350461

350617

350774

:<l0

121.350930

354087

351243

351400

351557

351714

351871

352029

354186

352343

..., tT113

1.352501

352659

352817

352975

3533133

353291

353449

353608

353767

353925

:<l

141.354084

354243

354492

354561

354721

354880

355040

355199

355359

355519

::l tT115

1.355679

355840

356000

356160

356321

356482

356642

356803

356964

357126

en

161.357287

357448

357610

357772

357933

358095

358257

358420

358582

358744

171.358907

359070

359232

359395

359558

359722

359885

360048

360212

360376

181.360539

360703

360867

361032

361196

361360

361525

361690

361854

362019

191.362185

362350

362515

362681

362846

363014

363178

363344

363510

363676

201.363842

364009

364176

364342

364509

364676

364843

365011

365178

365346

211.365513

365681

365849

366017

366185

366354

366522

366691

366859

367028

221.367197

368366

367535

367705

367874

368044

368214

368384

368554

368724

231.368894

369064

369235

369406

369576

369747

3699518

370090

370261

370432

241.370604

370776

370948

371120

371292

371464

371637

371809

371982

372155

251.372328

372501

372674

372847

373021

373194

373368

373542

373716

373890

N 0 (,;.

)

Tab

le8.

11C

onti

nued

NSucrosegl100g

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 ~

261.374065

374239

374414

374588

374763

374938

375113

375288

375464

375639

271.375815

375991

376167

376343

376519

376695

376872

377049

377225

377402

281.377579

377756

377934

378111

378289

378467

378644

378822

379001

379179

291.379357

379536

379715

379893

380072

380251

380431

380610

382593

382774

301.381149

381329

381509

381689

381870

382050

382231

382412

382593

382774

311.382955

383137

383318

383500

383682

383863

384046

384228

384410

384593

321.384775

384958

385141

385324

385507

385691

385874

386058

386242

386426

331.386610

386794

386978

387163

387348

387532

387717

387902

388088

388273

341.388459

388644

388830

389016

389202

389388

389575

389761

389948

390135

351.390322

390509

390696

390884

391071

391259

391447

391635

391823

392011

361.392200

392388

392577

392766

392955

393144

393334

393523

393713

393903

371.394092

394283

394473

394663

394854

395044

395235

395426

395617

395809

VJ

381.396000

396192

396383

396575

396767

396959

397152

397344

397537

397730

c:: ("')

391.397922

398116

398309

398502

398696

398889

399083

399277

399471

399666

;<l0

401.399860

400055

100249

400444

4000639

400834

401030

401225

401421

401617

VJ tTl

411.401813

402009

402205

402401

402598

402795

402992

403189

403386

403583

421.403781

403978

404176

404374

404572

404770

404969

405167

405366

405565

431.405764

405963

406163

406362

406562

406762

406961

407162

407362

407562

441.407763

407964

408165

408366

408567

408768

408970

409171

409373

409575

451.409777

409980

410182

410385

410588

410790

410994

411197

411400

411604

461.411807

412011

412215

412420

412624

412828

413033

413238

413433

413648

471.413853

414059

414265

414470

414676

414882

415089

415295

415502

415708

481.415915

416122

416230

416537

416744

416952

417160

417368

417576

417785

491.417993

418202

418411

418620

418829

419038

419247

419457

419667

419877

501.420087

420297

420508

420718

420929

421140

421351

421562

421774

421985

511.422197

422409

422621

422833

423046

423258

423471

423684

423897

424110

521.424323

424537

424750

424964

425178

425393

425607

425821

426036

426251

531.426466

426681

426896

427112

427328

427543

427759

427975

428192

42408

541.424625

428842

429059

429276

429493

429711

429928

430146

430364

430582

551.430800

431019

431238

431456

431675

431894

432114

432033

432553

432773

561.432993

433213

433433

433653

433874

434095

434316

434537

434758

434980

571.435201

435423

435645

435867

436089

436312

463534

436757

436980

437203

581.437427

437650

437874

438098

438322

438546

438770

438994

439219

439444

591.439669

439894

440119

440345

440571

440796

441022

441248

441475

441701

601.411928

442155

442382

442609

442836

443064

443292

443519

443747

443976

611.444204

444432

444661

444890

445119

445348

445578

445807

446037

446267

621.446497

446727

446957

447188

447419

447650

447881

448112

448343

448575

631.448807

449039

449271

449503

449736

449968

450201

450434

450667

450900

641.451134

453667

451601

451835

452069

452304

452538

452773

453008

453243

651.453478

453713

453949

454184

454420

454656

454893

455129

455365

455602

661.455839

456076

456313

456551

456788

457026

457264

457502

457740

457979

."67

1.458217

458456

458695

458934

459174

459413

472059

459893

460133

460373

:I:68

1.460613

460854

461094

461335

461576

461817

462059

462300

462542

462784

-< [J)69

1.463026

463268

463511

463753

463996

464239

464482

464725

464969

465212

r;70

1.465456

465700

465944

466188

466433

466678

466922

467167

467413

467658

;l> t'"'

."

711.467903

468149

468395

468641

468887

469134

469380

469627

469874

470121

::<:l 0

721.470368

470616

470863

471111

471359

471607

471855

472104

472352

472601

." tTl

731.472850

473099

473349

473598

473848

474098

474348

474598

474848

475099

::<:l :!

741.475349

475600

475851

476103

476354

476606

476857

477109

477361

477645

tTl

751.477866

478119

478371

478624

478878

479131

479384

479638

479892

480146

[J)

761.480400

480654

480909

481163

481418

481673

481929

482184

482440

482695

771.482951

483207

483463

483720

483976

484233

484490

484747

485005

485262

781.485520

485777

486035

486294

486552

486810

487069

487328

487587

487846

791.488105

488365

488625

488884

489144

489405

489665

489926

490186

490447

801.490708

490970

491231

491493

491754

492016

492278

492541

492803

493066

811.493328

493591

493855

494118

494381

494645

494909

495173

495437

495701

821.495966

496230

496495

496760

497026

497291

497556

497822

498088

498354

831.498620

498887

499153

499420

499687

499954

500221

500488

500576

501024

841.501292

501560

501828

502096

502365

502634

502903

503172

503441

503711

851.503980

N 0 Ul

206 SUCROSE

1979). These corrections are given in Table 8.12 between 15 and 30°C. Thepresence of extraneous saccharides in technical sugar solutions alsoreduces the accuracy of refractometers. For mixtures in aqueous sucrosesolutions and invert sugar, a formula for the correction of refractometric todry substance was proposed (ICUMSA, 1990).

8.4.6 Polarimetry

When a beam of plane polarized light is passed through optically active(chiral) solute such as a mono- or a disaccharide, it is bent to the right orleft, depending on the overall chiral properties of the solute (Shallenberger,1985). In practice, this is done in a polarimeter (saccharimeter in the sugarindustry).Sucrose differs from reducing sugars in that its specific rotation remainsconstant over time whereas reducing sugars exhibit mutarotation, i.e.change in rotation due to the establishment of a tautomeric equilibriumover a particular period. This property allows sucrose to be sensitivelyidentified and it is indeed a dependable parameter for assessing its purity.Application of polarimetry to sugar products is based on the determination of a ratio of optical rotation between the same and an aqueous sucrosesolution with reference concentration, i.e. the 'normal sugar solution'. Thebasic conditions of polarimetry (the so-called 'International Sugar Scale')and the existing method of measurement are extensively described inchapter 7.The new reference, lOooZ corresponding to

[a] 20.00°C = 40.777°C,546.2271

has been valid since 1 July, 1988. Saccharimeter scales and quartz controlplates calibrated with the former reference, lOooS for

[a] 20000°C =40.765°C589.4400 '

can still be used providing that the values obtained are reduced by 0.029%.Taking into account the new reference, the specific rotation value (in ° for aconcentration in g/cm3 and length of the tube in dm, the wavelength beingthat of spectrally filtered yellow sodium light D-line 589.4400 nm at 20°e)is, for sucrose weighed in standard air: [alJo = 66.590°C.

8.4.7 Thermal properties of aqueous sucrose solutions

8.4.7.1 Specific heat. Specific heat is the amount of heat required to raiseone degree the temperature of a unit weight of solution. As shown in Table8.13, the specific heat of sucrose solutions (in kJ/kg K) is lower than that of

Tab

le8.

12Temperaturecorrectionsbetween15and30°Cforrefractiveindexdeterminedat589nm(referencetemperature

=20°C)(Datafrom

ICUMSA,1989)

Temperature

Sucrose(g/100gofsolution)

CC)

05

1015

2025

3035

4045

5055

6065

7075

8085

Subt

ract

fro

mth

em

easu

red

valu

e

""15

0.29

0.31

0.32

0.33

0.34

0.35

0.36

0.37

0.37

0.38

0.38

0.38

0.38

0.38

0.38

0.38

0.37

0.37

:I:16

0.24

0.25

0.26

0.27

0.28

0.28

0.29

0.30

0.30

0.30

0.31

0.31

0.31

0.31

0.31

0.30

0.30

0.30

-< [/J17

0.18

0.19

0.20

0.20

0.21

0.21

0.22

0.22

0.23

0.23

0.23

0.23

0.23

0.23

0.23

0.23

0.23

0.22

ri18

0.12

0.13

0.13

0.14

0.14

0.14

0.15

0.15

0.15

0.15

0.15

0.15

0.15

0.15

0.15

0.15

0.15

0.15

> 1""19

0.06

0.06

0.Q70.Q70.07

0.Q70.Q70.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.07

"" '"A

dd

toth

em

easu

red

valu

e0 ""

210.06

0.07

0.Q70.Q70.07

0.07

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.07

tT1 '"22

0.13

0.14

0.14

0.14

0.15

0.15

0.15

0.15

0.16

0.16

0.16

0.16

0.16

0.16

0.15

0.15

0.15

0.15

.., til23

0.20

0.21

0.21

0.22

0.22

0.23

0.23

0.23

0.23

0.24

0.24

0.24

0.24

0.23

0.23

0.23

0.23

0.22

[/J

240.27

0.28

0.29

0.29

0.30

0.30

0.31

0.31

0.31

0.32

0.32

0.32

0.32

0.31

0.31

0.31

0.30

0.30

250.34

0.35

0.36

0.37

0.38

0.38

0.39

0.39

0.40

0.40

0.40

0.40

0.40

0.39

0.39

0.39

0.38

0.37

260.42

0.43

0.44

0.45

0.46

0.46

0.47

0.47

0.48

0.48

0.48

0.48

0.48

0.47

0.47

0.46

0.46

0.45

270.50

0.51

0.52

0.53

0.54

0.55

0.55

0.56

0.56

0.56

0.56

0.56

0.56

0.55

0.55

0.54

0.53

0.52

280.58

0.59

0.60

0.61

0.62

0.63

0.64

0.64

0.64

0.65

0.65

0.64

0.64

0.64

0.63

0.62

0.61

0.60

290.66

0.67

0.68

0.69

0.70

0.71

0.72

0.73

0.73

0.73

0.73

0.73

0.72

0.72

0.71

0.70

0.69

0.68

300.74

0.75

0.77

0.78

0.79

0.80

0.81

0.81

0.81

0.82

0.81

0.81

0.81

0.80

0.79

0.78

0.77

0.75

s

208 SUCROSE

Table 8.13 Specific heat (kJ/kg K) of sucrose and its solutions (From Lyle, 1957)

Temperature Water Concentration of sucrose solutions (g/ 100 g of solution) Sucrose(0C) crystal

10 20 30 40 50 60 70 80 90

0 4.187 3.936 3.684 3.433 3.182 2.931 2.680 2.428 2.177 1.926 1.16410 4.187 3.936 3.684 3.475 3.224 2.973 2.721 2.470 2.219 2.010 1.20220 4.191 3.936 3.726 3.475 3.224 3.014 2.763 2.554 2.303 2.052 1.23530 4.195 3.977 3.726 3.517 3.266 3.056 2.805 2.596 2.345 2.135 1.26940 4.199 3.977 3.726 3.517 3.308 3.098 2.847 2.638 2.428 2.219 1.30650 4.204 3.977 3.768 3.559 3.349 3.140 2.889 2.680 2.470 2.261 1.34060 4.212 3.977 3.768 3.559 3.349 3.140 2.973 2.763 2.554 2.345 1.17270 4.216 3.977 3.810 3.601 3.391 3.182 3.014 2.805 2.596 2.386 1.41180 4.224 3.977 3.810 3.601 3.433 3.224 3.056 2.847 2.680 2.470 1.44490 4.233 4.019 3.810 3.643 3.475 3.266 3.098 2.889 2.721 2.554 1.478100 4.241 4.019 3.852 3.643 3.475 3.308 3.140 2.973 2.763 2.596 1.516

water. These values may be defined as the ratio of the thermal capacity ofsucrose to that of water and they are of practical interest in heat transfercalculations for food processes.

8.4.7.2 Heat of solution. When crystallized sucrose is dissolved in water,thermal energy is absorbed and the temperature decreases. Depending ontemperature, size of crystal and the degree of crystallinity, the dissolutionof sucrose may be more or less endothermic. Van Hook (1981) reportedthat fine granulated sucrose shows an endothermal effect of 16.7 ± 0.8 Jigwhen 5.0 g of sucrose are dissolved in 50 g of water at room temperature.Other data quoted from the thesis of Roth (1976) are 4.75 ± 0.26 kJ/mol forcrystalline sucrose, which is in good agreement with the values of 4.73 kJImol given by Culp (1946). Freshly milled sugar has a heat of solution of3.52 ± 0.9 kJ/mol; a recrystallized sample after adsorption of water byamorphous sugar is endothermal to the extent of 3.86 ± 0.16 kJ/mol whileamorphous sugar shows an exothermic effect to the extent of -16.9 kJ/mol(Roth, 1976).

8.4.7.3 Heat of dilution. The heat of dilution is the amount of heatevolved when a sucrose solution is diluted with water. Values obtained byVallender and Perman (1931) when 1 g of water is added to differentconcentrations of sucrose at different temperatures are reported in Table8.14.

8.4.7.4 Heat of crystallization. Kilmartin and Van Hook (1950) reportedthat the enthalpy of crystallization (heat content of crystallized sucrose andsaturated sucrose solution minus heat content of supersaturated solution)is, respectively, 10.47 kJ/mol and 32.66 kJ/mol at 30 and 57°C. More


Table 8.14 Heats of dilution (1) of sucrose solutions at different temperatures (0C) and forvarious concentrations (gll00 g of solution) (From Vallender and Perman, 1931)

Temperature (OC) Concentration (g/100 g of solution) Heat of dillution per g ofwater added (1)

0 58.94 2.2256.98 1.8055.03 1.5951.27 1.1749.4 1.0547.39 0.7545.31 0.63

63.39 7.0320 61.82 6.36

60.32 5.8256.51 4.9855.05 4.4453.62 3.8552.2 3.6047.79 2.7246.49 2.3941.20 1.6740.07 1.5534.84 1.0930.18 0.8825.88 0.5422.39 0.3518.99 0.28

40 66.75 10.3863.96 8.1262.43 6.9960.81 6.259.26 5.8257.75 5.5756.26 4.954.8 4.6947.03 2.9345.78 2.6838.24 1.8837.02 1.6734.48 1.4231.66 1.0928.66 0.9623.86 0.7118.93 0.50

60 67.97 9.0466.23 8.3764.59 7.6263.23 7.3363.07 7.2461.77 6.8259.78 6.4158.90 5.9956.67 5.28

210

Table 8.14 Continued

Temperature (OC)

SUCROSE

Concentration (g/IOO g of solution) Heat of dillution per g ofwater added (1)

80

53.8352.5051.2449.9947.1444.6041.9539.3836.9133.2428.72

68.7966.3563.9461.5959.3350.6748.8243.8342.2336.1029.9123.52

4.904.614.314.023.393.062.472.262.011.511.17

8.467.837.336.876.204.774.103.313.062.261.550.80

Table 8.15 Heat required to vaporize water with heat available from crystallization for 100 gof solution. (From Nicol, 1973)

Heat available from crystallization

Sucrose (g per Heat required, latent ~H at nooe ~H at 120°C ~H at l300e100 g of solution) heat of vaporization (kJ) (kJ) (kJ)

(kJ)

86 31.687 29.4 26.988 27.1 27.389 24.9 20.9 27.690 22.6 16.2 21.1 27.991 20.3 16.4 21.392 18.1 16.6 21.693 15.8 16.7 21.8

recently, Nicol (1973) investigating the dehydration of sugar solutionsfound that in some cases the heat available from crystallization is sufficientto allow vaporization of water. This is reported in Table 8.15.


8.4.8 Increase in volume

When sucrose is dissolved in water at a constant temperature, an expansionof volume occurs. Increase in volume (in 10-3 m3) at 200e for aqueoussucrose solutions at different final concentrations (1-70% w/w) is given inTable 8.16. The data are adapted from Spengler et al., 1938; Bates andAssociates, 1942b; Norrish, 1967.

8.4.9 Boiling point

Because of its particular importance in sugar technology and otherprocesses like confectionery, much attention has been paid to boiling pointof pure and impure sugar solutions (Bates and Associates, 1942b; Norrish,1967; Nicol, 1968; Spengler et al., 1938; Vavrinecz, 1973; Sheng, 1990).

Table 8.16 Increase in volume when sucrose is dissolved in water at 20°C (From Bates andAssociates 1942a)

Resultant solution

kg of sucrose addedin 1 m3 of water

11.9923.9835.9747.9759.9671.9583.9495.93107.92119.92239.83359.75479.66599.58719.49839.41959.321079.241199.151319.071438.991558.901678.821798.731918.652038.562158.482278.392398.31

Sucrose in g/loo g

of solution

1.18622.34433.47604.58165.66226.71867.75158.76199.750310.717519.360226.477232.439937.508041.868845.660648.988151.931654.553956.904959.024660.945662.694564.293565.761167.112868.361869.519470.5953

Specific gravity(d2~~8

1.00281.00741.01181.06201.02051.02471.02891.03311.03711.04111.07811.11041.13871.16381.18611.20611.22401.24031.25501.26841.28071.29201.30241.31211.32101.32921.33701.34421.3509

Increase in volume(10-3 m3)

0.0270.0570.0830.1100.1400.1670.1970.2230.2540.2800.5600.8441.1241.4121.6961.9802.2682.5522.8403.1273.4153.7033.9944.2824.5704.8575.1495.4375.728

212 SUCROSE

Table 8.17 Boiling point elevation of sucrose solutions atatmospheric pressure (from Bates and Associates, 1942b)

g sucrose per100 g of water

501001502002503003504004505005506006507007508008509009501000105011001150120012501300135014001450150015501600

g sucrose per100 g of solution

33.3350.0060.0066.6771.4375.0077.7880.0081.8283.3384.6285.7186.6787.5088.2488.8989.4790.0090.4890.9191.3091.6792.0092.3192.5992.8693.1093.3393.5593.7593.9494.12

Boiling pointelevation Cq

0.741.792.984.255.576.888.189.4410.6511.8112.9313.9814.9815.9316.8417.6918.5119.2720.0120.7021.3621.9922.5823.1623.7024.2224.7125.1825.6426.0726.4926.88

Boiling point elevation (BPE) is defined as the difference between theboiling temperature of sugar solution and that of water at the sameabsolute pressure. BPE is pressure and concentration dependent. Valuesof BPE are reported in Table 8.17 for atmospheric pressure according toBates and Associates (1942b).Under vacuum, BPE may be calculated using a formula proposed byHugot (1987): BPE = 0.25 X DS X (30 + DS) X [1 - 0.54h/(229 - h))/(103.6 - DS), where DS is the % dry substance and h the vacuum inmmHg.

8.4.10 Freezing point

Like other polyhydroxycompounds, sucrose depresses the freezing point ofwater far below O°e. Depending on initial concentration, it may even


prevent a non negligible amount of water from freezing. The maximumfreeze concentrated amorphous sucrose solution was discussed in chapter4 of this book. The freezing point depression together with amorphizationof a fraction of ice confer to sucrose a cryoprotectant ability. This propertyis not only due to the specific hydration and structure maker effect waterorganization below O°C, but also to surface interactions between frozenproteins and cells.Values of freezing point depression of sucrose and other saccharides(fructose, glucose, lactose and maltose) are reported in Table 8.18according to Bubnik and Kadlec (chapter 5).Approximate estimation of freezing point depression can be obtainedusing the formula given by Hoynak and Bollenback (1966): freezing point(CO) = -(5.DS)/(85-DS) with DS (dry substance) in g % g of solution.

8.4.11 Water activity

When sucrose is dissolved in water, entropy is decreased as watermolecules become organized under the effect of the sugar. Watermolecules are less free to escape into the vapour phase and this is at theorigin of vapor pressure depression. As water activity is by definition theratio of the vapour pressure of solution and solvent (aw = PIPo), and byconvention aw = 1.0 for water, the increase in sucrose concentrationprovokes a decrease in aw . This parameter is of practical use in the foodindustry and sucrose is known as a good water activity depressor, playing inthis case a role of preservative for foods sensitive to bacterial spoilage.Water activity of sucrose solutions above 50% (w/w) are reportedaccording to Norrish (1967) in Table 8.19.Norrish (1966) also established a relation applicable for confectionerysyrups between water activity and the composition of the syrup:

In a w = In Xl + [(-Kz )lIZ X Xz + (-K3)lIZ X X3 + ...Fwhere Xl is the molar fraction of water; Xz, X3 are the molar fractions ofsolutes 2,3; Kz, K 3 are the activity coefficients of solutes 2,3; K = -2.6 forsucrose; Kvaries from -2.4 to 0.9 for glucose syrups with DE ranging from90 to 33, respectively.

8.4.12 Osmotic pressure

Osmotic pressure is another property of sucrose linked to its solute-solventinteractions in water. It is closely related to water activity by the relation.

RxTn = - --_- X In A w

Vwhere n is osmotic pressure, V the partial molar volume of water, Rand Tare gas constant and absolute temperature, respectively.

Table 8.18 Freezing point depression CC) of sucrose and other saccharides solutions

g solute/loo g solution Freezing point depression (0C)

Anhydrous Hydrated Sucrose Fructose* Glucose Lactose Maltosesolute solute

1.00 1.05 0.05 0.10 0.08 0.06 0.062.00 2.11 0.11 0.21 0.18 0.11 0.113.00 3.16 0.17 0.32 0.28 0.17 0.174.00 4.21 0.23 0.43 0.38 0.23 0.235.00 5.26 0.29 0.54 0.50 0.29 0.29

6.00 6.32 0.35 0.66 0.62 0.35 0.357.00 7.37 0.42 0.78 0.74 0.42 0.428.00 8.42 0.49 0.90 0.87 0.50 0.489.00 9.47 0.56 1.03 1.01 0.5510.00 10.53 0.63 1.16 1.15 0.62

11.00 11.58 0.71 1.29 1.29 0.6912.00 12.68 0.79 1.43 1.44 0.7713.00 13.68 0.87 1.57 1.59 0.8414.00 14.74 0.95 1.71 1.74 0.9215.00 15.79 1.03 1.86 1.90 1.00

16.00 16.84 1.12 2.01 2.06 1.0817.00 17.89 1.21 2.16 2.22 1.1718.00 18.95 1.30 2.32 2.38 1.2519.00 20.00 1.40 2.48 2.55 1.3420.00 21.05 1.49 2.64 2.71 1.43

22.00 23.16 1.70 3.05 3.05 1.6424.00 25.26 1.92 3.43 3.39 1.8526.00 27.37 2.16 3.82 3.74 2.0828.00 29.47 2.42 4.20 4.09 2.3430.00 31.58 2.71 4.45 2.62

32.00 33.68 3.02 4.81 2.9334.00 35.79 3.35 5.18 3.2536.00 37.89 3.72 5.56 3.6038.00 40.00 4.13 5.94 3.9940.00 42.10 4.58 6.35 4.41

42.00 44.21 5.07 6.76 4.8844.00 46.31 5.62 7.20 5.3546.00 48.44 6.22 7.6548.00 50.54 6.88 8.1450.00 52.65 7.61 8.65

52.00 54.76 8.4054.00 56.86 9.2856.00 58.97 10.2458.00 61.07 11.3060.00 63.18 12.45

62.00 65.29 13.7164.00 67.39 15.0966.00 69.50 16.5868.00 71.60 18.2170.00 73.71 19.97

*Hydrated solute column not shown for fructose.

Tab

le8.

