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CONTENTS
Sections Description Page No.
Title Page i
Dedication ii
Approval of the Viva-Voce Board iii
Certificate v
Declaration vii
Acknowledgement viii
Biodata ix
List of Tables xiv
List of Figures xvii
List of Symbols and Abbreviations xxi
Abstract xxiv
Chapter 1 Introduction 1
Chapter 2 Review of Literature 7
2.1 Composition of Onion 7
2.2 Processed Onion 8
2.3 Onion Varieties Suitable for Dehydration 9
2.4 Dehydration Practices for Onion 12
2.5 Vacuum Drying as a Potential Drying Method 16
2.6 Pretreatments 18
2.7 Quality Characteristics 19
2.7.1 Non-enzymatic browning 19
2.7.2 Flavour or pungency 20
2.7.3 Rehydration ratio 20
2.7.4 Scanning electron microscopy 21
2.7.5 Texture 21
2.7.6 Thermo-physical properties 21
2.8 Fuzzy Sensory Analysis 22
2.9 Storage Study 23
2.10 Mathematical Modelling 23
2.10.1 Modelling used for onion dehydration 24
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2.10.2 Modelling used for vacuum drying 27 2.10.3 Computational fluid dynamics modelling for
drying
28
2.10.4 CFD on vacuum drying 29
Chapter 3 Theoretical Considerations 33
3.1 Determination of Moisture Transport Mechanism 33 3.1.1 Constant diffusivity approach 35 3.1.2 Variable diffusivity approach 37 3.2 Modelling of Thin Layer Vacuum Drying 39 3.3 Numerical Modelling of the Vacuum Drying Process for
Onion
41
3.3.1 Description of the system 41
3.3.2 Physical model 42
3.3.3 Mathematical model 44
3.3.4 Computational fluid dynamics model formulation
49
3.3.5 Computing the solution 55
3.3.6. Model validation 55
Chapter 4 Materials and Methods 57
4.1 Sample Preparation 57
4.2 Vacuum Dryer 57
4.3 Experimental Set Up and Design 60
4.3.1 Design of experiment 60
4.3.2 Data analysis and optimization 61 4.4 Quality Evaluation of Dehydrated Onion 64 4.4.1 Determination of moisture content 64
4.4.2 Determination of colour 65
4.4.3 Determination of flavour 65
4.4.4 Determination of rehydration ratio 66 4.5 Sensory Evaluation of Vacuum Dried Onion Slices 67
4.5.1 Preparation of score card and training of the judges
67 4.5.2 Fuzzy comprehensive modelling of Sensory
scores
68
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4.6 Scanning Electron Microscope Analysis 74
4.7 Texture Analysis 76
4.8 True Density 77
4.9 Thermal Conductivity 78
4.10. Storage Study 79
Chapter 5 Results and Discussion 81
5.1 Drying Kinetics of Vacuum Dried Onion Slices 81
5.1.1 Moisture profile analysis 81
5.1.2 Drying rate variation 82
5.1.3 Moisture ratio variation 92
5.1.4 Modelling of drying characteristics 94 5.1.5 Determination of diffusivity value 101 5.2 Numerical Simulation of Drying Process of Onion Slice 109
5.2.1 Temperature profile of onion slice during drying
109 5.2.2 Pattern of isotherm in the slice 112 5.2.3 Moisture profile of onion slice during drying 114 5.2.4 Moisture concentration contour at different
time of drying
117 5.2.5 Comparison of experimental and simulated
data
119 5.3 Quality Analysis of Dried Onion Slices 122
5.3.1 Moisture content 122
5.3.2 Colour development 123
5.3.3 Flavour content 131
5.3.4 Rehydration ratio 137
5.4 Optimization of the Parameters of Vacuum Dried Onion Slices
138
5.4.1 Statistical analysis 139
5.4.2 Optimization 142
5.5 Fuzzy Sensory Analysis 145
5.5.1 Triplets for overall sensory scores of onion - tomato paste samples
145 5.5.2 Overall membership functions of sensory 147
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scores on standard fuzzy scale
5.5.3 Similarity values of onion-tomato paste samples and their ranking
147 5.5.4 Quality attributes ranking of onion-tomato
paste samples in general
149 5.5.5 Quality factor ranking of individual onion-
tomato paste samples
149 5.6 Characterization of the Final Vacuum Dried Onion 151
5.6.1 Microstructure analysis 151
5.6.2 Texture analysis 153
5.6.3 True density 155
5.6.4 Thermal conductivity 156
5.7 Storage Study 158
Chapter 6 Summary and Conclusions 163
References 171
Appendix A Specifications of the instruments used for study 183
Appendix B ANOVA for Quality Parameters 186
Appendix C MATLAB Program for Sensory Analysis using Fuzzy Logic 188 Appendix D Drying Data for Vacuum Drying of Onion 192 Appendix E Sensory Score Card for Fuzzy Logic Modelling 197
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LIST OF TABLES
Table Description Page No.
