ABSTRACT
The effect of sample thickness (5, 10 and 15 mm) and method of drying; hot air drying (50, 60 and 70 ⁰C), microwave drying (200, 300 and 400W), open sun and solar drying on the drying characteristics and kinetics of cooking banana slices were investigated. Results showed that cooking banana drying is a diffusion-controlled process. The rate of drying increased with increase in drying temperature, microwave power rating and solar intensity. The values of drying rate constant (k) and effective moisture diffusivity (Deff) increased with the increase in microwave power and drying temperature levels. The dependence on effective diffusivity coefficient were expressed by an Arrhenius type relationship. The results also showed that sample thickness, method of drying, drying air temperature and microwave power levels affected the drying rate and thus the drying time. It was observed that cooking banana slices dried completely within 3hrs – 11hrs:30 mins, 15 - 45 mins, 10 – 30hrs and 18 – 42 hrs under hot air drying, microwave drying, solar and open sun respectively. Irrespective of the drying methods, all the samples exhibited constant rate and falling rate period. Fifteen thin-layer mathematical drying models were fitted to the experimental data and compared with three statistical parameters, the coefficient of determination (R2), chi-square (X2) and root mean square error (RMSE). The Midilli and KucuK model (2002) was shown to have a better fit to the experimental data obtained from oven drying, microwave drying, solar and open sun drying respectively when compared to other tested models. Effective moisture diffusion coefficients (Deff) were determined utilizing Fick’s second law equation and the values of Deff found to be in range of 1.393 - 8.89 x 10-8 m2/s and 1.074 x 10-7 – 8.89x10-8 m2/s for oven and microwave drying respectively. The Arrhenius-type relationship that describes the temperature dependence of effective moisture diffusivity for oven drying was determined to be 23.599kJ/mol, 24.809 kJ/mol and 24.223kJ/mol for 5, 10 and 15 mm sample thicknesses respectively. Modified Arrhenius equation was applied to identify the activation energy of microwave drying and its value ranged from 18.619 to 11.940kJ/mol for 5, 10 and 15 mm sample thicknesses. Respective empirical equations were also generated from Midilli-Kucuk (2002) thin layer drying model that can predict the drying curves of cooking banana under the listed drying methods.
TABLE OF CONTENTS
Title Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgement v
Table of Contents vi
List of Tables vii
List of Figures viii
List of Plates ix
Abstract x
CHAPTER ONE: INTRODUCTION
1.1 Background of Study 1
1.2 Statement of Problem 3
1.3 Objectives of the Study 5
1.3.1 General objective of the study 5
1.3.2 Specific objectives of the study 5
1.4 Justification of the Study 5
1.5 Scope of the Study 7
CHAPTER TWO: LITERATURE REVIEW
2.1 Overview of Cooking Banana 8
2.1.1 Origin in Nigeria 9
2.1.2 Nutrition value and chemical composition of cooking banana 10
2.1.3 Utilization and processing of cooking banana 11
2.2 Basic Principles in Drying 12
2.2.1 Heat transfer 14
2.2.2 Mass transfer 14
2.3 Drying Rate Period 17
2.3.1 Initial drying period 18
2.3.2 Constant rate period 18
2.3.3 First falling drying rate period 19
2.3.4 Second falling drying rate period 19
2.4 Drying Rate 20
2.4.1 Parameters affecting the drying rate. 21
2.5 Drying Kinetics 28
2.6 Classification of Drying Methods 32
2.6.1 Natural air drying 33
2.6.2 Supplemental heat and low temperature drying 34
2.6.3 Forced air drying 34
2.6.4 Heated air drying 35
2.6.5 Unheated air drying 36
2.7 Solar Drying 37
2.7.1 Types of solar dryers 38
|2.8 Thin Layer Drying Models 41
2.8.1 Theoretical models 42
2.8.2 Semi-theoretical models 42
2.8.3 Empirical models 51
2.9 Microwave Drying 54
CHAPTER THREE: MATERIALS AND METHODS
3.1 Material Selection 58