19Activityofwaterinaqueoussucrosesolutions(FromNorrish,1967)

Sucroseby

Water

Sucroseby

Water

Sucroseby

Water

Sucroseby

Water

Sucroseby

Water

Sucroseby

Water

weight(%)

activity

weight(%)activity

weight(%)activity

weight(%)activity

weight(%)activity

weight(%)activity

50.0

0.936

58.0

0.907

66.0

0.862

74.0

0.786

82.0

0.645

90.0

0.366

50.2

0.935

58.2

0.906

66.2

0.860

74.2

0.783

82.2

0.640

90.2

0.356

50.4

0.935

58.4

0.905

66.4

0.859

74.4

0.781

82.4

0.635

90.4

0.347

50.6

0.935

58.6

0.904

66.6

0.857

74.6

0.778

82.6

0.630

90.6

0.337

50.8

0.933

58.8

0.903

66.8

0.856

74.8

0.775

82.8

0.625

90.8

0.327

51.0

0.933

59.0

0.902

67.0

0.854

75.0

0.773

83.0

0.620

91.0

0.317

51.2

0.932

59.2

0.901

67.2

0.853

75.2

0.770

83.2

0.614

91.2

0.314

"tl :t

51.4

0.932

59.4

0.901

67.4

0.851

75.4

0.767

83.4

0.609

91.4

0.297

...:51.6

0.931

59.6

0.900

67.6

0.850

75.6

0.763

83:6

0.603

91.6

0.287

CIl (=i

51.8

0.929

59.8

0.899

67.8

0.848

75.8

0.762

83.8

0.598

91.8

0.276

:> r'

0.929

0.592

92.0

0.266

"tl

52.0

60.0

0.898

68.0

0.846

76.0

0.759

84.0

'"52.2

0.929

60.2

0.897

68.2

0.845

76.2

0.756

84.2

0.586

92.2

0.256

0 "tl

52.4

0.927

60.4

0.896

68.4

0.842

76.4

0.752

84.4

0.580

92.4

0.245

ttl '"

52.6

0.927

60.6

0.895

68.6

0.842

76.6

0.746

84.6

0.574

92.6

0.235

::l52.8

0.927

60.8

0.89

468.8

0.840

76.8

0.746

84.8

0.568

92.8

0.224

ttl

CIl

53.0

0.926

61.0

0.894

69.0

0.838

77.0

0.743

85.0

0.562

93.0

0.213

53.2

0.926

61.2

0.891

69.2

0.836

77.2

0.740

85.2

0.555

93.2

0.203

53.4

0.925

61.4

0.890

69.4

0.835

77.4

0.739

85.4

0.549

93.4

0.193

53.6

0.924

61.6

0.889

69.6

0.833

77.6

0.735

85.6

0.542

93.6

0.182

53.8

0.924

61.8

0.888

69.8

0.831

77.8

0.732

85.8

0.535

93.8

0.172

54.0

0.923

62.0

0.887

70.0

0.829

78.0

0.727

86.0

0.529

94.0

0.161

54.2

0.922

62.2

0.886

70.2

0.827

78.2

0.723

86.2

0.522

94.2

0.151

54.4

0.922

62.4

0.886

70.4

0.823

78.4

0.720

86.4

0.516

94.4

0.141

54.6

0.921

62.6

0.884

70.6

0.823

78.6

0.716

86.6

0.507

94.6

0.131

54.8

0.920

62.8

0.882

70.8

0.821

78.8

0.713

86.8

0.500

94.8

0.122

tv ......

VI

tv .....0\

Tab

le8.

19C

onti

nued

Sucroseby

Water

Sucroseby

Water

Sucroseby

Water

Sucroseby

Water

Sucroseby

Water

Sucroseby

Water

weight

(%)

activity

weight

(%)activity

weight

(%)activity

weight

(%)activity

weight

(%)activity

weight

(%)activity

55.0

0.919

63.0

0.881

71.0

0.819

79.0

0.709

87.0

0.493

95.0

0.112

55.2

0.919

63.2

0.880

71.2

0.817

79.2

0.705

87.2

0.485

95.2

0.103

55.4

0.918

63.4

0.878

71.4

0.815

79.4

0.701

87.4

0.478

95.4

0.094

55.6

0.917

63.6

0.878

71.6

0.813

79.6

0.697

87.6

0.470

95.6

0.085

55.8

0.916

63.8

0.876

71.8

0.811

79.8

0.693

87.8

0.462

95.8

0.077

{/) c:::

0.875

72.0

0.810

80.0

0.689

("")

56.0

0.915

64.0

88.0

0.454

96.0

0.068

:o:l

56.2

0.915

64.2

0.874

72.2

0.807

80.2

0.685

88.2

0.445

96.2

0.061

0 {/)56.4

0.914

64.4

0.872

72.4

0.803

80.4

0.681

88.4

0.437

96.4

0.054

trl

56.6

0.913

64.6

0.871

72.6

0.802

80.6

0.677

88.6

0.429

96.6

0.047

56.8

0.912

64.8

0.870

72.8

0.800

80.8

0.672

88.8

0.420

96.8

0.040

57.0

0.911

65.0

0.869

73.0

0.798

81.0

0.668

89.0

0.41

197.0

0.033

57.2

0.911

65.2

0.867

73.2

0.795

81.2

0.664

89.2

0.402

97.2

0.029

57.4

0.910

65.4

0.866

73.4

0.793

81.4

0.659

89.4

0.393

97.4

0.024

57.6

0.909

65.6

0.865

73.6

0.791

81.6

0.654

89.6

0.384

97.6

0.020

57.8

0.908

65.8

0.863

73.8

0.788

81.8

0.650

89.8

0.375

97.8

0.016


Osmotic pressure of sucrose solutions in atmospheres for concentrationsin molalities (mol/kg water) ranging from 0.1 to 1.0 are reported in Table8.20 according to Heyrovska (1987).

8.4.13 Surface tension

This parameter accounts for the cohesion of water molecules in the liquidstate. Sucrose molecules, because of their compatible packing in water,slightly enhance the air interfacial tension of water, usually called surfacetension. The values of surface tension (mN/m) of sucrose solution at roomtemperature for concentrations ranging between 0 and 65% were adaptedfrom Landt (1934). They are given in Table 8.21.

8.4.14 Viscosity of sucrose solutions

Although the theory of viscosity is detailed in chapter 6 of this book, onlyfew data are given in that chapter. The authors (Mathlouthi and Genotelle)preferred to focus on practical aspects such as empirical relations for thecalculation of viscosity or the control of molasses exhaustion rather thanthe listing of the values of viscosity. These data are reported for puresucrose solutions in Table 8.22 according to ICUMSA (1979).

Table 8.21 Surface tension (N/m) at 21°C atvarious sucrose concentrations (g/100 g ofsolution) (From Landt, 1934)Table 8.20 Osmotic pressure of aqueous

sucrose solutions at 25°C (From Heyrovska,1987)

Sucrose(g/100 g of water)

369121518212427303336

Concentration(g/I00 g of solution)

Osmotic pressure(x 105 Pa) 0

52.17 104.56 156.95 209.33 2511.72 3014.11 3516.49 4018.89 4521.27 5023.66 5526.05 6028.43 65

Surface tension(N/m)

72.6872.9173.5174.1074.7075.3075.8976.49n0877.6878.2778.8779.4680.06

N ......

Tab

le8.22Viscosity(mPas)ofpuresucrosesolutions(FromICUMSA,1979)'

00

TemperatureeC)

Sucrose

(gil00g

010

2030

4050

6070

80ofsolution)'

203.782

2.642

1.945

1.493

1.184

0.97

0.81

0.68

0.59

213.977

2.768

2.031

1.555

1.231

1.00

0.84

0.71

0.61

224.187

2.904

2.124

1.622

1.281

1.04

0.87

0.73

0.63

234.415

30.50

2.224

1.692

1.333

1.09

0.90

0.76

0.65

244.661

3.208

2.331

1.769

1.390

1.13

0.93

0.79

0.67

254.931

3.380

2.447

1.852

1.451

1.17

0.97

0.82

0.70

265.223

3.565

2.573

1.941

1.516

1.22

1.01

0.85

0.72

275.542

3.767

2.708

2.037

1.587

1.28

1.05

0.88

0.75

C/l

285.889

3.986

2.855

2.140

1.663

1.34

1.10

0.92

0.78

c::29

6.271

4.225

3.015

2.251

1.744

1.40

1.14

0.96

0.81

(j '"0 C/l

306.692

4.487

3.187

2.373

1.833

1.47

1.20

1.00

0.85

tTl

317.148

4.772

3.376

2.504

1.927

1.54

1.25

1.04

0.88

327.653

5.084

3.581

2.645

2.029

1.61

1.31

1.09

0.92

338.214

5.428

3.806

2.799

2.141

1.69

1.37

1.14

0.96

348.841

5.808

4.052

2.967

2.260

1.78

1.44

1.19

1.00

359.543

6.230

4.323

3.150

2.390

1.87

1.51

1.25

1.05

3610.31

6.693

4.621

3.353

2.532

1.98

1.59

1.31

1.10

3711.19

7.212

4.950

3.573

2.687

2.09

1.67

1.37

1.15

3812.17

7.791

5.315

3.815

2.856

2.21

1.76

1.44

1.20

3913.27

8.436

5.718

4.082

3.039

2.35

1.86

1.52

1.26

4014.55

9.166

6.167

4.375

3.241

2.49

1.97

1.60

1.32

4116.00

9.992

6.71

4.701

3.461

2.65

2.08

1.68

1.39

4217.67

10.93

7.234

5.063

3.706

2.82

2.21

1.77

1.46

4319.58

11.98

7.867

5.467

3.977

3.01

2.35

1.88

1.54

4421.76

13.18

8.579

5.917

4.277

3.22

2.50

1.99

1.63

4524.29

14.55

9.383

6.421

4.611

3.46

2.66

2.11

1.71

4627.22

16.11

10.30

6.988

4.983

3.71

2.85

2.25

1.82

4730.60

17.91

11.33

7.628

5.400

4.00

3.05

2.40

1.93

4834.56

19.98

12.51

8.350

5.868

4.32

3.28

2.56

2.05

4939.22

22.39

13.87

9.171

6.395

4.68

3.53

2.74

2.19

5044.74

25.21

15.43

10.11

6.991

5.07

3.81

2.94

2.34

5151.29

28.48

17.24

11.18

7.669

5.52

4.12

3.17

2.50

5259.11

32.34

19.34

12.41

8.439

6.03

4.47

3.42

2.69

."53

68.51

36.91

21.79

13.84

9.321

6.61

4.87

3.70

2.89

:I:54

79.92

42.38

24.68

15.49

10.34

7.27

5.30

4.01

3.12

-< VJ

5593.86

48.90

28.07

17.42

11.50

8.02

5.81

4.36

3.37

is56

111.0

56.79

32.12

19.68

12.86

8.88

6.38

4.76

3.66

:> r'57

132.3

66.39

36.95

22.35

14.44

9.88

7.04

5.20

3.98

."

58159.0

78.51

48.78

25.51

16.29

11.1

7.80

5.72

4.34

:;I:l 0

59192.5

92.70

49.84

29.28

18.46

12.4

8.65

6.30

4.75

." tTl

:;I:l ..,

60235.7

110.9

58.49

33.82

21.04

14.0

9.66

6.98

5.20

til61

291.6

133.8

69.16

39.32

24.11

15.8

10.9

7.75

5.74

VJ

62364.6

163.0

82.42

46.02

27.80

17.9

12.2

8.63

6.35

63461.6

200.4

99.08

54.27

32.26

20.5

13.8

9.68

7.05

64591.5

249.0

120.1

64.48

37.69

23.7

15.7

10.9

7.87

65767.7

313.1

147.2

77.29

44.36

27.5

17.9

12.4

8.81

661013

398.5

182.2

93.45

52.51

32.1

20.6

14.1

9.93

671355

513.7

227.8

114.1

62.94

37.7

23.9

16.1

11.3

681846

672.1

288.5

140.7

75.97

44.7

27.9

18.4

12.8

692561

982.5

370.1

175.6

92.58

53.3

32.9

21.4

14.7

N ......'-0

N N 0

Tab

le8.22

Co

nti

nu

ed

Temperature(0C)

Sucrose

(gil00g

010

2030

4050

6070

80ofsolution)'

703628

1206

481.6

221.6

114.0

64.4

39.0

25.0

16.8

715253

1658

636.3

283.4

142.0

78.4

46.6

29.4

19.5

727792

2329

854.9

367.6

178.9

96.5

56.1

34.9

22.8

7311876

3340

1170

484.3

228.5·

121

68.4

41.7

26.9

7418639

4906

1631

648.5

296.0

152

84.1

50.3

32.0

7530207

7402

2328

884.8

389.5

193

105

61.4

38.3

'"76

-11400

3390

1270

535

247

131

76.7

45.9

C (j77

18200

5010

1800

735

324

170

95.5

56.2

;<l0

78-

30700

7870

2680

1050

441

221

122

70.5

'"79

52500

12400

4100

1500

603

292

156

88.5

tT1- -

80-

93300

20700

6280

2250

855

394

203

114

81178000

35700

10100

3450

1240

547

272

151

82-

351000

64700

17100

5550

1860

773

373

204

83-

750000

123000

30200

9230

2880

1140

519

283

84-

1.7

X106

250000

55000

16200

1700

1720

773

404

85-

4.21

X106

54100

111000

30000

8000

2740

1170

598

86-

9.44

X106

3.18

X10

6206000

53100

13100

4230

1710

857

'Thevaluesupto75gsucrose/100gofsolutionderivefromSwindells

etal

.(1958).Above75

gllO

Og.thedataarethoseofSchneider

etal

.(1

963)

.'Weightscorrectedtovacuum.

References


Anderson, G.L., Higbie, H. and SIegman, G. (1950) J. Am. Chern. Soc., 72, 3798.Bates, F.J. and Associates (1942a) Nat. Bur. of Standards, Circular C440, Washington, DC,SA, p. 642.

Bates, F.J. and Associates (1942b) Nat. But. of Standards, Circular C440, Washington, DC,USA, p. 365, 694.Bridgman, P.W. (1933), Proc. Am. Acad. Arts Sci., 68, 27.Brown, G.M., Levy, H.A. (1963) Science, 141,921-923.Brown, G.M. and Levy, H.A. (1973) Sucrose: precise determination of crystal andmolecular structure by neutron diffraction. Acta Crystallogr., 829, 790-797.Bubnik, Z. and Kadlec, P. (1992) Zuckerind., 117,345-350.Charles, D.F. (1960) Solubility of pure sucrose in water. Int. SugarJ., 62,126-131.Ciz, K. and Valter, V. (1967) Properties of sucrose. In: Sugar Technology (ed.Bretschneider, R.). SNTL Pub., Prague, Czech Republic, p. 33.Culp, E.e. (1946) Heat of solution: sucrose in water. Sugar, 41 (2),44,46.Hanson, J.e., Sieker, L.e. and Jensen, L.H. (1973) Acta Crystallogr., 829, 790-797.Helderman, W. (1927) Z. Phys. Chern., 130,396.Herzfeld, A. (1892) Z. Ver. Rubenzuckerind., 42,181.Heyrovska, R. (1987) The physical chemistry of aqueous solutions of sucrose. In Abstracts of

Lectures and Posters of the 8th Int. Symposium on Solute-Solvent Interactions (edsBarthel, J. and Schmeer, G). Univ. Regensburg, pp. 106-109.Hirschmiiller, H. (1953) Physical properties of sucrose. In Principles of Sugar Technology(Ed. Honig, P). Elsevier Pub., Amsterdam, L. The Netherlands, pp. 19-72.

Hoynak, P.S. and Bollenback, G.N. (1966) This is Liquid Sugar (nnd edn). Refined Syrups& Sugars Inc., New York, USA, p. 242.Hugot, E. (1987) La Sucrerie de Cannes (JIJrd edn). Lavoisier Pub., Paris, France.ICUMSA (1978) 17th Session, Montreal, S.12, Refractive Index, pp. 166-174.ICUMSA (1979) Sugar Analysis, General Methods, ICUMSA Pub., Peterborough, UK.ICUMSA (1982) 18th Session, Dublin, S.7, Density. ICUMSA Pub., Peterborough, UK,pp.84-86.ICUMSA (1990) 20th Session, Colorado Springs, S.II, Density. ICUMSA, Spieweck, F.,265-270

Jones, A.J., Hamish, P. and MacPhail, A.K. (1979) Sucrose: an assignment of the I3CNMR parameters by selective decoupling. Aust. 1. Chern., 32, 2763-2766.Kell, G.S. (1975) J. Chern. Engng. Data, 20, 97Kilmartin, E.J. and Van Hook, A. (1950) Sugar, 45,34.Landt, E. (1934) Naturwiss" 22, 809.Lopez-Chavez, A (1993) Solution properties and tastes of polyols. PhD thesis, ReadingUniv., Reading, UK.Lyle, O. (1957) Technology for Sugar Refinery Workers. Chapman & Hall Pub., London,UK, p. 629.Mathlouthi, M., Cholli, A.L. and Koenig, J.L. (1986) Spectroscopic study of the structureof sucrose in the amorphous state and in aqueous solutions. Carbohydr. Res., 147, 1-9.Narayana, R. (1950) Current sciences, 19, 276.Nicol, W.M. (1968) Boiling point elevation of pure sucrose solutions. Int. Sugar J., 70, 199202.Nicol, W.M. (1973) Sucrose dehydration by heat of crystallization. In Advances in

Preconcentration and Dehydration of Foods. Applied Science, London, UK, pp. 203-205.Norrish, R.S. (1966) An equation for the activity coefficients and equilibrium relativehumidities of water in confectionery syrups. J. Food Technol., 1,25.Norrish, R.S. (1967) Selected Tables of Phys. Properties of Sugar Solutions (Sci. Techn.Surveys, 51). The British Food Manuf. Ind. Res. Assoc., Leatherhead, Surrey, UK.Pancoast, H. and Junk, W. (1973) Handbook of Sugar, 1st Ed., AVI.Pavlik, B. (1939) Z. Kristallogr., Kristallgeom., Kristallphys. Chern., 100,414.Pearson, D. (1976) The Chemical Analysis of Food. Churchill Livingstone, Edinburgh, UK.

222 SUCROSE

Plato, F. (1900) Wiss. Abh. Kaisel., Normal Aichung Kommission, 2, Springer Verlag,Berlin, Germany, p. 140.

Plato, F. (1901) Wiss. Abh. Kaisel., Normal Aichung Kommission, Springer Verlag, Berlin,Germany.Roos, Y. (1993) Melting and glass transition of low molecular weight carbohydrates.

Carbohydr. Res., 238, 39-48.Roos, Y. and Karel, M. (1990) Differential scanning calorimetry. Study of phase transitionsaffecting quality of dehdyrated material. Biotechnol. Prog., 6, 159-163.Roth, D. (1976) Amorphisierung bei der zerkleinerung und rekristallisation als ursachen deragglomeration von puderzucker und verfahren zu deren vermeidung. PhD thesis,Karlsruhe, Germany.

Schneider, F., Schliephake, D. and Klimmek, A. (1963) Uber die viskositat von reinensaccharoselosungen. Zucker, 16, 465-473.Shallenberger, R.S. (1985) Monochromatic polarimetry. In Analysis of Food Carbohydrates(ed. Birch, G.G). Elsevier Applied Science, London, UK, pp. 41-59.

Shamil, S. (1988) Physical, chemical and sensory studies of sapid molecules. PhD thesis,University of Reading, Reading, UK.

Shamil, S., Birch, G.G., Mathlouthi, M. and Clifford, M.N. (1987) Chem. Senses, 12,397409.

Sheng, L.G. (1990) Calculation of the boiling point elevation of sugar solutions. Int. Sugar J.,92, 1100, 168-169.Smelik, A., Vasatko, 1., Dandar, A. and Matejova, 1. (1972) Zucker, 23,133,139,595.Spengler, 0., Boettger, S. and Werner, E. (1938) Z. Wirtschaftgr. Zuckerind., 88, 521.Swindells, 1.F., Snyder, C.F., Hardy, R.C. and Golden, P.E. (1958) Viscosities of sucrosesolutions at various temperatures, Tables of recalculated values. Nat. Bur. Stand., Suppl. toCire. No 440.

Vallender, R.B., and Perman, E.P. (1931) Trans. Faraday Soc., 27,124.Van Hook, A. (1981) Growth of sucrose crystals. Sugar Techno!. Rev., 8, 41-79.Vavrinecz, G. (1962) Z. Zuckerind., 12,481.Vavrinecz, G. (1973) Z. Zuckerind., 23, 10.Wagenbreth, H., Toth, H., Kozdon, A. and Emmerich, A. (1988) Phys. Techn. Bund.,

Mitteilungen, 98, 198.

9 Technological value of sucrose in food productsM.A. CLARKE

9.1 Introduction

The technological value of sucrose in food products is the value of sucroseas a component in foods and food products that results from the uniquecombination of physical and chemical, including sensory, properties foundin the sucrose molecule: common table sugar. In this chapter, productionand consumption figures on sucrose will be presented; utilization amountsin food categories and beverages will be outlined. Subsequent sections willdetail and describe the physical and chemical properties of sucrose that, intheir unusual multiplicity and strength, combine to give sucrose itsversatility as a universal sweetener.Early biotechnology, beginning perhaps thousands of years Be, wasbased on sucrose in certain plants especially for fermentation reactions;these will be reviewed in light of current sucrose-based fermentations inindustry today. The applications section of this chapter will list variousfood types in which sucrose is used, and will outline the functions ofsucrose in these foods, thereby providing examples of the technologicalvalue of sucrose.

9.1.1 Sources, production and consumption of sucrose

Sucrose, regular table sugar, is usually derived from sugar-cane or sugarbeet, the former grown in the tropics or semitropics, the latter in cooler ortemperate climates, both yielding sucrose at between 10 and 15% weight ofthe plant. Crop production and sucrose manufacture are describedextensively in the literature (Meade and Chen, 1985; Clarke and Godshall,1988; McGinnis, 1987). Juice extracted from the crop plant is purified andconcentrated to syrup; sucrose is crystallized and removed from the syrupcrystal mixture; the residual dark, heavy syrupy material is molasses ortreacle. White sugars are crystallized from very light colored syrups; brownsugars from dark colored syrups. Syrups intermediate in the process aresometimes sold as special dark syrups for food or beverage manufacture; alight colored partially inverted syrup (medium invert syrup) is frequentlyused in confectionery, canning, beverages and baking. High test molasses


224 SUCROSE

(cane juice molasses) is cane juice concentrated by evaporation withoutpurification; it is finding increasing use in food manufacture.Production and consumption of both sugar-cane and sugar-beet sourcedsucrose are listed, on a regional basis for the past several years, in Table9.1 (Licht, 1993). Ratio of cane:beet sourced sugar is indicated in the sametable.Category of consumption, or type of usage, figures are not available on a

global basis. Consumption categories for the US are listed in Table 9.2(Anon., 1993). Production of starch-based glucose syrups, though animportant (>50%) market factor in the US, totals less than 10% of sugarproduction, worldwide.

9.1.2 Comparative sweetness of sugar

Sweetness of sugar is discussed from the theoretical and food science pointsof view in chapters 2 and 10. Comparative sweetness of several

Table 9.1 Production, consumption and source of sucrose by region' (From Licht, 1993)

Region 1991-92 1992-93

Production Consumption % Cane Production Consumption % Cane

Western Europe 19.8 17.3 20.6 17.2Eastern Europe 10.0 16.2 9.8 15.4Africa 7.6 9.3 6.7 9.5North and CentralAmerica 20.6 16.5 19.4 16.6

South America 15.2 12.3 16.1 12.8Asia 38.6 38.7 34.0 40.3Oceania 3.4 1.1 4.9 1.1

Total 115.8 111.4 63.5 111.5 112.9 65.2

'In 1 000 000 tons, raw value, from September to August.

Table 9.2 Usage of sugars by category in the US, calendar year, 1000 tons (From Anon.,1993)

1991 1992Bakery, cereal 1632 1719Confectionery, chocolate 1276 1246Ice cream, dairy 439 429Beverages 204 164Canned, frozen foods 332 315All other food 623 649Non-food 89 69Wholesale and institutional 2177 2205Retail 1182 1230

Total' 8063 8259

'May not add up because some small categories are not included.

VALUE OF SUCROSE IN FOOD PRODUCTS

Table 9.3 Carbohydrate - sweetness ratings (FromYalpani, 1992)

225

Carbohydrate

SucroseGlucoseFructoseGalactoseMaltoseLactose50% Invert90% HFCS*42% HFCS95 DE corn syrup'42 DE corn syrupBrown sugarHoneyMalt syrup

AlsoMaltitolLactitolIsomaltXylitolSorbitolMannitolNeosugar

*HFCS is high fructose corn syrup.'DE is dextrose equivalent (instarches).

Sweetness

10075-80175324020

120-130120-160100-11075-8040-5085-9095-10525-30

9030-3928-45100604040-60

hydrolysed

carbohydrate sweeteners in various forms is listed in Table 9.3 (Yalpani,1992). Sweetness is influenced by form (solid or solution), concentration insolution and temperature and by many subjecive factors, e.g. color ofsample tested.Choice of sweetener in a food product depends not only on sweetnessand cost, but on a combination of chemical and physical propertiesaffecting texture, color and color development, moisture content, storagefactors, and packaging quality. The following sections of this chapter willdiscuss the chemical and physical properties of sucrose and their effect onfoods. These properties are listed in Table 9.4.

