bagher emadi thesis
DESCRIPTION
peelerTRANSCRIPT
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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
2.2.1 Introduction 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
6.3.1 Introduction 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
6.4.1 Introduction 92
6.4.2 Material of experiments 92
6.4.3 Results and discussion 94
6.4.4 Optimization and estimation of the responses 98
6.5 Peeling by using abrasive foams 98
6.5.1 Introduction 98
6.5.2 Material of experiments 99
6.5.3 Results and discussions 100
6.5.4 Optimization and estimation of the responses 104
6.6 Peeling by using abrasive-cutter brush 105
6.6.1 Introduction 105
6.6.2 Material of experiments 106
6.6.3 Results and discussion 108
6.6.4 Optimization and estimation of the responses 111
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 Results and discussion 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 Results and discussion 146
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
6.5 The contribution of independent variables to responses resulted
from using abrasive-pads 95
6.6 The effects of independent variables on responses resulted from
using abrasive pads 96
6.7 Abrasive foam and accessories 99
6.8 The contribution of independent variables to responses resulted
from using abrasive foams 102
6.9 The effects of independent variables on responses resulted from
using abrasive foams 103
6.10 Abrasive-cutter brush 107
6.11 The contribution of independent variables to responses resulted
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.
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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.
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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.
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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
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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.
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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
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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)
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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
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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%.
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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.
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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
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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-
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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
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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
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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.
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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).
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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
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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
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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)
hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library
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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