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COMBINED OSMOTle AND MICROWAVE DRYING OF STRAWBERRIES AND BLUEBERRIES

A Thesis submitted to The Faculty of Graduate Studies and Research of McGill University

by KamadenahaJJy Venkatachalapathy

In Partial Fulfilment of the Requirements for the Degree of

Doctor of Philosophy

Department of Agricultural and Biosystems Engineering Macdonald Campus of McGill University Ste-Anne-de-Bellevue, H9X 3V9 Quebec, Canada

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Canadi ABSTRACT

Kamadenahally Venkatachalapathy Ph.D. (Agr. and Biosystems Eng.)

Combined osmotic and microwave drying of strawberries and blueberries

This work was aimed at obtaining high quality dried strawberries using microwaves ta assist convection air of 2 mis at 30-45°C. Preliminary trials with whole strawberries were unsuccessfuL Fnrits would cook rather than dry at low microwave power levels, and burst at higher powers. This was due to the inhibition of moisture movement by the waxy cuticle. Sliced and pureed strawberries dried, but were of lower quality than freeze-dried. A treatment consisting of clipping the berries in a solution of ethyl oleate and sodium hydroxide was studied. Such treatments are used in industry to reduce the skin resistance ta moisture diffusion. Results showed that the treatment greatly enhanced the drying rates of whole berries in convection and microwave regimes. A 1% concentration of ethyl oleate was sufficient for maximum reduction of drying time, and it is possible that even lower concentrations could he used for strawberries. Rehydration was similar to that of the dipped andfreeze-dried samples, but the microwaved samples were a bit softer, and had less aroma, colour and flavour. Osmotic dehydration was then stuclied as a technique of binding flavours and aromas and of reducing the time required for finish drying with microwaves. These studies were performed on strawherries and blueberries. Results showed berries that were dipped and then osmotically dehydrated for 24 h in sucrase, yielded a microwave-dried final product that was equal ta the freeze-dried one in terms ofquality, and this, with a much

ü lower time for finish drying. The shrinkage ratio ofstrawberries has a straight line relation ta the moisture ratio. The reduction in equivalent diameter is well-described by a reciprocal logarithmic function. The results of these major aspects of the research suggest that microwave-drying could be a viable and more rapid alternative ta freeze drying when berries are first subjected to a pretreatement of ethyl oleate and partial dehydration by osmosis. 1t was aIso found that if microwave energy is applied in continuous mode, the initial applied power should not exceed 0.2 W g-t, otherwise burning may occur. 1t is also recommended that osmotic dehydration be limited to not much longer than 24 hours, since off-odours develop. The results apply to convective regimes with inlet air temperatures of 45°C and inlet velocity of 2 mis.

iti RÉsUMÉ

Kamadenahally Venkatachalapathy Ph.D. (Génie agricole et biosystèmes.) Séchage par osmose et par micro-ondes de la fraise et du bleuet

Un séchoir hybride combinant l'énergie micro-onde (2 450 l\fHz) et l'air chaud fut utilisé pour la production de fraises séchées à partir de fruit entiers, tranchés ou réduits en purée. TI n'a pas été possible de sécher les fruits entiers avec ce systèmes puisque les fnrits cuisaient et se fendillaient. Les fruits tranchés ou réduits en purée ont séché mais les produits finis avaient perdu leur arôme, leur saveur et leur couleur caractéristiques lorsque comparés à des fruits lyophilisés. Pour remédier à ce problème, un traitement par trempage dans une solution d'éthyle d'oléate et d'hydroxyde de sodium fut utilisée. Ce traitement utilisé par l'industrie pour facilité le passage de l'eau à travers l'enveloppe des fruits a permis d'accroître considérablement le taux de • séchage et il fut possible de sécher des fraises entières dans un séchoir à air chaud avec ou sans micro-ondes. Une concentration de 1% d'éthyle d'oléate a été suffisante pour maximiser la réduction du temps de séchage et il est fort probable que des concentrations encore plus faibles pourraient être utilisées pour les fraises. La réhydratation des échantillons étaient comparable à celle d'échantillons traités et lyophilisés. Parcontre, la qualité des échantillons séchés dans le système hybride était inférieure à la lyophilisation tant au niveau de la texture, que de l'arôme, de la saveur et de la couleur. Par la suite, la déshydratation partielle par osmose a été employé pour favoriser la rétention de l'arôme et de la saveur, et pour réduire les temps de séchage. Des essais furent effectués sur des fraises et des bleuets.

iv Les résultats ont démontré qu'un traitement par trempage dans une solution d'éthyle d'oléate et d'hydroxyde de sodium suivi d'un traitement de 24 heures de déshydratation partielle par osmose permettait d'accroître l'efficacité du séchage dans le système hybride. Les essais ont de plus indiqué que la qualité (arôme, saveur et couleur) des produits ainsi obtenus était comparable à celle d'échantillons obtenus par lyophilisation et que les temps de séchage étaient de beaucoup inférieurs à la lyophilisation. Les densités de puissance micro-ondes ne devraient pas excéder 0,2 W g-} pour optimiser la qualité des produits séchés. Afin d'éviter la formation d'odeurs désagréables, on recommande de limiter la déshydratation partielle par osmose à des temps n'excédant pas 24 heures. L'analyse des résultats a indiqué que le rapport d'affaissement du volume des fraises (résultant du séchage) était relié linéaire au rapport de la teneur en eau du produit, tandis que la réduction du diamètre équivalant du fruit pouvait être représentée par un relation logarithmique réciproque. Cette étude a démontré qu'un traitement par trempage dans une solution d'éthyle d'oléate et d'hydroxyde de sodium suivi d'un traitement de 24 heures de déshydratation partielle par osmose avant le séchage dans un système hybride combinant l'air chaud et les micro-ondes est une alternative viable et plus rapide que la lyophilisation pour la production de petits fruits séchés.

v This work is dedicated to my father late Mr. K. Venkataramaiah as a token of my gratitude for his great respect for education and educated people. ACKNOWLEDGMENTS

• 1 wish to express my deep gratitude to Professor Dr. G.S.V. Raghavan, Chair, Department ofAgricultural and Biosystems Engineering, McGill University for his initiation, encouragement and help which is responsible for my accomplishment of this work. With his effort and fore thought and continuous support academically, morally and in all walks of my life during my studies at McGill, extremely influenced me to accomplish this work. He inspired me in many ways in my life, his devotion to work and academic excellence, friendliness with his students are emulative. 1 personally henefitted a lot from him but for his initiation 1 would have not accomplished this work. 1thank and respect Mrs. Subhadra Raghavan for her support in encouraging him in all ms pursuits and for her great simplicity. May their tribe grow. 1 am grateful to the following faculty members, Prof Dr. Suzelle Barrington, Prof Dr. Jacques Andre Landry, Dept. of Agr and Biosystems Engineering, Prof Dr. A. Kushalappa, Dept. of Plant Science, Dr. V. Yaylayan, Dept of Food Science and Prof Dr. A.S. Mujumdar Dept. of chemical Engineering for their continuous advise and encouragement during my studies. Dr. Kushalappa not only helped me in my academic pursuits, but he and Mrs. Meena Kushalappa were helpful in manyaspects, which made my life comfortable, 1 am very grateful to them. Many people contributed directly and indirectly in realizing this work, 1 acknowledge with gratitude the continuous support given to me by Mr. Yvan Griepy and Ms. Valérie Orsat, whenever 1 had any problem 1 looked forward to their help and advise, they are great people to work with and always ready to help. Yvan was helpful in setting up the microwave drying unit, and in the measurement oftexture. Mr. Peter Alva a scientific

vii wizard was responsible incritica11y analyzing the results and discussing the possibilities and criticizing for the improvement of presenting the results in a seientifie way, he is a gentleman ta work with. 1 thank him immensely for his editorial eomments and help and for bis friendship. Mr. E. Noroozi of the Dept of Food Science was helpful in providing all the neeessary equipments for eondueting rehydration studies, 1 thank mm for all that help. Mr. Mark McBratney is a very interesting and a meticulous person to diseuss on various experiences of life and his help in eondueting the studies on strawberry temperature and shrinkage measurements during microwave drying was gratefully aeknowledged. Prof Dr. Chandra Madramoottoo was always enquiring about my progress ofwork and helped in providing the chromameter for measuring the color, 1 sincerely thank him. 1sincerely thank Dr. Samson Sotoeina1 for bis friendship and help in conducting freeze drying studies. 1 express my gratitude to Dr. Zaman Alikhani, very honest humble and a true friend for his advise and encouragement during my studies. 1 am very much indebted ta Prof Dr. H. Ramaswamy, Dept. of Food Science and Mrs. Lakshmi Ramaswamy, Prof Dr. Shiv Prasher, Dept. of Agricultural and Biosystems Engineering and Mrs. Sunitha Prasher, Prof Dr. Kisan Gunjal of Dept. Agricultural Economies and Mrs. Sharmila Gunjal for their help friendship and continuous support and encouragement. 1sincerely acknowledge the help and encouragement received during my studies from Dr. T.N. Tulasidas and Mrs. Kamala Das, Dr. B. Ranganna and Mrs. Rathna Ranganna, Mr. M. Ramachandra and Mrs. Hema Ramachandra, Mr. Prabhanjan and Mrs. Shubha Prabhanjan, Dr. G.S. Bhat and Mrs. Sashikala Bhat. 1 am thankful ta all my colleagues from the Department ofAgricultural Engineering UAS Bangalore for their help and encouragement. 1 specially thank for the support given ta me

viii during my studies by Dr. M. Chowde Gowda, Mr. S. Gopinath, Mr. D. Ganeshamurthy, Mr .G.N. Srinivas, Mr. D. Keshavamurthy, Mr. T.M. Basavakumar, Mr. H. Eswarappa, Mr. Ramakumar, Mr. Venkatesh SosIe, Mr. Venkatesh Meda, and Mr. K Harish 1 sincerely thank the Natura! Science and Engineering Research Cauncil of Canada (NSERC) and Canadian International Development Agency (CIDA) for their financial help during my studies and the University of Agricultural Sciences for the study leave. 1 am very grateful ta my late father K Venkataramaiah, whom 1 missed during bis last clays, who had high regard and respect for education and educated people, ms interest in education brought me to this stage. 1 thank my mother for her benedictions and my wife Dr. M. Lalitha, my daughters V. Gayathri, V. Suvarna and V. Suparna for their support and encouragement during my studies. Finally my contribution in this work is very trivial compared to Many people who directIy and indirectIy responsible and contributed and supported me in carrying out this work, 1saIute and thank aIl of them very sincerely.