9.2 Chemical properties of sucrose

9.2.1 Purity

The purity of sucrose, generally available at over 99.9% purity with themajor non-sugar present being water, provides a relatively low cost

226 SUCROSE

Table 9.4 Physical and chemical properties of sucrose

Physical

Bulk densityColligative propertiesCrystallinityDielectric constantOsmotic pressureSolubilityVapour pressureViscosityWater activity

Chemical

AntioxidantColorFermentationFlavorFlavor enhancementPuritySensory propertiesSweetness

Table 9.5 Microbiological specifications for sugars

(1) Canners standardsFlat sour spores:Average not more than 50 spores/lO gMaximum 75 spores/l0 g

Thermophilic anaerobic sporesShall be present in not more than 60% in five samples

Sulfite spoilage bacteriaShall be present in not more than 40% in five samples and in anyone sample tothe extent of not more than 5 spores/lO g

(2) Carbonated beverageNot more than200 mesophilic bacteria/lO g10 yeast/lO g10 molds/l0 g

(3) 'Bottlers' liquid sugarShall not contain more than:100 mesophilic bacteria/lO g (dry sugar)10 yeast/lO g (dry sugar)10 molds/l0 g (dry sugar)

ingredient with consistent chemical, physical and microbiological characteristics for the food and beverage industry. In Table 9.5 are outlinedmicrobiological specifications, in the USA, for sugars for various uses.

9.2.2 Solution reactions: inversion, degradation, Maillard and browningreactions

The principal reactions of sucrose, in food manufacture or preparation,occur as reactions in solution. Although the medium or substrate mayappear to be a solid, e.g. bread dough, cake mixture, the sugarsundergoing reactions are in solution form, dissolved in water. Reactionsare outlined in Table 9.6.


Table 9.6 Solution chemistry of sucrose

227

(1) Sucrose hydrolysis to glucose and fructose (invert)(2) Glucose and fructose reaction types(a) Acidic - HMF formation(b) Alkaline - Lactic acid formation

- Transformations, rearrangements(c) Acid and alkaline - Maillard reactions to form colors and flavors, condensations

The hydrolysis of sucrose, called 'inversion,' to an equimolar mixture ofglucose and fructose, called 'invert sugar' or 'invert' (because of theinversion from positive to negative, upon hydrolysis, of the polarimetricmeasurement used to quantify the sugars), is the initial reaction for most ofthe important reaction sequences of monosaccharides in food chemistry.Inversion can occur from low pH up to about pH 8.5. Subsequentreactions, which occur also in all syrups of glucose and/or fructose are, asoutlned in Table 9.6:

(a) Reactions in acidic medium, outlined in Figure 9.1 (Monte andMagna, 1981-82), which lead to formation of 5-hydroxymethylfurfural (HMF). HMF rapidly decomposes into dark-colored compounds, with off-flavors.

(b) Reactions in alkaline medium, outlined in Figure 9.2, include lacticacid formation by chemical means (rather than by fermentation),and the rearrangement of glucose to a mixture of mannose andfructose, which is often responsible for the reported presence offructose and mannose in food products that contain, in actuality,only glucose. An alkaline environment during extraction or hydrolysis procedures can cause the transformation of glucose to amixture of mannose and fructose by this mechanism.

(c) Maillard reactions: the reaction of a reducing sugar with an uamino group to form a condensation product that can subsequentlypolymerize into dark-colored compounds, is the basis of the'browning reaction' observed during baking and cooking processes.Alternate pathways of color, or melanoidin, formation after theinitial Maillard reaction are shown in Figure 9.3 (Monte and Magna,1981-82).

(d) Thermal degradation of sucrose and caramel formation. Thethermal decomposition of solid sucrose may be the exception to therule that the common food-related reactions occur in watersolution; however, moisture absorption by sucrose as it is heatedcan account for some thermal degradation along pathways ofsolution reactions. Multiple reactions, some anhydrous, someinvolving water are involved in the formation of the complex

228

H-C=OI

H-C-OHI

HO-C-HI

H-C-OHI

H-C-OHI

CH20H

H-C-OH"C-OHI

HO-C-HI

H-C-OHI

H-C-OHI

CH20H

D-GLUCOSE

1,2-ENOL

SUCROSE

H-C=OIC-OH"9-H 3-DEOXYALDOSE-2-ENE

H-C-OHI

H-9-0HCH20H

1l rearrangement

H-C=OIC=O"H-C-HI

H-C-OHI

H-C-OH,CH20H

H-C=oI

C=OI

H-C"C-HI

H-C-OHI

CH20H

cyclization

5-HYDROXYMETHYLFURFURAL

3-DEOXY-D-GLUCO-SULOSE

HEXOSULOSE-3-ENE

Figure 9.1 Formation of HMF from glucose. (From Monte and Magna (1981-82),reproduced with permission.)

mixture known as 'caramel'; these have been reviewed by Godshall(1986, 1988)

An outline of some of the reactions involved in thermal degradation ISshown in Figure 9.4.

VALUE OF SUCROSE IN FOOD PRODUCTS 229

H-C-OH"C-OHI

HO-C-HI

H-C-OHI

H-C-OHI

CH20H

H-C=OI

H-C-OHI

CH20H

+

H-C-OH"C-OHI

CH20H

l,2-ENEOL

GLYCERALDEHYDE

TRIOSE ENEDIOL

PYRUVALDEHYDEHYDRATE

'il'1JiJ~ 1!.@/lJINV ®~ &JlNqgVIJIJ"~I!.&J~IN®~

W~IJIJ ~©IX~IJIJ~'il'~OIJlJ 1N~~ININ~IJIJ@~INJIEIJIJ'il'

HO-9=O DL-LACTIC ACIDH-C-OH

I

CHa

9H20HC=OI

HO-9-H D-FRUCTOSEH-C-OH

IH-C-OH

I

CH20H

H-C=OI

HO-C-HI

HO-C-HI

H-C-OHI

H-C-OHI

CH20H

D-MANNOSE

H-C-OH"C-OHI

HO-C-HI

H-C-OHI

H-C-OHI

CH20H

ENEDIOL

H-C=OI

H-C-OHI

HO-C-HI

H-C-OHI

H-C-OHI

CH20H

D-GLUCOSE

Figure 9.2 Reactions in alkaline solution. (From Monte and Magna (1981-82), reproducedwith permission.)

230 SUCROSE

Aldose + Amino acids

1N-Substituted clycosylamine

1N-Substituted l-amino-l-deoxy-2-ketose

Pathway I / Pathway 2

3-Deoxyhexononeintermediates

Methyl dicarbonylintermediates

Reductonesdicarbonyls

+ Amines5-(Hydroxymethyl)2-Furaldehyde

Pigments or melanoidins

Figure 9.3 Pathways of melanoidin formation. (From Monte and Magna (1981-82),reproduced with permission.)

9.2.3 Sensory properties

The sweetness of simple carbohydrates is, of course, their outstandingcharacteristic property. Sweetness, and these sugars, are major aids topalatability, at comparatively low cost.Considerable discussion has been recently devoted to elucidating and

explaining sweetness (Mathlouthi et ai., 1993). Although final conclusionsare still in debate, it is clear that the human response to the sweetness ofsucrose differs from response to other sweet compounds. The relative


H2C---O

\:~L~

F~_6L..(6

OH

lH+

+

H+CH20H ~

~+o CH20H

non-specificOH ~H - degradation

11 ~~O~OH.CH20Hf Sucrose-OH \--Y'

OH

HOH2C 0 ( -O-Sucrose

vv~

HO-CH20H

OH

Figure 9.4 Mechanism of thermal degradation of sugars.

sweetness, compared to sucrose, of glucose is less than sucrose and offructose is more, depending on the form (solid or solution), degree ofcooking and concentration of the monosaccharide. Sweetness data fromTable 9.3 are further amplified in Table 9.7; flavor enhancement andeffects of sucrose on flavor extension and suppression are discussed byGodshall in the same article (Godshall, 1988) (see also chapter 10).Other sensory properties of sucrose are related to its ability to form avariety of crystal sizes, and to its bulk density, viscosity and solubilityproperties.

9.2.4 Color

Although sucrose, glucose and fructose are themselves white crystallinematerials, they are in part responsible for most of the yellow-to-browncolors developed in baked and processed foods. Reactions responsible forcolor formation include the following discussed above;

• thermal degradation of sugars, with condensation at low pH andcaramel formation;

• alkaline degradation of fructose, with subsequent condensation; and

232 SUCROSE

Table 9.7 Sweetness and flavor of selected carbohydrates in solution

Carbohydrate

MonosaccharidesGlucoseFructose

DisaccharidesSucroseMaltose (malt sugar, maltobiose)Lactose (milk sugar)Palatinose (isomaltulose, Iylose)Leucrose (glucose-I, 5-fructose)

Polyols (sugar alcohols, hydrogenated sugars)XylitolSorbitol (hydrogenated glucose) syrupyMaltitol (hydrogenated maltose)MannitolLactitol (hydrogenated lactose)

Mixtures and syrupsLycasin 80/55 (hydrogenated glucose syrup)Palatinit (Isomalt; I: 1 mix of glu-sorbitol

+ glu-mannitol)HFCS (high fructose corn syrup)Invert syrupMaltodextrin (DE < 20)Neosugar (fructo-oligosaccharides)

Sweetness

61,70130-180

10043,50

40, 26, 15-305050

100, 85-12050,63,706840,6530-40

75

45100-1601050+46--60

Flavor character

Sweet, bitter side tastePure sweet, fruity

Pure sweetSweet, syrupyFaint sweet, fruityPure sweet, masks bitterPure sweet

Sweet, coolingSweet, coolingSweetSweetClean, sweet

Sweet

Pure sweetSweetSweetBland to faintly sweetSweet

• Maillard reactions with primary amines and subsequent melanoidinformation.

9.2.5 Antioxidant properties

Sucrose has been reported to exhibit antioxidant properties in preventingdeterioration of flavor in canned fruit, and of rancidity in cookies. Sugarsolutions can inhibit rust formation on iron (relative to water) (Nicol,1991). The lowering of water activity by sucrose may be in part responsiblefor these observations. An extract of non-sugars in molasses has also beenfound to exhibit an antioxidant effect on whole grain products (Clarke,1991).

9.3 Physical properties of sucrose

9.3.1 Colligative properties

Freezing point depression, boiling point elevation, osmolality and effect onwater activity are all affected by the concentration of sugar in a product.


Freezing-point depression is an important property in ice creams, frozendesserts and sauces, and in freeze-dried foods. The effect of sucrose andglucose on freezing point depression is shown in Figure 9.5 (Best, 1992).All colligative properties are affected by the number of molecules presentin a given quantity, rather than by the weight of the quantity. Figure 9.6shows the effect of several sweeteners at varying concentrations on thefreezing point of a frozen dessert (Smith, 1990). Glucose and fructose,having about twice the number of molecules per gram (or other weight)that sucrose does, have a greater effect on freezing point depression andother colligative properties than does sucrose. This property must becombined with sweetness and viscosity properties in sweetener selectionfor frozen foods.The reduced vapor pressure caused by the sugars in solution has asignificant effect on increasing the boiling point of solutions as shown inFigure 9.7 (Jun'( and Pancoast, 1980). Boiling point elevation is importantin candy manufacture, and in sauces and condiments that may be added toa mixture to be cooked or processed. Boiling point elevation for sucroseconcentrations used in confectionery manufacture is reported in Table 8.18(see chapter 8). The increase in boiling point allows higher cookingtemperatures and subsequent slower formation of very small crystals oncooling (Flanyak, 1991).The high osmotic pressure generated by sucrose in solution (see chapter8, Table 8.21) is an important factor in preservation of foods, andprevention of microbial contamination (Flanyak, 1991) (Honig, 1953).High concentrations of sugars decrease the water activity and lower theequilibrium relative humidity, leaving insufficient water available to

32

LL0

+-'caQ.

OJ 31c'NQ)Q).....LL

30 +------,~----r---r_--___r_o 5 10 15 20

Solution concentration (% solids)

Figure 9.5 Freezing poinl depression of sucrose and glucose. (From Best (1992), reproducedby permission.)

234

oo

o

-80

SUCROSE

o 100

Sweetener Concentration Percent (w/w)

Figure 9.6 Freezing points of various concentrations of sweetners used in frozen dessertmanufacture. CSS, corn syrup solids; DE, dextrose equivalent; DS, disaccharides; FRU,fructose; MD, maltodextrins; MS, monosaccharides. (From Smith (1990), reproduced with

permission. )

B.PoE

30

25

20

15

10

5

%005070 75 80 85 90Figure 9.7 Boiling point elevation (BPE) by sucrose and 50% invert (for invert the values areuncorrected for barometer or possible superaturation factors.). (From Junk and Pancoast

(1980), reproduced by permission.)

VALUE OF SUCROSE IN FOOD PRODUcrS 235

sustain viable microorganisms. In acidic media, such as jams andpreserves, sucrose inverts to glucose and fructose thereby increasing themolal concentration of sugars per weight, and the lowering effect on wateractivity.The equilibrium relative humidity (ERH) , a measure of water activity, isthe ratio of the vapor pressure of atmosphere at a sugar crystal to the vaporpressure of water at the same temperature, or, it can be considered as therelative atmospheric humidity at which the sugar is in equilibrium with theair. There are three ways in which water is associated with a sucrosecrystal:

Free water (surface water) - varies with surface humidity and composition ofsyrup film on crystal

Bound water - converts to free water during conditioning

Inherent water - moisture pockets in crystal. From 0.01% for 0.5 mm crystalsto 0.4% for 3--4 mm crystals.

After sugar is crystallized, some of the bound water diffuses out of thecrystal and into the air during the conditioning process, when ambient air iscirculated through the sugar. If conditioning is insufficient, the water doesnot leave the sugar crystal and causes stickiness and subsequent caking orhardening of the sugar (Nicol, 1979). Figure 9.8 shows the difference inhumidity isotherms for sugar of different grain size. The availability ofsucrose in large, uniformly sized crystals is an important property inpreparation of dry mixes, for their stability and keeping properties.Fructose and glucose are also available in crystalline form, but only insmaller crystal sizes which, combined with their tendency to hygroscopicity,

\Wcter content

Ii/f/

/i/,/

~-------~-- .,./- -- - .~.-.-._.-._.-

......-.---r---r---.....--,.---.......--..,.,...-_.,......--.--_ERH10 30 50 70 90

Figure 9.8 Water vapor sorption isotherms (20°C) of white granulated sugar of differentquality: (--) poorer quality, smaller grain; (- - - -) intermediate; and (_._._) better

quality, larger grain.

236 SUCROSE

decrease their effectiveness on keeping qualities of dry mixes. The purityof large, uniform crystals of sucrose is an important factor in maintainingflavor properties of mixes.Equilibrium relative humidity factors are important for flow properties,and for resistance to infection by microorganisms. From Figure 9.8, it maybe seen that the equilibrium water content of sugar remains much the sameover the 20-70% relative humidity range; therefore, sugar is normally freeflowing in most temperate climates. Table 9.8 shows the minimum ERH atwhich microorganisms can grow; it is apparent that concentrated solutionsof sugar, as in jams, jellies and preserves, do not have sufficient availablemoisture to permit growth of microorganisms (Nicol, 1979), this is one wayin which sugars can act as a preservative. Equilibrium moisture absorptionof sucrose is shown in Table 9.9, compared to other commercial sweetenersand carbohydrates (Yalpani, 1992).

9.3. J. J Solubility. The solubility of carbohydrate solid sweeteners isshown in Figure 9.9. Sucrose is intermediate in solubility between glucoseand fructose. Rate of solution depends on particle size, rate of agitation,

Table 9.8 Minimum ERH allowing microorganismgrowth (From Nicol, 1979)

Type of microorganism

Common bacteriaCommon yeastsCommon moldsHalophilic bacteriaXerophilic moldsOsmotolerant yeasts

ERH(%)

918880756560

Table 9.9 Equilibrium moisture absorption of sucrose compared with other sweeteners (FromYalpani, 1992)

Sweetener Moisture absorption (%)

Sucrose, cryst.Glucose, cryst.Fructose, cryst.Invert sugarMaltose, solidLactose, anhydr.Raffinose, anhydr.

*RH, relative humidity at 20°C.

1 h

0.040.070.380.190.800.540.74

60% RH*

9 days

0.030.070.635.17.01.212.9

100% RH*

25 days

18.414.573.476.618.41.415.9

90

70

50

30


gsugar %gsolution

237

20 ltDtemperature

60 B

Figure 9.9 Solubility of some sugars. (From Nicol (1979), reproduced by permission.)

and degree of undersaturation. The high degree of solubility is essential inpreparation of tinned fruits, jams, jellies, preserves and syrups. Mixturesof sugars can give a higher dissolved solids concentration than an individualsugar, another important factor for low water activity in jams, syrups andpreserves, where the sucrose-invert mixture is the usual combination. Thehygroscopic nature of sugars is related to their solubility: the high degree ofsolubility of crystalline fructose correlates with its high degree of hygroscopicity or rate of water absorption. Crystalline fructose is frequentlymixed with sucrose, to prevent too much water pickup and inadvertentsolubilization of the fructose sweetener in storage or in dry mixes. Furtherinformation on solubility is found in chapter 5.

9.3.1.2 Viscosity. This property is discussed in detail in chapter 6. Theviscosity of solutions of simple carbohydrates varies with their concentration(Honig, 1953; Nicol, 1991); viscosity of their pure solutions is generallyNewtonian. Sucrose is intermediate in viscosity between the very highviscosity of glucose syrups and the high fructose syrups: one reason for thevery high, almost plastic viscosity of glucose syrups is their content ofunhydrolyzed starch and dextrin molecules. These compounds are at lowerconcentrations in the more highly processed high fructose syrups.Comparative viscosities of sucrose and fructose are shown in Table 9.10(Yalpani, 1992).

238 SUCROSE

Table 9.10 Viscosities of sucrose and fructose at 20°C (Yalpani, 1992)

Concentration (w/w%) Viscosity (cps)

Sucrose Fructose

203040506070

1.93.26.215.058.0480.0

1.82.95.611.034.0170.0

Table 9.11 Bulk density of sugars (From Junk and Pancoast, 1980)

Sugar type Typical values

kg/m3 16/ft3

Confectioners AA 833-881 52-55Sanding 801-833 50-52Manufacturer's or fine 785-833 49-52

Granulated'Bottler's or standard granulated 769-817 48-51Baker's special 785-849 49-53Sifted powdered 384-481 24-30Compacted powdered 609-721 38-45Agglomerated 320-384 20-24Soft (brown)-compacted 833-993 52--{)2

'Maximum value of bulk density for granulated sugar occurs at grain size 0.2 mm, and is93 kg/m 3 (no conglomerates).

9.3.1.3 Bulk density. The large, uniform particle size of sucrose makes itan ideal carrier for trace ingredients, e.g. color, flavor, as well as a diluentand bulking agent. These properties, combined with its solubility, havemade sucrose a popular ingredient in dry mixes. Bulk densities of typicalsugar are listed in Table 9.11 (Junk and Pancoast, 1980). Table 9.12 showsrelated information on sieve analysis of these typical commercial gradesalso variety of granulations (McGinnis, 1987).The humectancy properties of sucrose, or its resistance to change inwater absorption as referred to above, give sucrose-containing dry mixesdesirable keeping qualities. The humectant properties of sucrose also meanthat cakes, breads, and cookies made with sucrose show good resistance todrying out, and stay fresher longer.The moisture and bulking properties of sugar, particularly sucrose,affect the physical structure of baked products, cereals and confectioneryby three principal means.

Tab

le9.

12Comparativesieveanalysisofvariousgrainsizesandbrands(FromMcGinnis,1987)

Large

crystal

Manufacturers

Fine

Extra-fine

Bakers

Crushed

Powdered

USBSsievesize

specials

Coarsegranulated

granulated

granulated

granulated

special

grain

<Meshes

Openings

» t"'

perinch

c:::Inch

mu

AB

AB

C0

AB

AB

AB

AB

AB

AB

CtT1 0 "J1

120.06611680

5.7

rJJ

160.04691190

59.1

c::: (')20

0.0331

84027.1

9.318.011.3

5.0

3.1

0.0

0.0

:<l

300.0232

590

7.349.249.259.855.031.6

13.4

5.5

1.3

0.3

0.0

0.0

0 rJJ40

0.0165

420

0.537.624.025.634.056.7

72.7

44.5

18.4

11.9

9.3

3.0

tT1

500.0117

297

5.0

3.2

5.3

7.8

11.8

37.037.439.1

39.2

36.0

5.0

1.5

Z70

0.0083

210

1.9

10.529.1

32.633.541.045.048.5

30.417.3

"J1 0100

0.0059

149

1.912.2

13.7

13.015.037.035.540.837.1

0.5

0140

0.0041

105

10.012.022.0

19.1

1.80.0

0

200

0.0029

749.23.70.1

." :<l

325

0.0017

4434.2

9.3

1.1

0 0 c:::P

erce

ntag

efi

ner

than

last

scre

enre

port

edab

ove

(') ..,

0.3

3.9

3.8

0.1

0.7

0.8

0.2

0.6

1.6

2.4

6.0

5.0

3.0

3.0

6.826.554.487.099.8

rJJ

Mea

nap

ertu

re-

coef

fici

ento

fva

rian

ceMA

1271

660

680

668

625

553

480

420

325

312

300

275

210

165

CV

2221

2719

2116

2026

3231

3228

2728

IV V.l\0

240 SUCROSE

(1) The effect of sugars on gelatinization of starch in a mixture. Sugardelays onset of gelatinization to higher temperature, allowing abaked product more time to rise (Nicol, 1970).

(2) The effect of sugars on denaturation of protein, related to the waterbinding capacity of sugar and its ability to stabilize protein foam, asin meringues and egg white leavened, fat-free, cakes.

(3) The ability of sucrose to disperse its particles (in some cases,amorphous particles) throughout fat-phase confections, e.g.chocolate, to maintain bulk, even distribution of flavor, and stabilityto moisture and to microbial infection. The fact that sucrose canadapt the rheological properties of chocolate to the non-Newtonianflow characteristics that permit layering is vital for chocolatemanufacture. The finer the particle size of the sugar, the moreviscous the chocolate melt, for any given fat content (see Table9.12).

9.3.1.4 Dielectric constant. The dielectric constants of sucrose andmonosaccharides are much higher than those of complex carbohydrates,lipids, proteins, or other major food components as shown in Tables 9.13(Malmberg andMaryott, 1950) and 9.14 (Shukla andMisra, 1990). Dielectricconstant is one physical property of a food that affects its heating bymicrowave radiation. Because sucrose has, additionally, the specialproperty of forming a permanent dipole when hydrogen is bonded withwater, it is an especially suitable ingredient for formulation of microwaveable foods. It can be used to increase rate of heating, to allow surfaceheating only, or to formulate browning and crisping conditions (Shukla andMisra, 1990).

9.4 Applications: effects of sucrose in food processing

The following section will describe the effects that the chemical andphysical properties of sucrose, as outlined above, show in the variousbaked goods, confectionery and other foods in which sucrose plays animportant role, nutritionally, and otherwise.

9.4.1 Breads

Sugars provide the base for yeast fermentation and rising of bread. Sucroseincreases the gelatinization temperature of starch and allow higher rising.Browning reactions, including Maillard reactions and other forms of nonenzymatic browning, create the color of the product, and increase thebread flavor. Humectancy characteristics increase moisture retention andshelf life.


Table 9.13 Dielectric activity of major food constituentsat microwave frequencies (From Shukla and Misra, 1990)

241

Food constituents

Bound waterFree waterProteinLipidTriglyceridesPhospholipids

Complex carbohydrates: starchMonosaccharidesAssociated electrolytesIons

Relative activity

LowHighLow

LowMediumLowHighLowHigh

Table 9.14 Dielectric constants of aqueous dextrose and sucrose solutions (From Malmbergand Maryott, 1950)

Dielectric constant at

20°C 25°C 30°C

Glucose (wI %)0 80.38 78.54 76.765 79.17 77.37 75.6410 76.1415 76.56 74.80 73.1120 73.4330 72.13 70.46 68.8240 68.73 67.11 65.5650 64.90 63.39 61.91

Sucrose (wI %)0 80.38 78.54 70.7610 78.04 76.19 74.4320 75.45 73.65 71.9030 72.64 70.86 69.1340 69.45 67.72 66.0550 65.88 64.20 62.5760 61.80 60.19 58.64

9.4.2 Cakes

Cakes, in the USA, are of three main types: pound cakes, with littlechemical leavening, which contain 25-35% by weight sugar; layer cakes,leavened with baking powder or soda, which contain 23-30% sugars; andsponge or chiffon cakes, with egg as principal leavening, which contain 3037% sugars. Angel food cakes, leavened by egg whites only, contains 42%sugar by weight. Sucrose is the required sweetener for most cakes to enableair incorporation, through creaming with fat and maintaining air bubbles,

242 SUCROSE

and also for elevation of starch gelatinization temperature, and forpreservation and shelf-life because of humeclancy factors. Some 10-15%of sucrose can be replaced by glucose and/or fructose in syrup form (withadjustment to liquid content) to produce cakes with slightly less volumeand sweetness, and a heavy crust.A comparison of a standard cake made with and without sugar is shown

in Table 9.15. The cake made without sugar did not rise and was dry andtasteless (Tsang and Clarke, 1988). A slice from the cake without sugarcontained some 25% more calories than a similarly sized slice from thecake with sugar, because of the smaller size and concentration of fat in thesugarless cake.One recent development in this area is the developing market in theUSA for fat-free, or lowered fat, cakes and sweet breads, with formulations containing higher levels of sugar and oligosaccharides to supply bulksubstitute for the missing fat.

