Bagher Emadi Thesis

Experimental studies and modelling of innovative peeling processes for

tough-skinned vegetables

Bagher Emadi

M.Sc. Mechanics of Agricultural Machinery B.Sc. Agricultural Machinery

Thesis submitted as a requirement for the degree of

Doctor of Philosophy

School of Engineering Systems Faculty of Built Environment and Engineering

Queensland University of Technology 2005

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This dissertation is dedicated to my wife, Masoumeh, and my two lovely

children, Roya and Amir Reza

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Keywords peeling, mechanical peeling, abrasive peeling, mechanical properties, tough-skinned vegetables, model, mathematical model, peeling rate, peeling efficiency, peel losses, pumpkin, melon

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Abstract: Tough-skinned vegetables such as pumpkin and melon currently are peeled either

semi-automatically or automatically. The main limitation of both methods, especially

for varieties with an uneven surface, is high peeling losses.

Improvement of current mechanical peeling methods and development of new

mechanical methods for tough-skinned vegetables which are close to the ideal

peeling conditions using mechanical properties of the product were the main

objectives of this research.

This research has developed four innovative mechanical peeling methods on the basis

of the mechanical properties of tough-skinned vegetables. For the first time, an

abrasive-cutter brush has been introduced as the best peeling method of tough-skinned

vegetables. This device simultaneously applies abrasive and cutting forces to remove

the peel. The same peeling efficiency at concave and convex areas in addition to high

productivity are the main advantages of the developed method. The developed peeling

method is environmentally friendly, as it minimises water consumption and peeling

wastes.

The peeling process using this method has been simulated in a mathematical model

and the significant influencing parameters have been determined. The parameters are

related to either the product or peeler. Those parameters appeared as the coefficients

of a linear regression model. The coefficients have been determined for Jap and

Jarrahdale as two varieties of pumpkin. The mathematical model has been verified by

experimental results.

The successful implementation of this research has provided essential information for

the design and manufacture of a commercial peeler for tough-skinned vegetables. It is

anticipated that the abrasive-cutting method and the mathematical model will be put

into practical use in the food processing industry, enabling peeling of tough-skinned

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vegetables to be optimised and potentially saving the food industry millions of dollars

in tough-skinned vegetable peeling processes.

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Contents Keywords iii

Abstract iv

List of Symbols and Abbreviations xvi

Authorship xviii

Acknowledgement xix

1 Introduction 1.1 Significance and motivation of the research 1

1.2 Objectives of the research 4

1.3 Originality and major contribution of the thesis 4

1.3.1 Definition of tough-skinned vegetables 4

1.3.2 Determination of mechanical properties

of tough-skinned vegetables 5

1.3.3 Developing new mechanical peeling methods

for tough-skinned vegetables 5

1.3.4 Developing current peeling methods 5

1.3.5 Introducing the best mechanical peeling method

applicable in peeling industry for tough-skinned

vegetables 6

1.3.6 Mathematical modelling of mechanical peeling 6

1.3.7 Determination of design parameters of

tough-skinned vegetable peeler 6

1.4 Thesis organization 7

1.5 Publications (refereed) of the author arising from the PhD

Research 8

2 Literature review

2.1 Mechanical properties of fruits and vegetables and

methods of testing 11

2.1.1 Introduction 11

2.1.2 States of product to be tested 11

2.1.3 Compression test 12

2.1.4 Cutting test 13

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2.1.5 Shear strength test 13

2.1.6 Coefficient friction test 14

2.2 Peeling methods of fruits and vegetables 15


2.2.2 Mechanical peeling 15

2.2.2.1 Abrasive devices 16

2.2.2.2 Devices using drums 17

2.2.2.3 Devices using rollers 17

2.2.2.4 Knives or blades 17

2.2.2.5 Milling cutter 20

2.2.3 Thermal peeling 22

2.2.3.1 Flame (dry heat peeling) 23

2.2.3.2 Steam (wet heat peeling) 23

2.2.3.3 Thermal blast peeling 25

2.2.3.4 Freeze-thaw 25

2.2.3.5 Vapour explosion (Vacuum peeling) 26

2.2.4 Chemical peeling 26

2.2.4.1 Caustic (lye) peeling 26

2.2.4.2 Enzymic peeling 28

2.3 The current situation of peeling tough-skinned vegetables 30

2.4 Mathematical modelling of peeling process 30

2.5 Conclusions and discussion 32

2.6 Summary 33

3 Testing of mechanical properties of tough-skinned vegetables

3.1 Introduction 35

3.2 Design and construction of instrumentation for testing

vegetables properties 37

3.2.1 Cutter 37

3.2.2 Holder of unpeeled sample 37

3.2.3 Holder of skin sample 38

3.2.4 Indentor 38

3.2.4.1 Spherical end indentor 38

3.2.4.2 Flat end indentor 38

3.2.4.3 Cutting indentor 39

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3.2.5 Curvature meter 39

3.2.6 Friction coefficient tester 39

3.3 Testing methodology 39

3.3.1 Force-deformation test 41

3.3.2 Shear strength test 41

3.3.3 Cutting force test 42

3.3.4 Friction coefficient test 42

3.3.5 The relative contribution of skin to the

unpeeled mechanical properties 43

3.4 Results and discussion 43

3.4.1 Force-deformation relationship 44

3.4.2 Toughness 47

3.4.3 Cutting force 47

3.4.4 Maximum force of shear strength 48

3.4.5 Shear strength 49

3.4.6 Static coefficient of friction 50

3.4.7 The relative contribution of skin to

unpeeled mechanical properties 50

3.4.8 Application of investigated mechanical properties 53

3.5 Summary 55

4 Testing equipment for investigation of mechanical peeling methods

4.1 Introduction 56

4.2 Objectives of the design 57

4.2.1 Adaptability for investigation of different mechanical

peeling tools 57

4.2.2 Possibility of accommodation of different product size 57

4.2.3 Possibility of peeler head position adjustment 57

4.2.4 Possibility of peeler tool position adjustment 57

4.2.5 Possibility of rotation of peeler tool at

different angular velocities 58

4.2.6 Possibility of rotation of vegetable holder at

different angular velocities 58

4.2.7 Simplicity and low cost of manufacturing 58

4.3 Enforcement of the objectives 58

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4.3.1 Chassis and chamber 58

4.3.2 Vegetable holder 58

4.3.3 Peeler head 61

4.3.4 Attachments 63

4.4 Performance of the test rig 64

4.5 Summary 65

5 Preliminary trials of different mechanical peeling methods

5.1 Introduction 66

5.2 Trials of different tools 67

5.2.1 Wire brush 67

5.2.1.1 Rotary wire brush 67

5.2.1.2 Twisted wire brush 68

5.2.2 Ball chain tool 71

5.2.3 Milling cutter 72

5.2.4 Mower trimming lines 73

5.2.5 Abrasive ropes 74

5.2.6 Abrasive pads 74

5.2.7 Abrasive foams 75

5.2.8 Rope covered by spiral blade 75

5.2.9 Sandpaper belt 77

5.2.10 Abrasive plates 78

5.2.11 Abrasive-cutter brush 79

5.2.12 Abrasive bristle products 80

5.3 Conclusions 81

5.4 Summary 82

6 Experimental investigation of mechanical peeling methods

6.1 Introduction 83

6.2 The criteria of experiments 84

6.2.1 Peel losses 84

6.2.2 Peeling efficiency 84

6.2.3 Estimated responses 85

6.2.4 Data analysis 86

6.3 Peeling by using milling cutter 86


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6.3.2 Material of experiments 86