lX TABLE OF CONTENTS

Page CHAPTER 1 INTRODUCTION 1 1.1 Hypothesis 2 1.2 Objectives 3 1.3 Scope 4

CHAPTERH REVIEW OF LITERATURE 5 2.1 Introduction 5 2.1.1 Strawberries 7 2.1.1.1 Some physical properties of strawberries 7 2.1.1.2 Nutritional content of strawbenies 8 2.1.1.3 Strawberry flavour and aroma 10 2.1.2 Blueberries 12 2.1.2.1 Properties of Lowbush blueberries 13 2.1.2.2 Blueberry colour 14 2.2 Dehydration and drying 15 2.2.1 Drying of fruit 16 2.2.1.1 Specifie studies 17 2.2.2 Pretreatments for drying fruit 18 2.2.3 Osmotic dehydration 20 2.2.3.1 Energy consumption in osmotic dehydration 24 2.3 Shrinkage 24 2.4 Microwaves 26 2.4.1 Introduction 26 2.4.2 Advantages of Microwave Heating 27 2.4.3 Advantages of Microwave Drying 28

x 2.4.4 Interactions of Microwaves and Biological Materials 28 2.4.5 Microwave Drying of Agricultural Products 31 2.5 Drying Models 33 2.6 Quality Assessment 37 2.6.1 Colour Measurement 37 2.6.2 Texture Measurement 38 2.6.3 Rehydration 39 2.6.4 Sensory Evaluation 40

CHAPTERIII MATERIALS AND METHOnS 41 3.1 Microwave Drying Setups 41 3.1.1 Setup used in Preliminary Studies 41 3.1.2 Experimental Microwave Drying Unit 43 3.1.3 Initial Moisture Determinations 44 3.1.4 Data Acquisition System 47 3.2 Freeze drying 49 3.3 Osmotic Dehydration 49 3.4 Quality Evaluation of the Dried Product 49 3.4.1 Rehydration tests 49 3.4.2 Colour determination 51 3.4.3 Texture 52 3.4.4 Sensory evaluation 53 3.5 Experimental Design and Analyses 53

CHAPTER IV PRELIMINARY STUDIES ON MICROWAVE DRYING OF WHOLE, SLICED AND PUREED STRAWBERRIES 4.1 Introduction 54 4.2 Materials and Methods 54

xi 4.2.1 Initial Investigations 54 4.2.2 Microwave Drying of Whole Strawherries 55 4.2.3 Drying of Sliced Strawherries 56 4.2.4 Drying of Strawberry Puree 56 4.2.5 Freeze-Drying 57 4.2.6 Quality Evaluations 57 4.3 Results and Discussion 58 4.3.1 Drying of Slices 58 4.3.2 Drying Puree 60 4.3.3 Rehydration of Strawberry Slices 64 4.3.4 Quality and Colour Analysis 65 4.3.5 Chromacity 66 4.4 Conclusions 66 4.5 Connecting Statement to Chapter 5 68

CHAPTERV MICROWAVE DRYING AND SHRlNKAGE OF PRETREATED WHOLE STRAWBERRIES 5.1 Introduction 69 5.2 Materials and Methods 70 5.2.1 Microwave Drying 70 5.2.2 Freeze Drying 71 5.2.3 Shrinkage of Strawberries 71 5.2.4 Relative Drying Rate 72 5.3 Results and Discussion 72 5.3.1 Drying Kinetics 72 5.3.2 Rehydration Ratio 75 5.3.3 Toughness 78 5.3.4 Colour Difference With Respect to Fresh Berry 80 5.3.5 Shrinkage 81

XlI 5.4 Conclusions 87 5.5 Connecting Statement to Chapter 6 89

CHAPTER VI OSMOTIC AND MICROWAVE DRYlNG OF STRAWBERRlES 6.1 Introduction 90 6.2 Materials and Methods 90 6.2.1 Dipping Treatment 90 6.2.2 Osmotic Dehydration 91 6.2.3 Drying Experiments 91 6.2.4 Quality Assessment 91 6.3 Results and Discussion 92 6.3.1 Osmotic Dehydration 92 6.3.2 Drying Times 93 6.3.3 Empirical Model of Finish Drying With Microwaves 97 6.3.4 Quality analyses 101 6.4 Conclusions 106

CHAPTER VII OSMOTIC AND MICROWAVE DRYlNG OF BLUEBERRIES 7.1 Introduction 107 7.2 Materials and Methods 107 7.3 Results and Discussion 107 7.3.1 Osmotic Dehydration 107 7.3.2 Drying Kinetics 109 7.3.3 Empirical Model of Finish Drying With Microwaves 114 7.3.4 Quality 114 7.4 Conclusions 122

xiü CHAPTER VIII CONCLUSIONS, CONTRIBUTIONS TO KNOWLEDGE AND RECOMMENDATIONS FOR FURTHER WORK 8.1 Summary and Conclusions 123 8.2 Contributions to Knowledge 126 8.3 Recommendations for Further Studies 127

REFERENCES 129

APPENDICES Appendix A 147 Appendix B 149 Appendix C 151 Appendix D 154 Appendix E 157 Appendix F 169

xiv LIST OF FIGURES

Figure Page

3.1 Microwave drying setup used in the preliminary experiments. 42 3.2 Experimental microwave drying setup. 45 3.3 Photograph of the experimental microwave drying setup. 46 3.3a Lyo Tech Canada, Freeze drier (used in the experiments for drying strawberries and blueberries). 48 3.4 Data acquisition system connected to the drying setup. 46 3.5 The Minolta chroma meter (used in colour measurements). 50 3.6 Instron machine used in measuring the texture. 50 4.1 Dehydration of strawberry slices under microwave power levels 0,2,3 and 4 at inlet air temperatures 35°C. 62 4.2 Dehydration of strawberry puree under microwave • power levels 0, 1, 2, and 3 at inlet air temperature 35°C. 62 4.3 Dehydration of strawberry slices and puree under microwave power level 2 and inlet air temperature 35°C. 63 5.1 Convective and microwave drying of strawberries treated with 2% ethyl oleate and 0.5% sodium hydroxide, at different power levels. 76 5.2 Microwave drying of strawberries at 0.2 W/g power with different pretreatment levels. 76 5.3 Microwave drying of strawberries at power level 0.1 W/g correlated between shrinkage ratio and moisture ratio. 82 5.4 Microwave drying of strawberries at power level 0.2 W/g correlated between shrinkage ratio and moisture ratio. 82

xv 5.5 Microwave drying of strawberries at power level 0.1 W/g correlated between equivalent diameter and moisture ratio. 83 5.6 Microwave drying of strawberries at power level 0.2 W/g correlated between equivalent diameter and moisture ratio. 83 5.7 Surface (Tl) and centre (T2) temperatures of strawberry fruit during microwave drying at power level 0.1 W/g. 85 5.8 Surface (Tl) and centre (T2) temperatures of strawberry fruit during microwave drying at power levei 0.2 W/g. 85 6.1 Osmotic dehydration of treated and untreated strawherries. 96 6.2 Microwave drying of osmotically dehydrated strawherries at different power levels. 96 6.3 Microwave drying of strawberries at two power levels dehydrated at two osmotic levels (F:S). 98 6.4 Microwave drying rate of osmotically dehydrated strawberries at different power levels and inlet air temperatures of 35°C and 45°C. 98 • 6.5 Predicted moisture content of strawberries by the exponential model compared with the experimental values (PL: 0 W/g). 99 6.6 Comparison of the moisture content of strawberries predicted by the exponential model and the experimental values (PL: 0.1 W/g). 99 6.7 Predicted moisture content of strawberries by the exponential model compared with the experimental values (PL: 0.2 W/g). 100 7.1 Osmotic dehydration of untreated and treated blueberries at different durations of time. 111 7.2 Microwave drying of osmotically dehydrated blueberries at different power levels. 111

xvi 7.3 Microwave drying of blueberries osmotically dehydrated with F:S, 3:1 and 4:1 and dried at power levels 0.1 and 0.2 W/g. 112 7.4 Microwave drying rate of osmotically dehydrated blueberries at different power levels. 112 7.5 Predicted moisture content of blueberries by the exponential model compared with the experimental values (PL: 0 W/g). 115 7.6 Comparison of the moisture content of blueberries predicted by the exponential model and the experimental values (PL: 0.1 W/g). 115 7.7 Predicted moisture content of blueberries by the exponential model compared with the experimental values (PL: 0.2 W/g). 116