3.1.1 Experimental set-up and methodology 58
3.1.1 Oven drying method 58
3.1.2 Microwave drying method 59
3.1.3 Solar drying method 59
3.1.4 Open sun drying method 60
3.2 Experimental Design 61
3.2.1 Experimental design for oven drying 61
3.2.2 Experimental design for microwave drying 61
3.2.3 Experimental design for solar and sun drying 62
3.3 Moisture Content Determination 62
3.4 Determination of Moisture Ratio 62
3.5 Drying Rate Calculation 63
3.6 Determination of Effective Moisture Diffusivity 63
3.7 Determination of the Activation Energy 64
3.7.1 Oven drying 65
3.7.2 Microwave drying 65
3.8 Mathematical Modelling of Drying Kinetics 66
3.8.1 Statistical evaluation of drying models 66
CHAPTER FOUR: RESULT AND DISCUSSIONS
4.1 Preliminary Investigations on Drying Kinetics of Cooking Banana Slices 68
4.1.1 Initial moisture content determination 68
4.1.2 General Observations during oven drying 69
4.1.3 Effect of temperature on drying curves 69
4.1.4 Effect of temperature on drying rate 71
4.1.5 Effect of temperature on drying rate constant (K) 73
4.1.6 Effect of temperature on effective moisture diffusivity (deff) 75
4.1.7 Effect of temperature on activation energy 77
4.1.8 Mathematical modelling of oven drying kinetics 78
4.1.8.1 Validation of best fit model 80
4.1.8.2 Validation of generated empirical equations 81
4.2 Microwave drying kinetics of cooking banana slices 87
4.2.1 Effect of microwave power on drying curves 87
4.2.2 Effect of microwave power on drying rate 89
4.2.3 Effect of microwave power on drying rate constant (K) 91
4.2.4 Effect of microwave power on effective moisture diffusivity (deff) 94
4.2.5 Effect of microwave power on activation energy 94
4.2.6 Mathematical modelling of microwave drying kinetics 97
4.2.6.1 Validation of best fit model 99
4.2.6.2 Validation of generated empirical equations 100
4.3 Solar drying kinetics of cooking banana slices 106
4.3.1 Solar drying characteristics of cooking banana slices 109
4.3.2 Mathematical modelling of solar drying kinetics 110
4.3.2.1 Validation of best fit model 111
4.3.2.2 Validation of generated empirical equations 115
CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions 120
5.2 Recommendations 121
References 123
Appendices 136
LISTS OF TABLES
2.1 Taxonomical description of cooking banana 9
2.2 Advantages and limitations of microwave drying 55
3.1 Experiments done showing factorial design used (Oven drying) 61
3.2 Experiment done showing factorial design used (Microwave drying) 61
3.3 Experiment done showing Factorial design used
(Solar and open sun drying) 62
3.4 Thin layer mathematical models used 67
4.1 Drying rate constant (K) values (Oven drying) 75
4.2 Effective moisture diffusivity values (Oven drying) 77
4.3 Statistical results obtained from selected models 79
4.4 Statistical results of Midilli and Kucuk model (Oven drying) 80
4.5 Drying rate constant (K) values (Microwave drying) 93
4.6 Effective moisture diffusivity values (Microwave drying) 94
4.7 Statistical results obtained from the selected drying models (Microwave drying) 98
4.8 Statistical results of Midilli and Kucuk model (Microwave drying) 98
4.9 Statistical results obtained from the selected drying models
(Solar and open sun drying) 111
4.10 Statistical results of Midilli and Kucuk model
(Solar and open sun drying) 111
4.11 Statistical results of Newton model (Oven drying) 136
4.12 Statistical results of Page model (Oven drying) 136
4.13 Statistical results of modified Page model (Oven drying) 136
4.14 Statistical results of Henderson and Pabis model (Oven drying) 137
4.15 Statistical results of modified Henderson and Pabis model (Oven drying) 137
4.16 Statistical results of Midilli and Kucuk model (Oven drying) 137
4.17 Statistical results of modified Midilli model (Oven drying) 138
4.18 Statistical results of Logarithmic model (Oven drying) 138
4.19 Statistical results of Two-term model (Oven drying) 138