6.3.3 Results and discussion 88

6.3.4 Optimization and estimation of the responses 90

6.4 Peeling by using abrasive pads 92





6.5 Peeling by using abrasive foams 98



6.5.3 Results and discussions 100


6.6 Peeling by using abrasive-cutter brush 105





6.7 The comparison of the four innovative peeling methods 112

6.8 Potential industrial application of abrasive-cutter brush 113

6.9 Conclusions 113

6.10 Summary 115

7 Abrasive-cutter brush, full factorial experiments, and ANOVA

7.1 Introduction 116

7.2 Material of experiments 117

7.3 Peeling rate 119

7.4 Data analysis 119


7.5.1 The effect of p. speed on LnP. rate 122

7.5.1.1 The effect of p. speed on LnP.rate for

different levels of coarseness 123

7.5.1.2 The effect of p. speed on LnP.rate

in different locations of product 125

7.5.2 The effect of coarseness on LnP. rate 126

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7.5.2.1 The effect of coarseness on LnP.rate

at different p. speed 127

7.5.2.2 The effect of coarseness on LnP.rate

at different locations of pumpkin 128

7.5.3 The effect of location of products surface on LnP.rate 129

7.5.3.1 The effect of location on LnP.rate in

different coarseness of brush 130

7.5.3.2 The effect of location of products surface

on LnP.rate at different p. speed 131

7.6 Conclusions and discussion 132

7.7 Summary 134

8 Modelling of mechanical peeling as sum of consumed energy

in peeling process

8.1 Introduction 135

8.2 Theory of the model 136

8.2.1 The assumptions 136

8.2.2 Development of the model 136

8.2.3 Determination of the model coefficients 145

8.2.4 Model validation 145


8.3.1 Model coefficients 146

8.3.2 Model validation 149

8.3.3 Applicability of the model 150

8.4 Conclusions 151

8.5 Summary 151

9 Conclusions and perspectives

9.1 Thesis summary and conclusions 153

9.2 Directions for future research 155

Appendices 158

1.1 Multiple comparisons of the mean of the mechanical properties 158

1.2 Multiple Comparisons of contribution of skin to the

mechanical properties 171

1.3 Mechanical properties of varieties of melon and pumpkin

in three different states including skin, unpeeled, and flesh 176

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1.4 Relative contribution (%) of skin to different mechanical

properties for three pumpkin varieties including Jarrahdale,

Jap, and Butternut 177

1.5 Drawings of instrumentations 178

2.1 Test rig 184

3.1 Experimental results of using milling cutter 196

3.2 Experimental results of using abrasive pads 196

3.3 Experimental results of using abrasive foams 197

3.4 Experimental results of using abrasive-cutter brush 198

4.1 Normality assessment of peeling rate (g/min) of Jap and Jarrahdale varieties 199

4.2 Multi comparisons of the mean of LnP.rate among different

levels of independent variables 205

Bibliography 208

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List of Figures 1.1 The top view of pumpkin 3

2.1 Force-deformation curve 13

2.2 An industrial application of an abrasive roller peeler for tuberal

products such as potato 18

2.3 General feature of milling cutter in use 21

2.4 Enzymic peeled (right side) and manual (left side) peeled grapefruit 29

3.1 The instrumentations of testing mechanical properties of vegetables 40

3.2 Effects of force-deformation test (a-d) and relationship between

force (N) and deformation (mm) for melon and pumpkin in two cases

of skin and unpeeled (e-f) 45

3.3 Rupture force of skin and unpeeled states for different varieties

of pumpkin and melon 46

3.4 Toughness of skin and unpeeled states for different varieties of

pumpkin and melon 47

3.5 Cutting force of skin, flesh and unpeeled states for different

varieties of pumpkin and melon 48

3.6 The maximum shear strength force of skin, flesh and unpeeled

states for different varieties of pumpkin and melon 49

3.7 The shear strength of skin, flesh and unpeeled states for different

varieties of pumpkin and melon 50

3.8 The relative contribution (%) of skin to different mechanical

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properties of Pumpkin and Melon 52

4.1 Test rig 59

4.2 Product holder and two available positions 60

4.3 The two D.C. sources for vegetable holder and peeler head 60

4.4 Product holder 61

4.5 Details of the peeler head 62

4.6 Peeler head 62

4.7 Flap with holes in spiral pattern 63

4.8 The auxiliary peeler head as attachment 64

5.1 Rotary wire brush and its peeling effect on pumpkin 67

5.2 Twisted wire brush before and after loosening strands 68

5.3 Affected areas of pumpkin after using twisted wire brush 69

5.4 Improved design of artificial twisted brush in second stage

and its effects 70

5.5 The twisted wire brush in third stage and its peeling effect 71

5.6 Ball chain and its peeling effect after application 71

5.7 Different investigated lathe tools (a) and cutter in cylindrical

shape with triangular side section (b) 72

5.8 The effect of peeling by wedgy side cutter 73

5.9 Abrasive rope and its peeling effect on pumpkin 74

5.10 Different shapes of abrasive foam and their peeling effect

on pumpkin 76

5.11 Rope covered by spiral blade (a), and its peeling effect on pumpkin (b) 77

5.12 Sandpaper belt installed on test rig with and without pumpkin 77

5.13 Grater plate peeling unit and its effect on pumpkin 78

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5.14 Abrasive cutter brush and its effects on two stages 80

5.15 Abrasive bristle products 81

6.1 Disk shape milling cutter with triangular contour 88

6.2 The contribution of independent variables to responses resulted

from using milling cutter 89

6.3 The effects of independent variables on responses resulted from

using milling cutter 91

6.4 Abrasive peeler pads and accessories 93


from using abrasive-pads 95


using abrasive pads 96

6.7 Abrasive foam and accessories 99


from using abrasive foams 102


using abrasive foams 103

6.10 Abrasive-cutter brush 107


from using abrasive-cutter brush 109

6.12 The effects of independent variables on responses resulted

from using abrasive-cutter brush 110

7.1 The different type of stripes of coarseness used for fabrication

of abrasive-cutter brush 118

7.2 Different parts of product as levels of location variable 118

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7.3 The effect of mean p. speed on LnP.rate 124

7.4 The effect of p. speed on LnP.rate at different levels of coarseness 124

7.5 The effect of p. speed on LnP.rate at different in different

locations of pumpkin 125

7.6 The effect of mean coarseness on LnP.rate 127

7.7 The effect of coarseness on LnP.rate at different speed of

abrasive-cutter brush 128

7.8 The effect of coarseness on LnP.rate at different location

of product 129

7.9 The effect of mean location on LnP.rate 130

7.10 The effect of location on LnP.rate in different coarseness 131

7.11 The effect of location on LnP.rate at different p. speeds 132

8.1 The view of abrasive-cutter brush after penetration to the peel 137

8.2 The cross-sectional view of one protrusion

(two out of four teeth are shown) 138

8.3 Experimental versus predicted values of p. rate (gr/min) 149

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List of Tables 3.1 Static coefficient of friction of three varieties of pumpkin in the flesh,

unpeeled, and without periderm state on three different materials

including stainless steel, Teflon, and wood....51

6.1 Taguchi experimental design for independent variables and levels....88

6.2 Taguchi experimental design for independent variables and levels94

6.3 Taguchi experimental design for independent variables and levels...101

6.4 Taguchi experimental design for independent variables and levels...108

7.1 The results of frequencies analysis on peeling rate120

7.2 The results of frequencies analysis on LnP.rate.121

7.3 The results of Levenes test for homogeneity of variance of LnP.rate..121

7.4 ANOVA of the mean of LnP. rate among different levels of

independent variables 122

8.1 The results of multiple regression analysis for coefficients of two

mechanical peeling models........147

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List of Symbols and Abbreviations Pt total expenditure power, N. mm/min P1 expenditure power at fracture part of cutting, N. mm/min P2 expenditure power at forming part of cutting, N. mm/min c total peeling efficiency n the number of installed brushes on peeler head p the angular velocity of abrasive-cutter brush, rpm Ep penetration energy of abrasive-cutter brush, N. mm Ed the deflection energy of abrasive-cutter brush, N. mm K1 average shearing resistance per unit length of stroke, N/mm Vip linear penetration velocity of brushs teeth inside peel, mm/s t1 time of stroke, s 1 deflection of product, mm 2 the depth of average penetration, mm the ratio of toughness of product (Tp) to toughness of tool (Tt) Tp the toughness of product, N. mm Tt the toughness of abrasive-cutter brush, N. mm the density of protrusions on a brush, number/mm2 l1 the effective length (covered by abrasive strip) of brush, mm d1 the diameter of brush, mm the shear strength of product, N/mm2 d2 the diameter of protrusions hole, mm l2 the length of each tooth on protrusion, mm 1 the angle of teeth in protrusion, degree E the modulus of elasticity of the brush, N.mm-2 I the geometrical moment of inertia of the brush, mm4 3 the average deflection of the brush at fracture stage, mm L the whole length of brush, mm 3max the maximum deflection of brush in fracture stage, mm Vop the linear velocity of brushs teeth in scratching stage, mm/s Fc total cutting force, N Ff friction force, N Fd disintegration force on the structure of product, N Fe the spent force for elastic and plastic deformation, N Ef the expended friction energy, N. mm h the length of removed peel, mm K2 the friction coefficient the density of protrusion, number/mm2 the degree of unevenness of products surface d the dynamic coefficient of friction between the brushs tooth

and product Rv the total normal reaction, N Fde deflection force of brush, N N the normal reaction force to the weight of brush, N W1 the weight of one brush, g

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2 the angle between direction of the weight and direction of the line passes through the gravity centre of brush and is perpendicular to the surface of product in contact point, degree

4 the average deflection of brush in second stage of cutting, mm l3 the total projected lengths of protrusions teeth engaged in

cutting, mm K3 the coefficient of elastic and plastic force E2 the total required energy of peeling in second stage, N. mm K4 the coefficient of disintegration force K5 scratching coefficient in second stage, number/min v angular velocity of vegetable holder, rpm the number of scratches, number/min P. rate peeling rate, g/min LnP.rate the logarithmic transform of P. rate, g/min K6 transform coefficient of Pt to p. losses, g/N. mm v. speed the angular velocity of vegetable holder, rpm p. speed the angular velocity of peeler head, rpm peeling losses the substantial amount of usable vegetable flesh that is being

discarded because of peeling, % of weight of whole produce before peeling

peel losses the ratio of the weight of removed peel to the weight of whole produce before peeling divided by time of peeling, %/min

peeling efficiency the percentage peel that is removed from the initial skin per unit time, %/min

peeling rate the weight of removed peel divided by peeling time, g/min

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Authorship The work contained in this thesis has not been previously submitted for a degree or diploma at this or any other educational institute. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made. Signature: Date: ....