xvii UST OF TABLES

Table Page

2.1 Cost comparison of drying methods used in the food industry. 6

2.2 Major strawberry cultivars grown in North America in 1990. B

2.3 Nutrients in frozen strawberries per 100 g (Sweetened 4+1). 9

2.4 Volatile compounds (ng/g fresh mass/BO litres) in strawberry fruit during ripening. Il 2.5 Fruit quality measurements on some important cultivars. 12 2.6 Organic and Phenolic acids of lowbush blueberry cultivars. 13 2.7 Physical and Chemical Characteristics of lowbush blueberry cultivars at three different stages of maturity. 14 4.1 Drying times required for sHces to obtain a moisture content 0.2 kglkg (db) at different power leveis and inlet air temperatures. 59 4.2 Mean drying time of slices at different air temperatures. 59 4.3 Mean drying time of slices at different power levels. 59 4.4 Drying time required for puree to obtain a moisture content of 0.2 kglkg (db) at ditTerent power levels and inlet air temperatures. 60 4.5 Mean drying time of puree at different air temperatures. 61 4.6 Mean drying time of puree at different power levels. 61 4.7 Rehydration ratio and rehydration coefficient of microwave dried and freeze dried strawberry slices. 64 4.8 Mesns separation by Duncan's new multiple range test for the quality assessment of microwave (MW) and freeze-dried (FD) slices and puree by the judges. 65

xviii 4.9 Chromacity measurements (aIb) for microwave (MW) and freeze dried (FD) strawberry slices and puree. 66 5.1 Mean drying time of strawberries at different ethyl oleate concentrations. 73 5.2 Mean drying time of pretreated strawberries at difIerent power levels. 73 5.3 Relative drying rate of strawberries under different treatments in reaching a MC of 0.2 kg/kg (DB). 74 5.4 Rehydration ratios of microwave and freeze-dried strawberries with different chemical pretreatments. 75 5.5 Rehydration ratios of strawherries dried by different methods. 77 5.6 Rehydration ratios of strawherries at different ethyl oleate concentrations. 77 5.7 Texture (Toughness) of microwave and freeze dried strawberries treated with different ethyl oleate concentrations. 79 5.8 Toughness of strawberries at different EO concentrations. 79 5.9 Toughness of strawberries according to drying regime. 80 5.10 Colour measurements of microwave and freeze dried strawherries under different pretreatments. 80 5.11 Shrinkage ratio, equivalent diameter and change in volume during microwave drying of strawhenies at 0.1 W/g (top) and 0.2 W/g (bottom). 84 5.12 Constants for linear equations describing shrinkage ratio of strawherries, and for reciproca1logarithmic equations (RL) describing equivalent diameter, as functions of the moisture ratio under microwave drying at power levels 0.1 and 0.2 W/g. 86

xix 6.1 Means separations by Duncan's test for moisture removal at the two fruit to sugar ratios, treatment with EO/NaOH and time of dehydration. 92 6.2 Microwave drying time and relative drying rate of osmotically dehydrated strawberries. 94 6.3 Duncan groupings for Mean drying times at the two temperatures, frnit to sugar ratios and power levels. 95 6.4a Rehydration ratio and texture measurements of osmotically dehydrated and microwave dried strawberries. 102 6.4b Means separation of rehydration ratios at the experimental temperatures, fruit ta sugar ratios and power leveis. 103 6.4c Duncan's groupings for mean toughness at two temperatures, froit ta sugar ratios and drying regimes. 103 6.5a Color measurements of osmotically dehydrated and microwave and freeze dried strawberries. 104 • 6.5b Means separations of colour differences due to drying regime, froit to sugar ratio and inlet air temperature. 105 6.6 Sensory evaluation of osmotically dehydrated and microwave and freeze dried strawberries. 105 7.1 Duncan's groupings for MCEM (moisture content removed) from blueberries in osmotic dehydration under different F:S, pretreatments, and time. 108 7.2 Duncan's groupings for Mean drying time, at clifferent temperatures, F:S, and microwave power levels. 109 7.3 Drying time and relative drying rate of blueberries osmotically dehydrated and microwave dried at different microwave power levels. 110

xx 7.4 Rehydration and texture measurements of blueberries osmotically dehydrated followed by microwave or freeze dried, under different treatments. 113 7.5 Duncan's groupings for Mean Rehydration, at different temperatures, F:S, and microwave power levels. 117 7.6 Duncan's groupings for Mean Rehydration, at different temperatures, F:S, and microwave power levels. 117 7.7 Duncan's groupings for mean toughness, at different drying regimes. 118 7.8 Colour measurements of blueberries osmotically dehydrated then microwave or freeze dried under different treatments. 119 7.9 Duncan's groupings for Mean colour difference, at different temperatures, F:S, and drying regimes. 120 7.10 Duncan's groupings for Mean score by different judges. 121 7.11 Duncan's groupings for Mean score at different regimes. 121

xxi NOTATIONS a Chromacity coordinate (redness or greenness) B Sugar concentration % in equation 2.3 b Chromacity coordinate (Yellowness or blueness) regression prameter estimates equation 6.1 C Sugar concentration % in equation 2.1 Cp Specifie heat D Depth of penetration in cm D Diffusivity of moisture em2/see in equation 2.11 DB Dry basis De Equivalent diameter cm E Eleetric field strength Volts/m Color difference form the target color F Mass % in equation 2.3 F:S Fruit to sugar ratio (mass / mass) FD Freeze dry f Frequency Hz k Parameter of the drying regime in equation 2.12 kg kilo grams L Lightness or darkness (Chromacity coordinate) Mo Initial moisture content kg/kg M Moisture content at any time kg/kg MC Moisture content kg/kg MHz Mega Hertz (frequency) l\1Pa Mega Pascal MW Microwaves Equilibrium misture content kglkg initial moisture kg/kg

xxii Mass of rehydrated sample g mdh Mass ofdehydrated sample g Initial MC % (Wet basis) of the sample before drying MC% of the dry sample (wet basis) n Parameter of the drying regime in equation 2.12 Nana grams Microwave power level, Watts/g Radius, in m Shrinkage ratio Temperature oC Surface temperature oC Centre temperature oC time in h Drying time of control sample (min) Drying time of treated sample (min) v Volume at a given mositure content cm3 Initial volume m3 Bulk volume m3 at initial moisture content Bulk volume at moisture x m3 Dielectric constant F/cm . Dielectric loss factor F/cm E Complex dielectric constant Ào Free space wavelength cm p Density kglm3 ç Relative drying rate â Incremental change Subscripts b Bulk a Initial t target x at moisture x 1, 2 Surface, Centre c Control t treated, in equation 5.1

xxiv CHAPTERI

INTRODUCTION

The use of microwave energy for thermal treatment of agricultural commodities has been studied since the 1945's. Where hydroelectric power is available, microwave energy can reduce fossi! fuel needs in a wide variety ofthennal treatment applications in agriculture and other industries. The primary advantages of using radiative energy transfer rather than convective or conductive transfer are that: 1) more of the applied energy is converted to heat within the target material, and 2) heating is more rapid due to the fact that the surface-to-centre conduction stage is largely eliminated (Schiffinann., 1995). Rapid initial moisture 1055 has also been attributed to mechanical expulsion of moisture caused by the strong, yet short-lived vapour pressure gradient induced by volumetrie heating (Decareau, 1985; Kostaropoulos and Saravacos; 1995). Microwaves have been shown ta result in faster drying of materials such as corn while satisfying certain end-product quality constraints (seed quality corn - Shivhare et al., 1994; raisins - Tulasidas et al., 1994). Tulasidas et al. (1994) also showed that the specifie energy consumption for drying grapes into raisins by microwaves was at least 50% lower than by hot-air drying. Microwave drying has aIso been accepted in the food industry for processing applications including drying of pasta and snack {oods such as potata chips and cookies (Decareau and Peterson, 1986). AIthough such experimental work and real-life applications are encouraging, it is clear that microwaves are not suitable under all circumstances. High moisture materials with surface layers that resist moisture diffusion May burst when heated with microwaves due to internai

1 pressure buildup. Enzymatic processes necessary to flavour development May be too rapidly attenuated when microwaves are used for heating (eg. sweet potatoes - Sun et al., 1994). It is also difficult to obtain browning reactions when using microwaves ta cook meat or bakery products. Thus, one cannat îndiscriminately use microwaves ta process all commodities. Ta date there has been little work on the microwave drying of berry fruits such as strawberries (Fragaria ananassa) and blueberries (Vaccinium angustifolium). These high moisture commodities are important ta local rural economies as cash crops, and are important sources of vitamins and mineraIs to the populations of northern temperate climates. The development of more resistant and higher-yielding cultivars has resulted in production levels that outweigh the in-season demand. There is therefore a need ta develop markets for the dried product, which could be facilitated if a less expensive drying tecbnology could he developed. The industry quality standard for dried berries is presently the freeze-dried product, which is used as such in breakfast cereals, or in rehydrated state in bakery products, ice creams and other food preparations. Freeze-drying is generally accepted as the method which best preserves flavours, calour and arama. However, freeze-drying is energy-intensive and time-consuming. Thus, the possibility that microwave technology can be applied ta the production of high quality dried strawberries and blueberries shauld be considered.

1.1 HYPOTHESfS

The hypothesis entertained here is that it is possible ta produce high quality dried strawberries and blueberries using microwaves as the energy source. This implies that colour, arama and flavour must be retained ta a great an extent as possible, and that the rehydration characteristics are

2 good. Therefore, it is anticipated that industry techniques to reduce skin resistance ta diffusion and to stabilize aromas, flavours and pigments May be needed.

1.2 OBJECTIVES

The main objective of this study was to develop a microwave-based process to produce dehydrated strawbenies and blueberries having quality attributes equivalent to those of the freeze-dried product.