4.20 Statistical results of Two-term exponential model (Oven drying) 139
4.21 Statistical results of Demir et al. model (Oven drying) 139
4.22 Statistical results of Verma et al. model (Oven drying) 139
4.23 Statistical results of Approximation of diffusion model (Oven drying) 140
4.24 Statistical results of Hii et al. model (Oven drying) 140
4.25 Statistical results of Wang and Singh model (Oven drying) 140
4.26 Statistical results of Newton model (Microwave drying) 141
4.27 Statistical results of Page model (Microwave drying) 141
4.28 Statistical results of modified Page model (Microwave drying) 141
4.29 Statistical results of Henderson and Pabis model (Microwave drying) 142
4.30 Statistical results of modified Henderson and Pabis model (Microwave drying) 142
4.31 Statistical results of Midilli and Kucuk model (Microwave drying) 142
4.32 Statistical results of modified Midilli model (Microwave drying) 143
4.33 Statistical results of Logarithmic model (Microwave drying) 143
4.34 Statistical results of Two-term model (Microwave drying) 143
4.35 Statistical results of Two-term exponential model (Microwave drying) 144
4.36 Statistical results of Demir et al. model (Microwave drying) 144
4.37 Statistical results of Verma et al. model (Microwave drying) 144
4.38 Statistical results of Approximation of diffusion model
(Microwave drying) 145
4.39 Statistical results of Hii et al. model (Microwave drying) 145
4.40 Statistical results of Wang and Singh model (Microwave drying) 145
4.41 Statistical results of Newton model (Solar and Sun drying) 146
4.42 Statistical results of Page model (Solar and Sun drying) 146
4.43 Statistical results of modified Page model (Solar and Sun drying) 146
4.44 Statistical results of Henderson and Pabis model (Solar and Sun drying) 147
4.45 Statistical results of modified Henderson and Pabis model (Solar and Sun drying) 147
4.46 Statistical results of Midilli and Kucuk model (Solar and Sun drying) 147
4.47 Statistical results of modified Midilli model (Solar and Sun drying) 148
4.48 Statistical results of Logarithmic model (Solar and Sun drying) 148
4.49 Statistical results of Two-term model (Solar and Sun drying) 148
4.50 Statistical results of Two-term exponential model (Solar and Sun drying) 149
4.51 Statistical results of Demir et al. model (Solar and Sun drying) 149
4.52 Statistical results of Verma et al. model (Solar and Sun drying) 149
4.53 Statistical results of Approximation of diffusion model
(Solar and Sun drying) 150
4.54 Statistical results of Hii et al. model (Solar and Sun drying) 150
4.55 Statistical results of Wang and Singh model (Solar and Sun drying) 150
4.56 Average experimental drying data for thickness of 5mm (Oven drying) 151
4.57 Average experimental drying data for thickness of 10mm (Oven drying) 151
4.58 Average experimental drying data for thickness of 15mm (Oven drying) 152
4.59 Average experimental drying data for thickness of 5mm (Oven drying) 153
4.60 Average experimental drying data for thickness of 10mm (Oven drying) 153
4.61 Average experimental drying data for thickness of 15mm (Oven drying) 154
4.62 Average experimental drying data for thickness of 5mm (Oven drying) 155
4.63 Average experimental drying data for thickness of 10mm (Oven drying) 155
4.64 Average experimental drying data for thickness of 15mm (Oven drying) 155
4.65 Average experimental drying data for thickness of 5mm and 200W (Mw) 156
4.66 Average experimental drying data for thickness of 5mm and 300W (Mw) 156
4.67 Average experimental drying data for thickness of 5mm and 400W (Mw) 157
4.68 Average experimental drying data for thickness of 10mm and 200W (Mw) 157
4.69 Average experimental drying data for thickness of 10mm and 300W (Mw) 157
4.70 Average experimental drying data for thickness of 10mm and 400W (Mw) 158
4.71 Average experimental drying data for thickness of 15mm and 200W (Mw) 158