9.4.3 Cookies and sweet biscuits

Sucrose contributes flavor, taste, color, texture and tenderness to cookies,as in cakes and breads, and adds to shelf life. The type of sugar used canaffect the crumb structure, spread and softness of the cookie. Brown sugarsand molasses soften the crumbs, and extend shelf-life. Glucose andfructose produce more crust color at lower temperatures than sucrose;lactose, a disaccharide in milk products, has the effect of maintaining crustcolor. Coarser granulated sugar will produce a crisper cookie, and thespread of the cookie increases as sucrose levels are increased, or grain sizedecreased. The recently popular combination cookie with crisp crust andsoft interior can be made by combining an inner dough made withmonosaccharides and an outer dough made with sucrose. Over severaldays, moisture migrates from the outer shell to the inner dough creatingthe combination texture cookie.

Table 9.15 Comparison of cakes made with and without sugar (Tsang and Clarke, 1988)

Butter or margarine (g)Sugar (g)Eggs (g)Flour + spices (g)Cream (g)Total (g)

Energy (kcaltl00 g)Content (same size slices)

100 g of cakemade with sugar

252320293100

408 kcal = 5 slicesc. 82 kcal

100 g of cakemade without sugar

33

26374100

414 kcal = 3-4 slices104-138 kcal

'Ingredients other than sugar were identical.


9.4.4 Icings and frostings

Sugars are essential in these products for flavor, taste, texture - dependingon grain size of the sugar - and appearance. The purity and consistentcomposition of sucrose are important to the simple water-sugar icings mostcommonly used on baked goods.

9.4.5 Beverages

Sugar provides sweetness, flavor and mouthfeel in carbonated soft drinks,and in simulated juice-type beverages. Low microbial level, and low levelsof turbidity and non-sugars are important to protect the flavor andappearance of the beverage. High fructose corn syrup is used instead ofsucrose in almost all nutritively sweetened soft drinks in the USA, becauseHFCS is much cheaper than sucrose in the USA.

9.4.6 Jams, jellies and preserves

The preservation effect, resulting from lowered water activity and highosmotic pressure, is important here to maintain texture of fruit and toprevent microbial contamination. The properties of sweetness and flavorare also significant. Visual appearance of clear jellies and preserves isanother factor: some sweeteners, other than sucrose, cause haziness injellies. Inversion of sucrose, or use of invert syrups, prevents crystallizationin these products.

9.4.7 Confectionery

In chocolate products, sucrose is preferred because of its range of grain size(see Table 9.12), solubility, dispersant ability in fat, stability bulk andtexture. The rheological properties contributed by sucrose to the fatmixture are paramount to the flow and solid formation behavior ofchocolate. Sucrose has the ability to pass through an amorphous,microcrystalline form, at about 160°C (see chapter 4) which aids in therefining and tempering of chocolate. In milk chocolate, Maillard reactionsbetween reducing sugars and milk protein contribute to the 'caramel'flavor. In non-chocolate confectionery, such as hard, boiled sweets orcandy, the high viscosity of sugar solutions, and the solubility of sugarswhich allow them to remain in supersaturated solution, and provide astable syrup phase (Flanyak, 1991; Jeffrey, 1993) are of significance.Resistance to microbial infection is another key point. The addition of themore difficultly crystalline glucose allows preparation of a hard sweet with aglossy texture, with no crystalline sucrose.Gel-type sweets, both pectin and starch, require sugars for their bulk,

244 SUCROSE

sweetness and flavor, and viscosity to support the gel structure. Caramels,toffees and fudges require Maillard reaction products from reducing sugarsand milk (or other) proteins to produce the desired flavors.

9.4.8 Dairy products

Simple sugars give the necessary freezing point lowering to maintaintexture and product quality in ice creams and frozen desserts. Too Iowafreezing point means too little water may be frozen: an icy texture candevelop (Smith, 1990). Sucrose provides good dispersion among fats in icecreams, and increases viscosity in lower fat products. Sugars are used forsweetness, flavor, preservation (high osmotic pressure) and texture insweetened condensed milk. Flavor, sweetness and texture are added bysugars to yogurt products. Again, sugars are not fat substitutes, but arefinding use in lowered fat products to increase bulk, flavor and texture ortenderness.

9.4.9 Ready-to-eat breakfast cereals

Sugars added to cereals contribute flavor, color (browning characteristics)and sweetness, but also give important textural modifications. Sugars areadded to doughs at levels from 6 to 25%. Sugars act as binders, and can actto increase crispness, spread, and surface porosity in much the same way asin baked cookies. The greatest use of sugar in ready-to-eat cereals is as asurface coating; the solubility and crystallization properties permit clear orfrosted coatings that can serve as flavor or additive carriers, and thatincrease the shelf-life of the cereal. Content of invert sugar must becontrolled to prevent stickiness.

9.4.10 Meats

Sugar is often added in small quantities to cured, dried or preserved meats,especially to pork products. The purposes of the sugars are: as apreservative, to lower water activity, and as a flavor enhancer, to contrastwith salt and extend the meat flavor. Sugar is added at a level below 1%,contributing less than 1 g carbohydrate (or 4 kcal) to 100 g raw slab orsliced bacon with a total of 665 kcal (USDA, 1991).

9.4.11 Frozen and tinned vegetables

Sugars are sometimes added in small (sub-percent) levels to cooked tinnedand frozen vegetables to enhance flavor and help preserve color andtexture. A well-known example is the addition of sugar to baked beans: the


flavor and preservative effects of added sugar are significant to anyone whohas tasted plain boiled or baked navy or pea beans without sauce, and theregular baked beans in sauce. The addition of sugar-containing saucecauses a maximum increase of 0.8 g carbohydrate (or 3.2 kcal) per 100 g ofbaked beans (USDA, 1991).

9.5 Biochemical properties of sucrose

This section should perhaps more properly be called a brief summary offermentation reactions of sucrose. The microbiological reactions of sucrosethat are important in food processing have been listed, as specifications, inTable 9.5. The ability of sucrose to serve as a feedstock for microorganismsthat can convert sucrose chemically, through enzymic reactions, to avariety of food and organic chemicals, including foods and fuels, makessucrose a most facile and versatile substrate for fermentation.Sucrose in cane juice and molasses has been known as a feedstock for

fermentation to ethyl alcohol for at least 4000 years: two types of fermentedliquors from sugarcane sources are mentioned in the ancient India:-t texts,the Vedas (Paturau, 1989).The production of ethanol, for industrial and/or fuel use, by fermentationwith some species of the yeast Saccharomyces cerevisiae family is commonat raw sugar factories everywhere except in the US: molasses is generallythe fermenation substrate. The enormous ethanol fuel program in Brazilrequires more substrate than could be provided by residual backstrapmolasses alone, and so some cane is grown for production of juice forfermentation. A general practice at cane sugar factories in Sao Paulo stateis to take one crop of crystals (first or A strike) from the evaporated juiceand use the residual syrup, which contains two to three times the sucroselevels of final molasses, for fermentation. These mills produce sugar andalcohol only, producing no molasses.Production of beverage alcohol and rum, the latter still an art as much asa science in some countries, is general in sugarcane growing areas, withmost rum produced from molasses and a very small percentage of specialtyrums produced from cane juice. In some areas, beverage alcohol (or canespirits) or neutral flavor is produced to be sold as is or used as a basis forother (non-rum) liquors. Many descriptions of fermentation and rumproduction are to be found in the literature (Clarke and Godshall, 1988;Paturau, 1989).Fermentations similar to these for ethyl alcohol production also produceindustrial CO2 , often at distilleries or breweries in conjunction with themajor product. Liquid CO2 is used extensively in carbonated beverages.Some other fermentation processes that use molasses (or sucrose orglucose) as a feedstock, and the type of microorganisms commonly

246 SUCROSE

Table 9.16 Microorganisms used in common fermentations and the fermations products

Product

Acetic acid and vinegarButanol-acetoneDextranLactic acidCitric acidGlycerolBakers yeastFeed yeastsL-LysineItaconic acid

Common microorganism

Acetobacter acetiClostridium acetobutolycumLeuconostoc mesenteroides or dextranicumLactobacillus delbruckiiAspergillus nigerSaccharomyces ellipsoideusSaccharomyces cerevisiaeTorulopsis uti/isCorynebacterium glutamicusBrevibacterium spp. Aspergillus terreus

employed, are listed in Table 9.16 (Paturau, 1989). There are manyvariations on these basic fermentations.

References

Anon. (1993) Sugar and Sweetener Situation and Outlook Report. USDA Economic ResearchService.Best, D. (1992) Working with sweeteners. Prepared Foods, January, 50-52.Clarke, M.A. (1991) Non-sucrose carbohydrates. In Handbook of Sweeteners (eds Marie, S.and Piggott, J.R.). Blackie, London, UK.Clarke, M.A. and Godshall, M.A. (1988) Chemistry and Processing of Sugarbeet and

Sugarcane. Elsevier, Amsterdam, The Netherlands.Flanyak, J.R. (1991) Effects of sucrose in confectionery processes. The Manufacturing

Confectioner, October, 61-66.Godshall, M.A. (1986) Flavors from beet and cane sugar products. Proc. Sugar Proc. Res.,pp.210-228Godshall, M.A. (1988) The role of carbohydrates in flavor development. Food Technol.,November, 71-78.Honig, P. (1953) Principles of Sugar Technology (Vol. 1). Elsevier, London, UK.Jeffrey, M.S. (1993) Key functional properties of sucrose in chocolate and sugarconfectionery. Food Technol., January, 141-144.Junk, W.R. and Pancoast, H.M. (1980) Handbook ofSugars. AVI Publishing Co., Westport,CT, USA.Licht, F.O. (1993) World Sugar Statistics (54th edn). F.O. Licht, Ratzeburg, Germany.Malmberg, c.G. and Maryott, A.A. (1950) Dielectric constant of solutions. J. Res. Nat. Bur.

Standards, 45, 299-307Mathlouthi, M., Birch, G.G. and Kanters, J.A. (1993) Sweet Taste Chemoreception. ElsevierApplied Science, London, UK.McGinnis, R.A. (1987) Beet Sugar Technology (3rd edn). Beet Sugar DevelopmentFoundation, Denver, CO, USA.Meade, G.P. and Chen, J.c.P. eds. (1985) Cane Sugar Handbook (11th edn). WileyInterscience, New York, USA.Monte, W.C. and Magna, J.A. (1981-82) Flavor chemistry of sucrose. Sugar Techno/. Rev.,8,181-204.Nicol, W.M. (1979) Sucrose and food technology. In Sugar, Science and Technology (eds Birch,G.G. and Parker, K.J.). Applied Science Publishers, Ltd, London, UK, pp. 211-230.Nicol, W.M. (1979) Sucrose and food technology. In Sugar, Science and Technology (eds Birch,G.G. and Parker, K.J.). Applied Science Publishers, Ltd, London, UK, pp. 211-230.


Paturau, J .M. (1989). Byproducts of the Cane Sugar Industry (3rd edn). Elsevier Sugar SeriesNo. 11, Amsterdam, The Netherlands.Shukla, T.P. and Misra, D.K. (1990) Sugar in microwave cooking. In. Sugar: A User's Guide

to Sucrose (eds Pennington, N.L. and Baker, C.S.). AVI Publishing House, New York,USA.Smith, D.E. (1990) Sugar in dairy products. In Sugar: A User's Guide to Sucrose. (edsPennington, N.L. and Baker, C.W.). AVI Publishing House, New York, USA.NY, 331 pp.Tsang, W.S.C. and Clarke, M.A. (1988) Chemistry in sugar food processing. In Chemistry

and Processing of Sugarbeet and Sugarcane (eds Clarke, M.A. and Godshall, M.A.).Elsevier, Amsterdam, The Netherlands.USDA (1991) Handbook (No.8). Composition of Foods. USDA, Washington, DC, USA.Yalpani, M. (1992) Personal communication. Alpha-Beta Technology, Boston, MA, USA.

10 Role of sucrose in retention of aromaand enhancing the flavor of foodsM.A. GODSHALL

10.1 Introduction

Sucrose interacts with food ingredients and in processed foods in manydifferent ways. The major flavor function of sucrose is, certainly, tosweeten food, but sucrose also influences the flavor quality of foods indiverse other ways.Anecdotal and hearsay claims are made about the interaction of sucrosewith the flavor of foods, many of these in advertising copy for competingingredients. An example of this is the following statement that appeared inthe October 1993 issue of Food Engineering (Dillon, 1993) touting anothersweetener: ' ... sucrose often overshadows fruit and spicy flavors'. Is thistrue or untrue, and how do we find out? As we will find, later in thischapter, that statement is not true.The home cook has known for generations that a small amount ofsucrose will enhance the flavor of vegetables, meats and other foods thatare not normally sweet. These cooks have found that sucrose added in asubthreshold concentration (below the sweet detection level) will improvethe overall flavor.Commercial food processors have recognized the same phenomenon,and add sucrose (and sometimes other sweeteners) in small quantities toenhance and improve the flavor of many canned and processed meats,condiments and vegetables.

10.2 Sucrose and the other basic tastes

Four basic taste sensations are recognized in sensory studies - sweet, sour,salty and bitter. A fifth sensation, known as 'umami' is recognized byJapanese workers, but it is not yet well enough defined in the literature tobe considered a basic taste. Tastes differ from flavors or aromas in that a'taste' is sensed on the tongue and can still be identified when the nose isclosed off. Aromatics or volatiles are sensed with the olfactory apparatusand are very numerous, encompassing literally hundreds of sensations.'Flavor' is the term that encompasses the entire sensory experience of a


ROLE OF SUCROSE IN AROMA AND FLAVOR OF FOODS 249

food, and includes such elements as individual aromas and basic tastes aswell as other sensations, such as texture and chemical stimulation.In general, bitter, acid and salty tastes are suppressed in the presence ofsucrose, but the sweet taste of sucrose is not suppressed as much except athigh concentrations of the other tastants. The ability of sucrose to suppressthe other basic tastes, especially bitter and sour tastes, is responsible formuch of sugar's ability to 'round out' or 'smooth' the flavor of foods. Alltaste-taste interactions are concentration dependent.

10.2.1 Interactions with salty taste

A low concentration of sodium chloride enhances the sweetness of sucrose,and a high salt concentration depresses sweet taste. Sucrose, on the otherhand, depresses saltiness at all concentrations of sucrose (Kroeze, 1978;Gillette, 1985; DeGraff and Fritjers, 1989). Other salts, i.e. alkalineacetates and butyrates, also enhance the sweetness of disaccharides(Unilever NV, 1980). Moskowitz (1972) suggests that mixtures of sweetand salt develop an unblended or 'clashing' taste when the taste intensitiesare in a similar range because the two taste modalities are attempting todominate the taste perception.

10.2.2 Interaction with bitter taste

Some current theories on sweetness receptors postulate that a closerelationship exists between sweet and bitter taste receptors. Structurefunction studies have shown that some carbohydrates possess both bitterand sweet molecular regions, and may, therfore, be able to span both typesof receptors. As a consequence, the interactions between sucrose andbitter substances are complex and interesting.Many bitter foods (chocolate, coffee, tea, alcohol) are traditionallysweetened. Sucrose imparts 'balance' to bitter foods (Busch-Stockfisch andDomke, 1991). Sucrose moderates (suppresses) the bitterness of caffeineand coffee, but these do not suppress the sweetness of sucrose as much(Calvino et al., 1990).

10.2.3 Interaction with acid-sour taste

Sucrose effectively decreases the acidity of sour compounds. Hoppeshowed that the sourness of citric acid undergoes an exponential decreasewith increased sucrose concentration (Hoppe, 1981). An optimum sugaracid blend of 8% sucrose and 1.11% citric acid was recommended formaximizing beverage flavors (McBride and Johnson, 1987). Theseresearchers noted a compression pattern in that increasing concentrations

250 SUCROSE

of sugar and acid exerted a diminishingly small effect. In sugar-acidsolution of changing intensity, sugar clearly suppresses the intensity ofacid, but only the highest concentration of citric acid unequivocallysuppresses sweetness. In fact, sucrose is felt to suppress or moderate the'unpleasantness' of acids more so than it can that of sodium chloride(Frank and Archambo, 1986).Moskowitz (1972) has suggested that there is a compatibility between

sucrose and both bitter and sour tastes, and that a blended flavor maydevelop in almost all mixture proportions of sucrose-bitter and sucrosesour.

It is interesting to note that there is a report that carbonation did notaffect sweetness in a sucrose sweetened beverage at sucrose concentrationsranging from 2 to 16% (Yau and McDaniel, 1992). The personalobservation of the author is that a soft drink tastes sweeter once it has lostits carbonation and gone 'flat'.

10.2.4 Interaction with other sweeteners

The threshold concentration of sucrose is around 0.2-0.5% in aqueoussolution. Moderate interactive effects occur between sucrose and othercarbohydrate sweeteners (Moskowitz, 1974). A study by Van Der Heijdenet ai. (1983), showed that sucrose taste intensity is enhanced in increasingorder by xylitol, fructose and glucose; in turn, sucrose strongly enhancesthe sweetness of xylitol and fructose, but not glucose. Stone and Oliver(1969) found that a synergistic effect (i.e. the mixture is sweeter thanwould be expected from simple addition of the sweetness of the individualmixture components) of about 20-25% occurred with glucose-sucrosemixtures. The interaction is more pronounced at low concentrations; forexample, sucrose and fructose are strongly synergistic at 10% concentration, but only slightly synergistic at 20 and 30% (Partanen, 1988). Sucroseand glucose are also synergistic at 10% concentration, but becomesincreasingly more suppressive as concentrations are increased (Partanen,1988). Along the same lines, sucrose and corn syrup tend to be suppressive(Partanen, 1988). These relationships should be kept in mind whenformulating products that contain mixtures of natural sweeteners.With regard to the interaction of sucrose with high-intensity sweeteners,Kamen (1959) showed that sucrose and calcium cyclamate are synergisticat intermediate concentrations of sucrose (to correspond to 5.79% sucroseand 14.63% sucrose), but not interactive at lower (2.31% sucrose) or higher(37 .07% sucrose) concentrations.Sucrose is generally highly synergistic with high-intensity sweeteners,showing synergistic sweetness increases of 11% for mixtures with aspartame,14% with acesulfame, 15% for cyclamate, and 19% for saccharin (Frank etai., 1989a). Results such as these suggest that combinations of sucrose with


intense sweeteners may be beneficial in cutting down on some of theunpleasant bitter side-tastes that some of the intense sweeteners possess.

10.3 Retention of aromas

Flavor and aroma retention during the processing of dry food is of greatimportance. Food systems are extremely complex, and in order tounderstand some of the dynamics of ingredient and processing interactions,model systems are often used to simplify the factors. The literatureabounds with studies of model systems that use different sets ofcarbohydrates and polymers, different volatiles, and different processingconditions to try to understand the dynamics of flavor retention in foodprocessing. Sucrose is often included in these studies.Factors that affect the retention of volatiles in foods include such diverseelements as:

• type of drying system,• chemical nature of the volatile,• chemical nature of the non-volatile components,• concentration of each starting material,• time of drying• temperatures of processing, and• beginning and ending moisture content.

Model studies, in general, have shown that sucrose is effective in retainingvolatiles in dehydrated foods. Table lO.l, adapted from Flink and Karel(1970), shows that the order of volatile retention in a freeze-dried systemwas sucrose> maltose;:::: lactose> glucose» dextran-lO, for five volatile

Table 10.1 Retention of volatiles in freeze-dried model systems (Adapted from Flink andKarel, 1970)

Organic volatiles

AcetoneMethyl acetate2-Propanoln-Butanoltert-Butanol

Volatile retention in systems' containing the specifiednon-volatile solute (g/I00 g solids)

Glucose Maltose Sucrose Lactose Dextran-l0

0.99 2.01 2.30 1.83 0.030.67 2.29 2.51 2.20 0.042.11 2.71 3.02 2.71 0.301.26 2.27 2.83 2.50 0.701.93 3.10 3.27 3.15 2.96

•All systems had the initial composition (% by weight): non-volatile solute: 18.8; organicvolatile: 0.75; water: 80.45.

252 SUCROSE

compounds. Voilley et al. (1977) showed that sucrose retained acetone,octanoic acid and l-octanol more so than glucose or fructose; and thatretention of volatiles increased with sucrose concentration but not forfructose concentration. Part of this effect was attributed to the ability ofsucrose to retain the integrity of the microstructure. Similar findings forsucrose were reported by Sugisawa et al. (1973) - sucrose (and maltose)retained more of five alcohols and one ester than did levoglucosan, DE 20maltodextrin and gum arabic in a freeze-dried model system.

In a model system containing sucrose and pregelatinized starch frozen atdifferent temperatures prior to microwave freeze-drying, the retention of1-decanal and D-limonene was enhanced by the presence of sucrose (Chenetal., 1993).Dry sucrose binds on the order of 100 mmol/g each of ethanol, acetoneand propionic acid, a quantity felt to correspond to penetration of one totwo monolayers on the crystal surface (Rasmussen and Maier, 1974). Ofthe disaccharides, lactose, which possesses various crystalline andamorphous forms, is more efficient than sucrose for adsorbing largequantities of volatiles in the dry state, but lacks many of the other desirablequalities of sucrose (Ehler et al., 1979). Carbohydrates that are capable offorming inclusion compounds with flavor compounds, such as thecyclodextrins and starch, also adsorb volatiles more efficiently thansucrose.

10.3.1 Fixing volatiles with sucrose

During the 1970s, General Foods submitted a series of patents for fixingvolatile flavors in sucrose, using acetaldehyde as the model compound(Earle et al., 1972; Mitchell and Stahl, 1974; Mitchell, 1975; Chall et al.,1979; Malizia and Mitchell, 1979; Anon., 1980; Pitchon et al., 1980).Fixation occurred at levels of 0.1-0.5 wt%. Acetaldehyde was used as aflavor enhancer, providing a clean taste in dry gelatine dessert mixes.Several different techniques were used, but all exploited the ability ofsucrose to encapsulate volatiles in microregions by dehydration of a melt.

10.3.2 Co-crystallization

An American sugar producer has developed a process known as cocrystallization, in which rapid agitation of a supersaturated sugar solutionresults in aggregates of microsized crystals as cooling proceeds (Chen et al.,1988; Awad and Chen, 1993). The resulting structure is an open, lacyagglomerate well-suited to the incorporation of other flavors. A secondaryflavor ingredient can be added to the supersaturated syrup, resulting in anoxidatively stable, encapsulated flavor mixture. This process greatlyenhances the ability of sucrose to act as a flavor carrier, and has been


demonstrated with such ingredients as peanut butter, orange juice,chocolate, and essential oils.

10.3.3 Headspace effects - aromas in solution

Headspace analysis refers to the determination of the equilibriumconcentration of volatiles above a sample. Headspace analysis is significantbecause it will give an objective measurement of constituents thatcontribute to the aroma of a food. Ideally, headspace analysis can besupplemented and/or correlated with human sniffer panels, although littleof this type of work is actually carried out.The influence of a major solute, such as sucrose, on the volatility offlavor compounds depends on many factors, and can give contradictoryresults. In general, one finds that the partition coefficient of a volatilesubstance in solution increases with addition of a solute, due to a saltingout effect. However, sometimes other competing effects, such as intermolecular associations or even complexation between the volatile and thesolute, may decrease volatility (King, 1983). Therefore, up to a certainpoint, the aroma of a beverage could, theoretically, be intensified byincreasing the volatility of its trace aroma constituents by increasing thesucrose content.The volatility of compounds is, in fact, strongly influenced by increasingsucrose concentration. Massaldi and King (1973) noted that increasingsucrose concentration increased volatility of n-hexylacetate and decreasedthat of D-limonene and n-butylbenzene. Nawar (1971) found that acetonevolatility progressively increased and 2-heptanone and heptanal volatilityprogressively decreased in 20, 40 and 60% sucrose solutions. In a studythat compared odor analysis and GC headspace analysis, Ebeler et al.(1988) showed that while 5, 20, and 40% sucrose solutions did increase theheadspace of menthone and isoamyl acetate, the panelists could notdistinguish a significant difference in aroma, even though they coulddistinguish different concentrations of added volatiles in the range of1.25-20 ppm.Marinos-Kouris and Saravacos (1975) have published the activitycoefficients of nine organic compounds in 0, 20, 40, and 60% sucrosesolutions. These are shown in Figure 10.1. Table 10.2 shows the relativevolatilities of organic compounds in various concentrations of sucrose(Marinos-Kouris and Saravacos, 1975). What is noteworthy in this data isthat there is relatively little difference in the volatility of most compoundsin water (0% sucrose) and in 15% sucrose, which is about the maximumconcentration one would find in a beverage. Volatilities increase at a muchfaster rate after that.

254 SUCROSE

ethylacetate

102 f---+-----",...L+-----",..L--t-----I

10 F----=--t----+---+--i

o 20 40

0/0 sucrose

60102 0~--2..J.0---4..L0---6LO----l

% sucrose

Figure 10.1 Activity coefficients of organic aroma compounds in aqueous solutions (25°C).(From Marinos-Kouris and Saravacos (1975), reproduced with permission.)