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Acknowledgement The foremost gratitude to my God, Allah, who offered me this opportunity to learn

and progress. I appreciate Him for delighting me with kind parents who supported and

encouraged me. I am thankful to Him for blessing me with my wife, Masoumeh, who

showed patience and support during this research.

I would like to express my appreciation to my principal supervisor, Dr. Vladis Kosse,

for his time and valuable suggestions in supervising this thesis. I also thank my

associate supervisor, Prof. Prasad KDV Yarlagadda, for his support and suggestions.

The author wishes to thank Dr. Mahalinga Iyer, Dr. Prasad Gudimetla, and Dr. Kunle

Oloyede, School of Engineering Systems, QUT, for reading thesis and their valuable

comments.

I wish to express my gratitude to the government of Islamic Republic of Iran for

financial support through a PhD scholarship award.

I also wish to thank all former and present members of School of Engineering

Systems. Special thanks to Mr. Terry Beach, Mr. Mark Hayne, and Mr. Abdul Sharif

for their technical support.

Finally, I would like to thank my fellow students and colleagues for their company

and interesting discussions over the past three years.

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Chapter 1

Introduction

1.1 Significance and motivation of the research

The food and beverage sector was the largest sector of Australias manufacturing

industry in 2002-03, providing about 20% of total sales and services income. The

total income from sales and services for the Australian food processing industry was

estimated at $65.9 billion in the same year. The industry value added by the food

processing industry was recorded as $16.6 billion in 2002-03 (Department of

Agriculture, Fisheries and Forestry, NSW).

Peeling is an important preliminary stage of fruit and vegetable processing. The

quality and the final price of the processed product is highly dependant on this stage.

Manual peeling is possible for any kind of product but high losses and considerable

consumption of time and labour have encouraged the peeling industry to use other

methods. Mechanical, thermal, and chemical peelings are conventional methods

(Luh, and Woodroof., 1988), each of which has its own benefits and limitations.

Those methods apply mechanical tools, heat or cold, and lye respectively to peel off

the fruit and vegetable skins. As none of the current methods can satisfy all

requirements of producers and consumers, other kinds of peeling methods such as

enzymatic peeling have been developed. Changing the peeling technique has

changed the kind of merits and demerits but the problem is still unsolved. Getting

closer to the ideal peeling method is the aim of every researcher in this field. The

ideal peeling method possesses the following features (Radhakrishnaiah settee et

al., 1993):

-minimizes product losses

-peels to the extent dictated by the products

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-minimizes energy and chemical usage

-minimizes the pollution loads

-minimizes heat ring formation

Among the current peeling methods, mechanical methods can attract the satisfaction

of consumers as these methods possess some important benefits of the ideal

method such as the freshness of the peeled product. As the view of consumers is

important to the food processing industry researchers are encouraged to continue

the search for mechanical peeling methods that are closer to the ideal peeling, in

spite of the high losses so far experienced with these methods.

Approaching the ideal peeling method through trial and error is difficult and time

consuming. The design of new methods and improvement of current peeling

methods to achieve the ideal peeling conditions using physical and mechanical

properties of the product is one of the objectives of researchers (Ohwovoriole et al.,

1988). Depending on the proposed technique, different properties of products have

been applied to improve the efficiency of purposed peeling methods or devices.

However, despite all attempts, some fruits and vegetables, such as mangoes, are

commonly peeled manually (Radhakrishnaiah settee et al., 1993) and methods for

others such as pumpkin are far from the ideal peeling conditions.

Tough-skinned vegetables such as pumpkin and melon currently are peeled either

semi-automatically (i.e. Comet Food Ltd.) or automatically (i.e. Dornow Ltd.).

Comet Food Ltd. is a Brisbane-based organization visited by this researcher which

utilises semi-automatic peeling methods of pumpkin and melon. Circular shapes of

rotating graters are applied in the semi-automatic method. Segments of the product

are brought into contact with the grater by an operator. This process is tedious and

time consuming. In the latter method, whole pumpkins are passed through

automatic machines where the floor is covered by many rotator disks (carborundum

or blade). The main limitation of both methods especially for varieties with an

uneven surface is high peeling losses. For pumpkins as a case study, the minimum

peeling losses in an optimistic situation is about 35% (Department of State and

Regional Development, NSW). As Figure1.1.a shows, penetration to the inside of

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concave areas to peel off through grooves is accompanied by high removal of flesh

from convex areas in the current peeling methods.

a. Current peeling methods

b. The ideal peeling method

Fig.1.1.The top view of pumpkin

(a) Current peeling methods (b) ideal peeling method

The investigation of concepts for new mechanical peeling methods and

development of design recommendations for peeling of tough-skinned vegetables is

a challenging task in this research. The main objective is to achieve even peeling

Concave area Convex area Layer to be

removed

Layer to be removed

Concave area Convex area

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efficiency from different areas of uneven surfaced products with minimum peeling

losses (Figure1.1.b).

1.2 Objectives of the research

This research develops new mechanical methods and tools for peeling of tough-

skinned vegetables and recommends important design parameters for a peeler on

the basis of the mechanical properties of those products. The main objectives of the

research are as follows:

Measurement of the mechanical properties of tough-skinned vegetables, with pumpkin and melon as the case studies (three varieties each) (addressed

in Chapter 3).

Development of new innovative mechanical peeling methods and adaptation of existing methods for tough-skinned vegetables (addressed in Chapter 5).

Selection of the best peeling method close to the ideal peeling method (addressed in Chapter 6).

Simulation and mathematical modelling of the mechanical peeling process (addressed in Chapter 8).

Development of recommendations for design parameters of a mechanical peeler for tough-skinned vegetables (addressed in Chapters 3 and 8).

1.3 Originality and major contribution of the thesis

The originality and major contributions of the thesis are summarized in the

following sections.

1.3.1 Definition of tough-skinned vegetables

Despite common use of this term, the review of literature shows that there is no

clear definition for the term tough-skinned vegetable. As a result of this thesis, for

the first time, the tough-skinned vegetables could be scientifically defined. This

definition will be the first effort for classification of fruits and vegetables on the

basis of their mechanical properties.

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1.3.2 Determination of mechanical properties of tough-skinned

vegetables

For the first time, some mechanical properties of pumpkin and melon - as two kinds

of tough-skinned vegetables - will be determined. The three investigated varieties of

pumpkin are Jarrahdale, Jap and Butternut and the three investigated varieties of

melon are Rockmelon, Watermelon, and Honeydew. Rupture force, cutting force,

shear strength force, and shear strength will be measured. All properties will be

investigated in three states - skin, flesh, and unpeeled product - except rupture force

and toughness that will be studied in two states - skin and unpeeled product. The

static coefficient of friction also will be measured for three states of product -

unpeeled, without periderm and flesh. This property will be determined for three

different materials - stainless steel, Teflon and wood.

1.3.3 Developing new mechanical peeling methods for tough-

skinned vegetables

Inflexibility of current mechanical peeling methods that causing uneven peeling and

high peeling losses is one of the main current problems for the peeling industry

especially for tough-skinned vegetables. This thesis develops new mechanical

peeling devices suitable for tough-skinned vegetables and close to the conditions of

the ideal peeling method. The highlighted feature of these designed devices is that

they do not share the limitations of current peeling tools.