1.1 Yearwise Status of Area, Production, and Yield of Onion in India
2 2.1 Proximate Composition and Energy Values of Raw and
Dehydrated Onion
8 3.1 Models for Predicting Vacuum Drying Characteristics of Onion
Slices
40 3.2 Boundary Conditions Applied to the Vacuum Drying Model for
Heat Transfer in COMSOL
49 3.3 Boundary Conditions Applied to the Vacuum Drying Model for
Mass Transfer in COMSOL
49 3.4 Various Process Constants Assigned to the Drying Model in
COMSOL
53 3.5 Variable Expressions Assigned to the Drying model in
COMSOL
54 4.1 Experimental Design for Vacuum Drying of Onion Slice 61 4.2 Triplets Associated with Different Sensory Scales 69 5.1 Model Coefficients and Coefficient of Determination of
Polynomial Model for Drying Rate Data
92 5.2 Statistical Results Obtained from Various Thin Layer Drying
Models for Untreated Onion Slice
98 5.3 Statistical Results Obtained from Various Thin Layer Drying
Models for Treated Onion Slice
99 5.4 Estimated Values of Parameters of Page Model Used for Thin
Layer Drying Modelling of Onion Slices at Different Sample Thicknesses and Temperatures
100
5.5 Values of Constant Moisture Diffusivity for Vacuum Drying of Onion Slices
103 5.6 Percentage Error Values for Diffusivity Models 104 5.7 Reaction Rate Constant for NEB Kinetics and R2 Values for
VDOS
126 5.8 Activation Energy Values for Browning Kinetics of VDOS 129 5.9 Percentage Error in Predicted Optical Index 129 5.10 Reaction Rate Constant for Thiosulphinate Loss Kinetics and R2
Values
134
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5.11 Activation Energy Values for Flavour Kinetics of VDOS 135 5.12 Percentage Error in Predicted Flavour Loss 136 5.13 Experimental Results for Vacuum Drying of Onion Slices 138 5.14 ANOVA for the Selected Model and the Responses 140 5.15 Regression Coefficients of the Selected Polynomials Relating
the Responses with Variables
141 5.16 Sensory Scores and Triplets Associated with the Scores for
Quality Attributes of Onion-Tomato Paste Samples in General
145 5.17 Triplets for Sensory Scores for Each of the Quality Attributes of
the Samples
146 5.18 Similarity Values for the Three Samples Under Different Scale
Factors
148 5.19 Similarity Values for the Quality Attributes of Onion-Tomato
Paste Samples in General
149 5.20 Similarity Values and Ranking of Quality Attributes for
Individual Onion-Tomato Paste Sample
150 5.21 Textural Property of Untreated and Treated VDOS 155 5.22 True Density of Vacuum dried 5 mm Thick Treated Onion
Slices
156 5.23 Thermal Conductivity of Onion Slices Dried Under Optimized
Condition
157 5.24 Coefficient of Determination of the Fitted Model and Reaction
Rate Constants for Colour and Flavour Kinetics During Storage
159
B1 ANOVA for Moisture Content (Untreated) 186
B2 ANOVA for Moisture Content (Treated) 186
B3 ANOVA for Colour (Untreated) 186
B4 ANOVA for Colour (Treated) 186
B5 ANOVA for Flavour (Untreated) 187
B6 ANOVA for Flavour (Treated) 187
B7 ANOVA for Rehydration Ratio (Untreated) 187
B8 ANOVA for Rehydration Ratio (Treated) 187
D1 Moisture Content for 1 mm Thick Untreated Onion Slice 192 D2 Moisture Content for 1 mm Thick Treated Onion Slice 192 D3 Moisture Content for 3 mm Thick Untreated Onion Slice 192 D4 Moisture Content for 3 mm Thick Treated Onion Slice 193
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D5 Moisture Content for 5 mm Thick Untreated Onion Slice 194 D6 Moisture Content for 5 mm Thick Treated Onion Slice 195
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LIST OF FIGURES
Figure Description Page No.