Table 10.2 Relative volatilities of organic compounds in aqueous sucrose solution at 25°C(From Marinos-Kouris and Saravacos, 1975)

% Sucrose concentration

Volatile compound 0 15 35 60

Methyl anthranilate 1.02 1.27 2.15 4.96Methanol 8.3 8.9 9.8 14.5Ethanol 8.6 9.0 lOA 16.7n-Propanol 9.5 10.0 12.0 18.5n-Butanol 14.1 15.0 21.0 43.6N-Amyl alcohol 23.0 24.7 41.8 105.0n-Hexanol 31.0 36.0 62.0 195.0I-Butanone 76.0 96.0 112.0 181.03-Pentanone 77.0 85.0 121.0 272.0Ethyl acetate 225 265 368 986Ethyl butyrate 643 855 1620 6500


10.4 Modifying the taste of sucrose

The effect of the basic tastes on sucrose has already been discussed.Several compounds/ingredients also have the ability to either enhance ordiminish the taste of sucrose in food. From a psychological perspective, wealso find the phenomenon that the perceived sweet flavor is affected by thecolor of a food.

10.4.1 Enhancing the sweetness of sucrose

Maltol (2-methyl-3-hydroxy-pyran-4-one) is probably the best knownflavoring ingredient with the ability to enhance the sweetness of sucrose.Concentrations of 5-75 ppm were reported to enhance the sweetness ofsucrose to such an extent that up to 15% of the sucrose could be eliminated(Bouchard et al., 1968; Johnson, 1976). This effect is probably due to thesweet aroma of maltol, which can confuse inexperienced tasters (Binghamet al., 1990). This study also found that a subthreshold concentration ofmaltol (15 ppm) in sweetened lemonade did not significantly affect thesweet taste. Results such as these, that seem contradictory, indicate thatthe influence of maltol on sweetness perception is olfactory rather thangustatory, and depends on the concentration of malto\.Furaneol (2,5-dimethyl-3-hydroxy-4-oxo-4,5-dihydrofuran) is also

reported to enhance the sweetness of sucrose, such that a syrup containing9% sucrose and 12 ppm furaneol was judged by a taste panel to be as sweetas a syrup containing 10% sucrose (Pickenhagen and Ohloff, 1975).Both maltol and furaneol can be classified as caramel-type molecules,

which result from the degradation of carbohydrates. The presence ofcaramel compounds in brown sugar may account for the fact that brownsugar is often perceived as sweeter than regular white granulated sugar,even though it is of a lower purity (approx 90% versus 99%) (Godshall etal., 1984).An extract of the licorice plant (Glycyrrhiza glabra) , glycyrrhizin, has a

sweet taste 50 times sweeter than sucrose. A derivative, ammoniumglycyrrizinate, is reported to potentiate sweetness and has a stronglysynergistic effect with sucrose (MacAndrews & Forbes Company, 1966).This compound is commercially available in the US under the nameMagnasweet.Ethanol also enhances the sweetness of sucrose (Martin and Pangborn,1970). For example, a solution of 8% sucrose in 10% ethanol was judged tobe as sweet as an aqueous solution of 10% sucrose. Similarly, 7% sucrosein 20% ethanol appeared to be as sweet as a 10% sucrose solution.

256 SUCROSE

10.4.2 Hydrocolloids and perception of sweetness

Gums and thickeners play an important role in many sweetened foods, andsome of these have a pronounced effect on the sweetness of sucrose. Ingeneral, hydrocolloids decrease sweetness of sucrose, with a significantportion of the effect being attributed to viscosity. However, viscosity is notthe only determinant, as the type of hydrocolloid also has an effect, as forexample, in a study by Paulus and Haas (1980), guar gum was found tomore significantly decrease sweetness than tara seed gum, carob seed gum,and methyl cellulose, which had the least effect. In the same study, it wasfound that an increase of 1.5-2.5 times as much sucrose would be necessaryto induce the same sweetness in a solution of 1000 cPs viscosity whencompared to one of 1 cP. It should be noted, however, that in this study,very low concentrations of sucrose were used, to determine the effect onthreshold values.At higher concentrations of sucrose, it was found that guar gum reduced

sucrose sweetness about 25% (Launay and Pasquet, 1982). Pangborn et al.(1973) concluded that the sweetness intensity of sucrose begins to besignificantly depressed when the viscosity of the medium exceeds 12-16 cPoVaisey et al. (1969) found that gums with less viscosity drop as shear ratesincrease, such as guar and carboxymethylcellulose, tend to mask sweetnessperception more.The take-home message to these types of findings is that care must betaken when changing the thickeners and stabilizers in a product recipe,since the sucrose taste profile may change noticeably.

10.4.3 Temperature effects on sweet perception

Temperature generally affects the basic tastes in a similar manner - atapproximately physiological temperatures (32-38°), the sensitivity to sweettaste is maximum (Shimizu et al., 1959). At both low temperatures(5-1O°C) and high temperatures (4Q-60°C), sweetness of sucrose decreases(Paulus and Reisch, 1980). However, fructose solutions taste significantlysweeter than sucrose at 5°C but the reverse is true at 60°C, with fructoseshowing a marked decrease in sweetness with increases in temperature(Fricker and Gutschmidt, 1974). Data such as these suggest that foods andbeverages that are to be served warm or hot may require more sucrose andother taste substances to produce the desired taste intensity; also thatsucrose would be preferable than fructose as a sweetener in warmbeverages.

10.4.4 Masking the sweetness of sucrose

Arylalkanoic acids, in particular, 3,4-dimethoxyphenylacetic acid, canreduce the sweetness of sucrose by up to 80% when incorporated into


recipes (Lindley, 1984). Another compound, the sodium salt of paramethoxypropanoic acid (common name, lactisole), commercially availableunder the trade name Cypha, can be used in products at the 15-100 ppmlevel to reduce the perception of sweetness, while not changing thequantity of sucrose used (LaBell, 1989).Triterpene saponins in the leaves of Gymnema sylvestre and Ziziphus

jujuba will inhibit the sweet taste of sucrose for up to 1 h for Gymnemaand about 15 min for Ziziphus (Adams, 1985). Gymnema extracts reducedthe sweetness of all concentrations of sweet solutions by an average of 77%(Frank et al., 1992). The surfactant sodium dodecyl sulfate (not a foodingredient) also inhibits the perception of sweet taste, suggesting thatsurfactant properties of these inhibitors play a role. However, thesurfactants glycerol monostearate and lecithin increase both intensity andpersistence of sweet taste, so generalizations should be avoided(Ogunmoyela and Birch, 1982).

10.4.5 Interaction of sucrose-eolor-flavor

Studies have shown that color has a strong effect on the perception offlavor, especially of fruit-flavored beverages (Kotyla and Clydesdale,1978). Sweetness perception can be strongly affected by the type of color,especially red color. Johnson and Clydesdale (1982) showed that theperception of sweetness in a dark red solution was 2-10% higher than in alight red solution, even though the sucrose concentration in the dark redsolution was actually 1% lower than in the light red solution. They alsoshowed that perceived sweetness increased approximately 2-12% as redcolor intensity increased at the same time that a constant 4% sucroseconcentration was maintained (Johnson et al., 1983).While red color enhances sweetness, yellow color can cause a decreaseof as much as 2% in perceived sweetness (Kotyla, 1978). Green color alsois often equated with lower perceived sweetness. Roth et al. (1988)conclude that: 'the psychophysical relationship between sweetness andcolor is complex and related to the concentration of sucrose, colorintensity, and pleasantness effects'.

10.4.6 Iron-sucrose interactions

Sucrose, in common with other food carbohydrates, interacts with ironsalts in various ways that can significantly affect flavor. The taste of bothferric and ferrous iron salts is masked by complexing with sugar at high pH(above 7.5) and is enhanced at low pH (below 5.5) (Cross et al., 1985).Table 10.3 compares various food carbohydrates for their effect on theflavor threshold of iron salts. In a study on the effect of fortifying abeverage with various types of iron salts and sweetened with variouscarbohydrates, sucrose gave overall more acceptable scores than fructose,

258 SUCROSE

Table 10.3 Threshold concentration of iron salts (From Cross et al., 1985)

Threshold concentrationIron salt

Ferrous sulfate

Ferric ammoniumcitrate

Carbohydrate(equivalent to5% w/v Fructose)

NoneSorbitolGlucoseLactoseMaltoseFructoseSucrose

NoneSorbitolGlucoseLactoseMaltoseFructoseSucrose

Low pH«pH 5.5)

2.10.10.20.30.40.5I.5

7.72.54.22.66.13.73.9

High pH(>pH 7.5)

6.825.713.642.639.9330.026.2

45.0168.1134.064.0140.097.0248.7

glucose, sorbitol, maltose, and lactose, in its ability to mask the flavor offerric ammonium citrate, ferric nitrate, and ferrous sulfate (Cross andKearlsey, 1986). No sugar was effective in masking the flavor of the fourthsalt, ferric chloride. Interactions such as these have important implicationsin iron-fortification of foods.

10.5 Effect of sucrose in selected food systems

10.5.1 Coffee

As noted above, caffeine bitterness is more suppressed by sucrose thansucrose sweet taste is inhibited by caffeine. As sucrose concentrationincreases, it also slightly inhibits coffee flavor; this would suggest that thereis an optimum amount of sucrose for coffee sweetening (Calvino et ai.,1990). International interlaboratory taste tests have also suggested thatsome countries prefer higher levels of sugar in their coffee than others(Lundgren et ai., 1978).Addition of sugar to freeze-dried coffee extract was found to increase itsstorage stability in terms of flavor, aroma, and acidity (Nogueira deMoares Pitombo et ai., 1987).

10.5.2 Fatty systems

Fatty flavor is enhanced by sucrose in all but the highest fat formulations(36% or greater) (Pangborn and Tunaley, 1988), a fact which should bekept in mind when formulating low-fat foods, since these often lack the


desirable full flavor of the original food. Part of the effect of sucrose inenhancing fatty flavor was attributed to its effect on increasing viscosityand mouthfeel (Wiet et al., 1993). Reformulating fatty foods into low-fatversions must take into account not only the effect of the sweetener onflavor, but also the effect of gums and hydrocolloids added for bulking, asdiscussed in an earlier section.

10.5.3 Chocolate confectionery

Sucrose is the major ingredient in chocolate, comprising up to 50% of itstotal weight. Studies have shown that sucrose in uniquely suited to use inchocolate, which requires a dry sweetener with low hygroscopicity (Martin,1987). Sucrose also shows superior flavor-taste interactions over sweetenerssuch as fructose or sorbitol, as well as improved texture (Ogunmoyela andBirch, 1984). The sweet taste of sucrose in cocoa can be intensified andpersist longer by the addition of surfactants such as glycerol monostearateand lecithin (Ogunmoyela and Birch, 1982). At the same time, addedsurfactants could decrease overall chocolate flavor, a less than desirableresult (Birch and Ogunmoyela, 1980).Niediek (1975, 1981) has made extensive studies on the effect of'amorphous' sugar (finely ground or powdered sugar) on chocolate tastequality, and finds that the increased sorption properties of sucrose in theamorphous state can lead to off-flavors in chocolate if great care is nottaken. Niediek (1975, 1981) also makes the interesting observation thatpowdered or ground sugar always has a distinctly different taste fromgranular sugar.

10.5.4 Fruit flavors

Sucrose either does not affect fruity flavor (for example, sucrose did notchange the fruity flavor perception of either orange or strawberry flavor ina study by Wiseman and McDaniel (1991)) or enhances it. Usually, assucrose level increases, the perception of fruity flavor also increases, asstudies have shown enhancement of several fruit and berry flavors (Valdeset al., 1956; Von Sydow et al., 1974; Bonnans and Noble, 1993). Therefore,the statement quoted at the beginning of this article is shown to beuntrue - sucrose does not mask fruit flavors.Fruit flavors can also be interactive with sucrose and enhance theperception of sweetness in sucrose solutions as is seen with strawberryflavor (Frank et al., 1989b).

10.5.5 Effect of crystal size on mouthfeel in confections

Sucrose contributes to texture in many foods, but nowhere is its texturizingfunction more important than in grained confections, such as creams,

260 SUCROSE

chocolates, and fondants. The term 'mouthfeel' refers to the texture of afood in the mouth, and in confections, the size of the sucrose crystal is ofparamount importance to the overall flavor quality. The human palaterecognizes particles in the size range of 20-40 !-tm as smooth and creamy. Ifthe particles (i.e. sugar crystals) are any larger, the confection is perceivedas grainy and of low quality (Jeffrey, 1993).

10.5.6 Sucrose and cake crumb, crust and quality

Attempts to replace sucrose in bakery items, such as cakes, routinely showthe superiority of sucrose in development of texture, flavor and appearance.In one such study, the interior of cakes (crumb) as well as cake crusts madewith various sweeteners were judged. Sucrose was superior for the interior,followed in decreasing order of acceptability by HFCS (high fructose cornsyrup), fructose, fructose-glucose, and glucose (Thompson et al., 1980).Sucrose also resulted in highly acceptable crust flavor, followed, indecreasing order, by fructose, fructose-glucose, and HFCS. Sucrose hasalso been shown to be superior to fructose in sugar cookies and white cake,when substituted at the 100% level (Hardy et al., 1979). The studyreferenced above also illustrates another point: although fructose isconsiderably sweeter than sucrose in solution, it does not always show thisproperty in cooked or baked systems (Godshall, 1990).

References

Adams, M.A. (1985) Substances that modify the perception of sweetness. In Characterizationand Measurement of Flavor Compounds. (eds. Bills, D.D. and Mussinan, c.J. AmericanChemical Society Symposium Series 289, Washington, DC, USA, pp. 11-25.Anon. (1980) Modification of a volatile aroma component. Neth. App!. 7809,864 assigned toGeneral Foods Corp. Pub!. 1 Apr. 1980Awad, A. and Chen, A.C. (1993) A new generation of sucrose products made bycocrystallization. Food Technol., 47, 146-150.Bingham, A.F., Birch, G.G., Graaf, C. de, Behan, J.M. and PeTTing, K.D. (1990) Sensorystudies with sucrose-maltol mixtures. Chem. Senses, 15 (4), 447-456.Birch, G.G., and Ogunmoyela, O. (1980) Effect of surfactants on the taste and flavor ofdrinking chocolate J. Food Sci., 45 (4), 981-984Bonnans, S. and Noble, A.C. (1993) Effect of sweetener type and of sweetener and acidlevels on temporal perception of sweetness, sourness and fruitiness. Chem. Senses, 18 (3),273-283.Bouchard, E.F., Hetzel, C. and Olsen, R. (1968) Process of sweetening foods with maltoland sugar. US Patent 37-40, 3, 409, 441.Busch-Stockfisch, M. and Domke, A.Z. (1991) Sensory evaluation of the bitter taste ofamarogentin and its potential as a replacement for quinine in soft drinks. 1. Effect ofsucrose and citric acid. Lebensm. Unters. Forsch., 1992(1), 11-14.Calvino, A.M., Garcia-Medina, M.R. and Cometto-Muniz, J.E. (1990) Interactions incaffeine-sucrose and coffee-sucrose mixtures: evidence of taste and flavor suppression.Chem. Senses, 15(5),505-519.Chall, S.B., Pitchon, E. and Schulman, M. (1979) Sucrose-fixed volatile flavors. Can,Patent I, 057, 566. Applied 13 July 1976; assigned 3 July 1979.


Chen, A.e., Beiga, M.F. and Rizzuto, A.B. (1988) Cocrystallization: An encapsulationprocess. Food Technol., 42, 87-90.Chen, S.D., Ofoli, R.Y., Scott, E.P. and Asmussen, J. (1993) Volatile retention inmicrowave freeze-dried model foods. J. Food Sci., 58(5), 1157-1161.Cross, H., Pepper, T., Kearsley, M.W. and Birch, G.G. (1985) Mineral complexingproperties of food carbohydrates. Starke, 37(4), 132-135.Cross, H.L. and Kearlsey, M.W. (1986) Carbohydrate-iron interactions. In Interactions of

Food Components (ed. Birch, G.G. and Lindley, M.G.) Elsevier Applied SciencePUblishers, London, UK pp. 31-42.DeGraff, e. and Fritjers, J .E. R. (1989) Interrelationships among sweetness, saltiness andtotal taste intensity of sucrose, NaCI mixtures. Chem. Senses, 14(1),81-102.Dillon, P.M. (993) Sweet options. Food Eng., 104, 101-102.Earle, E.L., Pitchon, E., Schulman, M., Schulman, M. and Prasad, R. (1972) Process forfixing volatile enhancers in sucrose. US Patent 3,767,430 assigned to General Foods Corp.App!. 24 Feb 1972, pub!. 23 Oct 1973.Ebeler, S.E., Pangborn, R.M. and Jennings, W.G. (1988) Influence of dispersion mediumon aroma intensity and headspace concentration of menthone and isoamyl acetate. J.Agric. Food Chem., 36(4), 791-796.Ehlr, K.F., Bernhard, R.A. and Nickerson, T.A. (1979) Heats of adsorption of smallmolecules on various forms of lactose, sucrose, and glucose. J. Agric. Food Chem., 27(5),921-927.Flink, J. and Karel, M. (1970) Retention of organic volatiles in freeze dried solutions ofcarbohydrates. J. Agric. Food Chem., 18(2), 295-297.Frank, R.A. and Archambo, G. (1986) Intensity and hedonic judgments of taste mixtures:An information integration analysis. Chem. Senses, 11(4),427-438.Frank, R.A., Mize, S.J.S. and Carter, R. (1989) An assessment of binary mixtureinteractions for nine sweeteners. Chem. Senses, 14(5), 621-632.Frank, R.A., Ducheny, K. and Mize, S.J.S. (1989b) Strawberry odour, but not red color,enhances the sweetness of sucrose solutions. Chem. Senses, 14(3),371-377.Frank, R.A., Mize, S.J.S., Kennedy, L.M., Santos, H.C. de los and Green, S.J. (1993) Theeffect of Gymnema sylvestre extracts on the sweetness of eight sweeteners. Chem. Senses,17,461-479Fricker, A. and Gutschmidt, J. (1974) Sweetness of fructose and glucose under variousconditions. Symposia, Naturliche und synthetische Zusatzstoffe in der nahrung desMenschem. Dr. Dietrich Steinkopff Verlag, Darmstadt. Germany, pp. 143-153.

Gillette, M. (1985) Flavor effects of sodium chloride. Food Technol., 39(6), 47-52, 56.Godshall, M.A. (1990) Use of sucrose as a sweetener in foods. Cereal Foods World, 35(4),384-389.Godshall, M.A., Vinnett, e.H. and Chew, V. (1984) Sensory analysis of brown sugars andits correlation with chemical measurements. Proc. Sugar Processing Res. Con[.,Agricultural Research Service Publication No. ARS-49, New Orleans, USA, pp. 22-52.Hardy, S.L., Brennand, e.P., and Wyse, B.W. (1979) Fructose: Comparison with sucroseand sweetener in four products. J. Am. Dietet. Assoc., 74(1), 41-46.Hoppe, K. (1981) The taste interactions of citric acid with sucrose and sweeteners. Die

Nahrung., 25 (3), KI-K4.Jeffery, M.S. (1993) Key functional properties of sucrose in chocolate and sugarconfectionery. Food Technol., 47,141-144.Johnson, J.e. (1976) Specialized sugars for the food industry. Food Technol. Rev, 35, 297298.Johnson, J. and Clydesdale, F.M. (1982) Perceived sweetness and redness in colored sucrosesolutions. J. Food Sci., 47(3), 747-752.Johnson, J.L., Dzendolet, E. and Clydesdale, F.M.(1983) Psychophysical relationshipbetween sweetness and redness in strawberry-flavored drinks. J. Food Protection, 46(1),21-25.Kamen, J. (1959) Interaction of sucrose and calcium cyclamate on perceived intensity ofsweetness. Food Res., 24(3), 279-282.King, e.J. (1983) Physical and chemical properties governing volatilization of flavor andaroma components. In Physical Properties of Foods (eds Peleg, M. and Bagley, E.B. AVIPublishing Co, Westport, cr, USA, pp. 399-421.

262 SUCROSE

Kotyla, A.S. (1978) The psychophysical relationships between color and flavor of some fruitflavored beverages. PhD thesis, University of Massachusetts, USA.Kotyla, A.S. and Clydesdale, F.M. (1978) The psychophysical relationships between colorand flavor. CRC Crit. Rev. Food Sci. Nutr., 10, 303-321.Kroeze, J.H.A. (1978) The taste of sodium chloride: masking and adaptation. Chern. Senses

Flavour, 3(4), 443-449.LaBell, F. (1989) Sweetness reduction improves flavor delivery, functionality. Food Proc.,50, pp. 74-76.Launay, B. and Pasquet, E. (1982) Sucrose solutions with and without guar gum:Rheological properties and relative sweetness intensity. Prog. Food Nutr. Sci., 6, 247-258.Lindley, M.G. (1984) Method of inhibiting sweetness. UK Patent Appl. GB 2,139,470A,Nov. 14, 1984.Lundgren, B.et al. (1978) Taste discrimination vs hedonic response to sucrose in coffeebeverage. An interlaboratory study. Chern. Senses Flavor, 3(3), 249-265.

MacAndrews & Forbes Company (1966) US Patent #3,282,706.Malizia, P.D. and Mitchell, W.A. (1979) Sucrose volatile flavors. Can. Patent 1,066,945assigned to General Foods Corp. Pub!. 27 Nov. 1979.Marinos-Kouris, D. and Saravacos, G. (1975) Volatility of organic compounds in aqueoussucrose solutions. Proc. 5th Internat. Congo Chern. Engin, Chern. Equip. Design andAutomation, Prague, Czech Republic.Martin Jr, R.A. (1987) Chocolate. Adv. Food Sci., 31, 211-242.Martin, S. and Pangborn, R.M. (1970) Taste interaction of ethyl alcohol with sweet, salty,sour and bitter compounds. J. Sci. Food Agric., 21(12), 653-655.Massaldi, H.A. and King, c.J. (1973) Simple technique to determine solubilities ofsparingly soluble organics: Solubility and activity coefficients of D-limonene, nbutylbenzene, and n-hexyl acetate in water and sucrose solutions. J. Chern. Engng Data,18, 393-397.McBride, R.L. and Johnson, R.L. (1987) Perception of sugar-acid mixtures in lemon juicedrink. Int. J. Food Sci. Technol., 22(4), 399-408.Mitchell, W.A. (1975) Fixed volatile flavours and method. US Patent 3,898,247 assigned toGeneral Foods Corp. Appl. 5 Aug 1975, publ. 5 Aug 1975.Mitchell, W.A. and Stahl, H.D. (1974) Fixed volatile flavours and method for making same.US Patent 3, 787,592 assigned to General Foods Corp. Appl. 12 May 1970; poubl. 22 Jan1974.Moskowitz, H.R. (1072) Perceptual changes in taste mixtures. Perception & Psychophysics,11(4), 257-262.

Moskowitz, H.R. (1974) Models of additivity for sugar sweetness. In Sensation andMeasurement, (eds Moskowitz, H.R., Scharf, B. and Stevens, J.C.) D. Reidel PublishingCompany, Dordrecht, The Netherlands, pp. 379-388.Nawar, W.W. (1971) Some variables affecting composition of headspace aroma. J. Agric.

Food Chern., 19(6), 1057-1059.Niediek, E.A. (1975) Grinding and particle size analysis in food technology with particularreference to cocoa and sugar Proc. 6th Europ. Syrnp. Food: Engineer Food Qual., pp. 36Cr379. Society of Chemical Industry, London, UK.Niediek, E.A. (1981) Investigations on the influence of aroma sorption by sugars on the tastequalities of chocolate. Zucker-Susswaren Wirt., 34(2), 44, 53-57.Nogueira de Moares Pitombo, R., Colombo, A.J. and Vessoni Pena, T.C. (1987) Influenceof sugar addition on the content of volatile compounds and sensory properties of freezedried coffee samples. Revista de Farrnacia e Bioquirnica da Universidade de Sao Paulo,23(2), 125-132.Ogunmoyela, O.A.B. and Birch, G.G. (1982) Sucrose/surfactant interaction and sweet tasteof cocoa. Prog. Food Nutr. Sci, 6, 373-378.Ogunmoyela, O.A. and Birch, G.G. (1984) Sensory considerations in the replacement ofdark chocolate of sucrose by other carbohydrate sweeteners. J. Food Sci., 49(4), 10241027, 1056.

Pangborn, R.M. and Tunaley, A. (1988) Lipid-modification of perceived sweetness. Int.Conf. on Sweeteners, Sept 1988, Los Angeles, CA (abstr.).Pangborn, R.M., Trabue, I.M., and Szcezesniak, A.S. (1973) Effect of hydrocolloids onoral viscosity and basic taste intensities. J. Texture Stud., 4(2), 224-241.