1.3.4 Developing current peeling methods

Peeling by the use of a milling cutter has been successfully introduced by some

researchers (Cailliot and Serge, 1988; Gardiner et al., 1963; Boyce, Jose and Calif,

1961). Clogging as the main limitation prevented the industrial application of the

milling cutter as a peeling device. A new shape of milling cutter will be developed

by this study which can be used without the limitation of previous works. The

criteria of this research (evenness of peeling in different areas of product and higher

peeling efficiency) also will be considered in the developed method.

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1.3.5 Introducing the best mechanical peeling method applicable in

the peeling industry for tough-skinned vegetables

Developing and introducing an innovative mechanical peeling method for tough-

skinned vegetables is the main contribution of this study. The development of

method leads to the provision of necessary and industrially applicable knowledge

for the design and manufacture of the proposed peeler machine. The design

knowledge includes information about the peeling tool and product.

1.3.6 Mathematical modeling of mechanical peeling

No mathematical model for mechanical peeling has been known so far. For the first

time a mathematical model of mechanical peeling will be developed in this thesis.

The model simulates the cutting process by abrasive-cutter brush (developed in this

research). The applicability of the model for the peeling industry will be

emphasized. The variables of input and output of the model will be chosen from

important effective parameters related to the product and peeling tool. Those

variables should be easily measurable and empirically adjustable.

1.3.7 Determination of design parameters of tough-skinned vegetable peeler

Some necessary design parameters of tough-skinned vegetable peelers are

recommended by the obtained results of this study. These recommendations can be

used by the peeling industry to improve the performance of peeler equipment. The

research attempts to explain the role of these determined parameters and those

which require determination in the mechanical peeling processing.

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1.4 Thesis organization

The thesis consists of three main parts: investigation of mechanical properties of

tough-skinned vegetables, development of peeling methods and mathematical

modeling of the results. The content is organized in nine chapters as follows:

Chapter 1 is the introductory chapter that explains the motivation of the research,

and outlines the main objectives and major contributions of the thesis.

Chapter 2 reviews critically the existing peeling methods and background of the

research. This chapter also reviews work done on properties (especially mechanical)

of fruits and vegetables and the developed models in the area of fruit and vegetable

peeling.

Chapter 3 focuses on the mechanical properties of tough-skinned vegetables

including pumpkin and melon (three varieties each). It explains developed and

fabricated instrumentations, measuring methods, results and statistical comparisons

of the results. The mechanical properties, including rupture force, toughness, cutting

force, shear strength force, shear strength, and static coefficient of friction are

investigated in different specimen states.

Chapter 4 describes the test rig. Design and fabrication procedure are described,

specifications and advantages of the test rig that was used to investigate different

peeling methods and devices are also discussed.

Chapter 5 outlines all the attempts undertaken to explore suitable peeling methods

and devices. Although the devices might look simple, they were preliminary

prototypes that were made with regard to the specifications of the products and the

main defined objectives of the research.

Chapter 6 presents the experiments carried out for investigation of four different

peeling methods and devices for the Jap variety of pumpkin. The four different

investigated methods were abrasive pads, abrasive foams, milling cutter, and

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abrasive-cutter brush. Results and analysis of the results are comprehensively

explained in this chapter.

Chapter 7 reports the results and analysis of full factorial experiments conducted on

two varieties of pumpkin (Jap and Jarrahdale) by abrasive-cutter brush as the best

selected method. The influencing parameters that are related to the product and

abrasive-cutter brush have been comprehensively studied.

Chapter 8 describes mathematical modelling of mechanical peeling. The procedure

of mechanical peeling method using abrasive-cutter brush is simulated and the

influence of different parameters related to the product and peeler are modeled. The

value of the coefficients of the model is determined for Jap and Jarrahdale varieties

of pumpkin. The validation of the model is carried out using experimental data and

the results are discussed using the results of the study of mechanical properties of

tough-skinned vegetables.

Chapter 9 summarizes and concludes the thesis. It also proposes directions for

future studies related to this area.

1.5 Publications (refereed) of the author arising from the

PhD research

1) Emadi, B., Kosse, V., Yarlagadda, P. K. D. V., Mechanical properties of

pumpkin, International Journal of Food Properties, 8 (2), 277-287.

2) Emadi, B., Kosse, V., Yarlagadda, P. K. D. V., Design and manufacturing of test

rig for investigation of improved mechanical peeling methods of fruits and

vegetables, International conference on manufacturing and management: GCMM-

2004, pp.574-579, Vellore, India, Dec. 2004.

3) Emadi, B., Kosse, V., Yarlagadda, P. K. D. V., Mechanical properties of three

varieties of melon, Journal of Texture Studies, Submitted, 20/11/2005.

9

4) Emadi, B., Kosse, V., Yarlagadda, P. K. D. V., Experimental investigation of

abrasive peeling of pumpkin, 4th International congress on food technology, pp.118-

117, Athens, Greece,Feb.2005.

5) Emadi, B., Kosse, V., Yarlagadda, P. K. D. V., Relationship between mechanical

properties of pumpkin and skin thickness, 4th International congress on food

technology, pp.111-123, Athens, Greece, Feb. 2005.

6) Emadi, B., Kosse, V., Yarlagadda, P. K. D. V., Abrasive peeling of pumpkin,

part 1: using abrasive pads. Journal of Food Engineering, Submitted, 23/02/2005.

7) Emadi, B., Kosse, V., Yarlagadda, P. K. D. V., Abrasive peeling of pumpkin,

part 2: using abrasive foams. Journal of Food Engineering, Submitted, 22/02/2005.

8) Emadi, B., Kosse, V., Yarlagadda, P. K. D. V., Using abrasive disk as innovative

peeling method for vegetables with uneven surfaces, BEE Postgraduate Research

Conference, Brisbane, Australia, Dec. 2005.

10

Chapter 2

Literature review

Peeling of fruits and vegetables in theory and practice has been studied by many

researchers. These studies are very broad and cover various methods of peeling and

products to be peeled. Some products (such as potato) and methods of peeling (such

as chemical methods) are particular matters of interest and have attracted more

attention. The study of those products and methods has been extended to investigate

the physico-chemical behaviour of the product during the peeling process. There is

very little literature pertaining to the peeling of tough-skinned vegetables while

there has been little effort to quantify the tissue properties or any process modelling

of products such as pumpkin and melon.

From a mechanical peeling standpoint, there is evidence that more research is

needed to develop modelling approaches for many peeling operations, for example

the peeling of tough-skinned vegetables such as pumpkin.

This chapter sets out to identify and critically analyse all the previously published

literature with regard to the mechanical properties of fruits and vegetables, peeling

methods, and modelling of the peeling process.

11

2.1 Mechanical properties of fruits and vegetables

and methods of testing

2.1.1 Introduction

The study of the physical and mechanical properties of fruits and vegetables is very

important. It can improve the efficiency of processing equipment, especially peelers.

The agricultural products are subjected to either wanted or unwanted mechanical

loads after harvesting. During the process of mechanical peeling, the products are

loaded purposefully with wanted mechanical loads that are always accompanied by

unwanted loads. The unwanted mechanical loading (compression, impact, and

vibration) is the main reason for bruising of fruits and vegetables during post

harvesting operations (Brusewitz et al., 1991). Reducing the harmful effects of

unwanted loads and improving the effectiveness of wanted loads can be achieved by

knowledge of the mechanical properties (i.e. toughness, cutting force and shear

strength) of the products. Mechanical properties of products can also be used in

design of their mechanical peeler. Researchers have used various techniques to

investigate the mechanical properties of different produce. The following sections

address these issues.

2.1.2 States of product to be tested

The properties of products can be studied for different states including skin, flesh

and unpeeled (overall) state. Among different states of product to be tested,

determination of the mechanical properties of skin always poses a challenge. It can

be done by carrying out experiments on skin directly or indirectly. Several

researchers such as Grotte et al. (2001), Jackman and Stanley (1994), and Voisey et

al. (1970) have used the difference between unpeeled (overall) product and that of

product measured without skin (flesh) to indirectly obtain the result of the force-

deformation test of the skin. This experimental procedure has not been accepted by

others due to the likelihood of some errors in the result. For example, Thompson et

al. (1992) stated that to identify the contribution of the skin to the external puncture

force (in the case of cucumbers) by making measurements before and after skin

12

removal is accompanied by error. In addition, Jackman and Stanley (1992)

concluded that the difference of puncture load displacement between unpeeled

(overall) and without skin (flesh) product cannot provide the load displacement of

the skin itself. Jackman and Stanleys point was proved by an increase in effective

area of compression during puncture of the skin.