3.1 Drying Rate versus Free Moisture Content Curve 34 3.2 Falling Rate Drying Plot for Diffusion Mechanism and
Capillary Mechanism
34 3.3 Domain Geometry of Onion Slice Developed in COMSOL 50 3.4 Meshing of the Geometry in COMSOL with Triangular
Elements
51 4.1 Schematic Diagram of the Vacuum Dryer System 58
4.2 Pictorial View of Vacuum Dryer Set Up 59
4.3 UV-VIS Spectrophotometer (Model UV-1601) 66
4.4 Values of Triplets Associated with Triangular Membership Distribution Function for Five-Point Sensory Scale
69 4.5 Graphical Representation of Triplet (a b c) and its
Membership Function
71
4.6 Representation of Standard Fuzzy Scale 72
4.7 Scanning Electron Microscope (Model - JSM-5800LV) 76
4.8 Texture Analyser (Model TA-XT2i, UK) 77
4.9 Air Comparison Pycnometer 78
4.10 Thermal Conductivity Meter (Kemtherm QTM-D3) 78 5.1 Variation of Moisture Content for Different Thicknesses 83 5.2 Variation of Moisture Content for Different Temperatures 84 5.3 Variation of Drying Rate with Time for Different Thicknesses 85 5.4 Variation of Drying Rate with Time at 60 °C 86 5.5 Variation of Drying Rate with Moisture Content at Different
Thicknesses
87 5.6 Variation of Drying Rate with Moisture Content for Different
Temperatures
90 5.7 Moisture Ratio with Time at Different Slice Thicknesses for
VDOS
93 5.8 Variation of Moisture Ratio with Time at 60 °C Temperature 94 5.9 Moisture Ratio versus Time Plot for 1 mm Thick Onion Slices 95 5.10 Moisture Ratio versus Time Plot for 3 mm Thick Onion Slices 96 5.11 Moisture Ratio versus Time Plot for 5 mm Thick Onion Slices 97
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5.12 Comparison of Experimental and Predicted Moisture Ratio for Untreated and Treated Slices by the Page Model
101 5.13 ln(MR) versus Time Plot for 3 mm Thick Untreated Onion
Slices Dried at 50 °C
102 5.14 ln(MR) versus Time Plot for 3 mm Thick Treated Onion
Slices Dried at 50 °C
102 5.15 Diffusivity of Onion Slices versus Moisture Content at
Different Thicknesses
107 5.16 Diffusivity versus Moisture Content at 70°C Temperature 108 5.17 Temperature Profile at the Bottom Surface of the Onion Slice 110 5.18 Temperature Profile at the Side Surface of the Onion Slice 110 5.19 Temperature Profile at the Centre of the Onion Slice 111 5.20 Temperature Profile at the Top Surface of the Onion Slice 112 5.21 Temperature Profile after 30 min of Drying (1800 s) 113 5.22 Temperature Profile after 90 min of Drying (5400 s) 113 5.23 Temperature Profile after 180 min of Drying (10800 s) 114 5.24 Temperature Profile after 240 min of Drying (14400 s) 114 5.25 Moisture Concentration at Bottom (mid point) of Onion Slice 115 5.26 Moisture Concentration at Top (mid point) of Onion Slice 115 5.27 Moisture Concentration at Side (mid point) of Onion Slice 116 5.28 Moisture Concentration at Central Mid Point of Onion Slice 116 5.29 Moisture Concentration after 30 min (1800 s) 117 5.30 Moisture Concentration after 90 min (5400 s) 117 5.31 Moisture Concentration after 180 min (10800 s) 118 5.32 Moisture Concentration after 240 min (14400 s) 118 5.33 Diffusive Flux after 240 min of Drying (14400 s) 119 5.34 Temperature Profile at the Central Surface of 3 mm Thick