4.72 Average experimental drying data for thickness of 15mm and 300W (Mw) 158
4.73 Average experimental drying data for thickness of 15mm and 400W (Mw) 159
4.74 Average experimental drying data for thickness of 5mm (Solar drying) 159
4.75 Average experimental drying data for thickness of 10mm (Solar drying) 160
4.76 Average experimental drying data for thickness of 15mm (Solar drying) 160
LIST OF FIGURES
2.1 General drying rate curve 18
2.2 Constant and falling rate periods in thin-layer drying of high
moisture grain 28
4.1 Drying curve of moisture content against drying time for cooking
banana of 5mm thickness at 50ºC 68
4.2 Effect of temperature on the moisture ratio (M_t/M_o ) vs drying time at
slice thickness of 5mm. 70
4.3 Effect of temperature on the moisture ratio (M_t/M_o ) vs drying time at
slice thickness of 10mm. 70
4.4 Effect of temperature on the moistures ratio vs drying time at slice
thickness of 15mm 71
4.5 Effect of drying rate on drying time at different temperatures for slice
thickness of 5mm. 72
4.6 Effect of drying rate on drying time at different temperatures for slice
thickness of 10mm. 72
4.7 Effect of drying rate on drying time at different temperatures for slice
thickness of 15mm. 73
4.8 Log of moisture ratio of 5, 10 and 15mm thickness against drying
time of 50oc 74
4.9 Log of moisture ratio of 5, 10 and 15mm thickness against drying
time of 60oc 74
4.10 Log of moisure ratio of 5, 10 and 15mm thickness against drying
time of 70oc 75
4.11 Deff versus drying temperature at different levels of slice 76
4.12 Deff versus slice thicknesses at different levels of drying temperature 76
4.13 Ln(deff) versus 1/tabs of different levels of slice thicknesses 78
4.14 Comparison of experimental and predicted moisture ratio values by
Midilli and kucuk model for 5mm thickness 80
4.15 Comparison of experimental and predicted moisture ratio values by
Midilli and kucuk model for 10mm thickness 81
4.16 Comparison of experimental and predicted moisture ratio values by
Midilli and kucuk model for 15 mm thickness 81
4.17 Comparison of empirical MR with experimental MR for 5mm
thickness at 50⁰C 82
4.18 Comparison of empirical MR with experimental MR for 5mm
thickness at 60⁰C 83
4.19 Comparison of empirical MR with experimental MR for 5mm
thickness at 70⁰C 83
4.20 Comparison of empirical MR with experimental MR for 10mm
thickness at 50⁰C 84
4.21 Comparison of empirical MR with experimental MR for 10mm
thickness at 60⁰C 84
4.22 Comparison of empirical MR with experimental MR for 10mm
thickness at 70⁰C 85
4.23 Comparison of empirical MR with experimental MR for 15mm
thickness at 50⁰C 85
4.24 Comparison of empirical MR with experimental MR for 15mm
thickness at 60⁰C 86
4.25 Comparison of empirical MR with experimental MR for 15mm
thickness at 70⁰C 86
4.26 Moisture ratio vs drying time at various microwave powers for 5mm 88
4.27 Moisture ratio vs drying time at various microwave powers for 10mm 88
4.28 Moisture ratio vs drying time at various microwave powers for 15mm 89
4.29 Drying rate vs drying time at various microwave powers for 5mm 90
4.30 Drying rate vs drying time at various microwave powers for 10mm 90
4.31 Drying rate vs drying time at various microwave powers for 15mm 91
4.32 Relationship between LN MR and drying time (microwave drying)
for 5mm 92
4.33 Relationship between LN MR and drying time (microwave drying)
for 10mm 92
4.34 Relationship between LN MR and drying time (microwave drying)
for 15mm 93
4.35 Effective moisture diffusivity versus microwave power for
cooking banana 94
4.36 Arrhenius-type relationship between the values of Ln (Deff) versus sample
mass/power for 5mm 95
4.37 Arrhenius-type relationship between the values of Ln (Deff) versus sample
mass/power for 10mm 96
4.38 Arrhenius-type relationship between the values of Ln (Deff) versus sample
mass/power for 15mm 96
4.39 Comparison of experimental and predicted moisture ratio values by Midilli
