Ts-3 Effects of Drying Methods and Conditions on Drying Kinetics and Quality of Indian Gooseberry...

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EFFECTS OF DRYING METHODS AND CONDITIONS ON DRYING KINETICS

AND QUALITY OF INDIAN GOOSEBERRY FLAKE

MISS SIPORN METHAKHUP

A SPECIAL RESEARCH PROJECT SUBMITTED IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR

THE DEGREE OF MASTER OF ENGINEERING (FOOD ENGINEERING)

FACULTY OF ENGINEERING

KING MONGKUTS UNIVERSITY OF TECHNOLOGY THONBURI

2003


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Effects of Drying Methods and Conditions on Drying Kineticsand Quality of Indian Gooseberry Flake

Miss Siporn Methakhup B.Sc. (Food Technology)

A Special Research Project Submitted in Partial Fulfillmentof the Requirements for

the Degree of Master of Engineering (Food Engineering)Faculty of Engineering

King Mongkuts University of Technology Thonburi2003

Special Research Project Committee

. Chairman(Lect. Naphaporn Chiewchan, Ph.D.)

. Co-Chairman(Asst. Prof. Sakamon Devahastin, Ph.D.)

. Member(Asst. Prof. Tipaporn Yoovidhya, Ph.D.)

. Member

(Lect. Chairath Tangduangdee, Ph.D.)

. Member(Assoc. Prof. Somkiat Prachayawarakorn, Ph.D.)

Copyright reserved


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Special Research Project Title Effects of Drying Methods and Conditions onDrying Kinetics and Quality of Indian Gooseberry

FlakeSpecial Research Project Credits 6

Candidate Miss Siporn MethakhupSpecial Research Project Advisors Dr. Naphaporn Chiewchan

Asst. Prof. Dr. Sakamon DevahastinProgram Mater of EngineeringField of Study Food EngineeringDepartment Food EngineeringFaculty EngineeringB.E. 2546

Abstract

Vacuum drying and low-pressure superheated steam drying (LPSSD) of Indiangooseberry flake were carried out at various drying temperatures (65 and 75 oC) and

pressures (7, 10 and 13 kPa absolute pressure) to monitor the drying kinetics and qualitydegradation (in terms of ascorbic acid and color) of the dried product, which is aimed asan ingredient for an Indian gooseberry tea. In terms of drying kinetics, the dryingtemperature was found to have an effect on the moisture reduction of samples dried both

by vacuum drying and LPSSD. However, pressure seemed to have an obvious effectonly for LPSSD but only slightly in the case of vacuum drying. Moreover, it was foundthat the vacuum drying took shorter time to dry the product to the required moisturecontent than those of LPSSD at every drying condition. The use of a modified Pagesequation could adequately describe the drying behavior for every condition studied ( R

2

= 0.9334-0.9868). In terms of the quality of the dried product, it was found that thedrying temperature and pressure had almost no effect on both the color and ascorbicacid retention of products underwent LPSSD while only the drying temperature had asignificantly effect on the color and ascorbic acid retention of products underwentvacuum drying; except drying under vacuum at 75 oC and absolute pressure of 7 kPa,most samples underwent LPSSD had higher level of ascorbic acid and better colorretention than those underwent vacuum drying. When comparing the productsunderwent vacuum drying at 75 oC and absolute pressure of 7 kPa it was observed that

the level of ascorbic acid retention was similar to those measured in samples underwentLPSSD but higher than those measured in samples underwent vacuum drying at otherconditions. The total color difference value of this sample was, however, slightly higherthan those dried by LPSSD. Nevertheless, since the color changes are of no concern tothe consumers, this condition was proposed as the most favorable condition for dryingof Indian gooseberry flake in regard to minimum energy consumption.

Keywords : Ascorbic Acid / Color / Low-pressure Superheated Steam Drying / PagesEquation / Vacuum Drying


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6

. . .

. . 2546

65 75

7 10 13 ( )

Page ( R 2 = 0.9334-0.9868 )


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75 7

( )

75 7

: / / / Page /


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ACKNOWLEDGEMENTS

This special research would not be successful without contributions, both directly and

indirectly, of many persons. First of all, I would like to express my appreciation to my

advisors, Dr. Naphaporn Chiewchan and Asst. Prof. Dr. Sakamon Devahastin for their

advice and encouragement during the course of this project. I am also grateful to my

committee members, Asst. Prof. Dr. Tipaporn Yoovidhya, Dr. Chairath Tangduangdee

and Assoc. Prof. Dr. Somkiat Prachayawarakorn, for their valuable comments on my

presentation and for giving me suggestions as well.

Special thanks also go to Miss Peamsuk Suvarnakuta for her advice regarding the

operation of the dryer. I also wish to acknowledge the staff of the Food Engineering

Department for their assistance during the course of this study. Finally, I wish to thank

all friends in FEPS program and my family for their assistance and encouragement.


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CONTENTS

PAGE

ENGLISH ABSTRACT ii

THAI ABSTRACT iii

ACKNOWLEDGEMENTS v

CONTENTS vi

LIST OF TABLES viii

LIST OF FIGURES x

CHAPTER

1. INTRODUCTION 1

1.1 Background 1

1.2 Objectives 2

1.3 Scopes 2

1.4 Expected Benefit 3

2. LITERATURE REVIEW 4

2.1 Indian Gooseberry 4

2.2 Drying Theory 6

3. MATERIALS AND METHODS 26

3.1 Experimental Set-up 26

3.2 Experimental Design 28

3.3 Materials and Methods 29


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3.4 Moisture Content Determination 30

3.5 Total Ascorbic Acid Determination 31

3.6 Color Measurement 33

3.7 Statistical Analysis 33

4. RESULTS AND DISCUSSION 34

4.1 Drying Kinetics of Indian Gooseberry Flake 34

4.2 Quality Degradation of Indian Gooseberry Flake 41

4.3 Empirical Modeling of Drying Process 47

5. CONCLUSION AND RECOMMENDATION 49

5.1 Conclusions 49

5.2 Recommendation 50

REFERENCES 51

APPENDIX 59

A Calibration Curve for Total Ascorbic Acid Determination 59

B Experimental Data 61

C Statistical Analysis 80

CURRICULUM VITAE 84


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LIST OF TABLES

TABLE PAGE

2.1 Nutritional values of Indian gooseberry fruit 5

3.1 A 3-factor factorial design 28

4.1 Total ascorbic acid content of fresh and dried samples 42

4.2 Hunter parameters and total color difference ( E ) of sample 45

4.3 Parameters of Pages equation and calculated drying times for various 48

drying methods and conditions

B.1 Data of vacuum drying at 65 oC, absolute pressure of 7 kPa 62





B.6 Data of vacuum drying at 75oC, absolute pressure of 13 kPa 67

B.7 Data of LPSSD at 65 oC, absolute pressure of 7 kPa 68




B.11 Total ascorbic acid of retention Indian gooseberry flake dried at various 78

methods and conditions

B.12 Hunter parameters ( L, a, b ) and total color difference ( E ) of fresh and 79

dried Indian gooseberry flake

C.1 ANOVA for ascorbic acid of Indian gooseberry flake dried at various 81



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C.2 Dancan multiple test for ascorbic acid of Indian gooseberry flake dried at 81

various methods and conditions

C.3 ANOVA for total color difference of Indian gooseberry flake dried at 82

various methods and conditions

C.4 Dancan multiple test for total color difference of Indian gooseberry flake 83

dried at various methods and conditions


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LIST OF FIGURES

FIGURE PAGE

2.1 Typical drying curve and drying rate curve 9

2.2 Ascorbic acid 18

2.3 Degradation of ascorbic acid 20

2.4 Reaction mechanism of ascorbic acid coupled with DNPH 22

2.5 Diagram of Hunter color system 25

3.1 Schematic diagram of low-pressure superheated steam dryer and 27

associated units

4.1 Drying curves of Indian gooseberry flake undergoing vacuum drying 36

4.2 Drying curves of Indian gooseberry flake undergoing low-pressure 39

superheated steam drying

4.3 Comparison drying curves of Indian gooseberry flake 40

A.1 The relationship between absorbance value at 520 nm and concentration 60

of standard ascorbic acid solution

B.1 Linearized curve of Pages Equation for vacuum drying at 65 oC and 73

absolute pressure of 7 kPa








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B.7 Linearized curve of Pages Equation for LPSSD at 65 oC and absolute 76

pressure of 7 kPa


pressure of 7 kPa


pressure of 10 kPa


pressure of 13 kPa


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CHAPTER 1 INTRODUCTION

1.1 Background

Indian gooseberry ( Phyllanthus emblica Linn.) or Ma-khaam Pom in Thai

(Chatchavalchokchai, 1987) is indigenous in tropical Southeast Asia, including

Thailand and is known as a rich source of vitamin C. The fruit is commonly consumed

as a healthy food in both fresh and various preserved forms such as pickles, dried fruits,

and beverage products (Montri, 1998).