Partanen, T. (1988) Sugar interactions. Thesis No. 774, U. Helsinki.Paulus, K. and Haas, E-M. (1980) The influence of solvent viscosity on the threshold valuesof primary tastes. Chern. Senses, 5(1), 23-32.Paulus, K. and Reisch, A.M. (1980) The influence of temperature on the threshold values ofprimary tastes. Chern. Senses, 5(1), 11-21.Pickenhagen, W. and Ohloff, G. (1975) Composition for increasing the sweet taste ofproducts with reduced sugar content. German Patent 2,515,269, 23 Oct. 1975.Pitchon, E., Schulman, M. and Chall, S.B. (1980) Method for producing sucrose-fixedvolatile flavors. Brit. UK Patent 2,029,686, assigned to General Foods. Assigned 26 Mar.1980.Rasmussen, H. and Maier, H.G. (1974) Binding of volatile aroma compounds to mono- anddisaccharides. Chernie. Mikrobiol. Techno!. Lebensrn., 3, 119-124.Roth, H.A., Radle, L.J., Gifford, S.R. and Clydesdale, F.M. (1988) Psychophysicalrelationship between perceived sweetness and color in lemon- and lime-flavored drinks. J.Food Sci., 53(4),1116-1119,1162.

Shimizu, M., Yanase, T. and Higashihira, K. (1959) Relations between gustatory sense andtemperature of drinks. Kaseigaku Kenkyu, 6, 26-28.Stone, R. and Oliver, S.M. (1969) Measurement of the relative sweetness of selectedsweeteners and sweetener mixtures. J. Food Sci., 34(2),215-222.Sugisawa, H., Kobayashi, N. and Sakagami, A. (1973) Retention of volatile flavors in food.1. Flavor retention in the dried solution of carbohydrates. 2. Formation of micellarcolloid and its contribution to flavor retention in the dried solution of sugars. J. Food Sci.Techno!. (Tokyo), 20(8), 364-368.

Thompson, C.M., Mickelsen, 0., Schemmel, R., Funk, K. and Kakade, M.L. (1980) Tasteperception and flavor acceptance of cakes prepared with monosaccharides. Nutr. Rep. Int.,21(6), 913-922.Unilever, NV (1980) Process for increasing the sweetening power of oral preparationscontaining a disaccharide, NL Patent 78/7127, 3 Jan.Vaisey, M., Brunon, R. and Cooper, J. (1969) Some sensory effects of hydrocolloid sols onsweetness. 1. Food Sci., 34(5), 397-400.Valdes, R.M., Simone, M.J. and Hinreiner, E.H. (1956) Effect of sucrose and organic acidson apparent flavor intensity. II. Fruit nectars. Food Techno!., 10, 387-390.Van Der Heijden, A., Brussel, L.B.P., Heidema, J., Kosmeijer, J.G. and Peer, H.G.(1983) Interrelationships among synergism, potentiation, enhancement and expandedperceived intensity vs concentration. J. Food Sci., 48(4),1192-1196, 1207.Voilley, A., Simatos, D. and Loncin, M. (1977) Retention of volatile trace components infreeze-drying model systems. Lebensrn. Wiss. Techno!., 10(5), 285-289.Von Sydow, E., Moskowitz, H., Jacobs, H. and Meiselman, H. (1974) Odor-taste interactionin fruit juices. Lebens. Wiss. Techno!., 7, 9-16.Wiet, S.G., Ketelsen, S.M., Davis, T.R. and Beyts, P.K. (1993) Fat concentration affectssweetness and sensory profiles of sucrose, sucralose, and aspartame. J. Food Sci., 58(3),599-602,666.Wiseman, J.M. and McDaniel, M.R. (1991) Modification of fruit flavors by aspartame andsucrose. J. Food Sci., 56(6), 1668-1670.Yau, J.J.N. and McDaniel, M.R. (1992) Carbonation interactions with sweetness andsourness. J. Food Sci., 57(6), 1412-1416.

11 Sucrose: Its potential as a raw material for foodingredients and for chemicalsR. KHAN

11.1 Introduction

In Europe, sucrose (sugar) is isolated from sugar-beet (Beta vulgaris) andaccounts for nearly 40% of the total world production which in 1992exceeded 110 million tonnes. The remaining 60% of the world productionis manufactured from sugar-cane (Saccharum officinarum), grown in thetropics.Structurally and functionally, sucrose is a unique organic molecule and isthe most abundant of all sugars. In the past three decades, the progresstowards the understanding of the chemistry, structure, physical andfunctional properties of the sucrose molecule has been rapid. In 1965 therewere only about 15 well-characterised sucrose derivatives and today thereare more than three hundred well-identified sucrose compounds describedin the carbohydrate literature.This chapter will begin with a discussion on some of the fundamentalchemical aspects of sucrose, in particular the results which have made asignificant contribution towards the understanding of the structure andreactivity of the molecule. The discussion will then concentrate on some ofthe actual and potential commercial developments concerning foodingredients. In this area, sucrose-based products such as high-intensitysweeteners, low-calorie fats, bulking ingredients, low-intensity reducedcalorie sweeteners, and emulsifiers and surface active agents will bedescribed. Finally, some recent developments in sucrochemistry leading tointeresting chemicals will be presented.


POTENTIAL FOR FOOD INGREDIENTS AND CHEMICALS 265

Sucrose (1), j3-D-fructofuranosyl a-D-glucopyranoside, is a non-reducingdisaccharide, and the numbering of the carbon position in the molecule isas shown in the structure. It contains eight hydroxyl groups, three of whichare primary (C-6, 6', I') and the remaining five are secondary (C-2, 3, 3',4, 4'). The primary hydroxyl groups react preferentially, in particular thehydroxyl groups at 6 and 6' positions (James et ai., 1989).

11.2 Chemical reactivity

11.2.1 Reactivity towards trityiation reaction

Treatment of sucrose with four molar equivalents of chlorotriphenylmethane (trityl) in pyridine gave, after acetylation and chromatography, 6,1' ,6'-tri- (2) and 6,6'-di- (3) O-tritylsucroses in 50 and 30%,respectively (Hough et ai., 1972). Compound 2 is an intermediate in thesynthesis of sucralose (Hough et ai., 1979), a high-intensity sweetenerdeveloped and marketed by Tate & Lyle pic and Johnson & Johnson of theUSA. Detritylation of 2 with aqueous acetic acid followed by acetylmigration from C-4 to C-6 gave 4, which on chlorination with thionylchloride, pyridine and trichloroethane gave, after deacetylation, 4,1' ,6'trichloro-4,I' ,6'-trideoxygaiactosucrose (5, sucralose).

11.2.2 Cyclic acetaiation reactions

The most significant development in the chemistry of sucrose has been thesynthesis of cyclic actetals which had defied preparation, despite many

2 R = Ac, R' = Tr [-C(C6HS)31

3 R=R'=Ac4

5

266 SUCROSE

attempts, until 1974. The first synthesis of 4,6-0-benzylidenesucrose(Khan, 1974) (6,35% yield) was achieved from the reaction of sucrose witha,a-dibromotoluene in pyridine. Since then many new novel acetalatingreagents have been used to give a variety of sucrose acetals. Treatment ofsucrose with 2,2-dimethoxypropane, N,N-dimethylformamide andtoluene-p-sulphonic acid gave 4,6-0-isopropylidene (7) (Khan et at., 1978)and 4,6: l' ,2-di-O-isopropylidene (8) (Khan et at., 1975). The unique eightmembered 2,1'-cyclic acetal bridges the two rings in sucrose, is morestable to acid than the 4,6-acetal linkage and has been effective inproviding access to selective reactions at 2 and l' positions in sucrose.

A°t(" CHzOH

OH ~o HOC.H, 0

CHzOHOH

OH

6

0t(MX CHzOHOH 0

o HO

.. 0 OH ~H'OHOH

7

X~\~HO~ CHzOH

°V OOH

Me/\Me

8

The diphenylsililene cyclic acetals of sucrose were synthesised by using2,2-dimethoxydiphenylsilane, N,N-dimethylformamide and toluene-psulphonic acid to give the 2,1'- and 2,1':6,6'-di-O-(diphenylsilylene)derivatives in 45 and 10% yield, respectively (Jenner and Khan, 1980).

JJ.2.3 Selective esterification

The selective esterification reactions of sucrose have recently beenreviewed (James et at., 1989). 6-0-Acetylsucrose (9) has been synthesisedin 40% yield by direct acetylation of sucrose with acetic anhydride inpyridine at -40°C followed by column chromatography. This compoundhas been chlorinated to give sucralose. The concept of the direct protection ofthe 6-0H group of sucrose and then selective chlorination of the C-4,1' ,6'


positions has led to an economic process for the production of sucralose(Khan and Mufti, 1982).4,6-Cyclic orthoester derivatives of sucrose are intermediates for thesynthesis of 6-0-acylsucroses which are valuable compounds for thesynthesis of sucralose. Garegg et at. (1988) have described the use of thisapproach in their elegant synthesis of 6-0-acetyl-2,3,4-tri-0-«s)-3methylpentanoyl) sucrose, a precursor of tobacco flavour.Treatment of sucrose with trimethylorthoacetate, N,N,-dimethylformamide and toluene-p-sulphonic acid followed by acid hydrolysis gavethe 6-0-acetylsucrose (9) as the major and the 4-0-acetylsucrose as theminor component which underwent acetyl migration from C-4 to C-6,when treated with an organic base such as t-butylamine in N,Ndimethylformamide to give sucrose 6-acetate (Simpson, 1987). When thekinetic reagent 2,2-dimethoxyethene was used, 4,6-0-(1methoxyethyledene) sucrose (10), the intermediate for compound 9, wasobtained in near quantitative yield (Khan et at., 1992).

t(CHZOAC CHzOH

OH 0

o HO

HO OH ~CH'OHOH

9

Me 0t(.x OH o~CHZ°HO

o CHzOH

OHOH

10

The reactivity of sucrose towards pivaloyl (2,2-dimethylpropionyl)chloride has been thoroughly investigated (Chowdhary et at., 1984). The'reactivity profile' of sucrose towards pivaloylation has been shown to besignificantly different from other acid halides. For example, reaction ofsucrose with four molar equivalent of toluene-p-sulphonyl chloride inpyridine revealed, based on product isolation, the reactivity order of 0-60-6'>0-1'>0-2. Whereas a reactivity order for the pivaloylationreaction, under similar reaction conditions, was observed to be: 0-6 = 0-6'>0-1'>0-4. Two divergent routes to sucrose octapivalate by way of thisreaction have been suggested, each due to different reactivities of thepartially pivalated derivatives towards further acylation:

(a) 6,6'-OH>l'-OH>4'-OH>2-0H>4-0H>3-0H(b) 6,6'-OH>l'-OH>3'-OH>3-0H>4'-OH>2-0H and 4-0H

268 SUCROSE

11.2.4 SN2 displacement reactions

The first SN2 displacement reaction at C-2 posItion in sucrose and inmethyl a-o-glucopyranoside was achieved during the study of sulphurylchloride reaction with sucrose (Khan et al., 1980).The formation of a transition state at C-2 to allow an SN2 displacementreaction in methyl a-o-glucopyranoside was considered to be not possiblebecause of the unfavourable dipole-dipole interactions due to the ringoxygen and the C-l oxygen. However, when methyl 3-O-benzoyl-4,6-0benzylidene-a-o-glucopyranoside 2-chlorosulphate was treated withlithium chloride in hexamethylphosphoric triamide it gave, with inversionof configuration at C-2, the corresponding 2-chloro-manno derivative.Similar treatment of the sucrose hexaacetate 2,I'-bis (chlorosulphate) (11)with lithium chloride led to the 2,1'-manno derivative (12). Since thenthere have appeared many reports in the carbohydrate literature to supportthat an SN2 displacement reaction in an alkyl a-o-glucopyranoside ispossible.

II

11.3 Enzymic reactions

12

11.3.1 Lipase-catalysed acylation reactions

Enzymic acylation reactions offer considerable promise in the synthesis ofspecific ester derivatives of sucrose. For example, reaction of sucrose withan activated alkyl ester in N,N-dimethylformamide in the presence ofsubtilisin gave l'-O-acylsucrose, which on further treatment with anactivated fatty acid ester in acetone in the presence of lipase Chromobacterium viscosum afforded the I' ,6-diester derivative (Rive et al., 1988;Carrea et al., 1989).6-0-Acetyl-(9) and 4' ,6-di-O-acetyl-sucrose, important intermediatesfor sucralose, have been synthesised from sucrose using lipases. Treatmentof sucrose with isopropenylacetate in pyridine in the presence of Lipase PAmano gave, after chromatography, 9 (33%) and the 4' ,6-diacetate (8%).The latter compound has been obtained in 47% yield by the prolongtreatment, and has been converted to sucralose (5) (Dordick et al., 1992).


11.3.2 Selective deacylation reaction

Partially esterified derivatives of sucrose have been prepared usingenzymes. Enzymatic hydrolysis of sucrose octa-acetate (13) in a phosphateor citrate buffer has led to a series of compounds such as sucroseheptaacetate 4'-hydroxy, sucrose heptacetate 4-hydroxy, and sucrose tetraacetate 4,1' ,6' ,4'-tetrahydroxy (Bornemann et al., 1992) (14). Compound14 has been enzymically transformed to the 4'-butyrate derivative 15,which on chlorination followed by deacylation gave sucralose (Dordick etal., 1992) (5).

13 14

15

The hydrolysis of sucrose octa-acetate (13) with lipase Candidacylindracea has been reported to cause deacetylation selectively, unlikechemical deesterification reactions, in the pyranose moiety to give2,3,6,1' ,3' ,4' ,6'-hepta-, 2,3,4,1',3',4' ,6'-hepta-, 2,3,1',3',4' ,6'-hexa-,2,6,1' ,3',4' ,6' -hexa-, and 2,1' ,3',4' ,6'-penta- O-acetylsucroses (Gng et al.,1993).

11.4 Food ingredients

The recent chemical and biotechnological advances and the resultingcommercial opportunities have influenced the business strategy of thesugar industry. Most of the big sugar companies have now extended theirinterests beyond sugar into starch, high fructose corn syrup, synthetic and

270 SUCROSE

natural high-intensity sweeteners, and other related food and chemicalproducts. Some of these opportunities which sucrose presents will bediscussed.

11.4.1 High-intensity sweeteners

There are a number of high-inensity sweeteners based on sucrose but all ofthem are halodeoxy derivatives (Hough and Khan, 1989). 4,1',6'Trichloro-4,1' ,6'-trideoxygalactosucrose (5) with the trade name 'sucralose'is 650 times the sweetness of sucrose. It is being developed andmarketed by Tate & Lyle pic and Johnson & Johnson of the USA.Sucralose has been approved for use in food in Canada, Australia andRussia and is awaiting approval as a food and drink additive by the Foodand Drug Administration (FDA) of the USA and other health authoritiesall over the world.Sucralose is readily soluble in water (28% w/w, at 20oq, non-cariogenic,tastes like sucrose, and is heat stable. The relatively good sweetness qualityand its heat stability will be of particular value in cooking and bakingapplications. The market potential of sucralose can be assessed on the basisof the market size for aspartame which in 1985 was roughly US$700million. It is of interest to note that aspartame, unlike sucralose, is notstable to heat which excludes it from such markets as baking and cooking.

11.4.2 Emulsifiers and surface active compounds

Purified, food grade, sucrose fatty acid esters are being commerciallyproduced in Japan by Mitsubishi Kasei Food Corporation, Tokyo, and Dai!chi Kogyo Seiyaku, Kyoto. These esters are approved by the FDA forfood applications.The degree of fatty acid esterification in the sucrose molecule determinesits functional properties, for example, an average degree of esterificationof two will impart the mixture emulsifying characteristics and one estergroup per sucrose molecule will give the compound surface activeproperties (Parker et al., 1976). Sucrose esters are used in Japan inprocessed food for such functions as emulsification, and crystallisationinhibition, wheat flour improvers and lubrication. Sucrose monolauratehas been shown to inhibit the growth of Escherichia coli and other bacteria(Ando et al., 1983) with obvious advantages in food and drink products.

11.4.3 Low-calorie fat

Sucrose polyesters developed by Procter and Gamble under the brandname 'Olestra' are neither absorbed nor hydrolysed by pancreatic lipasesand are therefore classed as low-calorie fats (Mattson and Volpenheim,


1968). In addition, these products appear to reduce the level of cholesterolin humans. The polyesters containing six to eight fatty acid esters persucrose molecule are prepared by a solventless transesterification process.Sucrose is treated with ethyl ester of fatty acids in the presence of sodiummethoxide between 100-180°C for 14 h. The unreacted fatty acid estersand lower substituted sucrose esters are removed by enzymic hydrolysiswith lipase. Sucrose polyesters containing five or more fatty acid esters areresistant to lipases.'Olestra' has yet to be approved by the FDA for human consumption.

11.4.4 Non-cariogenic, reduced calorie, low-intensity sweeteners

n.4.4.1 Isomaltulose and [somalt. Isomaltulose (Palatinose®) and Isomalt (Palatinit®) are marketed as reduced calorie and non-cariogenic foodingredients. An efficient enzymic process for the production of isomaltulose, 6-0-(a-D-glucopyranosyl)-D-fructose, was first developed byTate & Lyle (Buck and Cheetham, 1979). Isomaltulose is produced on anindustrial scale from sucrose using immobilised a-glucosyl transferasefrom Protaminobacter rubrum. In Japan, palatinose is used as a noncariogenic sweetener. In Germany, the Sudzucker company also producesisomaltulose using a similar technology.Isomaltulose is a free-flowing, non-hygroscopic, crystalline (m.p.123-124°C) material. Its sweetness intensity is 42% that of sucrose.Isomaltulose is hydrolysed and absorbed in the small intestine and, likesucrose, has as energetic value of 4 kcal/g. It is not utilised by the microbialflora of the mouth and consequently no organic acids or polysaccharidesare formed. Hence, it has been considered as a non-cariogenic sweetener.Its annual production is roughly 10 000 tonnes/year.Isomaltulose is converted on a commercial scale to Isomalt (Palatinit) bya process which involves: catalytic hydrogenation (Raney-nickel catalyst),filtration and ion-exchange treatment to remove the catalyst, evaporation,and crystallisation to give a mixture of 6-0-(a-D-glucopyranosyl)-Dsorbitol and 6-0-(a-D-glucopyranosyl)-o-mannitol (Schiweck et al., 1991).Isomalt crystallises from water as dihydrate (m.p. 145-150°C). It has aneutral sweetness as compared with sucrose which has round and balancesweetness. The caloric value of isomalt is 2 kcal/g. It is claimed to be a noncariogenic and a suitable sweetener for diabetics.

11.4.4.2 Leucrose. Leucrose, 5-0-(a-D-glucopyranosyl)-fJ-D-fructo-pyranose (m.p. 156--158°C), is a keto-disaccharide. It appears to be a generalsecondary product of dextran-producing strains of bacteria. Pfeifer& Langen of Germany have developed a commercial process for the

272 SUCROSE

production of leucrose which involves the following: extraction of theenzyme dextransucrase from Leuconostoc mesenteroides bacteria, treatment with a 65% aqueous solution of sucrose and fructose (1:2, w/w) at25°C, separation of the product from fructose by ion-exchange columnchromatography, and crystallisation as leucrose monohydrate (Schwengers,1991).The structure of leucrose has been confirmed by 1Hand 13C NMR(Kamerling et at., 1972; De Bruyn et at., 1975) and by X-ray crystallography(Thiem et at., 1989).The sweetness intensity of leucrose is half of that of sucrose. It isconsiderably more stable towards acid hydrolysis than sucrose. Towardsenzymes such as sucrase, isomaltase and glucoamylase the hydrolysis isapproximately 54% slower than sucrose but the rate is still fast enough toensure that no leucrose enters the large intestine. It is not broken down bymicrobial flora of the mouth and therefore it is considered to be a noncariogenic sweetener.The higher cost of production of leucrose will not permit its use as asubstitute for sucrose in the low price segment of the market. However, itcan be used in speciality low-calorie food products, such as a replacementof high-calorie fat in chocolates or as an ingredient in non-cariogenic foodfor children.

1I.4.4.3 Fructootigosaccharides. Meioligo®, the trade name in Asia(formerly Neosugar®), also called Actilight® in Europe is a mixture of Dglucose, sucrose, and fructooligosaccharides (FOS) with one (1-kestose),two (nystose) and three fructofuranosyl residues linked by way offi-(1~2)bonds to the fructosyl moiety of sucrose (Hikada, 1982; Fuji and Komoto,1991). The production process for Neosugar® involves the microbialfermentation of sucrose using a fungal fructosyltransferase enzyme fromAspergillus niger.Neosugar® is claimed to be a non-cariogenic and reduced-calorie

sweetener promoting bifidogenus flora. Nystose, a trisaccharide and acomponent of Neosugar® has been shown not to provide D-glucose and Dfructose to the body which makes it a potential candidate for a bulksweetener in diets for diabetic patients.Neosugar® is commercially produced by Meiji Seika Company of

Japan. In Europe, it is being developed by a joint venture with EridaniaBeghin-Say, Beghin Meiji Industries. In America, although not yetapproved as a food additive, it is still being developed jointly by MeijiSeika and Golden Technologies Company.


11.4.5 Bulking ingredients

Sucrose, in addition to its sweetness, provides such important functionalproperties as viscosity, mouthfeel, flavour profile, and humectancy. Ifhigh-intensity sweeteners, which lack the bulking properties of sucrose, areto be used in the formulation of low-calorie food and drink products theywould require a low-calorie bulking ingredient. A low-calorie bulkingagent which can satisfactorily replace sucrose does not exist. Currently,there is interest in oligo- and polysaccharides based on sucrose and fructoseas they are claimed to be reduced calorie and non-cariogenic sweetenersand/or bulking materials. Some of these products which are derived fromsucrose will be discussed in this section.

11.4.5.1 Polyfructans. Polyfructans are polymers of fructose that occurin nature in two general forms characterised by the types of their glycosidiclinkages. Inulin, found in Jerusalem artichoke, chicory and many plants ofthe compositae family have ft-1,2-linked fructofuranose molecules and isnot very soluble in water. The second type, known as levans or fructans,hasft-(2~6) glycosidic linkages, with some branching through the CHzO-1position in the fructofuranose residues. Fructans are produced bymicrobial processes but have also been shown to occur in various grassesand some plants. They are known in sugar industry as microbial productsthat indicate sucrose loss and filtration problems in the beet sugar factory.They are associated with deteriorated beet and with frozen and thawedbeet.Microbial synthesis of a homogeneous polysaccharide containing fructosewith f3-(2~6) fructose backbone from sucrose by a strain of Bacilluspolymyxa (NRRL B-18475) has been achieved in >80% yield (Clarke etal., 1991). The structure of the polysaccharide has been established byNMR, and also by conventional techniques. It has been demonstrated thatthe polysaccharide has ft-(2~6) backbone with up to 12% branchingthrough ft-(1~2) linkage. The X-ray crystal structure revealed that thepolymer is amorphous in nature.The polysaccharide is non-hygroscopic in solid form, soluble in water,and is readily hydrolysed at high temperatures and acid pH to fructose. It isnot hydrolysed by amylase, dextranase or glucanases. Some degree ofhydrolysis has been achieved using crude pullulanase or Gramanase.

11.4.5.2 Fructoglucans. Acid-catalysed thermal polymerisation ofsucrose has been described to give a fructoglucan polymer in about 30%yield (Manley-Harris and Richards, 1993). The reaction is similar to theone used for the preparation of 'polydextrose', a low-calorie bulkingingredient manufactured from D-glucose. Treatment of anhydrous,amorphous, acidified (1% citric acid) sucrose under different temperature

274 SUCROSE

and pressure conditions afforded, after precIpItation by ethanol fromaqueous solution, the desired fructoglucan.The structure of the polymer has been established by methylationanalysis, gas-liquid chromatography-mass spectrometry and gel permeation chromatography. The polymer is highly branched and of lowmolecular weight, with the o-glucose and o-fructose ratio in about 2: 1. Theaverage degree of polymerisation for the major portion is about 25. Thereducing end groups include some o-glucopyranose linked at the 6position, plus possibly L-glucosan and dianhydrofructose. The fructoseresidues are mostly in furanose form and are mostly in the internal part ofthe molecule generally linked through the I-position. The internal 0

glucose residues are predominantly in pyranose form and are mostly linkedthrough the 6-position, also providing 2-, 3-, and 4-position for crosslinkages.There is commercial opportunity for carbohydrate-based polymers withreduced calorie, provided that the sensory and functional properties andcost are comparable to those of sucrose.

11.5 Chemicals from sucrose

Sucrose is used on a relatively large scale in countries like Brazil, India andPakistan for the production of industrial ethanol and chemicals therefrom,such as acetic acid and acetic anhydride. Similarly, the use of sugar in theproduction of commercially important microbial polysaccharides such asxanthan gum, gellan and alginates has already been demonstrated(Buchholz and Buttersack, 1988).In this chapter, the discussion will be limited to the applications of

sucrose in the production of polymers or polymer intermediates, surfactants, and additives for detergent powders.