2.1.3 Compression test

The compression (force-deformation) test is the basic and one of the most important

tests in the study of the mechanical properties of fruits and vegetables. The force-

deformation test shows the behaviour of the product under different levels of

compression forces. Some mechanical properties of different states of products

including skin, flesh, and unpeeled states can be determined by using this test

(Mohsenin, 1970). The test can be used for determination of an extended range of

mechanical properties such as modulus of elasticity, rupture force, rupture stress,

toughness, and firmness (Jackman and Stanley, 1994; Voisey and Lyall, 1965a;

Voisey and Lyall, 1965b; Grotte et al., 2001; Holt 1970; Voisey et al., 1970;

Behnasawy et al., 2004; Rybczynski and Dobrzanski, 1994). The same purposes can

be achieved by using the tensile test (Jackman and Stanley, 1994) but, in

comparison with the compression test, implementation of the tensile test is difficult

because of some limitations, such as difficulty in holding the skin specimens during

the test (Su and Humphries, 1972) and creation of premature tensile failure during

specimen preparation (Clevenger and Hamann, 1968; Thompson et al., 1992).

Two important properties resulting from the force-deformation test are rupture point

and toughness (Figure 2.1). Rupture point is a point on the force-deformation curve

at which the axially loaded specimen ruptures under a load (Mohsenin, 1970). The

work required to cause rupture in the product is known as the toughness (Mohsenin,

1970). Finney (1969) also defined the toughness as the area under the force-

deformation curve, recorded through the point of tissue rupture or failure (Figure

2.1). He explained the intercellular adhesion or cementing substances and cell wall

strength are factors quite likely to influence toughness of fruits and vegetables.

13

2.1.4 Cutting test

The cutting test is not as common as the compression test and is used to determine

the resistance of tissue to loading cutting force. Ohwovoriole et al. (1988) applied

this test to identify the necessary cutting force of unpeeled and peeled cassava tuber.

They used the resulting data to design a cassava peeler. A cutting edge in the form

of a sharpened 1.5 mm thick piece of sheet metal was placed between the plungers

of the Universal Testing Machine for that purpose.

Fig.2.1. Force-deformation curve (Finney, 1969)

2.1.5 Shear strength test

Shearing strength of the product is determined by shearing a plug from a slice of the

product. The shearing strength of the product indicates the degree to which the cells

are held together. Knowing shearing force (F), the diameter of the solid cylindrical

die with flat end (d), and the thickness of the slice (t); shearing strength (S) can be

determined (Mohsenin, 1961):

tdFS = (2.1)

Ohwovoriole et al. (1988) reported using the shear strength test to measure shear

stress of peeled and unpeeled cassava tuber. An 8.7 mm diameter rod was used on

hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library

14

an Instron machine to reveal the necessary data for cutting cassava tubers through

the peel. Their report does not mention the shape of the applied indentor.

2.1.6 Coefficient friction test

The coefficient friction test is applied to identify the coefficient of friction for

different states of product on various surface materials. The effective parameters on

these properties are the moisture content of the specimen and the kind of surface

material. Chung and Verma (1989) concluded that the surface material is more

effective on the dynamic than on the static coefficients of friction while Carman

(1996) reported a higher influence of moisture compared with surface material on

the static coefficient of friction. Chung and Verma (1989) concluded the ratio of

static and dynamic coefficient of friction remained almost invariant irrespective of

the moisture content of the sample and the type of surface material.

The coefficient of friction, either static or dynamic, has been measured using

different methods. Those coefficients can be measured by analog or digital systems.

Data obtained by a digital system are more accurate than those taken by an earlier

analog system (Chung and Verma, 1989).

In the analog system, specimens are placed on a table which is manually and slowly

tilted until movement of the specimen and the static coefficient of friction would be

the tangent of the slope angle of the table measured with a protractor. This method

has been used by some researchers (Oje and Ugbor, 1991; Bahnasawy et al., 2004;

Helmy, 1995; Saif and Bahnasaway, 2002; Ohwovoriole, 1988). The main benefit

of using analog systems is simplicity of the method.

The digital measuring system works based on a friction device (disk) modified by

Tsang-Mui-Chung et al. (1984) and improved by Chung and Verma (1989). The

latter authors used a personal computer for data acquisition. They applied the

following equations to calculate the static and dynamic coefficient of frictions:

qWT fas ./= (2-2) qWT fmd ./= (2-3)

15

where, s is static coefficient of friction, Ta is beginning value of torque, d is the

dynamic coefficient of friction, Tm is the average value of the torque, q is the length

of torque arm, and Wf is the weight of fruits to calculate the dynamic and static

coefficients of friction. The average value of the torque during the rotation of the

disk and the maximum value of torque obtained as the disc started to rotate were

used.

This method has been used by other researchers such as Marakoglu et al. (2005),

al r et al. (2005) and Gupta and Das (1998). They applied this method to

measure the friction coefficients of fresh blackthorn fruits, wild plum fruits, and

sunflower seed and kernel respectively.

2.2 Peeling methods of fruits and vegetables

2.2.1 Introduction

For some kinds of fruits and vegetables, such as mango, manual peeling is

commonly in use. The requirement to develop new methods and tools for peeling

that can be mechanised and automated has led to the versatile current peeling

methods, machinery and equipments. Peeling methods fall into three main groups:

mechanical, thermal and chemical peeling. Much research has been published

related to different methods of peeling and the range of publication is considerably

extensive regarding the variety of products and peeling methods. A literature review,

arranged on the basis of the technique used, along with examples of the latest works

of interest is given here.

2.2.2 Mechanical peeling

There is a variety of mechanical peelers designed to suit the peeling of either a

particular product or a group of products. In general, mechanical peelers are

classified on the basis of the type of mechanism that is incorporated into the peeling

system. Commercial mechanical peelers include abrasive devices, devices with

16

drums, rollers, knifes or blades, and milling cutters. Generally, the quantity of

losses in this kind of peeling is high, but the quality of the final peeled vegetables in

terms of features such as freshness is good. Those devices are briefly described

using related works of interest as follows:

2.2.2.1 Abrasive devices

The abrasive method can be implemented in a very simple way by using gloves

with an abrasive outside layer. Vegetables are rubbed by these gloves to remove the

skin. That is a common method for the peeling of potatoes in small amounts.

Somsen et al. (2004) have proved that manually peeling of potatoes using sandpaper

results in the lowest possible peel losses. They proved that these losses were

normally expected losses (wanted losses). Although some researchers realised that

the abrasive method has a lower quality compared to hand peeling (Barry-Ryan,

2000), it is still used commonly for root vegetables. These researchers criticized this

method as it bruises the underlying tissue to varying degrees and leaves the new

outer layer of cells damaged which leads to leakage of cellular fluids and

encourages microbial growth.

Jasper et al. (2001) patented a peeler equipped with a rough exterior surface. The

peeler abrades the outside surface of fruits and vegetables when it comes into

contact with the outside surface of the product. The surface roughness of the peeler

can be adjusted depending on the skin of the vegetable to be peeled. One of the

clear disadvantages of this device is that it can only be used in a domestic kitchen

environment.

Agrawal et al. (1982) discuss the development of an abrasive brush type ginger

peeling machine. Two continuous and abrasive vertical brush belts are the main

parts of the machine. Ginger, as a product with an irregular shape, passes between

these two belts while it moves in the opposite direction with a downward relative

velocity. The opposite direction of movement of the two belts causes an abrasive

action while the downward relative velocities provide the downward movement of

the product. Agrawal et al. have reported a peeling efficiency of about 75-85%.

17

2.2.2.2 Devices using drums

Singh (1995) describes a power-operated batch type potato peeler that includes a

peeling drum (670 mm in length and 450 mm in diameter) and a water-spraying unit.

The peeling drum with protrusions (2.5-3.0 mm) on the inside surface rotates and

removes skin from potatoes by means of abrasion. The space between two centres

of protrusions in rows and columns were 14 and 7 mm, respectively. A speed of 30

rev/min batch, load of 20 kg and time of 8 min were found to be the best

combination providing higher peeling efficiency and lower peel losses. As a result

of this combination, the peeling efficiency and peel losses were 78% and 6%,

respectively. A large amount of wastewater is a shortcoming of this method.

2.2.2.3 Devices using rollers

Suter (2002) developed a peeling machine for efficient and effective control of the

peeling operation. It applies a set of abrasive rollers (Figure 2.2). The rollers come

together in a longitudinal direction and the distance between them is adjustable. The

feeder feeds the rollers controllably on the basis of the sensed load inside the rollers

by a related sensor. Also, Zittel (1991) used a plurality of rotating abrasive rollers.