Indian gooseberry tea is an alternative product to instant beverage powder and

pasteurized juice. It is usually drunk for thirst-quenching. In Indian gooseberry tea

processing, drying is the main thermal treatment which affects the quality of product

such as its ascorbic acid content and color. This quality loss has an influence on the

consumer satisfaction.

Ascorbic acid is the water-soluble vitamin and sensitive to heat ( Erdman and Klein,

1982; Moser and Bendich, 1991) . The degradation of ascorbic acid can cause the quality

loss and color formation of product. The color formation can also occurred by other

ways such as browning and pigment degradation. Both of ascorbic acid degradation and

color formation are well appeared in the thermal processing. Therefore, ascorbic acid

content and color are important factors for fruit and vegetable products and are

subjected to appreciable change during the drying process.

Vacuum drying has been applied widely to dry various heat-sensitive products in which

qualities such as color, texture and various vitamins are deteriorated at elevated


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temperatures ( Drouzas and Schubert, 1996 ; Markowski and Bialobrzewski, 1997 ; Jaya

and Das, 2003 ). Recently, a novel concept of using low-pressure superheated steam

drying has been proposed as an alternative to drying heat-sensitive products ( Elustondo

et al., 2001; Devahastin et al., 2004 ) since it can combine the advantage of drying at

reduced temperature and pressure to those of conventional superheated steam drying

(Mujumdar and Devahastin, 2000 ). No data are available, either in terms of drying

kinetics and quality of Indian gooseberry undergoing either of these drying techniques,

however.

Due to the above-mentioned arguments, the aim of this work was to study the drying

kinetics as well as the quality loss of Indian gooseberry flake undergoing both vacuum

and low-pressure superheated steam drying at various conditions. The information

obtained could be used to design an appropriate drying process to minimize the quality

degradation of Indian gooseberry tea.

1.2 Objective

To study the effects of drying methods viz. vacuum drying and low-pressure

superheated steam drying and conditions on the drying kinetics and quality degradation

of Indian gooseberry flake.

1.3 Scopes

1. Studying the drying kinetics of Indian gooseberry flake at the drying temperatures

of 65 o and 75 oC and at absolute pressures ranging from 7-13 kPa using vacuum

drying and low- pressure superheated steam drying methods.


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2. Determining the quality of Indian gooseberry flake, in terms of ascorbic acid and

color degradation, undergoing the above-mentioned drying techniques and

conditions.

3. Determining the suitable condition for the drying of Indian gooseberry flake from

both drying kinetics and quality points of view.

1.4 Expected Benefit

The information from this work could be used as a guideline for the design of an Indian

gooseberry tea production process, which yields a high-quality dried product.


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CHAPTER 2 THEORY

This chapter provides the theory and literature related with this study. The first part

covers the general information on Indian gooseberry including its nutritional values and

the uses of the fruit. The second part describes the drying theory including the drying

kinetics and the drying methods as well as its effects on quality of the dried product.

2.1 Indian Gooseberry

2.1.1 General Information on Indian Gooseberry

Indian gooseberry or Ma-khaam Pom in Thai is classified as species Phyllanthus

emblica Linn. or Emblica officinalis Gaertn. (Chatchavalchokchai, 1987). It is a native

fruit of tropical South-Eastern Asia, particularly of central and southern India. The tree

is hardy, normally reaching a height of 8-12 meters. The fruit is light green at first and

becomes whitish, greenish-yellow or more rarely, brick-red as it matures. The fruit skin

is thin, translucent and adherent to the very crisp, juicy, concolorous flesh. The fruiting

season become fit for harvesting in December. However, they can be retained on the

tree up to March without any significant loss in quality or yield (Montri, 1998).

2.1.2 Nutritional Values

Indian gooseberry is one of the richest sources of natural ascorbic acid (vitamin C)

(Chatchavalchokchai, 1987; Montri, 1998). The ascorbic acid in the fruit is considered

highly stable, apparently protected by tannins (leucoanthocyanins) which can retard

oxidation reaction. The nutritional values of Indian gooseberry are listed in Table 2.1.


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Table 2.1 Nutritional values of Indian gooseberry fruit

Ingredient Value per 100 g of Ingredient Value per 100 g of

edible portion edible portion

Moisture 71.1-81.8 g Vitamin C 600-625 mg

Protein 0.07-0.75 g Carotene 0.1 mg

Fat 0.1-0.2 g Thiamine 0.03 mg

Carbohydrate 13.7-21.8 g Riboflavin 0.05 mg

Fibre 1.9-3.4 g Niacin 0.18 mg

Calcium 12.5-50 mg Tryptophan 3.0 mg

Phosphorus 20-260 mg Methionine 2.0 mg

Iron 0.48-1.2 mg Lyisine 17.0 mg

Ash 0.5-2.9 g Tannin 2.73 g

Source: Chatchavalchokchai (1987)

2.1.3 The Uses of Indian Gooseberry

Indian gooseberry has been used as medicine and food by people in various countries in

Asia (Montri, 1998). In China, the drink prepared from fruit extract is commonly

consumed and wine made from juice fermentation is seen in market. In Indonesia, fresh

fruit is added to import acidity to many dishes and is often used as a substitute for

tamarind. For India, fresh fruit is baked in tarts, added to other foods as seasoning and

the juice is used to flavor vinegar. Both ripe and half-ripe fruits are candied in whole

and also made into jam and other preserves, pickles and relishes. In Thailand, the fruit is

commonly consumed as fresh and in various preserved forms such as pasteurized juice,

beverage powder and dried fruit. It is also consumed as traditional medicine for


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expectorant, antipyretic, diuretic, antidiarrhoeal and antiscurvy (Chatchavalchokchai,

1987).

2.2 Drying Theory

Drying is traditionally defined as that unit operation which converts a liquid, solid or

semi-solid feed material into a solid product of significantly lower moisture content. In

most cases, drying involves the application of thermal energy, which causes water to

evaporate into the vapor phase. The requirments of thermal energy, phase change and a

solid final product distinguish this operation from mechanical dewatering, evaporation,

extractive distillation, adsorption and osmotic dewatering.

Drying is a complex process invoving simultaneous coupled, transient heat, mass and

momentum transport. These are often accompanied by chemical or biochemical

reactions and phase transformations, such as glass transition and crystallization, along

with the shrinkage.

Foods are dried commercially, starting either from their natural state (e.g. vegetables,

fruits, milk, spices, grains) or after processing (e.g. instant coffee, whey, soup mixes,

non-dairy creamers). The production of a processed food may sometimes involve drying

at several stages in the operation. In some cases, pre-treatment of the food product may

be necessary prior to drying.

In addition to preserving the product and extending its shelf life, drying may be carried

out to accomplish one or more of the following additional objectives:

- obtain desired physical form (e.g. powder, flakes, granules);


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- obtain desired color, flavor or texture;

- reduce volume or weight for transportation;

- produce new products which would not otherwise be feasible.

In drying heat may be supplied from the drying medium to the drying product by

convection (direct dryers), conduction (contact or indirect dryers) and radiation or

volumetrically by placing the wet material in a microwave or radio frequency

electromagnetic field (Mujumdar and Devahastin, 2000). In some cases, heat transfer

can occur as a result of a combination of these methods, either in parallel or

simultaneously.

2.2.1 Drying Kinetics

Drying kinetics is the description of the changes of moisture content of material during

drying. It can be expressed as a drying curve or drying rate curve which is shown in

Figure 2.1.

Drying curve (Figure 2.1 a) can be obtained experimentally by plotting the free

moisture content versus drying time. This plot can be converted into a drying rate curve

(Figure 2.1 b) by calculating the derivative of the curve over time as shown in Equation

2.1. From these two types of curve it is seen that drying is divided into two distinct

portions. The first is the constant rate period, in which unbound water is removed (line

BC). Water evaporates as if there is no solid present, and its rate of evaporation is not

dependent on the material being dried. In this stage of drying the rate-controlling step is

the diffusion of the water vapor across the air-moisture interface. This period continues

until water from the interior is no longer available at the surface of food material. Point


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C distinguishes the constant rate period from the subsequent falling rate period and is

called the critical moisture content. The surface of the solid is no longer wet. The falling

rate period has two sections as is seen in the figure. From C to D, the wet areas on the

surface of the drying material become completely dry. When the surface is dry (point D),

the evaporation front continues moving toward the center of the solid. This is shown by

the curve from D to E. The water that is being removed from the center of the solid

moves to the surfaces as a vapor. Although the amount of water removed in the falling

rate period is relatively small, it can take considerably longer time than in the constant

rate period. The heat transmission now consists of heat transfer to the surface and heat

conduction in the product.