11.5.1 Synthetic polymers based on sucrose

11.5.1.1 Polyurethanes. The non-reducing and polyhydroxy functionalities of sucrose make it suitable for conversion to polyfunctional polyols forpolyurethane manufacture (Frisch and Kresta, 1977; Fuzesi, 1977; Meathand Booth, 1977). Sucrose is not directly used as polyol in polyurethanemanufacture because it gives brittle foams. Thus sucrose is first etherifiedwith polyhydroxypropyl group in order to confer miscibility with diisocyanate and fluorocarbon blowing agent and to provide the molecularspacing necessary to impart such useful structural characteristics asstrength and flexibility to the finished foam.The use of sucrose in polyurethanes provides one of its most important


large-scale outlets in the chemical industry. In Europe, during 1984 and1985, roughly 9400 tonnes of sucrose was used in the manufacture ofpolyurethane foams (Khan and Jones, 1988).

11.5.1.2 Reactive sucrose derivatives. A series of reactive sucrosederivatives as intermediates to a variety of different polymers has recentlybeen reported. They are still only a chemical curiosity and theircommercial potential has yet to be established.New sucrose derivatives with reactive esters and ethers have beensynthesised. These derivatives are neither well defined nor are they singlecompounds. Invariably, they are mixtures of isomers with average degreesof substitution. However, they have led to interesting polymers or polymerintermediates such as sucrose methylacrylate gels, chelating resins, sucrosederivatives with carbonic acid amide groups or N-methylated groups ascondensation components for formaldehyde, sucrose derivatives withphotoactive groups, and sucrose derivatives with primary amino groupsand their fatty acid amides (Gruber and Greber, 1991).Monomethylacryloyl and vinylbenzyl derivatives of sucrose have beenprepared as intermediates for polymers (Sachinvala et ai., 1991) andpreparation of a range of copolymers of styrene and O-methylacryloylsucrose has been described (Jhurry et ai., 1992). Reactive intermediates, 40- and 6-0-monoacryloyl derivatives have been synthesised by selectiveacid-catalysed hydrolysis of 4,6-0-(1-ethoxy-2-propenylidene) sucrose.Polymerisation and copolymerisation with styrene has been reported(Fanton et ai., 1993).

11.5.2 Detergents

11.5.2.1 Surfactants. Sucrose mono-fatty acid esters have applications indetergents, cosmetics, and pharmaceutical formulations. These esters areproduced in Japan on a commercial scale. The reaction of sucrose with atriglyceride or a methyl ester of a fatty acid has normally been carried outin an aprotic solvent in the presence of a basic catalyst. In a solventlessprocess, sucrose is heated with stirring with tallow in the presence ofpotassium carbonate at 140°C to afford a mixture containing roughly 27%of sucrose monoesters, 3% of the higher esters and mono-, di-, andtriglycerides, and 30% of soaps. The crude mixture exhibits excellentsurface active properties (Parker et ai., 1976).Major industrial applications for non-toxic and biocompatible sucrosebased surfactants can be visualised in the detergent industry, in the area ofcosmetics, and in food and feed formulations. The market for low-costbiodegradable surfactants in the treatment of oil spills is also considerable.

276 SUCROSE

1l.5.2.2 Bleaching boosters. Sodium perborate is generally used as ableaching agent in detergent formulations. However, in order to performwell at temperatures as low as 40°C it requires an activator such astetra-acetyl ethylene diamine. The activator, an organic compound with atleast one acyl group, reacts with the perhydroxy ion of the bleaching agentin order to release the required peroxyacid.

It has been demonstrated that, instead of tetra-acetyl ethylene diamine,sucrose polyacetate can be used as an effective bleaching booster indetergent formulations (Mentech et al., 1993).

11.5.2.3 Detergent builders. Environmentally friendly detergent buildershave commercial potential. Sodium tripolyphosphate which is the bestmetal ion chelator is unfriendly to the environment. Citric acid and itssodium salt are biocompatible and have found application in liquiddetergent formulations.Carbohydrate based products such as oxidised starches, gluconic acid,glucaric acid, and sucronic acid are interesting materials as detergentbuilders or co-builders (to support and enhance the performance ofzeolites). However, the present cost of production of these materials is toohigh to be commercially attractive.

References

Ando, Y., Sunagawa, H., Tsuzuki, T. and Kameyama, K. (1983) Effects of sucrose estersof fatty acids on the growth of spores of Clostridium botulinum and Clostridium perfringens.Report of the Hokkaido Institute of Hygiene, 33, 1-7; Chern. Abstr., ]00, 188595h.Bornemann, S., Cassells, J.M., Combes, c.L., Dordick, J.S. and Hacking, A.J. (1992)Enzymic deacylation of sugars. UK Patent 2 224 504 A.Buchholz, K. and Buttersack, C. (1988) Sucrose. In A European Research Strategy to

Correct the Imbalances in Agricultural Production (eds Schliephake, D. and Kramer, P.).DECHEMA, Frankfurt am Main, Germany, pp. 55-88.Buck, C. and Cheetham, P.S.J. (1979) Production of isomaltulose. UK Patent 2 063 268.Carrea, G., Riva, S., Secundo, F. and Danieli, B. (1989) Enzymatic synthesis of various 1'O-sucrose and 1-P-fructose esters. J. Chern. Soc. Perkin Trans., T, 1057-1061.Chowdhary, M.S., Hough, L. and Richardson, A.C. (1984) Selective pivaloylation ofsucrose. J. Chern. Soc. Perkin Trans., T, 419-427.Clarke, M.A., Baily, A.V., Roberts, E.J. and Tsang, W.S. (1991) Polyfructrose: a newmicrobial polysaccharide. In Carbohydrates as Organic Raw Materials (ed Lichtenthaler,F.W.). VCH, Weinheim, Germany, pp. 169-181.De Bruyn, A., Van Beeuman, J., Anteunis, M. and Verhegge, G. (1975) Proton NMR study ofsome D-aldohexopyranose-D-fructos(id)es in water, Bull. Soc. Chim. Belg., 84, 799-811.Dordick, J.S., Hacking, A.J. and Khan, R. (1992) Selective acylation of sugars. US Patent5 128248.Fanton, E., Fayet, c., Gelas, G., Deffieux, A., Fontanille, M. and Jhurry, D. (1993)Synthesis of 4-0- and 6-0-monoacryloyl derivatives of sucrose by selective hydrolysis of 4,6-0-(I-ethoxyl-2-propyledene) sucrose, polymerisation and copolymerisation with styrene.Carbohydr. Res., 240, 143-152.Frisch, K.C. and Kresta, J.E. (1977) An overview of sugars in urethanes. Am. Chern. Soc.

Symp. Ser., 41, 238-256.


Fuji, S. and Komoto, M. (I991) Novel cabohydrate sweeteners in Japan. Zuckerind., 116,197-200.Fuzesi, S. (1977) Sucrose-based rigid l'fethanes in furniture applications. Am. Chem. Soc.

Symp. Ser., 41, 264-273.Garegg, P.J., Oscarson, S. and Ritzen, H. (1988) Partially esterified sucrose derivatives:synthesis of 6-0-acetyl-2,3,4-tri-O-[(s)-3-methylpentanoyl] sucrose, a naturally occurringflavour precursor of tobacco. Carbohydr. Res., 181,89-96.Gruber, H. and Greber, G. (1991) Reactive sucrose derivatives. In Carbohydrates as

Organic Raw Materials (ed Lichtenthaler, F.W.). VCH, Weinheim, Germany, pp. 95-116.Hikada, H. (1982) Neosugar - manufacturing and properties. Proceedings of 1st Neosugar

Res. Conference, Tokyo, Japan, pp. 3-13.Hough, L. and Khan, R. (1989) Enhancement of sweetness of sucrose by conversion intochlorodeoxy derivatives. In Progress in Sweeteners (ed Grenby, T.H.). Elsevier AppliedSciences, London, UK, pp. 97-120.Hough, L., Mufti, K.S. and Khan, R. (1972) 6,6'-Di-O-trityl-sucrose, Carbohydr. Res., 21,144-147.Hough, L., Phadnis, S.P., Khan, R. and Jenner, M.R. (1979) Sweeteners, UK Patent 1 543167.

James, C.E., Hough, L. and Khan, R. (1989) Sucrose and its derivatives. In Fortschritte derChemie organischer Naturstoffe, (eds Herz, W., et al.), Springer-Verlag, Vienna, Austria,pp.117-184.Jenner, M.R. and Khan, R. (1980) Use of dimethoxydiphenylsilane, N,N-dimethylformamide, and toluene-p-sulphonic acid as a novel acetalating reagent. JCS Chem.Commun.,50-51.Jhurry, D., Deffieux, A., Fontanille, M., Betremieux, I., Mentech, J. and Descotes, G.(1992) Sucrose-based polymers, linear polymers with sucrose side-chains. Makromol.Chem., 193,2997-3007.Kamerling, J.P. DeBie, M.J.A. and Vliegenthart, J.F.G. (1972) A PMR study of theanomeric protons in permethylsilyl oligosaccharides, a determination of the configurationof the glycosidic bond. Tetrahedron, 28, 3037-3047.Khan, R. (1974) Synthesis of 4,6-0-benzylidenesucrose. Carbohydr. Res., 32, 375-379.Khan, R. and Jones, H.F. (1988) Sucrose chemistry: its position as a raw material for thechemical industry. In Chemistry and Processing of SugarBeet and Sugarcane (eds Clark,M.A. and Godshall, M.A.). Elsevier, Amsterdam, The Netherlands, 367-388.Khan, R. and Mufti, K.S. (t975) Synthesis and reactions of l' ,2:4,6-di-O-isopropylidenesucrose. Carbohydr. Res., 43, 247-253.Khan, R. and Mufti, K.S. (1982) Process for the preparation of sucralose. UK Patent 2 079749.Khan, R., Mufti, K.S. and Jenner, M.R. (1978) Synthesis and reactions of 4,6-acetals ofsucrose. Carbohydr. Res., 65, 109-113.Khan, R., Jenner, M.R. and Lindseth, H. (1980) The first replacement of a chlorosulphonyloxy group by chlorine at C-2 in methyl a-D-glucopyranoside and sucrosederivatives. Carbohydr. Res., 78,173-183.Khan, R., Pelter, A., Smith, K. and Zhao, J. (1992) Process for the preparation of sucrose6-acetate. UK Patent Application 9210675.6.Manley-Harris, M. and Richards, G.N. (1993) A novel fructoglucan from the thermalpolymerisation of sucrose. Carbohydr. Res., 240, 183-196.Mattson, F.H. and Volpenheim, R.A. (1968) Low-calorie fat containing food compositions.US Patent 3 600 186.Meath, A.R. and Booth, L.D. (1977) Sucrose and modified sucrose polyols in rigid urethanefoam. Am. Chem. Soc. Symp. Ser., 41, 257-263.Mentech, J., Beck, K. and Burzio, F. (1993) Sucrose derivatives as bleaching boosters forthe detergent industry. In Carbohydrates as Organic Raw Materials II (ed. Descotes, G.)VCH, Weinheim, Germany, pp. 185-201.Ong, G.-T. Chang, K-Y., Wu, SoH. and Wang, K-T. (1993) Selective deacylation on theglucosyl moiety of octa-O-acetylsucrose by enzymic hydrolysis: formation of 2,1' ,3' ,4' ,6'penta-O-acetylsucrose. Carbohydr. Res., 241, 327-333.Parker, K.J., Khan, R. and Mufti, K.S. (1976) Sucrose esters. US Patent 3 996 206.

278 SUCROSE

Riva, S., Chopineau, J., Kieboom, A.P.G. and Klibanov, A.M. (1988) Protease-catalysedregioselective esterification of sugars and related compounds in anhydrous dimethylformamide.1. Am. Chern. Soc., 110, 589.Sachinvala, N.D., Niemczura, W.P. and Litt, M.H. (1991) Monomers from sucrose.

Carbohydr. Res., 218, 237-245.Schiweck, H., Munir, M., Rapp, K.M., Schneider, B. and Vogel, M. (1991) Sucrose as anindustrial bulk chemical. In Carbohydrate as Organic Raw Materials (ed Lichtenthaler,.W.). VCH Verlagsgesellschaft, Weinheim, Germany, pp. 57-94.

Schwengers, D. (1991) Leucrose, a ketodisaccharide of industrial design. Jn Carbohydrate asOrganic Raw Materials (ed Lichtenthaler, F.W.). VCH Verlagsgesellschaft, Weinheim,Germany, pp. 183-195.Simpson, P.J. (1987) 4,6-0rthoesters of sucrose and their use in the formation of sucrose-6acetate. UK Patent 2 195632 A.

Thiem, J., Kleeberg, M. and Klaska, K.H. (1989) Neue synthese und kristallstruktur derleucrose. Carbohydr. Res., 189,65-77.

12 Sucrose and osmotic dehydrationA.L. RAOULT-WACK, G. RIOS and S. GUILBERT

12.1 Introduction

The osmotic dehydration process, previously reviewed by Ponting et ai.(1966), Le Maguer (1988), and Raoult-Wack et ai. (1992), consists ofsoaking moisture-rich foods in concentrated solution, which creates twocross mass transfers (Ponting et ai., 1966; Karel, 1975; Hawkes and Flink,1978).

(1) An important water outflow, from product to solution. The foodproduct may lose up to 70% of water (g water/lOO g initial product)at moderate temperature (3G-50oq, away from oxygen, and withoutphase change, within a time duration of 1-3 h.

(2) A solute transfer, from solution to product. It is thus possible toinsert the desired amount of preserving agent, any solute ofnutritional interest, or sensory quality improver into the product.Hence, this technique appears to be all the more promising, in that itmakes it possible to achieve a formulation of a food item without anydamage to its integrity, which can be called 'direct formulation'(Raoult-Wack et ai., 1992).

A product's own solutes leaching may also be observed (Ponting et ai.,1966; Ponting, 1973; Dixon and len, 1977; Lerici et ai., 1977; Adambounouand Castaigne, 1983; Heng et ai., 1990; Vial et ai., 1990). Although in lowquantity, this loss is essential regarding the final product's organoleptic(acidity for instance) and nutritional qualities (mainly vitaminic andmineral). In most cases, the lower the leaching, the better the quality. Insome cases however, leaching is expected, for instance to improve thesensory qualities of plum wine by reducing excessive acidity and astringency(Vyas et ai., 1989; Moutounet et ai., 1991).

12.2 General presentation of osmotic dehydration

The dewatering effect is usually conceived as an osmosis phenomenonthrough the cell semi-permeable (i.e. water permeable/solute repellent)membranes. This is the reason why such processes have often been called


280 SUCROSE

'osmotic' dehydration (Ponting et ai., 1966). Proof has been given thatsemi-permeable membranes are not a necessary condition for high waterloss, with only marginal sugar pickup (Raoult et ai., 1989; Raoult-Wack etai., 1991a). This result is all the more interesting since one must often workwith items in which tissue structures have been damaged by ripening orchemical or heat pretreatments, or freezing. Therefore, it was suggested torefer to such processes as 'dewatering and impregnation soaking processes'.(DIS) (Wack et ai., 1992). The mass transfer is schematically representedin Figure 12.1.Most applications deal with fruits and vegetables in sucrose or othersugar solutions (dextrose, lactose, corn syrups). But solutions of sodiumchloride, sorbitol, glycerin, ethanol, polyols and other water activitylowering agents may also be used, frequently blended with sucrose(Hawkes and Flink, 1978; Contreras and Smyrl, 1981; Darbonne, andBain, 1991). The solutes used should be neither toxic nor expensive andshould be highly soluble in order to give highly concentrated solutions, but,if possible, not strongly viscous. They should also be compatible with thephysico-chemical (pH, structure, etc.) and organoleptic (favor) foodcharacteristics. These agents, usually called water activity lowering agents,and their properties were reviewed by Guilbert (1992).Soaking of fruits in concentrated sucrose solutions has traditionally beenused for candying (Campi, 1985), and semi-candying (Ponting et ai., 1966;Heng et ai., 1990). Candying consists in favouring the penetration ofsucrose into the product. This is achieved by preliminary tissue processing,such as chemical or heat treatments (e.g. blanching), and the implementation of a low concentration difference between the product and thesolution. The semi-candying process has been more recently developed. Itconsists of adding a final warm-air-drying to a candying phase which isreduced by a half compared with a classical candying. This process hasbeen used for 'dried fruit' production, particularly in South-East Asiaticcountries (Wack, 1990).

concentrated solution

WATER

SOLUTE(S)

...................... A ................................ ,......................................... ,........... A .. A A........................ ,............................................... ,

A ...... ,.................................................. .......... ,.................... ,............... ""A ........................ .... ...... .. .. A A A A I....................................... ..................... ................. .- ........:::::::::::::::............................................ .................... ........... ................ ...

product

product's own solutes ~ - - - - .•;:;::::::::(minerals. sugars. organic acids...) ~

Figure 12.1 Schematic drawing of mass transfer in soaking processes.

SUCROSE AND OSMOTIC DEHYDRATION 281

Recently, there has been increasing interest in soaking treatments offood items in highly concentrated solutions, in order to simultaneouslyachieve a significant dewatering and a controlled formulation of the solidfood, provided that adequate control of mass transfer is achieved. Waterloss and solute gain kinetics depend on the natural tissue properties(possibly affected by heat or chemical pretreatments, of freezing) andoperating variables, such as specific surface area of food pieces, temperature, time duration, mode of phase contacting, concentration andcomposition of the solution. To date, most studies have been carried out atatmospheric pressure. However, recent publications deal with the effect ofpressure on mass transfer (particularly water loss) in fruit (Shi and Fito,1993a, b). In this chapter, particular attention will be devoted to theprocess variables related to the concentrated solution, i.e. concentrationand composition, at atmospheric pressure (section 12.3).The soaking process does not generally produce stable products. It hasto be used as a preprocessing step before a complementary processing,such as drying, smoking, pasteurizing, canning, freezing, frying, oraddition of preservative agents. Various combinations were reviewed byRaoult-Wack et al. (1992). Drying is the most widely used complementaryprocess, generally leading to 'intermediate moisture foods', or dried foods,intended to be directly eaten out of hand (snacks, etc.) or to beincorporated in industrial cooked dishes, pastry, breakfast cereals, dairyproducts (yogurts, ice creams), or to be rehydrated.The interest for the introduction of an osmotic dehydration preprocessinginto a conventional stabilizing process mainly relies on the particularlygood nutritional (vitaminic and mineral) and organoleptic qualities(aroma, colour, texture) of the final product (Ponting et al., 1966). Thesecharacteristics are to be ascribed notably to moderate operating temperatures (Ponting, 1973; Uzuegbu and Ukaka, 1987), lower loss of volatilecomponents (Ponting, 1973; Bongirwar and Sreenivasan, 1977; Jezek andSmyrl, 1980) (since the process is operated in liquid phase) and above all tothe effects (direct or indirect) related to the introduced solute (section12.4).Introducing an osmotic dehydration preliminary step into the conventional stabilizing process may also lead to substantial energy savings(Lenart and Lewicki, 1988; Collignan et al., 1992; Lewicki and Lenart,1992). In fact the product is processed in liquid phase, giving generallygood heat and mass exchange coefficients, and water is removed from theproduct without phase change (Bolin et al., 1983). Globally, a DISpreliminary step decreases time of convection drying necessary to obtainwater activity between 0.6 and 0.8 (intermediate moisture food) (Lenart,1988), and also the water load to the drier, or the freezer to otherdehydrating systems (Huxsoll, 1982). Hence, an osmotic dehydration stepcombined with convection drying can increase the throughput of the drying

282 SUCROSE

production line. The throughput can be increased four fold if the dryersurface is fixed; under a regular industrial production conditions, an energycost reduction of 40-50% can be foreseen (Lenart and Lewicki, 1988).However, the evaluation of the economical interest of osmotic dehydrationmust take into account the costs related to the concentrated solution. Fromthis point of view, the management of total amount of concentratedsolutions employed may constitute a key factor to the process viability (seesection 12.5).

12.3 Operating variables related to the sucrose concentrated solution

The objective here is to show how the control of operating variablesdirectly related to the concentrated solution, in the particular case ofsucrose, makes it possible to widen the range of possible applications ofosmotic dehydration.Osmotic dehydration is generally favoured by the implementation ofhighly concentrated solutions (50-75 g of solute/lOO g solution) and smallpieces of food (e.g. 1 or 2 cm3). In most common operating conditions,mass transfer mainly occurs during the first 2 h for water loss, and duringthe first 30 min for solute gain. Then, mass transfer rates becomeprogressively lower and water loss stops whereas solute gain goes onincreasing regularly. Hence, the product tends to gain back weight, andlong time duration provides solute-rich products (Karel, 1975). Figure 12.2represents water loss and solute gain observed after a 3 h of processing, ona model food (agar gel) immersed and agitated in sucrose solutions ofvarious concentrations: 20,30,40,50,60 or 67% w/w (Raoult-Wack et al.,1991a). This figure clearly underlines the key role of the concentration ofthe soaking solution. Controlling concentration allows definition of an'impregnation' situation, when solute gain is greater than water loss, whichis the case of candying or semi-candying, and a 'dewatering' situation in theopposite case. Hence, a wide range of applications may be achieved,characterized by various water loss to solute gain ratios.The composition of the concentrated solution is another key factor ofosmotic dehydration .Solutesmay be used as dewatering and/or impregnatingagents, and these requirements are generally conflicting. For instance,impregnation is favoured by low-molecular-weight solutes whereas dewatering is enhanced by high-molecular-weight solutes (Raoult-Wack etal., 1992). Therefore, the use of blends comprised to two solutes or morehave been proposed, which may provide respective advantages of eachsolute (Islam and Flink, 1982; Lenart and Flink, 1984a). The prediction ofthe effect of one given blend is not easy, due to solute interactions. Hence,it is often necessary to define by experiment which blend of solutes is thebest suited to a given product. As an illustration, sucrose has often been

SUCROSE AND OSMOTIC DEHYDRATION

WL, SG (gliOO g initial product)

283

Figure 12.2 Evolution of (.) water loss and (e) sucrose gain obtained at t = 180 min as afunction of the initial concentration difference between the model food and the solution(50°C). (From Raoult-Wack et al. (1991a), reproduced by permission of Marcel Dekker Inc.)

used together with other solutes in mixed blends, for instance salt/sucrose(50/10 or 45/15 g of each per 100 g solution), for vegetable processing(Islam and Flink, 1982; Lenart and Flink, 1984a).More recently, sucrose-salt blends were used, to achieve simultaneousdewatering and salting of lean and fat fish (Collignan and Raoult-Wack,1992; Collignan et al., 1992). It was shown that a high dewatering effectcould be obtained, due to the high concentration levels implemented withmixed blends as compared to binary salt solutions (traditionally used),which was reinforced by sucrose-salt interactions. Moreover, sucrose-saltinteractions proved to hinder salt entrance. As an illustration, Figure 12.3gives the response surface for salt gain (noted StG, expressed in g/100 ginitial product) as a function of salt concentration (noted Cst, expressed ing/litre water) and sucrose concentration (noted Csu, expressed in g/litrewater), in the case of cod fillets soaked in salt-sucrose solution (at 10°C,for 4 h 54 min). Figure 12.3 shows that in the range of low sucroseconcentrations, salt gain increases linearly as a function of salt concentration. For high sucrose concentration, salt gain is independent from saltconcentration and remains low.This recent advance made it possible to improve traditional processing offish in that (1) the traditional salting/drying sequence could be reducedthanks to one single dewatering and impregnation soaking process, and(2) it is possible to obtain low salt content dried fish, with minimal sucrosecontent (Collignan and Raoult-Wack, 1994).

284

StG(g/100g initialproduct)

9.5

7.125

4.75

2.375

o

SUCROSE

Csu (gIl water)

350

262.5

175

87.5 Cst (gIl water)

Figure 12.3 Osmotic dehydration of cod fillets soaked in salt-sucrose solution (lOOC, 4 h 54min): response surface StG = f(Cst, Csu). (From Collignan and Raoult-Wack (1992),

reproduced by permission.)