The machine had a frame with a pair of end plates that rotatively carried the

longitudinal rollers. Each roller is powered by an individual motor, which is

coupled to that roller. However, those inventors neither achieved higher efficiency

nor could reduce peeling losses because their patents focused mainly on control and

drive mechanisms.

2.2.2.4 Knives or blades

Tardif and He (1999) released a machine equipped with blades to peel vegetables.

The vegetable, which is located in the hollow base of the machine, can be rotated by

a threaded rod on the top. The rod is rotated manually by a handle. A blade, which

is coupled to the supporting rod and urged by a spring, moves towards the vegetable

to be peeled. While the vegetable is rotated, the blade removes the peel.

18

Harding (2001) described an apparatus for peeling a convex surface of a section of a

fruit or vegetable. The machine is attached to a U-shaped peeling blade and a feeder.

The feeder grips the fruit or vegetable at a position about opposite the apex of the

peeling blade. Fruits or vegetables have to pass in front of a peeling blade being

guided by at least one guide. He also patented a melon peeler in 2000. The semi-

manual peeler includes a curved peeling blade which presents a curved surface to

the convex surface of the section of produce. It facilitates peeling of curved sections

of products but can not follow the unevennesses of surface.

Fig.2.2. An industrial application of an abrasive roller peeler for tuberal

products such as potato

(Dornow Food Technology GmbH, 2004)

Gingras (2001) presented equipment for the peeling of vegetables of a round, oval

or elongated shape such as cucumbers. The machine is equipped with a frame

including an adjustable hole to receive and let pass the vegetable to be peeled. The

frame also carries several knives that can be slid toward the corner of a hole.

Ridler (2000) described a peeling apparatus including a traversed blade, which

continuously and intermittently rotates in the opposite direction to a rotating

vegetable. The apparatus is controlled and powered manually. The user rotates the


19

vegetable that is mounted on a slender arbour by one hand and controls the peeling

blade by another hand at the same time.

Martin (2000) reported a peeling machine equipped with a lower and upper holding

assembly connected to a frame securing and rotating the vegetable to be peeled. A

carriage assembly including a cutting assembly is engaged with the end of a second

air cylinder. The extension of the second air cylinder pushes the cutting assembly

against the vegetable as the carriage assembly moves upwards; as a result peeling is

done.

Protte (1999) described a peeling machine for stalk-like vegetables, including a

plurality of knife stations that are successively arranged along the vegetables

moving inside the machine. There are pluralities of pairs of feed rollers, and every

pair is supported between successive knife stations.

Sommer (1997) discussed a device for the peeling of elongated vegetables

especially asparagus. The device comprises housing equipped with a passage

designed to permit a stick of asparagus to be inserted. There are several peeling

blades inside the housing, which are orientated in various directions of the passage

and act on the stick of asparagus. One blade as a minimum can move crosswise to

the elongated direction of the passage and presses flexibly towards the stick of

asparagus.

Rauschning (2001) expounded a device for the peeling of root vegetables. The

device includes a container equipped with at least two rotating discs at its bottom.

These discs have a grating or cutting surface on their upper side.

Odigboh (1976) revealed the development, design and construction of a continuous

process mechanical cassava peeler. The machine includes two cylinders which are

located parallel with 20 mm space and inclined at 15 to the horizontal plane. The

surface of the driver cylinder is covered by knives and rotates clockwise at 200

rev/min. The driven cylinder which has a roughened surface, also rotates clockwise

at 88 rev/min. The cassava pieces with 100 mm length are fed lengthwise to the

spaces between cylinders. Products are being peeled off while they rotate anti-

20

clockwise and move down. Low efficiency of peeling especially for small sizes of

roots (40 mm and less) and inability to be set up for roots of specific sizes are

reported as disadvantages. Although the continuous option of the apparatus is

considered to be one advantage, the necessity to cut roots to pieces of about 100

mm length is a limitation.

Srivastava et al. (1997) reported the design and development of an onion-peeling

machine which uses four scoring blades assisted by compressed air jets to slit the

outer layers of the onion skin. The skin will be loosened and dislodged from the

bulb by the compressed air under the peel. The compressed air penetrates under the

skin through the created slits by scoring blades. A pair of high-speed saw blades is

used to cut the ends of the onion. Srivastava et al. (1997) reported peeling losses as

17%.

2.2.2.5 Milling cutter

Cailliot and Serge (1988) claimed that peeling using the milling cutter is one of the

known methods for spherical shape products (Figure 2.3). In this method, one or

more fixed or rotary peeling tools (i.e. knife) with at least one cutting edge take the

peel off the product in a similar way to manual peeling. In the first stage of this

method, a fixed knife or blade was used to peel a rotary spherical product. As the

knife had no flexibility, it could not follow the irregular shape of the product

exactly and in particular it could not penetrate to the inside of thin grooves. The

history of using a milling cutter goes back to Boyce et al. (1961) who used a milling

cutter in the form of a very flat milling cutter, having a large number of cutting

teeth distributed over a considerably large diameter, in order to produce small chips

of peel, the discharge of which is left to chance. The big diameter of the cutter and

the shape of the teeth (like spoons) were two reasons that did not allow the cutter to

properly follow the shape of the product.

To remedy the production of a continuous peel and resulting clogging, Gardiner et

al. (1963) tested a milling cutter with a cylindrical cutting edge, combined with a

disc which supports the cylindrical cutting edge and which was provided with

apertures sharpened in the plane of the disc, so as to cut the ribbon of peel

21

transversely into smaller portions making it easier to discharge it. Although the

problem of clogging was solved, the peeling production was not sufficient. Similar

limitations were experienced by Polk (1972) who used a large rotary milled edge and rotary vegetable holder.

Couture and Allard (1979) invented a cutting head comprising a blade strip bent

longitudinally into a generally cylindrical shape. The peeler head was pivotally

connected to the body to follow the irregular shape of the product. The machine

included means for moving the cutting head along the supported vegetable in

contact therewith as the vegetable is rotated with the cutting edge and a continuous

strip of peel is cut around the vegetable.

Fig.2.3. General feature of milling cutter in use

(Boyce, San Jose and Calif, 1961)

As the cutting head had no rotation itself and had to follow the shape of the rotating

vegetable, the chance of it getting stuck especially for sharp irregular shapes was

high. Cailliot and Serge (1988) noted the disadvantages of this appliance such as


22

producing a continuos peel and clogging the peeling tool in an automatic appliance.

Cailliot and Serge (1988) claimed one vertical cutter type in their patent. The

diameter of the rotary cutter is small and equipped with two teeth to give balance.

20000 rpm for the rotary cutter gives a speed that is equal to tangential speed of at

least 20 m/s and needs low torque. The depth of penetration is limited by the choice

of a small diameter cutter for example 20 mm. The extra length of each tooth from

outside the diameter of the cutter defines the depth of penetration in this plan.

Cailliot et al. claimed to solve the common problem (clogging) but it seems the low

number of teeth and the shape of the teeth that come out from the surface of the

plate cause clogging for irregular shapes of product such as pumpkin with irregular,

thin and deep grooves. Tardif and He (1999) in another trial used a similar method

to Couture and Allard (1979) with a simple knife (non-rotating). It was found that

low flexibility will definitely lead to clogging as well. All attempts which have been

carried out so far were unsuccessful in solving the clogging problem especially for

products with an uneven surface. It is believed that the high number of teeth in

special shapes without any convex section may reduce the chance of clogging to

zero.

2.2.3 Thermal peeling

Thermal peeling as well as chemical peeling is used for thick-skinned vegetables.

This method can be performed by wet heat (steam) or dry heat (flame, infra red, hot

gases). Floros and Chinnan (1988a) reported that the widespread application of

steam peeling is due to its high level of automation, precise control of time,

temperature and pressure by electronic devices to minimize peeling losses, and due

to the reduced environmental pollution as compared to chemical peeling. This

method of peeling - especially dry heat - causes a cauterizing of the surface, wound

areas, and small pieces of charred skin, which if not removed, give a poor

appearance to vegetables, especially canned ones (Weaver et al., 1980). Different

types of thermal peeling are described below with reference to related works of

interest.

23

2.2.3.1 Flame (dry heat) peeling

Some vegetables such as pepper can be peeled by dry heat (flame). In this method,

vegetables are exposed to direct flame (for about 1 min at 1000C) or hot gases in

rotary tube flame peelers. Heat causes steam to develop under skins and this puffs

the skins up so that they can be washed away with water. Each heat treatment

should be immediately followed by cooling in water.

Weaver et al. (1980) report a flame application for the peeling of tomatoes. Dual

Maxon gas-fired burners are mounted at the top of a live-roller conveyor at the

height of 6 in. The affecting time of the flame is controlled by adjusting the speed

of the conveyor. Tomatoes are affected by the flame or infrared irradiation alone or

this is accompanied with boiling water or steam at an atmospheric pressure at 100C.