The specifie objectives pursued as the work evolved, were: i) Evaluate microwave-drying of whole and pureed fruit in terms of drying kinetics and product quality.

H) Study possible improvements due ta sodium hydroxide/ethyl oleate and determine optimum conditions of microwave operation with this pretreatment.

iii) Determine the osmotic dehydration rates of untreated and pretreated strawberries and blueberries in sucrase and the microwave-drying kinetics of osmotically dehydrated berries.

iv) Perform quality evaluations of the berries from (iii) and compare the products ta pretreated freeze-dried samples according ta texture, colour, taste and rehydration.

v) Determine and model shrinkage ofpretreated microwave-dried

3 strawberries in tenns of power level and moisture ratio.

• vi) Generate empirical models to predict the drying kinetics of osmotically dehydrated strawberries and blueberries in tenns of the microwave power applied.

1.3 SCOPE

This study is limited to blueberries and strawberries. The models were purely empirical, although the model described by Tulasidas et al. (1994), which also accûunts for shrinkage, could be tested for strawberries, since shrinkage/moisture relationships were elaborated in this study. This study did not iDclude the evaluation of storage time or keeping quality of the microwave-dried fruit. Although it was recognized that infusion of sugars could affect the drying behaviour and perhaps quality attributes ta a certain extent, no • attempt was made to conduct the relevant chemical analyses. Microscopie photography was not used to study the time trends in dissolution of waxy cuticle or in formation of surface micropores due to the dipping solution. No attempt was made to process or analyze the syrups recovered from the osmotic dehydration step. Although microwaves were applied in dutY cycle mode in the first experiments, no attempt to study the drying kinetics in pulsed mode was made in the subsequent experiments, even though this option could have been executed with the new microwave apparatus.

4 CHAPTER II

REVIEW OF UTERATURE This literature review will briefly caver some basic properties of strawberries and b1ueberries, principles of microwaves and microwave drying of agricultural materials, as weIl as osmotic dehydration. A short section on modelling is a1so ineluded sinee empirical models of microwave drying were generated from the data; however, modelling of microwave drying was not a primary objective of this thesis since this aspect was previous1y investigated for grain by Shivhare (1991) and grapes by Tulasidas (1997).

2.1 INTRODUCTION Strawberries and blueberries are cash crops which are produced mainly in Northern Temperate Regions. They are excellent sources of vitamins and mineraIs and are consumed to a large extent in fresh condition. However, the Cresh product is usually Iocally available over a very limited time and production far outweighs fresh demand. Blueberries May he cold stored for up to six weeks, whereas fresh strawberries have a storage life of only 5-7 days. Of the 2.1 million tons of strawberries produced annually, 70% are marketed fresh. Blueberry production is only about 85,000 tonnes, 90% being produced in North America. Houghly 50% are marketed fresh, whereas about 33% are preserved frozen. Bath of these fruits are used in a wide variety of food products including yogurts, pastries, muffins, snack foods, ice creams, cereals, baby food, concentrate, and juice drinks. Dried berries are consumed mainly in products where they will be rehydrated either prior to consumption, such as breakfast cereals, or during

5 preparation, sncb as pancake and muffin mixes. The main advantage of using the dried berries in baked goods is that they do not "bleed" juice, and thus give a more aesthetic overall produet; however, the drying method used must yield a produet with good rehydration characteristics, as weIl as retention of colour, flavour, aroma and nutrients. These and other berries are usually freeze·dried sinee this technique gives the best quality, even though freeze-drying is the most expensive method used in the agri-food industry (Table 2.1). Freeze-dried strawberries and blueberries are available in severaI forms . whole, sliced, diced, leathers - and vary widely in priee aceording ta size, availability and source. For example, freeze-dried strawberries currentlyare sold in the range of30-60 US$ per kg, whereas freeze-dried eultivated blueberries sell for about 35 US$ and wild blueberries sell for about 12 SUS per kg (pers. comm.- Oregon Freeze Dry).

Table 2.1. Cost eomparison of drying methods used in the food industry.

Drying Method Approximate cast

U.S cents / kg

Drom 2 - 2.2

Air 2.2- 3.3

Spray 1.5- 3.3

Foam-mat 5 • 6.6

Vacuum PuB' 6.6- 8.8

Freeze 11- 22

Source: (Heldman and Lund, 1992)

6 Although freeze-dried quality is considered to he the hest, freeze-dried products have sorne disadvantages. These include high cost of the freeze.. • dried product, hygroscopy of the product requiring hermetic storage containers, low bulk density because the fruit does not shrink during the process. Hence storage, packaging and transportation of the freeze dried berries is more expensive.

2.1.1 Strawberries World production of strawberries is estimated to be 2.1 million tonnes, the main contributors, (descending arder of production) being the USA, Spain, Japan, Italy, Korea, Poland, Russia, France, Turkey, the United Kingdom and Germany (FAO, 1995). Of the 750,000 tonnes of strawberries produced in the United States every year, about 450,000 tonnes are grown in California (Pszczola, and Donald., 1995; FAO, 1995). Yields among the Many cultivars range from 20 ta 40 tonnes per hectare, the average yield being about 25 tonnes per hectare (Pszczola, 1995). North American cultivars by region are listed in Table 2.2. Other cultivars are used in Europe.

2.1.1.1 Some physical properties of strawbenies At DoC and 90-95% RH, strawberries consist of89.9% waterand 8.3% 3 l solids. Their density is 1033 kg/m , specifie heat is 3.849 kJ.kg· , thermal conductivity is 1.3450 W.m1m 2 K and the thermal diffusivity (27° ta -18°e) is 1.47 X 10.1 m2/s (Hardenbirg et aL, 1990; Rahman, 1995; Arthey and Ashurst, 1996).

7 Table 2.2. Major strawherry cultivars grown in North America in 1990.

• Region Cultivar

Califomia Chandler, Selva, Pijaro, Commander, Kent Sheehy, Swede. Florida Selva, Pajaro, Dover Pacific North West Totem, Benton, Glooscap (BC, WA, OR) Upper Midwest Honey, Kent, Glooscap (ND, SD, MN, WI, MI) Eastern Canada Veestar, Kent, Glooscap, Honeoye, Blomidon (Ont, Quebec, MP) Northeastem US Earliglow, Honeoye, Kent, Allstar ewv, MD, NJ, VT, NH, l'dA, ME) • Lower Midwest Earliglow, Raritan, Redchief, Allstar, (NE IA,MO,IL, IN ML, OH) Surecrop, Delite Southem U.S. Chandler, Earliglow, Cardinal, Apollo Canadian cultivars (recent) Acadia, Annapolis, Blomidon, Bounty, Coronwalis, Goosecap, Governor Simco, Kent, Mic Mac, Mira

(Source: Chandler, 1991)

2.1.1.2 Nutritional content of strawberries Even in frozen form, strawberries are important sources of severa! dietary components, as shown in Table 2.3.

8 Table 2.3. Nutrients in frozen strawberries per 100 g CSweetened 4+1).

Nutrients Unit Whole Whole Sliced Unsweetened Sweetened Sweetened

Water g 89.53 71.63 70.53

Ash g 0.37 0.29 0.30

Calories kcal 36.19 106.40 111.42

Sodium mg 2.49 2.19 3.30

Potassium mg 142.40 114.30 135.67

Carbohydrates g 9.40 27.50 28.08

Dietary Fibre g 1.43 1.14 1.29

Sugars g 6.53 25.21 29.38

Fructose g 3.43 2.74 3.77

Glucose g 2.90 2.32 3.50

Sucrose g 0.27 19.42 17.97

Protein g 0.55 0.44 0.48

Vitamin C mg 40.50 32.40 30.35

Calcium mg 19.67 15.93 13.97

Phosphorus mg 19.46 15.96 15.54

Magnesium mg 10.32 8.26 10.30

Source: California Strawberry Commission, Watsonville Ca.

9 According to FDA definitions, processed strawberries are sodiumfree, fat and cholesterol-free and low in calories. They are a source of Vitamin C, potassium, folacin, and dietary fibre. Approximately 50% of this total dietary fibre is water-soluble-pectin. Frozen unsweetened strawberries typically contain about 6.5% sugar (3% fructose, 3% glucose, and 0.5% sucrase). The typical aroma of strawberries resides in the ail fraction. The colour pigment (cyanidin 3-glucoside) is said ta remain very stable after freezing, but May be influenced by pH, polyphenol oxidase activity, ascorbic acid and total phenols. The water activity of processed, straight-pack strawberries is 0.85-0.95. The pH ofprocessed berries ranges from 3.3-3.6. Citric acid is the primary acid (Pszczola, 1995).

2.1.1.3 Strawberry tlavour and aroma One of the Most challenging areas in strawberry research is the assessment of fruit aroma and flavour, the important flavour impact compound is furaneol. Strawberry aroma is extremely complex, consisting af 35 to 200 volatile compounds. Most of the ripe strawherry aroma originates from methyl esters of methyl alcohol in the fruit. Enzymes convert the esters into Many volatile compounds. Ofthese, seven volatiles whase production depend on stage ofripeness, appear to be correlated with strawherry arama(Table 2.4). The relative concentrations ofthese volatiles aIso vary widely among cultivars (Perkins-Veazie and Collins., 1995).

Goad strawberry flavour is a cambination of arama, sweetness and acidity. Soluble solids content (SSC), which includes sugars, acids, and other substances dissolved in the cell sap, is the Most commonly used measure of sweetness. About 80-90% of the SSC is due to the sugars: glucose, fructose and sucrose. The SSC in strawberries increases continuously during fruit growth and ripening, from 5% in small green

10 • Table 2.4. Volatile compounds (ng/g fresh mass/80 litres) in strawberry fruit during ripening.