The drying rate in the falling rate period is controlled by diffusion of moisture from the

inside to the surface and then mass transfer from the surface. During this stage some of

the moisture bound by sorption is being removed.

As the moisture concentration is lowered by drying, the rate of internal movement of

moisture decreases. The rate of drying falls even more rapidly than before and continues

to drop until the moisture content falls down to the equilibrium value for the prevailing

air humidity and then drying stops.


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Figure 2.1 Typical drying curve and drying rate curve (Okos et al., 1992)

(a) Drying curve (free moisture versus time)

(b) Drying rate curve (drying rate versus free moisture content)


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For calculation, evaporation rate of moisture and moisture content can be calculated

using Equation 2.1 and 2.2, respectively;

dt dX

A

M

N s=

or dt

dX

A

M f s (2.1)

where N is the rate of water evaporation (kg m -2 h-1)

A is the evaporation area (m 2)

M s is the mass of bone-dry solid (kg)

X is the dry-basis moisture content (kg/kg)

X f is the free moisture content, X f = X - X* (kg/kg)

X* is the equilibrium moisture content (kg/kg)

t is the drying time (h)

Dry basis moisture content: 100%

=

b

bt

W

W W MC (2.2.1)

Wet basis moisture content: 100%

=

t

bt

W

W W MC (2.2.2)

where MC is the moisture content (kg/kg)

W t is the weight of sample at time t (kg)

W b is the mass of bone-dry solid (kg)

The drying curve may also be plotted between the moisture ratio and drying time.

Moisture ratio is defined as:

eqi

eqt

M M

M M MR

= (2.3)


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where MR is the moisture ratio

M t is the moisture content at time t (kg/kg)

M eq is the equilibrium moisture content (kg/kg)

M i is the initial moisture content (kg/kg)

The drying kinetics of materials may be described adequately using both materials and

drying medium properties, and also the knowledge of the interface mass and heat

transfer coefficients (Karathanos and Belessiotis, 1999). However, it is sometimes much

more convenient to use a simple, empirial or semi-empirical model, to describe the

drying kinetics of material. This is espectially the case for foods. In such cases, the

model may consist of a drying constant, which is a combination of both transport

properties and interface transfer coeffiecients. The models are avaliable in many forms

and may be written in a thin-layer form as exemplified in Equation 2.4.

)( eqt t M M K

dt

dM = (2.4)

where M t is the material moisture content at time t (kg/kg)

M eq is the equilibrium moisture content (kg/kg)

t is the time (s)

Equation 2.4 suggests that during the falling rate period of drying of porous hygroscopic

materials, the drying rate is proportional to the instantaneous difference between the

material moisture content and the expected material moisture content when it is in

equilibrium with the drying medium. It is assumed that the material layer is thin enough

and the drying medium velocity is high, so that the conditions of the drying medium

(humidity, temperature) are constant throughout the material.


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The thin-layer equation (Equation 2.4) describes the drying phenomena in a unified way,

regardless of the controlling mechanism. The thin-layer equation has been used for the

estimation and prediction of drying times for several products and for generalization of

drying curves (Karathanos and Belessiotis, 1999). The drying constant is a suitable

quantity for the purposes of process design, optimization and in cases where a large

number of iterative model calculations are needed. This is due to the fact that the drying

constant embodied all the transport properties into a simple exponential function. The

solution of Equation 2.4 results in:

)exp( Kt M M

M M MR

eqi

eqt

=

= (2.5)

where K is the drying constant in 1/s. The limitation of Equation 2.5 in the prediction of

the drying curve has necessitated the introduction of a second drying parameter, the

product constant N .

Equation 2.5 was modified by introducing an empirical parameter N , and the resulting

equation is known as the Modified Pages Equation:

)exp( N

eqi

eqt Kt

M M

M M MR =

= (2.6)

The parameter N reflects the degree of non-linearity of the drying curve (Cronin and

Kearney, 1998). The parameters K and N depend on the product type, temperature and

drying conditions. This modified equation can also be presented in a linearized form

(Ramesh and Rao, 1996; Cronin and Kearney, 1998; Karathanos and Belessiotis, 1999)

as:

)ln()ln()lnln( t N K MR += (2.7)

This is of the form y = mx+C , the equation of a straight line. The graph of ln [-ln( MR)]

along the Y axis and ln( t ) on the X axis will give the slope N , and Y intercept as ln( K ).


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By knowing the moisture ratios at various intervals of time, K and N values at a

particular temperature can be determined.

In earlier works, Pages equation had been used by many researchers to present thin-

layer drying of various food products dried in the falling drying rate period such as

cooked rice (Ramesh and Rao, 1996), carrots (Cronin and Kearney, 1998), currants,

sultanas, figs, and plums (Karathanos and Belessiotis, 1999), mint leaves (Park et al.,

2002), papaya (El-Aouar et al., 2003).

2.2.2 Drying Methods

Drying method is one of the factors affecting the drying kinetics and quality of food

products. Hot air drying is the most common mode of thermal dehydration. However, it

causes the degradation of sensitive components leading to the losses of organoleptic and

other properties of the dried products. Vacuum drying and superheated steam drying,

espectially the one that operates at reduced pressures, are two methods that have proven

to be good means to avoid such problem. The characteristics of these drying techniques

are reviwed briefly as follows.

2.2.2.1 Vacuum Drying

In vacuum drying, the boiling point of water is lowered below 100 oC by reducing the

pressure. The degree of vacuum and the temperature for drying depend on the

sensitivity of the material to drying rate and temperatures. At constant temperature and

pressure, the drying time varies; depending on the kind of fruit, initial moisture and size,

but is generally 4 to 16 hours (Brown et al., 1964).


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However, vacuum drying is one of the most expensive methods of drying. So, they

often serve as a secondary dryer. The moisture content of high moisture food is reduced

to 20-25% by a conventional method, such as hot air drying and then vacuum is applied

to bring the moisture down to 1-3% (Sokhansanj and Jayas, 1995).

Because of reducing pressure, transfer of heat depends on methods other than

convection. Radiation and/or conduction are other modes; however, conduction may not

be efficient because the drying materials shrink, thus reducing the contact area. This

method is not very common in the food industry because its high costs. However, it has

been applied for dehydration of citrus juices, apple flakes, and various heat-sensitive

products in which the ascorbic acid retention is important (Sokhansanj and Jayas, 1995).

Drouzas and Schubert (1996) studied the drying of banana using microwave vacuum

drying at pressures of 15 to 300 mbar and at magnetron level of 150 to 850 W. The

results showed that there is no significant variation was observed as far as the drying

rate under different pressure level is concerned. Moreover, the worth quality (as

examined by taste, aroma, smell and rehydration tests) results were also obtained for

high values of pressure.

Markowski and Bialobrzewski (1997) studied the drying kinetics of celery slice (10 mm

thick, 57 mm diameter) using air drying in vacuum chamber. Experiments were carriedout at temperatures of 25 o to 50 oC and pressure inside the chamber was maintained at

10+0.2 kPa. The drying rate versus product water content curves showed no constant

drying rate period in all cases. The amount of water absorbed by celery slice previously

dried with a vacuum drier was measured. The samples dried at lower temperature level


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absorbed more water than those dried at higher temperatures. The results proved that

vacuum drying could be used for preservation of high quality celery slices in terms of

color and flavor.

Jaya and Das (2003) studied the vacuum drying of mango pulp at varying conditions of

pulp thickness (2,3, and 4 mm) and vacuum chamber plate temperature (65 o, 70 o, and

75oC) was carried out under 30-50 mm of mercury absolute pressure. A model based on

moisture diffusivity was found to give close prediction to moisture content of the pulp

at different times of drying with correlation coefficient varying between 0.98-0.99 for

pure mango pulp and pulp with ingredients. Color change of reconstituted pulp made

from mango powder was found to depend more on pulp thickness than plate

temperature. For getting low color change vacuum drying should be carried at

maximum pulp thickness of 2.6 mm and vacuum chamber plate temperature of 72.3 oC.