Each heat treatment was immediately followed with a cooling stage by exposure of

the product to water. A rapid splitting of the skin with very little charring is

achieved in thirty seconds of infrared radiation, but it cannot improve the peeling of

green-shoulder tissue on many cultivars. Weaver et al. state that flame peeling can

efficiently remove skin over green shoulders and immature green or yellow areas of

the fruit.

Davies (1996) discusses a vegetable peeling apparatus that has a heating station.

The heating at this station can be carried out by infrared radiation and that is

sufficient to at least partially lift the skin from the flesh.

2.2.3.2 Steam (wet heat) peeling

To eliminate charring, but to keep the effects of the high-temperature of the infrared

or flame, superheated steam is used. The steam pressure that is used in wet heat is

about 10 atm and it leads to the softening of skins and underlying tissues. When the

pressure is suddenly released, steam under the skin expands and causes the skin to

puff and crack. Then the skin is washed away with jets of water at high pressure (up

to 12 atm).

24

Kunz (1978) patented a method and device for peeling pumpkin by using wet steam.

While the pumpkins are shifting on endless conveyors, they are cut into halves and

are placed with the pulp facing downwards. Then they are exposed to pressurised

wet steam for a short time followed by a water cleaning step. Kunzs device uses

water sprays from the top to remove the skin and water sprays from the bottom to

remove pips and pip pulp. Weaver et al. (1980) applied the superheated steam

method for peeling of tomatoes as well. Fruits were affected directly by the flow of

steam in an open-mesh basket. Tomatoes were exposed two or three times to

superheated steam at three different levels of steam pressure and temperature. Each

heat treatment was instantaneously followed by cooling with water at about 22C.

Most efficient peel removal was achieved by using steam at temperatures and flow

rates of 425 to 480C and 12-15 lb steam per ft2 min respectively.

Smith (1984) developed a method of superheated steam peeling of apples by

refining conventional caustic and steam peeling methods. He used a batch-type

laboratory pilot-model steam peeler of bu (8.8 litres) capacity. His pilot-model

accepted either saturated steam at 100 psig (7 kg/cm2) or superheated steam at 100

psig (7kg/cm2) at mean inlet temperatures of 371C. He found that steam peeling

with saturated steam followed by flash cooling by injection of water increased

yields, saved labour, eliminated the need for expensive caustic solutions and

caustic-solution disposal, and finally, resulted in high quality apples for further

processing. Peeled yields in excess of 95% were attained in peeling treatment using

superheated steam with or without water injection. Furthermore, as the thermal

conductivity of superheated steam is considerably lower than that of saturated steam

at the same pressure, the amount of heat penetration into the flesh of fruits should

be controlled more easily.

Floros and Chinnan (1988a) explained that in single stage steam peeling, the

mesocarp cells would separate from the rest of the fruit. A large portion of edible

fruit will be washed away during the pressurised cold water treatment, which

follows the steam treatment. Many attempts have been made to date to improve the

efficiency of steam peeling for several commodities and therefore, they developed

a multi-stage process for steam peeling of pimiento peppers. Each process included

several repeated cycles at a constant temperature of 215 C and steam pressure of

25

480 KPa. Each cycle was 10 seconds long (except the last which was 5 seconds).

Steam was supplied and pressure built_up for the first 5 seconds. For the next 5

seconds, the steam was disconnected and the door of the chamber was opened, the

temperature was considerably reduced and allowed the pressure to drop

immediately to atmospheric. They observed the effect of various treatments on the

fruit surface by scanning electron microscopy. They also observed considerable

reduction of losses compared to single stage peeling. The reason was the short

consecutive heat treatments that supply sufficient heat to break down the outer

layers of the mesocarp cells with minimum effect on the next layers.

2.2.3.3 Thermal blast peeling

In a patented process developed by Harris and Smith (1986), vegetables and fruits

are placed in a closed and elevated pressure vessel. The products are affected by

infrared heat from the vessel wall and conductive heat from the superheated steam

atmosphere. The heat treatment leads to an increased plasticity of the skin tissues

caused by drying. The plasticized tissues will increase the resistance of peel to

rupture so steam can spread laterally to build the peeling area under the skin. This

stage is too short for heat to penetrate to the edible portion. After heat treatment,

pressure is reduced to atmospheric pressure by instantly opening the vessel. An

explosion leads to blowing up the product from the vessel and blasting the peel up

simultaneously as a result of the instantly and highly energized moisture under skin.

They applied this peeling method to many fruits and vegetables and observed better

results than lye and saturated steam peeling. For example, they tested this method

for Alagold pumpkin under 343.33C within 45 minutes and got 89.4 percent yield

by weight. They could reduce peeling losses for this variety of pumpkin from 28%

to about 11% for saturated steam and thermal blast peeling respectively.

2.2.3.4 Freeze-thaw

Brown et al. (1970), Thomas et al. (1976), Goud (1983), and Woodroof and Luh

(1988) attempted to eliminate the use of caustic solutions in the peeling of tomatoes

by the use of the freeze-thaw method. In this method tomatoes are immersed in

liquid nitrogen for 5-15 seconds, and then thawed in warm water at 66C for 30

26

seconds to loosen the peel. The loss was about 5-7% but this method was not

effective on immature yellow and green shoulder tissues. It was mentioned that the

method is applicable for peaches as well.

2.2.3.5 Vapour explosion (vacuum peeling)

Drooge et al. (1999) tested the vapour explosion method for removing the skins of

fruits and vegetables by explosive vaporization of the moisture under the skin of

fruits and vegetables. They placed the vegetable in a peeling vessel, and the

pressure in the vessel was rapidly reduced (below atmospheric pressure), leading to

explosive vaporization of the moisture. Drooge et al. suggested that it is possible to

reduce the air pressure and to cool the vegetable before the vapour explosion.

Kliamow et al. (1977) called this method vacuum peeling. They applied vacuum at

600-700 mm Hg to tear the peel off tomatoes. They reported high peeling efficiency,

retention of high fruit quality and low energy consumption as well as cost for this

method.

2.2.4 Chemical peeling

To reduce the losses during mechanical and thermal peeling methods, chemical

peeling has been considered. In this method, skins can be softened from the

underlying tissues by submerging vegetables in hot alkali solution. The quantity of

solution and the period of time are different for different kinds and varieties of

vegetables. Generally, lye may be used at a concentration of about 0.5-3%, at about

93C (2000 F), for a short period of time (0.5-3 min). The loosened skins are

washed away by high velocity jets of water or compressed air. This method of

peeling reduces the losses but it has harmful effects on the flesh of vegetables and

also is not environmentally friendly. Different kinds of chemical peeling are briefly

described with reference to related works of interest in the following section.

2.2.4.1 Caustic (lye) peeling

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Floros et al. (1987) evaluated the effect of lye concentration (4 to 12% NaOH),

process temperature (80 to 100C) and time (1.5 to 6.5 min) on the yield, peeling

loss and unpeeled skin, in a lye peeling process of pimiento peppers. They

optimized the process to achieve maximum removal of the skin and minimum loss

of edible fruit. They revealed that the high lye concentration (12% NaOH)

accompanied with short processing time (1.6 to 2 min) at a moderate temperature of

around 90C should yield an optimum process with removal of all of the skin and

peeling losses as low as 20%.

Floros and Chinnan (1988b) found that the double-stage process was more effective

than the conventional single-stage operation. They tested a double-stage lye

(NaOH) peeling process involving pre-treatment (concentration, c1; temperature, T1;

time, t1), holding time (th), and post-treatment (c2, T2, t2). The effect of seven factors

on four responses (unpeeled skin, peeling loss, product yield, and texture) was

studied. Processing times and lye concentrations were the most important factors,

while processing temperatures and holding times had no significant effect on the

peeling operation. A mild pre-treatment with 3.2% NaOH for 130 seconds

combined with a relatively strong post-treatment of 8% NaOH for 60 seconds at

84C and the holding time of 45 seconds were found to result in an optimum

process.

Garrote et al. (1993) surveyed the effect of NaOH concentration (4-20%), process

temperature (55-95C) and time (1-7 min) on the yield, peeling quality, unpeeled

skin and total usage of NaOH. They also evaluated titratable NaOH in the potato

tissue, NaOH penetration and heat ring depth. The best peeling quality, maximum

yield and minimum total usage of NaOH resulted with the following conditions:

concentration, 11-13%; time, 5-5.70 minutes and temperature, 90-95C. The

maximum temperature for which the heat ring and NaOH penetration depth were

equal was 72C where, at 20% NaOH and 7 minutes, peeling quality was very good

and the heat ring was eliminated.