Compound Ripeness Stage

Green White Pink Red

Ethyl Hexanoate 18.7 82.2 110.2 392.3

Ethyl Butanoate 18.1 81.9 88.4 317.2

l\'Iethyl Butanoate 2.9 61.2 54.4 251.4

Methyl Hexanoate 0.0 30.5 32.6 116.8

Hexyl Acetate 7.5 107.4 26.8 61.5

Ethyl Propionate 0.0 9.9 7.7 30.2

3-Hexenyl Acetate 0.0 13.5 9.0 4.6

(Source: Perkins, et al. 1995)

fruit to 6-9% in red berries, and is dependent upon cultivar and environmental conditions (Perkins et al., 1995; Kader, 1991). Sorne flavour characteristics of various cultivars are given in Table 2.5.

11 Table 2.5. Fnrit quality measurements on sorne important cultivars • Cultivar Ascorbic acid Soluble Titratable SS/acid (mg/100 g) Solids % Acid % Ratio

Chandler 51 7.3 0.69 10.6

Seascape 46 7.0 0.74 9.5

Douglas 43 6.8 0.72 9.4

Irvin 41 6.9 0.66 10.4

Oso Grande 40 5.7 0.52 Il.0

Capitola 39 6.7 0.91 7.4

Selva 33 7.1 0.59 12.0

(Source: Perkins, et al., 1995; Kader, 1991).

2.1.2 Blueberries Lowhush blueberries (Vaccinium angustifoliumJ are native to Eastern Canada and the Northeast United States, whereas highbush blueberries (Vaccinium corymbosum) are native to Europe. North America is the world's largest producer ofblueberries, accounting for nearly 90% ofworld production. The lowbush froit is small compared to highbush orrabbitteye blueberries (Vaccinium ashei). The total area ofcultivated blueberries planted in North America is 18,088 ha, 80% of which are currently bearing. Approximately 19,800 ha of bluebenies are commercially cultivated worldwide (Eck, 1988), yields averaging 4-5 tonnes per hectare. • 12 2.1.2.1 Properties of Lowbush blueberries The lowbush blueberry averages about 0.414 glberry with a dry matter content of 15.14% and soluble solids of 10.74%. It can he stored at a temperature of o-soc and responds weIl at oxygen and carbon dioxide concentrations of 5-10% and lS-20% respectively. The shelf life is limited by fungal spoilage. For long term storage, the fruit is frozen or mixed with syrup and flash frozen. The specifie heat of the lowbush blueberry is 3.60 kJ/kgOK (Somogyi and Ramaswamy, 1996).

Table 2.6. Organic and Phenolie aeids of lowbush blueberry cultivars

Acids (%) Cultivar Mean

Maturity (Ripe) Blomidon Cumberland Fundy %

Chlorogenic 13.36 9.35 10.07 10.93 Citrie 33.32 36.30 35.30 34.97 Malic 30.52 32.95 30.12 31.20 Quinie 19.05 1B.36 21.42 19.61 Acetic 2.B1 1.80 1.90 2.17 Caffeie 0.51 0.66 0.82 0.66 p-Coumaric 0.28 0.18 0.19 0.22

Ferulie 0.00 0.23 0.00 O.OB Shikimie 0.15 0.16 0.17 0.16

(Source: Kalt and McDonald. 1996.)

13 2.1.2.2 Blueberry colour Blueberry colour is an important quality factor influencing fresh market value and the suitability ofthe berries forprocessing. Their intense

Table 2.7. Physical and Chemical Characteristics oflowbush blueberry cultivars at three different stages of maturity. Berry Dry fu- Solu- Glac- Total mnes able ouse,

Fresh Matter Solids Fruct acid -ose

Maturity Cultivar (g/berry) % (N) % % peq/g dry

Unripe Blamidon 0.217 13.80 97.52 7.52 3.69 1.96 Cumberland 0.220 14.62 72.28 8.71 5.42 2.00 Fundy 0.305 12.63 95.68 7.70 4.29 2.22 Mean 0.247 13.68 88.49 7.98 4.47 2.06 Ripe Blomidon 0.292 15.95 79.17 10.72 7.40 1.59 Cumberland 0.344 16.23 49.00 12.18 8.73 1.43 Fundy 0.357 14.03 72.76 10.55 7.04 1.90 Mean 0.331 15.40 66.98 11.15 7.72 1.64 Ovenipe Blomidon 0.631 16.81 58.45 13.13 11.04 1.40 Cumberland 0.600 17.32 49.94 14.22 10.87 1.36 Fundy 0.761 14.89 58.74 11.92 9.15 1.68 Mean 0.664 16.34 55.71 13.09 10.35 1.48 Grand Mean 0.414 15.14 70.39 10.74 7.51 1.73

(Source: Kalt and McDonald. 1996.)

14 blue ta red colour, and high pigment content, make them valuable as food colorants ingredients for foods. Blueberries derive their bold colouringfrom the high content of anthocyanin. Anthocyanin is a soluble pigment that imparts colours ranging from blue ta shades ofred. The intensity of pigmentation increases during the first six days of colour change at the early stage of maturation. The phenolic acids (Table 2.6) and physical and chemical characteristics (Table 2.7) of sorne cultivars of blueberry are presented.

2.2. DEHYDRATIONAND DRYlNG Drying and dehydration are the remova! of moisture from a given material. Although these tenns are often interchanged, it is perhaps necessary to make a distinction since "drying" implies the process of moisture removal due ta simultaneous heat and mass transfer (ie. thermal drying). Drying as such, therefore refers primarily to the remova! of moisture in the vapour phase, whereas dehydration is a more encompassing term and includes methods of moisture remova! that can he done without addition of heat Ceg. compression, reverse osmosis, filtration, etc.). Since drying is based on phase change, the theoretical minimum energy required ta remove moisture by drying is about 2.5 MJlkg moisture, equivalent to the latent heat of vaporization of water. However, heat recovery from the released vapour, as weIl as initial steps ofmechanical or osmotic dehydration can reduce the specifie energy required substantially. In the food processing industry, specific energy requirements for dehydration are usually in the range of 1 to 2 MJ/kg moisture removed (Rizvi 1995) In drying, heat supplied from the environment conducts into the material and increases the vapour pressure within the material. As long

15 as the vapour can diffuse to the surface ofthe material and the surrounding air is Dot saturated, tm8 moisture is taken up and carried away • convectively. To sorne extent, the vapour-pressure difference between the air and the surface of the material can induce outward liquid flow. The heat may be supplied to the material surface by convection or conduction, from where it is conducted towards the interior. Radiation, such as microwaves, has the advantage ofpenetrating the surface and interacting volumetrically with water Molecules to generate heat internally, thus eliminating the surface-ta-centre conduction stage. This leads ta much faster heating rates, and can create a pressure-driven liquid flow in sorne circumstances (Ratti and Mujumdar, 1996).

2.2.1 Drying of fruit Three basic techniques are used to dry fruits: 1) Sun drying; 2) Atmospheric drying in batch (kiln, tower, and cabinet dryers) or continuous (tunnel, belt, belt-trough, fluidized hed, explosion pufi", foam-mat spray, drom and microwave heated) mode, 3) Suhatmospheric dehydration (vacuum, shelfldnun and freeze dryers). Low temperaturel10w energy processes such as osmotic dehydration and microwave drying have recently received more attention (Jayaraman and Das Gupta, 1992). The main problems in fruit drying are damage ta sensory characteristics and 108s of nutritional components due to long exposure to high temperatures (Van Arsdel et al. 1973; Fellows, 1988). These include loss ofaroma volatiles, oxidation ofpigments and vitamins, due to and case hardening in certain products. Case-hardening is a common defect ofdried fruits and is caused by drying that is tao rapid compared ta the rate of diffusion of moisture through the product. In such conditions, the outer layers overdry and inhibit moisture diffusion, leaving the interior wet. Since strawberries and blueberries are high moisture fnrits, are very

16 fragile, and are grown extensively in temperate and cool climates, sun drying is not usually possible. These fruits are usually preserved in frozen form, or freeze-dried. Osmotic dehydration is used to reduce the energy required for freeze-drying which is the preferred method for smaII fnrit in North America. A varietyofnovel methods have recently been investigated for drying these fruits. Among them are vacuum drying, microwave drying, and combinations of freeze and microwave drying, vacuum and microwave drying, osmotic and vacuum drying and osmotic and freeze drying (Alvarez et al. 1995; Hemphill and Martin, 1992; Sullivan, et al. 1982; Yang et al. 1987; Kim and Toledo, 1987; Yang and Atallah, 1985). These studies are oriented to finding alternatives to freeze-drying or to reducingthe costs and energy requirements for freeze-drying, under the constraint of quality maintenance.