2.2.2.2 Superheated Steam Drying

Superheated steam drying (SSD) involves the use of superheated steam in a direct

(convective) dryer in place of hot air, combustion, or flue gases as the drying medium to

supply heat for drying and to carry off the evaporated moisture. Any direct or

direct/indirect dryer can be operated as an SSD, in principle (Mujumdar, 1995). It has

long been recognized as a drying method that leads to nonpolluting and safe drying at

low energy consumption. In food industry, the SSD provides many advantages that arethe absense of oxidative reactions (e.g., enzymatic browning, lipid oxidation) due to

lack of oxygen, high drying rates in both constant and falling rate periods, depending on

steam temperature and pressure, and its ability to yield a higher porosity dried product

due to an evolution of steam within the product. Moreover, SSD strips more of the acids


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that contribute to an undersirable taste or aroma of the products (Devahastin et al.,

2004).

However, for drying operation, the product temperature necessarily exceeds the

saturated temperature of steam at the corresponding operation pressure. For products

that may undergo undesirable physical transformations such as melting or chemical

transformations such as hydrolysis at elevated temperature, a low pressure operation is

desirable (Mujumdar and Devashastin, 2000). Lowering the dryer operating pressure is

a feasible option that not only preserves the quality of dried product, but may also

enhance the drying rates as well (Devahastin et al., 2004).

Elustondo et al. (2001) studied sub-atmospheric pressure superheated steam drying of

foodstuffs both experimentally and theoretically. Wood slabs, shrimps, bananas, apples,

potatoes and cassava slices were dried using the steam pressures of 10,000-20,000 Pa,

the steam temperatures of 60 o-90 oC and the steam circulating velocities of 2-6 m/s. A

semi-empirical mathematical model was also developed based on a theoretical drying

mechanism, which assumed that the water removal was carried out by evaporation in a

moving boundary allowing the vapor to flow through the dry layer built as drying

proceeded to predict the drying characteristics of foodstuffs undergoing this drying

operation. A simplified expression, which has two experimentally determined

parameters, was derived and used to predicte the drying rate of tested samples. A model proposed was found to predict the drying kinetics resonably well. No mention about the

dried product quality is given, however.


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Devahastin, et al. (2004) used carrot cubes as a model heat-sensitive material,

experimental investigations were conducted to examine the drying kinetics and various

quality parameters of the dried product undergoing both low-pressure superheated steam

and vacuum drying. Effects of operating parameters such as pressure (absolute of 7-10

kPa) and temperature (60 o-80 oC) on the drying characteristics as well as quality

attributes, i.e., volume, shrinkage, apparent density, color and rehydration behavior, of

the dried product underwent the two drying processes were also evaluated and

compared. Although low-pressure steam drying required longer dwell time to achieve

the same final moisture content than vacuum drying, some of the quality attributes were

superior to those obtained in vacuum drying.

2.2.3 Effect of Drying on Quality of Food Products

Drying, the added heat and exposure times of the product at elevated temperatures affect

three quality degradations of the food products (Sokhansanj and Jayas, 1995). These

three qualities are in terms of chemical quality such as browning reaction, lipid

oxidation, and color loss; physical quality such as rehydration, solubility, and texture;

and nutritional quality such as vitamin loss, protein loss and microbial survival. In this

study, however, only ascorbic acid degradation and color change of the product as

affected by the drying processes are considered. The details of the effect of drying on

ascorbic acid and color deterioration of food are described in the following sub-sections.

2.2.3.1 Ascorbic Acid

Ascorbic acid or vitamin C is the water-soluble vitamin and sensitive to heat (Erdman

and Klein, 1982; Moser and Bendich, 1991). The structure of ascorbic acid is given in

Figure 2.2. Vitamin C is available in a wide variety of natural products but is present in


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18

significant quantities in vegetables and fruits. Plants rapidly synthesize L-ascorbic acid

from carbohydrates and the variations occur in its content due to the different species of

plants, ripeness, place of origin, storage conditions and handling (Ottaway, 1993).

As ascorbic acid is susceptible to heat, it is difficult to retain it during the dehydration of

foods (Erdman and Klein, 1982). The loss of ascorbic acid is dependent on many factors

including the presence and type of heavy metals, such as copper and iron, light, pH,

water activity level in the product, dissolved oxygen, and the drying temperature

(Villota and Hawkes, 1992; Ottaway, 1993).

Figure 2.2 Ascorbic acid

(Moser and Bendich, 1991)

The relevant factors that affect the degradation of ascorbic acid involved with the

present study are pH, O 2 and drying temperature as well drying time. The amount of O 2

and drying temperature and time varied with the drying condition while the pH value

varied with the sample itself. The effects of O 2, temperature and pH on the degradation

of ascorbic acid are described as follows.

The degradation of ascorbic acid affected by oxygen is divided into 2 mechanisms,

which are aerobic and anaerobic destructions (Erdman and Klein, 1982; Wedzicha,


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19

1984; Villota and Hawkes, 1992; Lee and Nagy, 1996). An anaerobic destruction of

ascorbic acid is generally believed to proceed at a slower rate than an aerobic

degradation, probably only one-tenth or one-thousandth of aerobic degradation rates

(Lee and Nagy, 1996). The aerobic oxidation of ascorbic acid occurs rapidly when

metal catalysts, particularly copper or iron, or enzymes such as ascorbic acid oxidase,

polyphenol oxidase, peroxidase and cytochrome oxidase are present. An anaerobic

destruction of ascorbic acid may proceed by a variety of mechanisms that have been

postulated but not verified (Erdman and Klein, 1982). The overall schematic diagram

of ascorbic acid destruction of both mechanisms is shown in Figure 2.3.

Under an aerobic condition, L-ascorbic acid in foods is easily oxidized to form dehydro-

L-ascorbic acid; both forms are likely to be present in equilibrium in foods (Ottaway,

1993). The vitamin C activity of L-ascorbic acid and its oxidized form, dehydroascorbic

acid, is the same. In fresh foods the reduced form is the major one present, but cooking,

processing and storage increase the proportions of the oxidized form.

From Figure 2.3, it can be seen that dehydroascorbic acid can be reduced to L-ascorbic

acid by chemical agents such as hydrogen sulfide or enzymatically by dehydroascorbic

acid reductase. The conversion of dehydroascorbic acid to diketogluonic acid is

irreversible and occurs both aerobically and anaerobically, particularly during heating.

The oxidation of reduced ascorbic acid may result in the formation of furfural, which isreactive aldehyde, by decarboxylation and dehydration. When furfural passes through

polymerization, the formations of dark-colored pigments are resulted. These dark-

colored compounds affect the color and flavor of certain foods, such as citrus juices, and

decrease their nutritive values.


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20

L-ascorbic acid

Anaerobic +2H + -2H + Aerobic

Delactonization Dehydroascorbic acid

H2O

2,3- Diketogluonic acid

-H 2O CO 2 CO 2

Deoxypentosone Xylosone

Furfural

+ Amino acids

Brown pigments Reductones

Figure 2.3 Degradation of ascorbic acid

(Erdman and Klein, 1982)

The effects of both drying temperature and time on ascorbic acid degradation can be

presented as in terms of the thermal resistance ( D121), which is time at 121 oC to decrease

concentration of ascorbic acid by 90%. The D 121 of ascorbic acid is around 100 minutes

depending on a w, pH, and other factors (Erdman and Klein, 1982). However, Maroulis

and Saravacos (2003) reported that D121 and z value (thermal resistance factor) which is

the temperature rise required to reduce the decimal reduction time by 90% (one logcycle) is 931 minutes and 17.8 oC, respectively.


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21

Erdman and Klein (1982) reported that drying of carrot and tomato slices at 47 oC for

16-24 hours in a forced-draft oven to a water activity of 0.33 resulted in no loss of the

total ascorbic acid for carrot and less than 20% loss for tomato.

Zanoni et al. (1999) studied the drying of tomato halves at temperatures of 80 oC and

110 oC in a cabinet air dryer. Their results showed that drying treatment at 110 oC for 350

minutes could reduce ascorbic acid by 100%. For drying at 80 oC, the retention of

ascorbic acid was 10% at drying time of 420 minutes.

For the effect of pH, the rate of ascorbic acid degradation in aqueous solutions is pH

dependent with the maximum rate at about pH 4. Moura et al. (1994) determined the

optimum pH value for ascorbic acid degradation which has to maintain prior to spray-

drying. The sample used in the study was pure water solution containing ascorbic acid

for which its pH was adjusted to be in the range of 2.5-5.0 using 37% HCl. The

degradation was determined at 20 oC+0.5 oC. It was found that the rate of ascorbic acid

oxidation was pH dependent, showing a maximum at pH 5.0 and minimum at a pH

range of 2.5 to 3.0.