Walter et al. (1982) investigated the effects of heat penetration on sweet potato

tissue under three lye-peeling treatments. Heat-mediated, starch gelatinization, cell

wall separation, chromoplast disruption, and enzymatic discoloration were

evaluated in different conditions according to the peeling treatment. Starch

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gelatinization, cell wall separation, and chromoplast disruption reduced in the order:

15 minute peel; 30 minute pre-soak (water 78-83C); followed by a 6 minute peel.

Discoloration occurred in significant amounts only in the 6 minute peel because

heat penetration was sufficient to disrupt lacticifer organization but insufficient to

inactivate the polyphenol oxidising (PPO) enzyme. The 30 minute pre-soak and 6

minute peel treatment resulted in the best product.

Floros et al. (1987) observed microstructural changes of pimiento peppers, which

were treated with varying degrees of NaOH (lye) solutions (1, 4, and 9%),

maintained at 80C, for different times (1, 2, and 3 minutes). They found that NaOH

removes the epicuticular and cuticular waxes, diffuses uniformly into the fruit

where it breaks down epidermal and hypodermal cell walls, and solubilizes the

middle lamella causing separation of the skin. In severe treatments the lye also

dissolves the parenchyma cells of the mesocarp resulting in considerable loss during

processing.

Walter et al. (1982) used the different treatments of peeling sweet potatoes in a

boiling, NaOH solution. The different treatments were: 6 minute peel (6p), 20

minute pre-soak in water (55C) followed by a 6 minute peel (20S), 30 minute pre-

soak in water (80C), followed by a 6 minute peel (30S), 15 minute peel (15p). The

area of tissue which was affected by heat was excited and analysed for o-

dihydroxyphenols (DP) and carotene destruction and sugar formation. The data

showed that roots peeled by 6P or 20S treatments could discolour as a result of the

PPO-DP reaction. 15P and 30S did not show discoloration because both treatments

are vigorous enough to inactivate the PPO system. All treatments except 6P caused

the inactivation of amylolytic enzymes. Carotenoid destruction was not detected.

2.2.4.2 Enzymic peeling

The adherence of peel to the fruits is done by pectin, cellulose and hemicellulose as

the polysaccharides (Toker and Bayindirli, 2003). Therefore, using corresponding

glycohydrolases to treat the product will lead to enzymic peeling. Janser (1996) has

claimed better texture and appearance for product after enzymatic peeling because

of fewer amounts of broken segments and juice losses. Researchers (Ben-Shalom et

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al., 1986; Rouhana and Mannheim, 1994; Soffer and Mannheim, 1996; Pretel et al.,

1997) have proved suitability of this method for peeling citrus fruits. Prakash et al.

(2001) studied enzymic peeling of Indian tough nature grapefruit by vacuum

infusion (Figure 2.4). They used two commercial peeling enzymes coded as Brand

A and Brand B. Brand A produced by Aspergillus Niger, involves a mixture

of pectinases and cellulase, while Brand B produced by Niger and Trichoderma

Reesi, contains pectinases, cellulase and hemi cellulase. Their conclusion was that:

a scalding time of 2 min in a boiling water bath, scoring the peel with four

radial lines, immersion in an mentioned enzyme bath containing enzymes

at 1mll-1, vacuum infusion at 760 mmHg for 1 min and incubating the fruit in the enzyme bath for 12 min at ambient temperature (302C),

followed by hand-peeling under running tap water, were found to be

necessary for easy peeling of Indian tough nature grapefruit.

In continuing the research for peeling of citrus fruits, some trials have been carried

out to assess the feasibility of this method for stone fruits such as apricot, nectarines,

and peaches (Toker and Bayindirli, 2003; Janser, 1996).

Fig.2.4. Enzymic peeled (right side) and manual (left side) peeled

grapefruit

(Prakash et al., 2001)


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2.3 The current situation of peeling tough-skinned

vegetables

The current situation of tough-skinned vegetable peeling was assessed by this

researcher during an industrial visit to Comet Food Pty. Ltd., Brisbane. This

company carries out fruits and vegetables processing involving peeling. During this

visit technologies that are used in peeling of different vegetables were observed. In

particular, pumpkin is chopped into segments by pivoted hand operated knife, and

then, each segment is rubbed against rotating grater drum. Because of the

unevenness of surface of some pumpkin varieties such as Jap and Jarrahdale (Figure

1.1), this method of peeling has high peeling losses. This limitation is common for

the peeling of tough-skinned vegetables. For example, one of the famous companies

which is active in design and manufacturing of food processing machinery is

Dornow Food Technology GmbH, Germany. Dornow has introduced automated

peelers as the latest peeler for the Hokkaido pumpkin variety which has an even

surface. Whole pumpkins are passed continuously through the machine. The floor

of the machine is equipped with many rotating disks. These disks could be

carborundum or blade. The main limitation of those peelers is also that they have no

flexibility to follow the uneven surface of some varieties of pumpkin such as

Jarrahdale and Jap (Figure 1.1). This limitation causes high peeling losses. The

value of peeling losses for pumpkin is not stated by the company but using similar

machines for peeling potatoes can produce peeling losses from 2.4% to 24%

(Dornow Food Technology GmbH, Year). The rate of peeling losses depends on the

degree of desired peeling. In this case, complete peeling to remove all eyes from

potatoes leads to 24% peeling losses.

2.4 Mathematical modelling of peeling processes

The mathematical modelling of peeling processes has been largely limited to the

chemical peeling process and, only rarely, to the thermal peeling process. Chemical

peeling is a complex phenomenon which involves mass diffusion and chemical

reactions. The general approach to the problem is the assumption that the rate of

peeling is a function of the lye concentration, temperature and treatment time, as

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well as other variables intrinsic to the product such as form and geometry, ripeness,

peel thickness, and type or variety (Barreiro et al. 1995). The relationship among

the above parameters must be identified for each product to attain the maximum

efficiency and minimum losses during the peeling process.

There are numerous research efforts that have established relationship amongst lye

concentration, temperature and time, and practical results are available for the

chemical peeling of various fruits and vegetables, including products such as

peaches (Olsen, 1941; Lankler and Morgan, 1944), pimiento peppers (Floros and

Chinnan, 1987,1988b), and so on. Most of the previous by mentioned

investigations have been conducted empirically to determine the peeling conditions.

For example, Athanasopoulos and Vagias (1987) adjusted the results for peeling

mandarin segments to a zero and first order reaction for peeling temperature and lye

concentration respectively. Also Floros and Chinnan (1987, 1988b) used a response

surface methodology to optimize the peeling process of pimiento peppers based on

empirical peeling data in one and two stage processes.

Barreiro et al. (1995) developed a mathematical model for the chemical peeling

process of foods with spherical shapes. They used the concept of the unreacted core

model for this purpose. They established some equations for the prediction of

peeling times, weight losses and texture changes during the peeling of guava as a

function of the variables involved in the peeling process. Chavez et al. (1996)

applied a mathematical model to describe the chemical peeling process. They

intended to identify the minimum processing losses of vegetables, energy and

NaOH solution consumption. They used the shrinking core model and the second

Fick`s law to formulate the mathematical model on the basis of mechanisms

included in the peeling process of potatoes. They compared their results to

experimental data and observed good agreement.

Few publications could be found on mathematical modelling of thermal peeling

methods. Somsen et al. (2004) developed two models on the basis of experimental

data of steam peeling for three varieties of potato. They successfully predicted the

heat ring and peel losses as the function of some independent variables such as size,

variety, conditioning temperature, steam pressure, and steam exposure time.

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No publication on mathematical modelling of mechanical peeling methods or

processes has been found by the date of writing the thesis.

2.5 Conclusions and discussion

The foregoing sections have elaborated on the current state-of-the-art technology in

peeling processes, highlighting some affective mechanical properties of products in

mechanical peeling, the various methods applicable to different food produce

especially tough-skinned vegetables such as pumpkin.

It is evident that each peeling method has its merits and limitations. Some fruits and

vegetables are not well adapted to thermal peeling because this method may cause a

cauterizing of the surface and wound areas. In the dry heat method, small pieces of

charred skin, which are not removed, give a poor appearance to the canned product.

A large amount of wastewater and considerable loss of flesh are other important

disadvantages of this method. It is necessary to find other methods of thermal

peeling that reduce the time required to expose vegetables to heat and subsequently

reduce flesh damage. Floros and Chinn

Bagher Emadi Thesis

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