2.2.1.1. Specifie studies Surprisingly, there is very little published work on drying of strawberries although there is a fair amount on the drying of blueberries. Lowbush blueberries were dehydrated by Yang and Atallah (1985) to moisture contents of 16-25% using four clifferent methods: forced air, vacuum oven, freeze dry and micro-convection methods. The quality attributes of dried blueberries were determined. The levels of vitamin A, C and niacin found in the dried berries were 10w compared to fresh berries. The forced air drying method gives berries with lower soluble solids, colour retention, rehydration and bulk density. The micro..convection method achieved the desired moisture level within the least time. However, the quality of the dried berries was poor in comparison with berries dried by other methods. Freeze drying gave the highest retention of the other important components such as soluble solids and colour and led to the

17 highest rehydration ratio and lowest bulk density. Berries dried by the vacuum aven method were aIso high in soluble solids and colour retention. The authors suggested the combination of vacuum oven and freeze drying methods ta bring down the time and cast, sinee the these methods led to a good quality product. They a1so suggested that the results of tms study could be applied to smaH fruits like raspberries, cranberries, strawberries and blackberries. Rabiteye blueberries were dried using a high temperature fluidized bed drier (HTFB) by Kim and Toledo (1987). A 15 m1sec air velocity was required for fluidization and at 170°C the moisture content was reduced from 5.8 kg/kg to 0.7 kglkg after 8 min. After osmotic dehydration in sucrose, the moisture content was 1.3 kg/kg. With a 4 min treatment in the HTFB at 150°C, the moisture content reduced ta 0.28 kg/kg. The HTFB simultaneously dried and puffed the berries, resulting in reduced bulk density compared to berries produced using conventional drying. Osmotic dehydration prior ta HTFB imparted a raisin-like texture to the product. A hybrid process of osmotic dehydration and freeze drying was investigated by Yang et al. (1987) ta produce a raisin-type lowbush blueberry product. Using a berry:sugar ratio of 3:1 or 4:1 for osmotic dehydration, followed by a sequence of thorough rinsing, freeze drying with abrupt release of vacuum, and a thermal conditioning, it was possible ta produce a raisin-type blueberry product. The final product had good texture, flavour, overall acceptability, and predicted shelflives of 16 and 64 months at 25°C and SoC storage respectively.

2.2.2 Pretreatments for drying fruit Fruits and vegetables are subjected to certain pretreatments in arder to facilitate drying and minimize adverse changes during drying and subsequent storage of the products. Alkaline dips are used particularly ta

18 improve moisture diffusion through the cuticle. Sulphating and blanching are used to inhibit the growth of microorganisms and to deactivate enzymes. The alkaline dip involves immersion of the product in an alkaline solution prior to drying and is used primarily for fruits that are dried whole, especially prunes and grapes. A sodium carbonate or Iye solution (0.5% or less) is usually used at a temperature ranging from 93.3°C to 100ee (Salunkhe and Desai, 1984). It facilitates drying by forming fine cracks in the skin. Alkalies such as potassium carbonate used in the Australian procedure, were found ta he not effective in reducing drying time. Dleate esters constitute the active ingredients of commercial dip

solutions used for grapes. They accelerate moisture 1055 by causing the wax piatelets on the grape skin to dissociate, thus facilitating moisture diffusion. Pooting and McBean (1970) studied the pretreatment of waxy fruits like cherries, blueberries, prunes and grapes. The most effective dipping materials for increasing drying rate were found ta be the ethyl esters of fatty acids. Ethyl oleate was the most convenient ta handle and was effective. Dipping the waxy fmits for a few seconds in a cold aqueous emulsion ofethyl oleate or other suitable compound reduced drying time in most cases to one..half or less that required for water dipped control. Suarez et al. (1984) air dried sweet and immature field corn kemels without treatment or after dipping for 30 sec in a cold aqueous suspension of 1% ethyl oleate. Dipping in ethyl oleate resulted in a great increase in the drying rate of kernels. This was attributed to the action ofethyl oleate on the waxy cuticle of corn kemel which resulted in reduced cuticular resistance to water loss. Shelled com was dried by Williams (1989) using a bin batch drying without and with 2.7,10.8, and 43.2% aqueous solutions of ethyl oleate. The application of 10.8% and 43.2% solution resulted in

19 increases in drying rates of 15 and 28%, respectively. Saravacos et al. (1988) studied the effect of ethyl oleate on the air drying rates of starches • and seedless grapes. Incorporation of ethyl oleate in unheated pastes of high-amylose starch significantly increased the drying rate during initial drying but had little effect on granular and gelatinized starches. Dipping grapes in ethyl oleate emulsions increased significantly the drying rate throughout the drying periode Rahman and Perera (1996) pretreated cherries by dipping in sodium metabisulphate, sodium hydroxide, citric acid, tartaric acid, ethyl oleate, potassium carbonate solution each at 2% (w/w) concentration. Ethyloleate \vas round to he the best pretreatment for drying. Tulasidas et al. (1994) aIso used ethyl oleate in a microwave drying study of grapes. Although they obtained good quality raisins without it, ethyl oleate reduced the drying time to some extent and led to better quality. Nevertheless, the quality ofthe untreated raisins was good enough to lead to the conclusion that ethyl oleate treatment was not absolutely • necessary. Because of the similarity of the grape and the blueberry, this study was taken into consideration during the work presented here. Sulphur dioxide treatments are widely used in fruit and vegetable drying as sulphur dioxide is by the most effective additive to avoid nonenzymatic browning (Dunbar, 1986). The addition of sulfites during drying considered to he a safe additive to incorporate into fruit and vegetahle products up to certain limits. However, recently there are reports on the hypersensitivity of a few inidividuals ta the ingested sulfite, (Jayaraman and Das Gupta, 1992).

2.2.3 Osmotic dehydratioD The concentration of food products by means ofproduct immersion in a hypertonie solution (i.e., sugar, salt, sorbitol, or glycerol) is known as

20 osmotic dehydration (Raoult-Wack et al. 1989; Raoult-Wack et al. 1991a). For fruits it is defined as the partial dehydration of fruits tbrough the process of osmosis which essentially involves immersion in sugar or sugar solution for a given period of time. Osmotic dehydration is used as an initial processing step before hot air, solar, freeze, or vacuum drying (Barbosa-Canovas and Vega-Mercado, 1996). It is a useful means of reducing the processing time and energy consumption ofdrying. Water loss to the extent of30-50% of the fruits is attainable and this is dependent on the strength of the sugar solution. Although it adds to the overall drying time, there is the advantage ofbindingflavour compounds and colour which would otherwise be lost on beating (Yang et al. 1987) and can improve other sensory characteristics (Jayaraman and Das Gupta, 1992). Strawberries were osmotically dehydrated in a batch recirculation system by Yang and Le Maguer (1992). Physical and chemical characteristics of two cultivars were investigated. Sugars of strawberries, glucose, fructose and sucrose content were obtained at periodic intervals for strawberry osmotically dehydrated in 63% sucrose syrup at 2S0C and 50°C. Sugar accumulation occurred steadily in strawberries dehydrated at SO°C, while the increase occurred only during the first 2 h of dehydration at 25°C. No significant difference was found in the weight 10ss and moisture content between cultivars during osmotic dehydration. Colour tended to decrease at higher immersion temperatures. In practical applications, 63% of sucrose at 25°C for 2 h was found to he the best processing combination; it removed over 40% ofmoisture from strawberries but accumulated less than 1 mg/g of sucrase for both cultivars. Alvarez et al. (1995) studied the effects of blanching and osmotic pretreatment of strawberries on kinetics of moisture transport during air dehydration. It was found that the effective diffusion coefficient of water in strawberries was strongly affected by heat pretreatment, but glucose

21 dipping after blanching caused no additional effect. The authors obtained an effective diffusivity coefficient of5.76±1.35 x 10-6 cm2/s for raw fruit and of5.43±0.65 x 10-6 cm2/s when the fruit was immersed in 51% w/w glucose monohydrate solution without previous blanching. The rate and efficiency of the process are dependent on such parameters as the kind and concentration ofthe osmoactive substance: the weight ratio of the solution to the food; the kind of osmosed material, its size, and shape; temperature and pressure; and pretreatment of the material prior ta osmosis. The rate of osmosis increases with an increase of concentration of the osmoactive substance (Farkas and Lazar, 1969). Temperature has a substantial effect on the course ofosmotic dehydration. It effects not only the rate of the process but aIso influences the chemical composition and properties ofthe product. Increased temperature increases the rate of chemieal reactions and mass transfer. Viscosity of the hypertonie solution is lowered and the diffusion coefficient of water increases with the increase of temperature. However, increased temperature promotes penetration of osmotic substances ioto the tissue (Pavasovie et al, 1986) Crystalline osmoactive substances are used at fruit ta substance ratios of 1:1 to 6:1 for fruits. For substances investigations were done at weight ratios 1:1 to 6:6. It is recommended that osmotic dehydration of fruits and vegetables be done with weight ratios of4:1 to 5:1 of product ta osmoactive substance (Lewicki and Lenart, 1995). Magee et al. (1983) developed a model for solute diffusion during osmotic dehydration ofapples. It was based on solids gain divided by water content m. m was expressed as:

(2.1)

22 where k is a rate constant, t is time and mo is the initial mass. The rate constant kwas expressed in tenns of temperature and sucrase • concentration, C: (2.2)

The average activation energy of the process was 2B.2 kJ mole·).

Videv et al. (1990) developed the following empirical equation to predict the rate of osmosis F during dehydration of apple slices:

(~) (23) F=31.8-o.307B-(O.56-o.016B)t-2.10 • -1(T-o.3)0.54-o.0042St .

Where F =decrease in mass %, B is the sugar concentration, T is the temperature and t is time. This expression was valid for B=60-75%, T=40... BO°C and t= 0.5-4.5 h. The increased interest in like-fresh food products makes osmotic dehydration a good option for food preservation. The quality of dried products treated by osmotic dehydratian folIowed by another dehydration technique (Le., freeze drying, vacuum drying, air drying) suggests that future food processing plants will include the possibility of implementing combined dehydration approaches (Barbosa-Canovas and Vega-Mercado, 1996). The osmotic syrup the byproduct of the osmotic dehydration can be used in the preparation of food products like candies, pies and also can be used as a flavouring substitute in food preparations.