Untreated Onion Slice
119 5.35 Temperature Profile at the Top Surface of 3 mm Thick
Untreated Onion Slice
120 5.36 Temperature Profile of at Side Surface of 3 mm Thick
Untreated Onion Slice
120 5.37 Moisture Concentration of 3 mm Thick Untreated Onion Slice
During Drying at Central Mid Point
121 5.38 Moisture Concentration at Different Locations of 3 mm Thick 122
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Untreated Onion Slice During Drying
5.39 Moisture Content of VDOS at Different Operational Conditions
123 5.40 Colour Change (OI) of VDOS at Different Operational
Conditions
125 5.41 NEB (OI/g sample) in 1 mm, 3 mm and 5 mm Thick Untreated
VDOS at Various Temperatures
127 5.42 Arrhenius Plot for NEB in Untreated VDOS 128 5.43 Actual and Predicted Optical Index (OI/g sample) for VDOS 130 5.44 Thiosulfinate Concentration ( mol/g sample) in VDOS 131 5.45 Thiosulfinate Loss ( mol/g sample) in VDOS 132 5.46 Change in Thiosulfinate for Untreated VDOS 133 5.47 Change in Thiosulfinate for Treated VDOS 134 5.48 Arrhenius Plot for Thiosulfinate Loss ( mol/g sample) in
Untreated VDOS
135 5.49 Actual and Predicted Flavour Loss for 3 mm Untreated Onion
Slices
136 5.50 Actual and Predicted Flavour Loss for 3 mm Treated Onion
Slices
136 5.51 Rehydration Ratio of VDOS at Different Operational
Conditions
137 5.52 Contour Plots of Moisture Content for (a) Untreated and (b)
Treated VDOS
143 5.53 Contour Plots of Colour Change for (a) Untreated and (b)
Treated VDOS
143 5.54 Contour Plots of Flavour Content for (a) Untreated and (b)
Treated VDOS
144 5.55 Contour Plots of Rehydration Ratio for (a) Untreated and (b)
Treated VDOS
144 5.56 Microstructural View of Onion Slices Produced Using SEM 152 5.57 Texture Profile for 5 mm Thick Untreated Onion Slices Dried
at 60 °C under 50 mm Hg Pressure
154 5.58 Texture Profile for 5 mm Thick Treated Onion Slices Dried at
60 °C under 50 mm Hg Pressure
154 5.59 Variation in True Density with Moisture Content 156 5.60 Thermal Conductivity Variation of Onion Slices with Moisture 158
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Content
5.61 Colour Change in Dried Onion Slices During Storage 158 5.62 Flavour Change in Dried Onion Slices During Storage 159 5.63 Colour Change Behaviour of Actual and Predicted Optical
Index at a Water Activity of 0.33
160 5.64 Flavour Loss Pattern of Actual and Predicted Thiosulfinate
Content at a Water Activity of 0.33
161
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LIST OF SYMBOLS AND ABBREVIATIONS
% Percentage
°C Degree Celcius
2D, 3D Two dimensional, Three dimensional
A Absorbance
a, b, c, d, A, B, C Coefficients of the equations
Adeq Adequate
Adj Adjusted
ADOGA American Dehydrated Onion and Ginger Association ANOVA Analysis of variance
ASHRAE American Society for Heating Refrigeration and Air Conditioning
avg Average
aw Water activity
b Cell path used, cm
c Moisture concentration of the domain at any instant, mol.m-3 c_0 Initial moisture concentration of onion slice, kg.m-3
c_b Bulk moisture concentration in the ambient, kg.m-3 C_m Specific moisture capacity