For the determination of ascorbic acid in foods, the technique is based on the ability of

the acid to be oxidized or to act as a reducing agent. Roe and Kuether (1943) suggested

the determination of vitamin C on the basis of coupling of 2,4-dinitrophenylhydrazine(DNPH) with the ketonic groups of dehydroascorbic acid (DHAA) and diketogulonic

acid (DKGA). This technique has previously used to determine total ascorbic acid in

various food materials (Damrongnukool, 2000). In most plant foods, the predominant

form of ascorbic acid is the reduced compound. However, during thermal processing or


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22

storage, the amount of DHAA increases substantially as percent of total ascorbic acid

(TAA). Therefore, the measurement of DHAA should not be neglected (Erdman and

Klein, 1982). Moreover, this technique can prevent interference from reducing agents,

such as sulfhydryl compounds, reductones, tannins, betanin and reduced metals (Fe, Sn,

and Cu), which are often present in foods. Damrongnukool (2000) proved that the

determination of total ascorbic acid in fruit juices using DNPH gave high asccuracy and

precision results.

The schematic diagram of the determination of total ascorbic acid is shown in Figure

2.4. Ascorbic acid is oxidized to dehydroascorbic acid and diketogulonic acid which

then reacted with 2,4-dinitrophenylhydrazine (DNPH). The osazone formed is extracted

in sulfuric acid yielding a red solution whose intensity is proportional to the total

ascorbic acid concentration.

Figure 2.4 Reaction mechanism of ascorbic acid coupled with DNPH

(Damrongnukool, 2000)


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23

2.2.3.2 Color

Color is an important quality attribute of foods to most consumers. It is an index of the

inherent good quality of foods and the association of color with the acceptability of food

is universal. One color-related problem that is always encountered during dehydration

and long-term storage of dehydrated fruits and vegetables is the discoloration due to

browning. Browning in foods is of two types: enzymatic and nonenzymatic. For

enzymatic browning in fruits and vegetables, enzyme polyphenoloxidase (PPO) can be

inactivated at temperatures above 60 oC. Moreover, in citrus fruits the ascorbic acid and

its isomers and derivatives act as inhibitors of enzymatic browning (Roig et al., 1999).

Wakayama (1995) studied the effect of temperature on PPO activity in Japanese apple.

It was found that for Fuji apple, the relative PPO activity decreased from 49% to 13% as

the temperatures increased from 50 oC to 60 oC and the enzyme was reduced to an

undetectable level at temperature above 70 oC.

Nonenzymatic browning consists of three types, also known as Maillard reaction,

caramelization and ascorbic acid degradation. It has been reported that Maillard

condensation and oxidation of ascorbic acid are the causes of browning in fruits and

their derivatives (Barreiro et al. 1997; Lozano and Ibarz, 1997; Maskan, 2001).

Ascorbic acid browning is the spontaneous thermal decomposition of ascorbic acid

under both aerobic and anaerobic conditions and either in the presence or absence ofamino-compound (Wedzicha, 1984).


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24

Maillard reaction is often the limiting factor in dehydration of foods particular of those

with intermediate moisture contents. The Maillard reaction occurs when foods are

heated-treated. Parameters affecting the Maillard reaction are primarily temperature and

duration of the heat treatment (Chua et al., 2002).

Other than browning, many reactions can affect color during thermal processing of

fruits and their derivatives. Among them, the most common are pigment degradation,

especially carotenoids and chlorophyll (Barreiro et al. 1997; Lozano and Ibarz, 1997;

Maskan, 2001, Maskan et al., 2002). Other factors affecting color include fruit pH,

acidity, processing temperature and duration, fruit cultivar and heavy metal

contamination (Maskan, 2001).

The color measurement is normally done in an indirect way to estimate the color

changes of foods since it is simpler and faster than the chemical analysis (Maskan, 2001;

Maskan et al., 2002). Hunter Lab system is one type of measuring color systems. It has

proven valuable in describing visual color deterioration and providing useful

information for quality control in various fruits and vegetables during drying such as

tomato (Zanoni et al., 1999), kiwifriut (Maskan, 2001), banana and guava (Chua et al.,

2002) and mango pulp (Jaya and Das, 2003). The color parameters are expressed as L

(whiteness or brightness/darkness), a (redness/greenness) and b (yellowness/blueness).

The color diagram of Hunter color system is shown in Figure 2.5.

There are other parameters derived from Hunter L-, a-, b- scale viz. the total color

difference ( E ) which is the saturation index or chroma that indicates color saturation


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and is proportional to its intensity. It is a combination of parameters L-, a -, b-values as

shown in Equation 2.8.

Figure 2.5 Diagram of Hunter color system

(MacDougall, 2001)

2/1222 ])()()[( ba L E ++= (2.8)

where E is the saturation index or chroma

L is the difference of lightness ( L-Lo)

a is the difference of redness ( a-a o)

b is the difference of yellowness ( b-b o)


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CHAPTER 3 MATERIALS AND METHODS

This chapter explains the experimental set-up including the steps of experiment

performed in this study. The experimental design and the statistical analysis are also

explained as well as the procedure of quality determination.

3.1 Experimental Set-up

A schematic diagram of the low-pressure superheated steam dryer and its accessories is

shown in Figure 3.1. The dryer consists of a stainless steel drying chamber, insulated

carefully with rock wool, with an inner dimension of 45 45 45 cm 3; a steam reservoir,

which received the steam from the boiler and maintained its pressure at around 200 kPa

(gage); and a liquid ring vacuum pump (Nash, model ET32030, Germany), which was

used to maintain the vacuum in the drying chamber. Steam trap was installed to reduce

the excess steam condensation in the reservoir. An electric heater, rated at 1.5 kW,

which was controlled by a PID controller (Omron, model E5CN, Japan) was installed in

the drying chamber to control the steam temperature and to minimize the condensation

of steam in the drying chamber during the start-up period was reduced considerably. A

variable-speed electric fan was used to disperse steam throughout the drying chamber.

The steam inlet was made into a cone shape and was covered with a screen to also help

distribution of the steam in the chamber. The sample holder was made of a stainless

steel screen with dimensions of 12 12 cm 3. The change of the weight of the sample

was detected continuously (at 30 seconds intervals) using a load cell (Minebea, model

Ucg-3 kg, Japan), which was installed in a smaller chamber connected to the drying

chamber by a flexible hose (in order to maintain the same vacuum pressure as that in the

drying chamber), and also to an indirector and recorder (AND A&D Co., model AD


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27

4329, Japan). The temperatures of the steam and of the drying sample were also

measured continuously using type K thermocouples, which were connected to an

expand board (Omega Engineering, model no. EXP-32, USA). Thermocouple signals

were then multiplexed to a data acquisition card (Omega Engineering, model no. CIO-

DAS16Jr., USA) installed in a PC. LABTECH NOTEBOOK software (version 12.1,

Laboratory Technologies Corp., USA) was then used to read and record the temperature

data.

Legend

1, boiler; 2, steam valve; 3, steam reservoir; 4, pressure gauge; 5, steam trap; 6, steam

regulator; 7, drying chamber; 8, steam inlet and distributor; 9, electric fan; 10, sample

holder; 11, electric heater; 12, on-line temperature sensor and logger; 13, vacuum break-

up valve; 14, insulator; 15, on-line weight indicator and logger; 16, vacuum pump; 17,

PC with installed data acquisition card

Figure 3.1 Schematic diagram of low - pressure superheated steam dryer and associated

units

6

4

5

2 1

16

13

3 7

14

4 12

TEMP

15

9

10

8

17 11

12


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3.2 Experimental Design

The effects of three factors, drying method, temperature and pressure of drying, on

drying kinetics and quality degradation of Indian gooseberry flake were investigated in

this study. The hypothesis of this work were 1) drying method had an effect on drying

kinetics and quality degradation of Indian gooseberry flake and 2) Temperature and/or

pressure had an effect on drying kinetics and quality degradation of Indian gooseberry

flake. The experiments were conducted for 2 types of drying method, which are

vacuum and low-pressure superheated steam drying (LPSSD). The drying conditions

were experimented at 2 levels of temperatures, i.e. 65 and 75 oC and 3 levels of absolute

pressures, i.e. 7, 10 and 13 kPa.