23 2.2.4.1 Energy consumption in osmotic dehydration It is estimated that dissolution of osmoactive substance in a hypertonie solution needs energy of 1 kJ/kg of water removed from the material. Hence this process affects energy consumption in osmotic dehydration negligibly. The use of energy for syrup mixing or circulation is estimated to be 17.2, 10.0, and 4.3 kJlkg of water removed at temperatures 20°C, 30°C and 400 e respectively. To keep the process running at a desired temperature, a supply ofheat is necessary. Depending on the amount of water removed from the material, the heat supply amounts to 180-240 kJ/kg at 300 e and 380-500 kJ/kg of water removed at 40°C (Lewicki and Lenart 1995).

2.3 SHRlNKAGE An important physical change that occurs during thermal drying is shrinkage of the product, which is caused by microstructural changes due to moisture gradients. Shrinkage influences the transport properties of individual particles as weIl as the porosity and thickness ofthe packed beds in dryers. Thus, shrinkage must be accounted for in process modelling, design and control. High moisture materials such as small fruit, shrink to about 25% of their original size. Experimental data has shown that shrinkage is mainly a function of moisture content for a wide variety offood products (Tulasidas et al. 1994; Kilpatrick et aL 1955; Lazar and Franks 1971; Suzuki et al. 1976; Lozano et al. 1983). Lozano et al. (1983) described the bulk shrinkage coefficient by:

(2.4)

24 Where ~=Bulk shrinkage coefficient (Lozano et al. 1983) 3 Vb=Bulk volume m at moisture content x 3 Vb= Bulk volume m at initial moisture

Bulk volume at each moisture content was calculated from the bulk density and the corresponding sample weight W.

(2.5)

3 Vb = Bulk volume, W =Sample mass kg, Pb =Bulk density kg/m The bulk shrinkage coefficient as a function of moisture contents gives an idea how the food material shrinks as a function of moisture content. Lozano et al. (1983) also modelled the relative volume change as a linear function of the relative moisture content (MIMa), where Mo is the initial moisture content. The sarne approach was used by Tulasidas et al. (1994) to provide input parameters to a detailed model describing the drying kinetics of grapes in a microwave environment, as they shrink ta raisins. Their approach led to excellent agreement between observed and simulated values of moisture content as a function of time. In practice, shrinkage is also dependent on drying conditions CVan Arsdel, 1973; Hatti, 1991). Slow drying may lead to uniform shrinkage, whereas very rapid drying May result in Iower shrinkage due ta induction of a permanent tension that preserves the original dimensions to sorne extent; however, cracks and voids can develop intemally. In freeze-drying, the shape of the product is retained. These voids are also responsible for high rehydration rates. SeveraI investigators have investigated the shrinkage offood grains

25 (Lang et al. 1993) where the moisture content is far less than the fruits. There are a few data on porosity, bulk density and bulk shrinkage coefficient as a function of moisture content. Kilpatrick et al. (1955) studied volume shrinkage of potatoes and other vegetables as drying proceeds. Charm (1978) reported on volumetric contractions of meat and potatoes. Chirife (1969) provided data for apples and potatoes. Suzuki et al. (1976) investigated the shrinkage in dehydration of root vegetables. Shrinkage and porosity of apple tissue at different moisture contents were reported by Lozano et al. (1980). Tulasidas (1994) studied the shrinkage of grapes under convective and microwave drying atmospheres and correlated with the moisture content. Shrinkage characteristics of individual particles and changes of flXed-bed volume and bed porosity during dehydration of apples, potato and carrots were presented and the dependency on water content and drying conditions were analyzed by Hatti (1994), a simple model for correlating shrinkage of an individual particle with water content and air velocity was compared.

2.4. MlCROWAVES 2.4.1.Introduction Microwaves are electromagnetic waves in the 1 mm to 1 m waveband, corresponding to frequencies of 300 l\fiIz to 300 GHz. Dielectric heating occurs between 1 MHz and 100 MHz, whereas microwave heating oecurs between 300 and MHz and 300 GHz. Only certain frequencies are allowed for industrial, scientific and Medical applications, sinee the microwave band has been exploited primarily for communication and military applications. The frequeney allocations for different purposes are made by the International Telecommunications Union (ITU) and certainfrequencies are alloeated ta particular countries. For example 915 MHz is allowed ooly in United States and North America. The frequency allocations for various

26 uses are listed in Decareau (1985). The use of microwave energy has been considered to he a suitahle • approach for coping with certain drawbacks of conventional methods of thermal treatment offoods (Decareau and Peterson, 1986). The following advantages are associated with microwaves in heating and drying.

2.4.2. Advantages ofMicrowave Heating i) Since the transfer of energy is radiative, heating is instantaneous. Furthermore, heat is generated within the material, and not just conducted towards the centre, thus, internaI temperature gradients tend to he smaller. ii) Microwave energy couples directly to the material being heated. The transfer of energy ta the air, waHs of the aven, conveyor or other parts, is minimal since their dielectric constants are very small. This can lead to significant energy savings. • iii) Efficient and accurate control ofhesting rates can he achieved by controlling the output power of the generator. iv) Since the moisture flow is partly pressure·driven from the interior, there is no receding moisture front as in convection, which eliminates case hardening. This is favourable for sorne applications, but is considered a negative aspect in cooking or baking siDce crust formation and surface browning do not occur unless special measures are taken. v) Many chemical and physical reactions are promoted by the heat generation by microwaves, leading ta puffing, drying, melting, protein denaturation, starch gelatinization.

27 2.4.3. Advantages ofMicrowave Drying i) In conventional drying, moisture is initially flashed off from • the surface and the remaining water diffuses to the surface slowly. In the microwave situation, internai heat generation leads to an increase in internaI vapour generation, which promotes liquid flow towards the surface, and aIso leads to higher internaI temperatures, both of which increase the drying rate. ii) In microwave drying, there is great potential for energy savings, due to the speed of drying and lower specifie energy needs in the case of high-Ioss materials. iii) Drying times can be shortened by 50% or more depending on the product and the drying conditions such as power levels and temperature. iv) Microwave drying equipment occupies less space and reduces the handIing time. v) Improves the productquality and in some cases eliminates case hardening, internaI stresses and other problems like cracks. vi) Cost savings May be realized through energy savings, increased throughput, labour reduction, reduced heat load in the plant, increased efficiency and reduced maintenance costs.

The following sections review theprinciples ofmicrowave heatingand ofequations relevant ta the understanding ofmicrowave heatingand drying kinetics.

2.4.4. Interactions of microwaves and biological materials The interactions of microwaves with target materials are usually described in terms ofthe dielectric properties ofthe material, the dielectric

28 constant E' and the dielectric 108s e", which are the real and imaginary components of the relative complex permittivity e·. The complex dielectric constant e May be expressed as

e· = e' - j e/l (2.6)

were j =f-1, The 10ss tangent is defined by: tan Ô = E"/E'. These properties can be measured at various frequeneies, and they are not constants: they are dependent on the temperature, moisture content, composition and particle density of the materia1. In a processing situation, the bulk dielectric properties, rather than the particIe dieleetric properties are of interest, sinee the microwaves will also interact with the air in the inter-particle space. Although the bulk dielectric properties can be roughly estimated from the dielectric properties of the particles and air, the exchange of moisture between the particles and the air space make aecurate prediction difficult and dependent on airflow eharacteristics and distribution which are scale dependent. The dielectric constant is analogous ta a eapacitance, sinee it is a measure of the material's ability to store microwave energy. The dieleetric 10ss is analogous to a resistance as it represents the material's ability to dissipate absorbed energy as heat. The ratio of the material's dielectric 10ss ta its dielectric constant is defined as its dissipation factor or 10ss tangent (tan ô). Materials can he classified as high 10ss or low 10ss. Among the substances associated with foods and agricultural products, water is the greatest absorber and dissipator of microwave energy (e'=78, E"=12), and is the component most responsible for the dielectric properties of a complex substance as a whole. Dipolar liquids and monomeric constituents interact with microwaves to a greater extent than do polymers such as lignin and

29 cellulose. The dielectric properties are the basis for estimating important values sucb as the heat generation rate and the penetration depth. The principal mechanisms of energy dissipation as heat are due to frictional effects of dipolar rotation and ion migration resulting from the response to the alternating electric field. The power absorption or specific absorption rate of a material depends on various factors including frequency, temperature, the magnitude of electric field in tissue, density, and dielectric constants of free space and tissue (Rosenberg and Boegl, 1987). When microwaves penetrate matter they are attenuated exponentially. The penetration depth is inversely proportional to the frequency and to the dielectric constant:

(2.7)

Where D = is the penetration depth in cm Â.o =free space wavelength cm Ife· is low then the above equation May he simplified to

D=_D_AR (2.8) 21te"

This equation is reasonahly accurate for Most foods even though Many have relatively higher e· values. From these equations it is obvious that materials with high dielectric constants and 10ss factors will have smaller penetration depths than those with lower values. Ifthe material is very thick, the entire cross section cannot be heated by microwave irradiation and conduction becomes involved in heating the centre. The heating rate ofa material is also a function ofthe dielectric 108s

30 factor, and is given by: • AT k f E'l e" (2.9) At p Cp

where, fis the frequency of the applied microwave field, E is the electric 1 field strength inside the sample CV m· ), p is the density and Cp is the specifie heat. This equation is an extension of that for power dissipated within the material as given by Metaxas and Meredith (1983).