and Kucuk model for 5mm thickness (Microwave drying) 99
4.40 Comparison of experimental and predicted moisture ratio values by Midilli
and Kucuk model for 10mm thickness (Microwave drying) 100
4.41 Comparison of experimental and predicted moisture ratio values by Midilli
and Kucuk model for 15mm thickness (Microwave drying) 100
4.42 Comparison of empirical MR with experimental MR for 5mm thickness at 200W 101
4.43 Comparison of empirical MR with experimental MR for 5mm thickness at 300W 102
4.44 Comparison of empirical MR with experimental MR for 5mm
thickness at 400W 102
4.45 Comparison of empirical MR with experimental MR for 10mm
thickness at 200W 103
4.46 Comparison of empirical MR with experimental MR for 10mm
thickness at 300W 103
4.47 Comparison of empirical MR with experimental MR for 10mm
thickness at 400W 104
4.48 Comparison of empirical MR with experimental MR for 15mm
thickness at 200W 104
4.49 Comparison of empirical MR with experimental MR for 15mm
thickness at 300W 109
4.50 Comparison of empirical MR with experimental MR for 15mm
thickness at 400W 109
4.51 Drying curves for cooking banana (Solar drying). 106
4.52 Drying curves for cooking banana (Open sun drying). 106
4.53 Drying rate curves for cooking banana (Solar drying). 107
4.54 Drying rate curves for cooking banana (Open sun drying). 107
4.55 Drying parameter values for thickness of 5mm (Solar drying) 108
4.56 Drying parameter values for thickness of 10mm (Solar drying) 108
4.57 Drying parameter values for thickness of 15mm (Solar drying) 109
4.58 Comparison of experimental and predicted moisture ratio values by
Midilli-Kucuk model for 5mm thickness for solar drying 112
4.59 Comparison of experimental and predicted moisture ratio values by
Midilli-Kucuk model for 10mm thickness for solar drying 112
4.60 Comparison of experimental and predicted moisture ratio values by
Midilli-Kucuk model for 15mm thickness for solar drying 113
4.61 Comparison of experimental and predicted moisture ratio values by Midilli-Kucuk model for 5mm thickness for open sun drying 113
4.62 Comparison of experimental and predicted moisture ratio values by
Midilli-Kucuk model for 10mm thickness for open sun drying 114
4.63 Comparison of experimental and predicted moisture ratio values by
Midilli-Kucuk model for 15mm thickness for open sun drying 114
4.64 Comparison of empirical MR with experimental MR for 5mm thickness
for solar drying 116
4.65 Comparison of empirical MR with experimental MR for 10mm thickness
for solar drying 116
4.66 Comparison of empirical MR with experimental MR for 15mm thickness
for solar drying 117
4.67 Comparison of empirical MR with experimental MR for 5mm thickness
for open sun drying 118
4.68 Comparison of empirical MR with experimental MR for 10mm thickness
for open sun drying 118
4.69 Comparison of empirical MR with experimental MR for 15mm thickness
for open sun drying 119
LIST OF PLATES
1.1 Typical cooking banana (Musa Bluggoe:ABB) 1
3.1 A pictorial view of the convective dryer used for the experiment 162
3.2 A pictorial view of the microwave oven used for the experiment 162
3.3 A pictorial view of the active solar dryer used for the experiment 163
4.1 Dried sliced cooking banana by oven drying method 163
4.2 Dried sliced cooking banana by microwave drying method 164
4.3 Dried sliced cooking banana by solar dryer method 164
4.4 Preparation of the samples prior to drying 165
CHAPTER ONE
INTRODUCTION
1. 1 Background of Study
Cooking bananas (Musa spp., ABB genome) are derived generally from the hybridization of Musa accumulata and Musa balbisiana (Stove and Summonds 1987; Robinson, 1996) and are rather similar to unripe desert banana (M. Cavendish acculata) but appears often large in exterior appearance. Most of the world’s cooking bananas are eaten either raw, in their ripe state, or cooked form or the remaining proportion is processed in order to obtain a storable product (Robinson, 1996).