The overall of experiment was divided into 2 parts. The first part was to study the

effects of drying methods and conditions on drying kinetics of Indian gooseberry flake

to determine the drying time for reducing of moisture content of Indian gooseberry flake

to 7.5% dry basis. The second part was to study the effects of drying methods and

conditions on quality degradation of dried Indian gooseberry flake which had moisture

content of 7.5% dry basis.

Table 3.1 A 3-factor factorial design

Absolute pressure (kPa)

Temp. Vacuum drying LPSSD

(oC) 7 10 13 7 10 13

65 y 1111 , y1112 y1121 , y1122 y1131 , y 1132 y2111, y2112 y2121 , y2122 y2131 , y2132

75 y 1211 , y1212 y1221 , y1222 y1231 , y 1232 y2211 , y2212 y2221 , y2222 y2231 , y2232


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A 3- factor factorial design was used in scheduling of the experiments with two

replicates in each case as shown in Table 3.1. The level of significance was determined

at confident level of 95%.

3.3 Materials and Methods

Fresh Indian gooseberry fruits were purchased from a local market and stored in

refrigerator at 5 oC. After rinsing the fruits by tap water and dewatered by a cloth, the

seeds were then removed using a stainless steel knife. The flesh was cut into small

pieces and blended using blender (Moulinex, AS184, Germany) for 1 minute. After that

40 grams of the prepared sample were spread in perforated tray (9.5 X 9.5 cm.) which

was made from aluminum foil. After that the tray containing flake samples were dried

by vacuum and low-pressure superheated steam drying at temperatures of 65 and 75 oC

and absolute pressures of 7, 10 and 13 kPa. During drying, weight of samples was

recorded every 10 min. The drying was operated until the weight of samples reached

equilibrium. Then the samples were dried in hot air oven (Memmert, model ULM 600

II, Germany) at 105 oC until the weights, which was measured by digital balance

(Sartorious, model RC 250S, Germany), were constant in order to obtain dry masses.

The modified Pages equation was used to fit the experimental data to calculate drying

time for reducing the moisture content of Indian gooseberry flake to 7.5% dry basis.

After that the new sets of prepared samples, which had pH value (measured by pH

meter, Tv Rheinland, type CG 841, West Germany) in the ranges of 2.2-2.7, were

dried at the same conditions as drying kinetics study until moisture content reach 7.5%

using time obtained from modified Pages equation. This final moisture content was

selected since it is the maximum final moisture content of black tea ( TISI, 1983 ).


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Moisture content (AOAC, 1990), ascorbic acid and color of fresh and dried Indian

gooseberry flake were measured All experiments were performed in duplicate.

3.4 Moisture Content Determination

Procedure

Moisture can was cleaned and dried in hot air oven (Memmert, model ULM 600 II,

Germany) for 2 h, then cooled in desiccator and recorded the weight which was

measured by digital balance (Sartorious, model RC 250S, Germany). The Indian

gooseberry samples were weight for 2 grams into the moisture can and dried in hot air

oven at 105 oC for overnight until the weight was constant. The moisture can containing

sample was cooled in desiccator. After that the weight of can and sample was

determined. Weight of dried sample was then calculated to determine its moisture

content.

Calculation

The percentage of moisture content is calculated as follows:

% Moisture content 100*12

32

ww

ww

= (3.1)

Where w 1 is initial weight of moisture can (g)

w 2 is weight of moisture can containing sample before drying (g)

w 3 is weigh of moisture can containing sample after drying (g)


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3.5 Total Ascorbic Acid Determination

Procedure

1. Extraction

Two grams of fresh Indian gooseberry flake was added to 50 g of 10% metaphosphoric

acid (HPO 3) (AR Grade, Carlo Erba, Italy). Then the slurry was blended using blender

(Moulinex, AS184, Germany) for 2-3 minutes. The 20 grams of the slurry was diluted

with mixed acid (5% metaphosphoric acid + 10% acetic acid (CH 3COOH,) (AR Grade,

J.T.Baker, USA)) to 100 ml in volumetric flask. After that the diluted slurry was filtered

by buchner funnel and Whatman filter paper no.42.

2. Oxidation to dehydroascorbic acid

50 ml of aliquots of the filtrates were shaken with 1 gram of acid-washed charcoal (C)

(AR Grade, Carlo Erba, Italy) and filtered by buchner funnel and vacuum pump

(M104X, USA). 20 ml of the filtered solution was pipette into 50 ml volumetric flask

which contained 20 ml of 2% of thiourea (NH 2CSNH 2) (AR Grade, Carlo Erba, Italy).

The mixers were then shaken and diluted with 5% metaphosphoric acid to make 50 ml

of solution in volumetric flask.

3. Formation of osazone

Each 4 ml of solution from previous was pipette into 4 test tubes. The first tube was

considered as a blank. Three other tubes were added with 1 ml of 2% 2,4-

dinitrophenylhydrazine or DNPH ((NO 2)2C6H3 NHNH 2) (AR Grade, Carlo Erba, Italy).

Then all of test tubes were soaked in water bath (Heto-Holten A/S, model DK-3450,

Denmark) at 37 oC for 3 hours.


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4. Formation of soluble pigment (osazone)

Every test tube obtained from previous was soaked in ice bath and slowly added drop by

drop with 85% of sulfuric acid (H 2SO 4) (AR Grade, Merck, Germany) using dropper.

Then 1 ml of 2% DNPH was added to the blank tube while it was soaked in an ice bath.

All test tubes were shaken and kept at room temperature for 30 minutes. The absorbance

of the resulting red solution was measured at 520 nm by a spectrophotometer

(Shimadzu, UV-2101 PC, Japan). Each sample was measured 2 replicates and the data

were presented in an average.

Calculation

The total ascorbic acid (TAA) in Indian gooseberry flake is calculated as follows;

Weight fraction of ascorbic acid; )(*250

*

21

31

ww

wwW

+= g/ml

Where w 1 is weight of sample (g)

w 2 is weight of 10% HPO 3 solution (g)

w 3 is weight of slurry (g)

Ascorbic acid read from standard curve X g/ml

Ascorbic acid per ml of each solution of sample W g

Total ascorbic acid of 100 g sampleW

X

10= mg (3.2)

The percentage retention of total ascorbic acid (TAA) is calculated as following

equation ( Ramesh et al., 2001 );


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%Retention = (Wt. of sample after dried)(Conc. of TAA in the sample after dried) *100

(Wt. of sample before dried)(Conc. of TAA in the sample before dried)

(3.3)

3.6 Color Measurement

Color changes of fresh and dried Indian gooseberry flake were analyzed by measuring

the reflectance using a colorimeter (JUKI, model JP7100, Japan). 2 o North skylight was

used as the light source. The colorimeter was calibrated against standard white plate for

powder ( L = 91.78, a = -0.28, b = 0.07) before the sample measurement. A glass cylinder

containing fresh/dried Indian gooseberry flake was placed above the light source and

covered with a lid. Three Hunter parameters, namely L (lightness), a

(redness/greenness), and b (yellowness/blueness) were measured. Each sample was

measured 2 replicates and the data were presented in an average.

3.7 Statistical Analysis

The designed experiment was completely random. The data was analyzed and presented

as mean values with standard deviations. Differences between means value were

established using Duncans multiple range test. Values were considered at 95%

significant (


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CHAPTER 4 RESULTS AND DISCUSSION

This chapter presents the results and discussion of the study of drying kinetics and

quality degradation of Indian gooseberry flake undergoing various drying methods and

conditions. The quality degradation was reported in terms of the color changes and the

ascorbic acid degradation.

4.1 Drying Kinetics of Indian Gooseberry Flake

Fresh Indian gooseberry flake with an initial moisture content in the range of 380 to 470

% dry basis (80 to 82.5 % wet basis) was dried until its equilibrium moisture content

was reached in both a low-pressure superheated steam dryer (LPSSD) and a vacuum

dryer. The drying curves of Indian gooseberry flake dried at various conditions and the

data fitted according to the well-known Pages equation are shown in Figures 4.1 to 4.3.