C_p, Cp Specific heat of onion cb Bulk concentration, mol.m-3 CFD Computational Fluid Dynamics CV Coefficient of variation
cz Concentration gradient components dc/dz D Diffusion coefficient, m2/s
D_m Surface moisture diffusivity, m2.s-1 db Dry basis, kg water.(kg dry solid) -1 De Effective moisture diffusivity, m2.s-1
DOF Degrees of freedom
e.g. Exempli gratia, for example Ea Activation energy, kJ.mol-1 et. al. Et alibi, and others
exp Exponential
eqn Equation
F0 Fourier number
FAO Food and Agriculture Organisation
FEM Finite element method
FMC Final moisture content
g Grams
h Heat transfer coefficient, W.m-2.K-1 h_m Mass transfer coefficiemt, kg.m-2.s-1
h_Ta Heat transfer coefficient of chamber air, W.m-2.K-1
Hg Mercury
i.e. Ed set, that is
ICAR Indian Council of Agricultural Research ITC International Trade Centre
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K Kelvin
k Thermal conductivity tensor, W.m-1.K-1 k_c Mass transfer coefficient, m.s-1
k_m Moisture conductivity of onion slice, kg.m-1.s-1 k_T Thermal conductivity of vacuum dried onion kc Reaction rate constant for colour kinetics
kcal Kilo Calorie
kd Drying rate constant
kf Reaction rate constant for flavour kinetics
kg Kilo Grams
kPa Kilo Pascal
l, x Thickness of slice, m
lda Latent heat of vaporization, J.kg-1
m Meter
M0 Initial moisture content, kg water.(kg dry solid)-1
Md Moisture content at any instant in dry basis, kg water.(kg dry solid)-1 Me Equilibrium moisture content, kg water.(kg dry solid)-1
min Minutes
mm Millimeter
MR Moisture ratio
Mv Mass flux vector, mol.m-2.s-1
mw Wet basis moisture content, kg water.(kg total matter)-1 Mw Moisture content in percent wet basis
N Force, kg.m.s-2
n Normal vector of the boundary N0 Number of observation taken
Mv0 Inward mass flux normal to the boundary, mol.m-2.s-1
NAFED National Agricultural Cooperative Marketing Federation of India
NEB Non-Enzymatic Browning
NHRDF National Horticultural Research and Development Foundation
nm Nanometer
OI Optical index
PDE Partial differential equation
PE Percentage error
pred Predicted
q Heat flux vector, W.m-2
Q Heat source (or sink), W.m-3
q0 Inward heat flux vector normal to the boundary, W.m-2 R2 Coefficient of determination
rho, ρ Bulk density of vacuum dried onion RMSE Root mean square error
RR Rehydration ratio
Rt Reaction rate, mol.m-3.s-1
s Seconds
SEM Scanning electron microscopy Temperature difference
T
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t Time, s
T Temperature at any instant, K T_0 Initial temperature of onion slice, K T_a Chamber air temperature, K
T_b Bottom temperature of onion slice, K
TC Thiosulphinate concentration of the solution, μmol.g-1
Theo Theoretical
Tinf External temperature of domain, K
TL Thiosulphinate loss
Tr Transmittance
VDOS Vacuum drying of onion slices
viz. Videlicet, namely
w Weight of sample, g
wb Wet basis, kg water.(kg total matter) -1 Ye Experimental values of response Yp Predicted value of the response
z Number of constants in the model (eqn 3.18 )
ε Molar absorptivity of thiolsulphinate solution at 254 nm, g.μmol-
1.cm-1
λ Characteristics roots of the corresponding regression equation
μ Micro
χ2 Reduced chi-square