12.4 Influence of sucrose impregnation on the end-product quality

The solute is inserted into the product during the soaking treatment hasboth direct and indirect effects on the end-product quality. In fact, it isresponsible for direct modifications of the organoleptic quality of theprocessed product, but also influences the product behaviour duringosmotic dehydration and further processing or storage. As an example ofdirect modifications, the presence of the introduced sucrose in the productwas found to increase the sugar to acid ratio (Dixon and Jen, 1977),improve the texture (Lerici et at., 1983; Torregiani et at., 1988; Paoletti etat., 1990) and the stability of the pigments during drying and storage(Collignan and Raoult-Wack, 1994). It is thus possible to limit theintroduction of S02 into reactive products (Ponting, 1966; Dixon et at.,1976; Crivelli et at., 1989). The presence of the introduced solute may alsoenhance the product suitability to rehydration (Mazza, 1983), and have aprotective effect on natural tissue structure during further drying, freezing,or freeze-drying, by limiting collapse and cellular disruption (Lee et ai.,1967; Bolin and Huxoll, 1993).The influence of the introduced solute on the product behaviour also


depends on the solute distribution within the product, and the physicalstate of the impregnated food. During the soaking treatment of a model gelor a raw plant tissue in sucrose solutions, solute remains located in a 2 or3 mm depth superficial layer in the product (Bolin et ai., 1983; Lenart andFlink, 1984b; Raoult-Wack et ai., 1991c; Marcotte and Le Magner, 1991,1992), e.g. during 8 h in the case of potatoes soaked in 60% w/w sucrosesolution at 20cC (Lenart and Flink, 1984b), whereas water loss occursdeeply in the product. According to some authors, sucrose entrance wouldbe limited to extracellular spaces (Hawkes and Flink, 1978; Bolin et ai.,1983). Microscopic observations confirmed that sucrose can diffusethrough the cell wall, and then stays in the space between the wall and themembrane (Isse and Schubert, 1992). The formation of this concentratedlayer can be a key factor to control mass transfer during osmoticdehydration, in the sense of favouring water loss and limiting soluteimpregnation (Raoult-Wack et ai., 1991a, b). Moreover, this layer mayalso reduce hydrosoluble solute losses, such as ascorbic acid (Vial et ai.,1990) or fructose (Saurel, 1993). The tissue impregnation, and particularlythe presence of the concentrated sucrose layer, may also influence theproduct behaviour during the complementary processing. As an illustration,it has been widely observed that drying rates are lower after an osmotictreatment, but the behaviour of solute-impregnated products during airdrying is not yet fully understood (Lenart and Flink, 1984a; Lenart andLewicki, 1988; Collignan et ai., 1992).The physical state of the food solids (mainly polymer matrix, ownsugars, and impregnating sugars) is of particular importance for aromaretention and for textural and chemical changes during final drying andstorage. During relatively rapid drying of foods which have beenpreviously subjected to osmotic dehydration, amorphous sugars aregenerally formed, due to high initial solid content, high viscosity andpresence of other compounds with anticrystallizing properties (such asorganic acids and polymers). During storage, crystallization rate of sugarsin the product is determined by (1) crystallization characteristics of sugars,and (2) temperature and moisture content.Since it is possible, to a certain extent, to 'formulate' the food andparticularly the food surface by osmotic dehydration, the balance betweenvarious sugars with desired crystallization characteristics can be adjusted.For example, a soft texture can be obtained by adding sugars or otheringredients such as glucose, invert sugar or maltodextrins, which delays orprevents crystallization. This technique has currently been used in thecandying and semi-candying industry. On the contrary, a surface impregnation with sucrose will favor surface crystallization, thus decreasing, for thesame water activity, the product stickiness, and giving a product with acrisp surface and a soft internal part.For a given composition and water content, the physical state of foods is

286 SUCROSE

related to temperature, above the glass transition temperature (Tg) (seechapter 4). For this temperature range - typically from Tg to (Tg +100 K) - the WLF equation (Williams et aI., 1955) gives a good descriptionof the dependence of the viscosity of food solids on [T - Tg] (Solsanto andWilliams, 1981), and hence of transitions and transformations related to themobility of sugar molecules, such as crystallization, collapse and stickiness(Levine and Slade, 1989; Roos and Karel, 1993). The dependence on timeis exponential and as a result, there is a rapid rate of transition when (T Tg) is large (Roos and Karel, 1993). It is generally the case for pre-soakedproducts, which are moderately dried (to reach a final water contentranging from 12 to 16%, which corresponds to water activities 0.55 to0.65), in order to get a soft texture.The main evolution observed during storage is sugar crystallization,which occurs faster in hermetically sealed containers, at constant moisturecontent, than in open containers or in relatively permeable packagingmaterials, when the water activity of product remains constant or decreases(Vuataz, 1988). This can be attributed to the release of water (which occursduring crystallization at constant water content) and related increase of (T- Tg ) of the amorphous portion of the material, which causes accelerationof crystallization (Roos and Karel, 1993).

12.5 Control management of the sucrose concentrated solution

Optimization of the concentrated sucrose solution has two main objectives:the reduction of syrup volumes involved and control of recycling of syrup.During the soaking process, the concentrated solution undergoes composition changes related to the produce evolution, which looses water andsolutes, and gains sucrose from the solution. For osmotic dehydrationprocesses characterized by high water loss, the most important change isthe dilution of the concentrated solution; however, changes related toproduct's leaching (colour, acids, sugar, minerals, vitamins) should not beneglected since they may influence the product quality.Most laboratory studies are carried out with a large excess of solutionvolume, so as to ensure negligible variations of the solution composition,which makes the interpretation and modelling easier. The weight ratio,solution to product, is generally between 10 and 20. However, as far asindustrial applications are concerned, this ratio has obviously to be as lowas possible to decrease production costs. The influence of the weight ratiosolution to product (experimental values; 2, 6 or 20) on the processperformances was recently studied by Saurel (1993) on model gel cubessoaked in sucrose solution. This study showed that it is possible to act ontemperature or concentration in order to compensate the dilution of theconcentrated solution, when a low ratio was used.


Giroux (1992) studied the automation of a continuous device for osmoticdehydration. Results showed that on-line measurements of the electricalconductivity and density of the concentrated sucrose solution allow indirectcontrol of mass transfer (water loss and solute gain) within the soakedproduct. Such a control is possible only when the ratio of solution toproduct is low enough to provide significant changes in solution composition.Recycling the solution is actually the second stage of osmotic dehydration.

In the case of binary sucrose solution, recycling can be achieved byreconcentration. As far as energy savings are concerned, this stage is notlimiting as it could take advantage from the performing evaporationtechniques like multi-effect evaporators or mechanical steam recompression(Huxsoll, 1982; Bolin et al., 1983). It may be pointed out that to someextent, the concept of multiple effect dehydration is indirectly applied tothe piece-form product, as noticed by Huxsoll (1982). Solution recyclingmay also be achieved through addition of dry sugar (O'Mahony et al.,1986), provided that further industrial valorization of concentrated sugarsolutions (e.g. for candies, or table syrup) is possible. In the case of mixedblends, recycling is more complex, since the respective proportions of eachsolute should be respected.

12.6 Conclusion

Sucrose can be used, alone or blended with other solutes in concentratedsolutions, to simultaneously achieve dewatering and direct formulation ofwater-rich foods by 'osmotic dehydration', before any complementaryprocessing. The control of the combined process, and hence the quality(stability, and organoleptic characteristics) of the end-product, appear tobe tightly related to the control of the total amount of sucrose inserted inthe food, its distribution within the foods, as well as its physical state.These parameters are mainly influenced by the composition and concentration of the soaking solution. Moreover, on-line measurements on thesoaking solution can provide indirect continuous control of mass transferbetween the product and the solution.Recent trends in the field of osmotic dehydration consist of using mixed

solutions, which makes it possible to widen up the application range of theprocess to vegetable and animal products, whereas it was formerly limitedto semi-candying of fruit. These trends are enhancing the need for furtherunderstanding and control of the soaking solution properties, mainlydetermined by composition, solute interactions, and concentration.

288

References

SUCROSE

Adambounou, T.L. and Castaigne, F. (1983) Deshydration par osmose des bananes etdetermination des courbes de sorption isotherme Lebensm. Wiss. Technol. 16, 230--252.Bolin, H.R. and Huxsoll, c.c. (1993) Partial drying of cut pears to improve freeze/thawtexture. 1. Food Sci., 58(2), 357-360.Bolin, H.R., Huxsoll, c.c., Jackson, R. and Ng, K.C. (1983) Effect of osmosis agents andconcentration on fruit quality. 1. Food Sci., 48, 202-205.Bongirwar, D.R. and Sreenivasan, A. (1977) Studies of osmotic dehydration of banana. J.

Food Sci. Technol., 14, 404-112.Campi, C. (1985) La conservazione della frutla, base essenziale nel processo di canditura.

Industrie Alimentari, 767-798.Collignan, A. and Raoult-Wack, A.L. (1992) Dewatering through immersion in sugar/saltconcentrated solutions at low temperature; an interesting alternative for animal foodstuffstabilisation. In Drying 92 (ed. Mujumdar, A.S.). Elsevier, London, UK, pp. 1887-1897.

Collignan, A. and Raoult-Wack, A.L. (1994) Dewatering and salting of cod by immersion inconcentrated sugar-salt solutions. Lebensm.-Wiss. u.-Technol., 27, 259-264.Collignan, A., Raoult-Wack, A.L. and Themelin, A. (1992) Energy study of food processingby osmotic dehydration and air-drying. Agric. Engng 1., 1(3), 125-135.

Contreras, J.E. and Smyrl, T.G. (1981) An evaluation of osmotic concentration of applerings using corn syrup solids solutions. Can. Inst. Food Sci. Technol. J., 14,301-314.

Crivelli, G., Torreggiani, D., Senesi, E., Forni, E., Bertolo, G. and Maestrelli, A. (1989)Researches on the osmotic dehydration of apricots. Annali del/'Instituto Sperimentale per laValorizzazione Tecnologica dei Prodotti Agricoli, 20, 47-56.Darbonne, L. and Bain, J. (1991) Process for dehydration of edible plants. French patent,FR 89-{)8956 (890704).Dixon, G.M. and Jen, J.J. (1977). A research note - changes of sugar and acids of osmoticdried apple slices. J. Food Sci., 42 (4), 1126-1/27.Dixon, G.M., Jen, J.1. and Paynter, V.A. (1976) Tasty apple slices result from combinedosmotic-dehydration, and vacuum-drying process. Food Product Development, 10 (7),6Q--66.Giroux, F. (1992) Conception et realisation d'un procede automatise de deshydrationimpregnation par immersion. PhD Thesis, ENSIA-SIARC, Montpellier, France.

Guilbert, S. (1992; Additifs et ageents depresseurs de I'activite de I'eau. In Additifs etAuxiliaries de Fabrication Utilises dand les Industries Alimentaires (ed. Multon, J.L.)Lavoisier Tee. et Doc, Paris, France, pp. 225-256.Hawkes, J. and Flink, J.M. (1978) Osmotic concentration of fruit slices prior to freezedehydration. 1. Food Proc. Press., 2, 265-284.Heng, W., Guilbert, S. and Cuo, J.L. (1990) Osmotic dehydration of papaya: influence ofprocess variables on the quality. Sciences des Aliments, 10, 831-848.

Huxsoll, c.c. (1982) Reducing the refrigeration load by partial concentration of foods priorto freezing. Food Technol., 5,98-102.Islam, M.N. and Flink, J. N. (1982) Dehydration of potatoe. II. Osmotic concentration and itseffect on air drying behaviour. J. Food Technol., 17,387-403.Isse, M.G. and Schubert, H. (1992) Osmotic dehydration of mango: mass transfer betweenmango and syrup. In Proc. of 4th World Congress of Chemical Engineering, (ed. Behrens,D.). Dechema, Frankfurt, pp. 738-745.

Jezek, E. and Smyrl, T.G.(1980) Volatile changes accompanying dehydration of apples bythe Osmovac process. Can. Inst. Food Sci. Technol. J., 13(1), 43-44.Karel, M. (1975) Osmotic drying. Principles of Food Science (Part II) (ed. Fennema, O.R.).Marcel Dekker, NY, USA, pp. 348-351.Lee, C.Y., Salunkhe, O.K. and Nury, F.S. (1967) Some chemical and histological changesin dehydrated apples. 1. Sci. Food Agric., 18, 89-93.Le Maguer, M. (1988) Osmotic dehydration: review and future directions. In Proc. of

Symposium on Progress in Food Preservation Processes, CERIA Brussels, 1,283-309.Lenart, A. (1988) Sucrose as a Factor Modifying Osmo-convection Drying of Apples. WarsawAgricultural University Press, Warsaw, Poland, pp. 1-84.


Lenart, A. and Flink, 1.M. (1984a) Osmotic concentration of potatoes. I - Criteria for theend-point of the osmotic process. J. Food Technol., 19, 45---{j3.Lenart, A. and Rink, 1.M. (1984b) Osmotic concentration of potatoes. II Spatial distributionof the osmotic effect. J. Food Technol., 19,65-89.Lenart, A., Lewicki, P.P., (1988) Energy consumption during osmotic and convectiondrying of plant tisue. Acta Alimentaria Polonica, 14(1),65-72.Lerici, CR., Pepe, M. and Pinnavaia, G. (1977) La disidratazione della frutta medianteosmosi. Industria Conserve, 52, 125-129.Lerici, CR., Pinnavaia, G., Dalla Rosa, M. and Mastrocola, D. (1983) Applicazionedell'osmosi diretta nella desitratazione della fruta. Industrie Aliment., 3, 184--190.Levine, M. and Slade, L. (1989) Interpreting the behavior of low moisture foods. ]n Water

and Food Quality (ed. Hardman, T.H.). Elsevier Applied Science, London, UK, pp. 71134.

Lewicki, P.P. and Lenart, A. (1992) Energy consumption during osmoconvection drying offruits and vegetables. In Drying of Solids (ed. Mujumdar, A.S.). International SciencePublisher, New York, USA, pp. 354--366.Marcotte, M. and Le Maguer, M. (1991) Repartition of water in plant tissues subjected toosmotic processes. J. Food Proc. Engng., 13, 297-320.Marcotte, M. and Le Maguer, M. (1992) Mass transfer in cellular tissues. Part II. Computersimulations vs experimental data. 1. Food. Engng., 17, 177-199.Mazza, G. (1983) Dehydration of carrots. Effects of pre-drying treatments on moisturetransport and product quality. J. Food Technol., 18, 113-123.

Moutounet, M., Roux, C and Mourgues, 1. (1991) Dehydratation osmotique du raisindestine a la vinifacartion. Revue Franr;aise d'Oenologie, 31 (128),39-43.O'Mahony, 1.S., Kahn, M.L. and Adapa, S.N. (1986) Fruit infusion using a syrup which hasbeen subjected to enzyme treatment and concentrated. US Patent, 4626434.

Paoletti, F., Lombardi, M., Menesatti, P. and Bertone, A., (1990) Evaluation, par un testde compression, de I'effet de la deshydration osmotique sur la consistance de la pulpe depomme Golden. Industrie Aliment., 29 (284), 658-660.Ponting, 1.D. (1973) Osmotic dehydration of fruits - Recent modifications and applications.

Process Biochem., 20, 18-20.Ponting, 1.D., Walters, G.G., Forrey, R.R., lackson, R. and Stanley, W.I., (1966)Osmotic dehydration of fruits. Food. Technol., 20, 125-128. .Raoult, A.L., Lafont, F., Rios, G. and Guilbert, S. (1989) Osmotic dehydration: study ofmass transfer in terms of engineering properties, ]n Drying 89 (Mujumadar, A.S. andRoques, M.). Hemisphere Publishing Corporation, New York, USA, pp. 487-495.Raoult-Wack, A.L., Guilbert, S., Le Maguer, M. and Rios, G. (199Ia) Simultaneous waterand solute transport in shrinking media - Part I: application to dewatering andimpregnation soaking process analysis (osmotic dehydration). Drying Technol., 9 (3),589---{j12.Raoult,-Wack, A.L., Petitdemange, F., Giroux, F., Rios, G. Guilbert, S. and Lebert, A.(1991b) Simultaneous water and solute transport in shrinking media - Part 2: a compartmental model for the control of dewatering and impregnation soaking processes. DryingTechnol., 9 (3), 613---{j30.

Raoult-Wack, A.L., Botz, 0., Guilbert, S. and Rios, G., (199Ic) Simultaneous water andsolute transport in shrinking media - Part 3: a tentative analysis of the spatial distribution ofthe impregnating solute in the model gel. Drying Technol., 9 (3), 63Q---{j42.Raoult-Wack, A.L., Guilbert, S., Lenart, A. (1992) Recent advances in drying throughimmersion in concentrated solutions. ]n Drying of Solids (ed. Mujumdar, A.S.)International Science Publisher, New York, USA, pp. 21-51.Roos, Y. and Karel, M. (1993) Effects of glass transition dynamic phenomena in sugarcontaining food systems. In The Glassy State in Foods (eds Blanshard, 1.M.V. andLillford, P.l.). Nottingham University Press, Nottingham, UK, pp, 207-222.

Saurel, R. (1993) Contribution a I'etude des transferts de matiere en DehydratationImpregnation par immersion de produits biologiques. PhD Thesis University of MontpellierII, France.

Shi, X.Q. and Fito, P.M. (1993a) Mass transfer in vacuum osmotic dehydration of fruits: amathematical model. Lebnsm. Wiss. Technol., 26(6), l---{j.

290 SUCROSE

Shi, X.Q. and Fito, P.M. (1993b) Vacuum osmotic dehydration of fruits. Crying Technol.,11 (6), 1429-1442.

Soesanto, T. and Williams, M.L. (1981) Volumetric interpretation of viscosity for concentrated and dilute sugar solutions. J. Phys. Chem., 85, 3338-3341.

Torregiani, D., Maltini, E., Bertolo, G. and Mingardo, F. (1988) Frozen intermediatemoisture fruits: studies on techniques and products properties. In Proc. of Symposium onProgress in Food Preservation Processes, CERIA Brussels, pp. 71-78.Uzuegbu, J.O. and Ukeka, C. (1987) Osmotic dehydration as a method of preserving fruitsto minimize ascorbic acid loss. 1. Sci. Food Agric., 1(3), 187-188.Vial, C., Guilbert, S. and Cuo, J. (1990) Osmotic dehydration of Kiwi-fruits: Influence ofprocess variables on the color and ascorbic acid content. Sciences des Aliments, 11,63-84.Vuataz, G. (1988) Preservation of skim-milk powders: Role of water activity andtemperature in lactose crystallization and lysine loss. [n Food Preservation by Water activityControl (ed. Seow, C.C.) Elsevier Applied Science, Amsterdam, The Netherlands, pp. 73101.

Vyas, K.K., Sharma, R.C. and Joshi, V.K. (1989) Application of osmotic technique inplum wine fermentation: effect on physico-chemical and sensory qualities. J. Food Sci.Technol., 26 (3), 126-128.

Wack, A.L. (1990) La filiere thallandaise des fruits semi-confits. Bulletin TPA, 2, 9-10.Williams, M.L., Landel, R.F. and Ferry, J.D. (1955) The temperature dependence ofrelaxation mechanisms in amorphous polymers and other glass-forming liquids. J. AmChem. Soc., 77, 3701-3707.

Index

activity coefficient 103agglomeration 95amorphous quenched melt 92amorphous solid sucroseglass transition of 86-8rearrangement in 89specific heat of 190amorphous sucrosedensity of 190properties of 190amorphous sugar 75, 97thermal properties of 84amorphous sugar transformationmoisture dependence of 89temperature dependence of 91antioxidant properties 232apparent specific volume 200, 202aqueous solutions 191aroma retention 251Arrhenius 134ash 178

B coefficients 135bacteriological methods 181Berlin Institute's Method 168beverages 243bleaching boosters 276boiled sweets 95boiling point 211breads 240breakfast cereals 244bulk density 238bulking ingredients 273

cake 241crumb 260crust 260caking 95cations 141chemical reactivity 265chocolate confectionery 259chromatography 169:r.as-liquid 169

I C NMR spectra 190co-crystallisation 252Codex Alimentarius 182coffee 258colour 179, 231commodity 1,9

compressibility 188computational methods 21concentrated amorphous solution 78behaviour of 85disorder in 76order in 76-7concentrated solutions 136concentration units 191conductivity 178confectionery 95, 243conformation 13-14solution 26conformational flexibility 25, 27conformational variability 22conglomerates 36, 38consumption 2, 4control management 286convection drying 281cookies 242crystalcolour 70quality 66, 69size 259surface 66crystalline sugarstability of 95crystallinity 75crystallisation 56-{)3cooling 63, 66evaporation 60process 33, 61-2rate 59technique 58temperature 59time 62-3cubes 96cyclic acetalation 265

dairy products 244deacylation reaction 269density 188detergent builders 276detergents 275dextran 142dextrans 46dielectric constant 240differential thermal analysis 85dilute solutions 135domestic raws 4

292

Einstein 128equation 127, 132relation 128electrical properties 188emulsifiers 270energetic aspects 1energy, renewable 2enzymatic methods 173, 176enzymic reactions 268equilibrium relative humidity (ERH)235--6esterification 266European Regulation 79/796 182

fatty systems 258fixing volatiles 252flow behaviour 127, 139fondants 95food ingredients 269food processing 240free volume 134-5freeze-dried sugarcollapse of 91freezing point 212-3fructoglucans 273fructo-oligosaccharides (FOS) 272fruit flavour 259furaneol 255

glass transition 85, 190glassy molten sample 82glassy state 79granulometry 188growth 53-7crystal 53, 57rate 56

headspace effects 253heat of crystallisation 208heat of dilution 208heat of solution 208heavy metals 179heterogeneous phases 142relations 139high fructose corn syrups 9high-intensity sweeteners 270high performance liquid chromatography(HPLC) 171-2Hildebrand 133homogeneous phasesrelations 138Huggins constant 135hydrocolloids 256hydrogen bonding 16

icings and frostings 243immobilised enzymes 176

INDEX

impure solution 48solubility 110, 114, 115impurities 39,40,41,69effect of 39inclusions 69increase of volume 211infrared 157Fourier-tranform (Ff-IR) 157near (NIR) 157spectroscopy 157spectrum 159inorganic compounds 47inorganic non-sugars 46instability 76interaction 250iron-sucrose 257sucrose-eolour-flavour 257International Sugar Scale 161inversion 163isoglucose 9isotope dilution 167

jams 243jellies 243Jones and Dole equation 130

Knight and Allen. 168

laboratory methods 144Lane and Eynon 168liquid, fluidity of 133low-calorie fat 270low-intensity sweeteners 271Luff-Schoorl 169

macromoleculescomponents 141organic 141Maillard reactions 227massecuites 139, 140, 143meats 244melting point 187microbiology 180microcystalline structure 90,91microorganisms 180mobility of water 91modifying taste 255molasses 140, 150exhaustion 149, 150Quentin 149standard viscosity of 150monoclinic crystal 186monosaccharides 41morphology 34crystal 34, 35mouthfeel 259

Neosugar® 272

INDEX 293

neutron 14neutron diffraction 13NIR spectrophotometry 166NIR spectroscopy 167NMR spectroscopy 20, 159NMR spectrum 82nomenclature 12nonaqueous solventsolubility in 118nucleation 49, 51, 52, 53rate 51nucleus 50, 51radius 51nutritional aspects

oligosaccharides 41, 43optical rotation 27orientations 15orifices lSIosmotic dehydration 279,281osmotic pressure 213,217

packing 18, 19pharmacopaeia 182, 183phase equilibrium diagram 107, 108pipes lSIpoisoning 41polarimetry 206polarisation 162, 163, 164polarography 166polyfructans 273polysaccharides 44polyurethanes 274potassium chloride 46powdered sugarcaking of 96storage of 96power law 139preserves 243price 4, 9pseudo-plastic 140pulsed amperometric detection 173pure solution 48purity 176, 178

raffinose 44Raman spectroscopy 159reactive sucrose derivatives 275recrystallisation 89, 90, 93reducing sugars 168Refractive Index 202refractometry 164, 165, 166regulations 181rheological behaviour 126, 127, 139, 141

saccharides 120, 121solubility of 118salt-sucrose solution 283

saturation coefficient 115, 117sensory properties 230shapeellipsoidal 131factor 131shear rate 126shear stress 126shelf-life 95silylation technique 170SN2 displacement 268S02 179soaking process 281soaking treatments 281solubility 47,48,49, 122sucrose 48, 49solution reactions 226specific heat 189specific rotation 161stability 76standards 181statistical data 2structure 11, 76, 77sucralose 270production of 267synthesis of 267sucrosebiochemical properties of 245chemical characterisation of 155chemicals from 274consumption of 223-4density of solutions 200dry-milled 82extruded 82freeze-dried 80, 81, 82impregnation 284, 285methods of titration of 160molecule 33physical properties of 186, 232polyesters 270, 271production of 223radioactive 167solubility 101, 105, 106, 121, 191, 194sources of 223spray-dried 80, 82technological value of 223titration 175sucrose solutioncomposition of 101concentration of 101, 103, 104freezing of 78sucrose-salt blends 283sucrose-water systemthermal behaviour of 85sugardry-milled 83extruded 83, 84hydration numbers of 86sugar glasses 79

294

sugar solutionsviscosity of 151supersaturated solutions 108supersolubility 86surface active compounds 270surface tension 217sweet perception 256sweet-taste chemoreception 29sweetness 224, 225sweetness masking 256synergistic effect 250synthetic polymers 274

tasteacid-sour 249basic 248bitter 249salty 249temperature 104, 105, 106ternary system 118thermal degradation 227thermal properties 206thermal treatment 93thin-layer chromatography 156thin-layer separation 156three-component mixture 112three-component triangle diagram 110three-dimensional structures 28tissue impregnation 285trade Itwins 35crystals 36

INDEX

vegetables 244vibrational spectroscopy 156viscometerscapillary 144Couette type 145falling-ball 145orifice 146pipeflow 146rotating 144vibrating 146viscosity 58, 126, 131, 237dependence on temperature 148effect of 147, 149, 151impure solutions 138inherent 130intrinsic 129, 132, 135massecuites 143of molasses 150reduced differential 130reference 150relations 127role of 147of sucrose solutions 217of sugar solutions 151viscosity-eoncentration relations 128, 131viscosity-temperature relations 132

washing 71, 91, 213WLF equation 134

X-ray 13, 14crystallini ty 187

Sucrose_ Properties and Applications

Documents

Transcript of Sucrose_ Properties and Applications