Generally, drying at higher temperature gives shorter time to reach the equilibrium

moisture content than drying at lower temperatures. Figure 4.1 shows the drying curves

of Indian gooseberry dried by vacuum drying. The dehydration times for reaching the

equilibrium moisture content (4.2-5.7% dry basis) were approximately 200, 210 and 230

minutes when using the drying temperature of 65 oC at absolute pressures of 7, 10 and

13 kPa, respectively. For drying at 75 oC at the same levels of absolute pressure, the

drying times to reach the equilibrium moisture content (2.4-3.8% dry basis) were

reduced about roughly 20%, namely, 160, 170 and 190 minutes, respectively. This is

due to the fact that drying at higher temperature implies a larger driving force for heat

transfer, which is, in this case, the temperature difference between the drying medium

and the temperature close to the wet-bulb temperature since the chamber was not at low-

enough pressure and the effect of convection (due to the fan) was still significantly


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35

present. In addition, higher drying temperature leads to the higher values of moisture

diffusivity. It has also been reported by Prabhanjan et al. (1995) that the higher drying

temperatures provided a larger water vapor pressure deficit or the difference between

the saturated water vapor pressure and partial pressure of water vapor in air at a given

temperature, which is one of the driving forces for drying. Similar behavior was

observed by Jaya and Das (2003).

It can be seen also in Figures 4.1 a and 4.1 b that higher rates of drying were obtained

when the absolute pressure of the dryer was decreased. This is because decreasing of

absolute pressure results in a lower boiling point of water. The decrease of a boiling

point of water resulted in an increase of the driving force for the outward moisture

diffusion process. Hence, escaping of moisture molecules from the drying product

became easier and faster.


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36

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250

Time (min)

M o i s t u r e r a t

i o

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250Time (min)

M o i s t u r e r a

t i o

Figure 4.1 Drying curves of Indian gooseberry flake undergoing vacuum drying at

temperatures:

(a) 65 oC, absolute pressures 7 kPa ( ), 10 kPa ( +) and 13 kPa ()

(b) 75 oC, absolute pressures 7 kPa ( ), 10 kPa ( ) and 13 kPa ( )

Solid line ( ) represents curve fitting using modified Pages equation

b

a


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37

Figure 4.2 shows the drying behavior of Indian gooseberry flake undergoing low-

pressure superheated steam drying (LPSSD) at similar conditions to the vacuum drying.

Since the preliminary results demonstrated that after drying the samples for more than 8

hours, the equilibrium moisture content could not be obtained when drying at 65 oC and

absolute pressures of 10 and 13 kPa. Therefore, for drying at 65 oC the experiments were

conducted only at the absolute pressure of 7 kPa.

It can be seen from this figure that during the first few minutes of superheated steam

drying, there was an increase in moisture content of the sample due to the steam

condensation on its surface. The occurrence of condensed water could be explained by

the gas law that the pressure is proportional to the temperature of the gas (Mujumdar

and Menon, 1995). In the warming up period, when the saturated steam that was fed to

the drying chamber flashed due to the low pressure environment in drying chamber, the

corresponding temperature of saturated steam was reduced to lower than 100 oC. If the

temperature of the drying chamber (and of the product) was lower than that of steam,

the water condensed. Moreover, it could be seen that the initial gain in moisture content

varied with the drying temperature and pressure; the samples could gain more moisture

at lower drying temperature and higher absolute pressure. This is due to the fact that

increasing of the drying temperature could accelerate the rising of temperature in the

drying chamber. Therefore, the steam was accelerated to obtain the superheated state

faster than at lower drying temperatures. Such results are similar to the works reported

by Tang and Cenkowski (2000) and Devahastin et al. (2004). For the effect of pressure

on initial steam condensation, it is expected that at lower operating pressure, the

saturated steam would flash into the superheated steam of higher degree of superheated.

The amount of initial steam condensation is therefore minimized.


50/96


51/96

39

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350 400 450

Time (min)

M o i s t u r e r a t i o

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350 400 450

Time (min)

M o i s t u r e r a

t i o

Figure 4.2 Drying curves of Indian gooseberry flake undergoing low-pressure

superheated steam drying at temperatures:

(a) 65 oC, absolute pressures 7 kPa ( )

(b) 75 oC , absolute pressures 7 kPa ( ), 10 kPa ( ), and 13 kPa ( )


a

b


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40

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350 400 450

Time (min)

M o i s t u r e r a t

i o

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350 400 450

Time (min)

M o i s t u r e r a

t i o

Figure 4.3 Comparison drying curves of Indian gooseberry flake at temperatures:

(a) 65 oC, absolute pressures of 7 kPa ( ), 10 kPa ( +), 13 kPa () for vacuum

drying and 7 kPa ( ) for LPSSD

(b) 75 oC, absolute pressures of 7 kPa ( ), 10 kPa ( ) and 13 kPa ( ) for

vacuum drying and 7 kPa ( ), 10 kPa ( ), and 13 kPa ( ) for LPSSD


b

a


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4.2 Empirical Modeling of Drying Process

To describe the drying behavior of Indian gooseberry flake undergoing both types of

drying processes as well known Pages equation (Equation 4.1) was used:

)exp( N

eqi

eqt Kt

M M

M M MR =

= (4.1)

Table 4.1 lists the drying constants obtained by application of modified Pages equation

to the experimental data. A good agreement was found between the experimental and

fitted values with the R2 values of 0.9334-0.9862. The values of K , representing the

transport properties of sample undergoing vacuum drying and LPSSD, were in the

ranges of 2.40 10 -3 to 5.02 10 -3 min -1 and 1.30 10 -3 to 1.89 10 -3 min -1, respectively. It

can be seen that K values for vacuum drying were higher than LPSSD. This is because

the moisture reduction rate of superheated steam drying was lower than that of vacuum

drying at the conditions tested in this work.

Considering the two drying processes at the same temperature, parameter K of both

LPSSD and vacuum drying decreased as the absolute pressure increased. This is due to

the fact that lower absolute pressure gives lower boiling point of water. Thus, the

evaporation of water from the sample was enhanced. At the same pressure, parameter K

increased as temperature increased because an increase of the drying temperature

increased the driving force of heat and mass transfer (through an increase of the value of

moisture diffusivity).


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Table 4.1 Parameters of Pages equation and calculated drying times for various drying


Drying Condition Modified Page's parameter R 2 Drying time

method T ( oC) P ab (kPa) K (10 -3 min -1) N(7.5% d.b.)

(min)VD 1 65 7 2.86 1.40 0.9862 170

10 2.40 1.41 0.9593 19013 2.29 1.37 0.9806 200

75 7 5.02 1.32 0.9804 14510 4.24 1.36 0.9334 15013 3.33 1.38 0.9868 160

LPSSD 2 65 7 1.30 1.33 0.9507 40010 - - - -13 - - - -

75 7 1.89 1.35 0.9618 28010 1.59 1.35 0.9799 32013 1.45 1.34 0.9687 370

1 VD stands for vacuum drying

2 LPSSD stands for low-pressure superheated steam drying

For parameter N , the degree of non-linearity of the drying curve (Cronin and Kearney,

1998) which can be defined as a product constant, it varied from 1.32 to 1.41 for

vacuum drying and 1.33 to 1.35 for LPSSD. From Table 4.1, it is seen that the N value

did not possess a clear trend. The results for parameters K and N were similar as those

found by Karathanos and Belessiotis (1999), Park et al. (2002) and El-Aouar et al.

(2003).

4.3 Quality Degradation of Indian Gooseberry Flake

The fresh Indian gooseberry flake which had pH in the range of 2.2 to 2.7 and initial

moisture contents in the range of 410 to 480 % dry basis were dried using two different

drying methods at various conditions to obtain the final moisture contents in the range


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of 7.1 to 7.5% dry basis. This final moisture content was lower than the maximum

allowable moisture content of black tea which is 7.5% dry basis (TISI, 1983). The

quality of dried product in terms of ascorbic acid content and color was then

determined.

Table 4.2 shows the total ascorbic acid content in fresh and dried Indian gooseberry

flake samples. The average initial ascorbic acid content was about 1.04 g/100 g sample.

The change of ascorbic acid was expressed as percentage of retention. All dried Indian

gooseberry flake tended to lose some ascorbic acid as compared to the fresh one. The

ascorbic acid retentions were in the ranges of 64 to 94% for vacuum drying and 93 to

96% for LPSSD. For vacuum drying, it was found that the ascorbic acid retention of the

sample increased as the drying temperature increased. This may be due to the shorter

drying time required to dry the samples to the desired moisture content (Jayaraman and

Gupta, 1995; Maharaj and Sankat, 1996). However, the pressure had only a little effect

on the ascorbic acid retention. This may be explained by the fact that the drying times

were not much affected by the operating pressure and that the level of oxygen content

(which caused the aerobic degradation of vitamin C) was not much different at different

pressures.

For LPSSD, the ascorbic acid retention was not significantly different at different

drying conditions even though the drying times were different. The results implied that

a very low amount of oxygen was available in the drying system and thus presented no

effect on the ascorbic acid degradation during drying.


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Table 4.2 Total ascorbic acid content of fresh and dried samples a

Drying Condition Ascorbic acid (g/100g) %Retention

method T ( oC) P ab (kPa) Fresh Dried

VD1

65 7 1.08+0.07 3.67+0.13 71.52ab

+1.9710 1.06+0.09 3.50+0.25 66.89 a+2.51

13 0.94+0.05 3.07+0.30 64.84 a+6.01

75 7 0.96+0.02 3.84+0.18 94.46 cd+2.57

10 0.99+0.02 3.72+0.11 89.46 c+2.78

13 0.98+0.11 3.34+0.03 78.13 b+2.83

LPSSD 2 65 7 1.05+0.06 3.99+0.22 93.46 cd+1.5810 - - -13 - - -

75 7 1.06+0.04 4.04+0.08 95.35d+3.49

10 1.09+0.08 4.03+0.11 95.67 d+2.10

13 1.04+0.08 3.99+0.11 94.96 cd+2.14a Mean +S.D. ( n=2). Means in the same column having a same letter are not

significantly different ( < 0.05).



Generally, an increasing level of ascorbic acid degradation is resulted from slower

drying methods (Nindo et al., 2003). From part the kinetics results, it was found that

drying by LPSSD took longer time than by vacuum drying at the same drying

conditions. However, Table 4.2 shows that LPSSD could preserve ascorbic acid better

than vacuum drying. This is because the level of ascorbic acid oxidation in the vacuum

drying system was higher than in LPSSSD due to the higher level of oxygen in the

system. The ability of the superheated steam drying system has, in fact, been reported

earlier by other investigators (e.g., Moreira, 2001)


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For the statistical analysis of the results, drying in LPSSD at every condition and drying

in the vacuum drying system at 75 oC, and absolute pressures of 7 and 10 kPa gave

insignificantly different level of ascorbic acid retention. Among these drying methods

and conditions, vacuum drying at 75 oC and absolute pressure of 7 kPa was the best

because it gave the highest level of ascorbic acid retention and consumed shortest

drying time.

Effects of drying methods and conditions on the color of the sample were also

determined. Table 4.3 shows the Hunter parameters ( L, a, b ) and the total color

difference ( E ) of Indian gooseberry flake. It was found that LPSSD and vacuum

drying at every condition resulted in a decrease of an L value and an increase of an a

value of dried sample compared with the fresh one. However, b value of the dried

sample was similar to that of fresh sample. These results implied that the browning

reaction and pigment destruction occurred in the dried sample (Maskan, 2001; Maskan

et al., 2002). However, the overview of color changes of sample could be observed from

the E value.

In the vacuum drying system, drying at 65 oC and an absolute pressure of 7 kPa could

preserve the color of the sample better than at other conditions. This is due to the fact

that the main cause of color changes in vacuum drying was chlorophyll degradation and

nonenzymatic browning reaction, which was the Maillard reaction and ascorbic acid

oxidation since oxygen and light, which are the causes of these reactions existed at the

lowest level in the system. Because Maillard reaction depends on temperature and

duration of the heat treatment (Chua et al., 2002), drying at lower temperatures gave

better color retention than drying at higher temperatures. However, there were no


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significant differences between drying at 65 oC, absolute pressures of 10 and 13 kPa and

drying at 75 oC.

For LPSSD, it was found that there were no significant differences in terms of color

between different drying conditions. In this case, the main cause of color change was

the Maillard reaction. Other causes of color changes were the ascorbic acid and

chlorophyll degradation. However, these degradations had only a small effect on the

color of the sample when compared with Maillard reaction since the system had only a

small amount of oxygen.

When considering the color retention of sample between two different drying methods,

it was found that the LPSSD could retain the color better than the vacuum drying

system. This is because the degree of ascorbic acid and chlorophyll degradation of

LPSSD was lower than that of the vacuum drying system.

The color results presented here are somewhat different from those reported by other

investigators studying superheated steam drying system. Iyota et al. (2002) reported that

potatoes dried in superheated steam at temperature ranging from 170 o to 240 oC were

glossier and reddish than dried in hot air drying. However, this is due to the effect of

temperature on color and amount of soluble polysaccharides was less in this work.

Moreover, in this study Indian gooseberry was used as a sample, it had a small amount

of amino acid and carbohydrate when compared with potato in study of Iyota et al.

(2002). Therefore, less effect of Maillard reaction of Indian gooseberry than that of

potato drying.


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Table 4.3 Hunter parameters and total color difference ( E ) of sample a

Drying Condition L a bmethod T ( oC) P abs (kPa) Fresh Dried Fresh Dried Fresh Dried

VD 1 65 7 63.25+0.04 58.35+0.04 -1.6+0.09 0.62+0.08 18.65+0.13 18.39+0.08 10 67.51+0.05 61.99+0.22 -1.24+0.01 -0.36+0.01 18.75+0.18 18.35+0.16 13 67.05+0.04 60.82+0.23 -1.35+0.01 0.15+0.02 18.16+0.26 18.09+0.52

75 7 63.78+0.12 60.25+0.26 -1.57+0.02 -0.14+0.02 17.73+0.12 18.13+0.11 10 64.86+0.98 60.37+0.04 -2.17+0.2 0.07+0.47 18.28+0.04 18.70+0.07 13 63.69+2.27 59.52+2.58 -2.22+0.42 1.11+1.05 18.42+0.76 17.82+0.43

LPSSD 2 65 7 67.64+1.12 64.80+1.05 -1.26+0.58 -0.43+1.29 18.59+0.12 17.86+0.15 10 - - - - - - 13 - - - - - -

75 7 67.91+0.03 65.47+0.25 -2.37+0.89 -1.12+0.03 17.28+0.34 16.67+0.11 10 65.89+2.36 63.92+1.46 -1.81+0.47 -0.65+0.10 17.62+1.07 16.93+0.67 13 67.70+0.99 65.18+3.69 -1.83+0.1 -0.69+0.52 17.60+1.52 17.25+0.70

a Mean +S.D. ( n=2). Means in the same column having a same letter are not significantly different ( < 0.05).




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However, for Indian gooseberry tea production, dried Indian gooseberry flake was filled

in a tea bag before it was soaked into the water by the customer. Therefore, the color of

dried product was not concerned in this study. In contrast, the difference conclusion was

proposed if the color of dried product could not be neglected.

From the results of ascorbic acid and color retention of product, it could be concluded

that the drying in vacuum system at temperature of 75 oC, absolute pressure of 7 kPa was

proposed to be the suitable method and condition for drying of Indian gooseberry flake.

This is because it gave highest ascorbic acid retention while the color does not much

change from natural sample. Moreover the vacuum drying at 75 oC, absolute pressure 7

kPa also consumed shortest drying time which may lead to lowest operating cost.


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CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

The effects of drying methods and conditions on drying kinetics and quality degradation

of Indian gooseberry flake have been examined in this study. In terms of drying

kinetics, the drying temperature was found to have an effect on the moisture reduction

of samples dried both by vacuum drying and LPSSD. However, pressure seemed to

have an obvious effect only for LPSSD but only slightly in the case of vacuum drying.

Moreover, it was found that the vacuum drying took shorter time to dry the product to

the required moisture content than those of LPSSD at every drying condition. The

drying curve of Indian gooseberry flake could be well fitted by the modified Pages

equation. For both low-pressure superheated steam and vacuum drying, the drying

constant, K , was increased as temperature increased and absolute pressure decreased

while product constant, N, was not affected by these factors. Parameter K of vacuum

drying was higher than low- pressure superheated steam drying.

The quality studies showed that except the vacuum drying at 75 oC and absolute

pressures of 7 kPa amd 10 kPa, LPSSD could retain ascorbic acid better than the

vacuum drying. In addition, LPSSD could preserve the color of the sample better than

the vacuum drying at every drying condition tested. For vacuum drying, temperature

had a significant effect on ascorbic acid content and color of the product while absolute

pressure did not significantly affect the quality. For low-pressure superheated steam

drying, on the other hand, the drying conditions did not affect the ascorbic acid and

color of dried product.


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It could be concluded that drying in vacuum system at temperature of 75 oC, absolute

pressure of 7 kPa was proposed to be the suitable method and condition for drying of

Indian gooseberry flake. It gave highest ascorbic acid retention while the color did not

change a large from na

Ts-3 Effects of Drying Methods and Conditions on Drying Kinetics and Quality of Indian Gooseberry...

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