2.4.5 Microwave Drying ofAgricultural Products Chin et al. (1985) used a microwave oven ta dehydrate tomato products to determine the total solids and compared the results with that of vacuum aven dried tomato products. The results obtained by the • microwave oven drying procedure were equivalent to those obtained by the vacuum aven procedure. Because of the inherent speed and ease of use, they recommended that the microwave aven drying method be considered as an alternative ta the official vacuum aven method. Al-Duri and McIntyre (1991) used convection aven, microwave aven and combined microwave / convection aven for cornparing drying kinetics of milk and milk products and fresh pasta. They concluded that for low moisture products convection aven is not advantageous as the diffusion process is slow. But in microwave operated aven the drying rate increased remarkably with power input, the product was uniformly dried, the microwave oven is suitable for low moisture content heat sensitive products like pasta. Combined mode ofhot air with microwave power increased the drying rate but high temperatures are Dot recommended for obtaininggood • 31 quality products. Hemphill and Martin (1992), used microwave oven drying method for • determining total solids of strawberries, and compared the results with the freeze drying, and hot air oven drying methods, since total solids are an important quality for processing. Law power microwave drying method values oftotal solids correlates weIl with those determined by eitherfreeze drying or aven drying. Bouraoui et al. (1994) employed convective and combined microwave and convective drying to dry potato slices. Effective moisture diffusivity profiles were calculated using Fick's diffusion model in one dimension. Statistical analysis showed diffusivity to increase with increasing internaI temperature but to decrease (while microwave drying) with increasing moisture content. Microwave, convective drying was compared with convective drying. They concluded that microwave drying has potential for better quality dried product while considerably reducing drying duration. Microwave dried product had a better rehydration compared to convective • dried product. Tulasidas etal. (1993), used convection and combination ofconvection and microwave drying in a modified microwave oven to dry Thomson seedless grapes to raisins at different inlet air temperatures. The fruits pretreated with an alkaline solution of ethyl oleate resulted in better quality. It was reported that the microwave drying reduced drying time with a comparable product quality even without chemical pretreatment. It was found that an inlet air temperature of 50°C to be optimum for microwave drying. Prabhanjan et al. (1995) studied microwave assisted convective drying of carrot cubes in a modified domestic microwave oven with provision for the passage of air at constant flow rate and given air temperature. Drying time to achieve the desired moisture level and • 32 rehydration characteristics ofthe product were used to compare microwave assisted drying with the convective method. They concluded from the study • that Microwave drying resulted in substantial decrease (25-90%) in the drying time and the product quality was better when dried at the lower power leveI. Drouzas and Schubert (1996) investigated the microwave vacuum drying of banana slices. The drying process was examined by introducing pulse generated microwave power in banana samples. The product temperature was kept below a maximum level, so that the final product should not be bumed by hot spots during microwave drying. The product quality was evaluated by taste, aroma, smell and rehydration tests and found to he of excellent quality. Yongsawatidigul and Gunasekaran (Part 1 and II, 1996) used microwave-vacuum drying method to dry cranherries and studied the continuous and pulsed application of microwave energy. They came ta a conclusion that pulsed microwave power is more efficient than continuous • application. Shorter power ON time and longer power OFF time provided more favourable drying efficiency in pulsed mode. In part II, they evaluated the calour, texture, water activity of microwave-vacuum dried berried in comparison with hot-air dried berries. They found that the microwave-vacuum dried berries had redder and softer texture than those dried by the conventional hot air methad. It was concluded that the microwave-vacuum dried berries were comparable in quality with that of hot air dried berries.

2.5. DRYING MODELS

The moisture transfer ID foods is a subject of considerable importance. The mechanisms of moisture transport are numerous and often complexe Transport phenomena are usually classmed as resulting • 33 from pressure diffusion, thermal diffusion, forced diffusion, and ordinary diffusion (Van Arsdel, 1973). Often a diffusion transport mechanism is assumed and the rate of moisture movement is described by an effective

diffusivity value, Deff, no matter which mechanism is really involved in moisture movement. There is an extensive body of literature concerning the drying of agricultural products. Theoretical, semi-theoretical and empirical models have been developed for drying in convective, conductive and radiative regimes. Although attempts are being made ta develop more accurate models, such efforts are not always necessary in practical applications. Simpler two-parameter models based on exponential decay or modifications thereof, cannot follow typical two ... stage drying kinetics fully (constant rate period followed by falling rate period); however, they can often predict moisture ratios to within 10% or better, and can therefore be useful for comparison purposes. The rate constants can be expressed in tenns of operating conditions if desired, and should perform reasonably weIl if the range of conditions is small or if the operating conditions affect drying parameters in an easily expressible functional relationship. Among the problems inherent in modelling of drying processes of agricultural materials, are those associated with the nature of the material to be dried. Biological materials often undergo chemical and structural changes during application of heat, and these can alter properties such as thermal conductivity, moisture difTusivity and other characteristics which are often assumed ta be constants in drying models. Moisture itself May be Cree or bound, and the movement offree water May aIso be inhibited by the presence of osmoactive substances such as salts and sugars. Since drying of agricultural materials has traditionally involved convection or conduction, theoretical modelling efforts have been based on moisture diffusion, using Fick's Second Law of diffusion:

34 &m = V (D V m) (2.10) ~t

where, m is the moisture content (%), t is time (h) and D is the moisture 2 t diffusivity (cm h- ).

Crank (1975) gave the analytical solution to this equation for a homogeneous, isotropie sphere with constant diffusion coefficient, as:

ln -M. (2.11) mIl -In.

where, me is the equilibrium moisture content, ma is the initial moisture content, D is the moisture diffusivity, n is an integer and R is the radius.

Although analytical solutions have been developed for other regular geometries, and although modifications to deal with moisture and temperature dependencies of D and sometimes inappropriate boundary conditions, Shivhare (1991) found this model to be inadequate in representing the microwave-drying situation for low moisture commodities snch as corn, even though shrinkage is negligible. He proposed a modified moisture ratio based on varying surface moisture, which was expressed as a function of time and the free moisture (1Ilu ... me). The modified model certainly gave reasonable fits ta drying curves for corn st different power levels and also fit microwave drying data for wheat presented by Prabhanjan et al. (1992); however, it was not based on microwave heating mechanisms, other than due to the fact that the diffusivity, D was

35 estimated from actual microwave drying trials. Later, Tulasidas et al. (1997) developed a comprehensive seml theoretical model ofdrying of high moisture particulates, which was based on the continuum approach. The mass transfer equation for a fixed coordinate system (Crank, 1975) is rewritten to account for shrinkage using concepts presented by Crapiste et al. (1988a,b). Terms for evaporation, electric field strength and heat generation (based on dielectric properties and absorbed microwave power) and convective heat transfer were aIso essential components of the model. This model was quite accurate at predicting moisture 10ss with time under various conditions and performed betterthan the modified logarithmic model (Page, 1949), or Page's equation, which is often used to estimate drying:

M-M MR = ___C = exp (- k tn) (2.12) Mo - Me

where k, n are parameters associated with a particular drying regime and are estimated by non-lïnear regression from experimental data. The error ofprediction ofthe experimental drying curves by the Page equation varied from 0 to 8% relative to that of the comprehensive simulation model (Tulasidas et al. 1997). However, in some cases, the experimental data and the predicted data (Page's and comprehensive model) were almost identical. Thus, although it may be more satisfying ta develop a model that May be interpreted in terms of the physical phenomena involved in microwave drying, the improvement in accuracy may only be small and not of extensive practical significance. Moreover, the more comprehensive model requires substantially more experimental data, including moisture-shrinkage relations, dielectric properties, and estimates of effective diffusivity. • 36 2.6. QUALITYASSESSMENT 2.6.1 Colour Measurement Colour as seen by the human eye is an interpretation by the brain of the character of light coming from an abject, it is possible to define a foods colour in a purely physical sense in terms of the physical attributes of the food. A more satisfactory approach is to define colour as objectively as possible and interpret the output in terms ofhow the human eye see colour (Francis, 1995). The calculation of the XYZ data of a food spectra was common practice earlier but it was cumbersome, and this led ta the development of tristimulus colorimeters. The concept is simple, one needs a light source and three glass filters with transmittance spectra that duplicate the X, Y, and Z curves and a photocell. With this arrangement one can get an XYZ reading that represents the colour of the sample. With this basic arrangement and different filter-photocell combinations as weIl as different axes in space diverse types of colorimeters are being produced and used. This system has sorne drawbacks in that it was not visually uniform, one unit of colour rneasurement in one area of the solid was not visually different from the sarne unit in another area. A number of attempts were made to calculate a colour solid that was visually uniform in all areas, but it was concluded that the task wss impossible. However, sorne came close and three systems seern to be gaining priority. One is the CIE XYZ system, and the second is the Judd-Hunter Lab solide The later represents a colour solid in which L is lightness or darkness, +a is redness, -8 is greenness, +b is yellowness, and -b is blueness. A third scale known as CIELAB, with parameters L·, s·, b- (Hunter and Harold, 1987) appears to he gaining in popularity. Bssed on the above principles many instruments were developed to measure the colour. Current instromentation ranges from relativelysimple

37 designs to sophisticated colorimeter-computer combinations. Photovolt colorimeter is a simple colorimeter consisting of a measurement unit and eight exposure heads for different applications. The Minolta Chroma meters are similar, with five specialized exposure units. Among the more sophisticated instruments, the Hunterlab Labscan is actually a spectrophotometer combined with a computer. With these instruments measurement of colour is a relatively mature science, but operator ingenuity is still required to measure samples snch that meaningful data can be obtained.

2.6.2.Texture measurement Texture, appearance, and flavour are the three major components involved in food acceptability (Boume, 1978). Therefore an accurate method for determining food texture is of vital importance to the food scientist and industry. The main goal of texture studies is to devise one or more mechanical tests with the capacity ta replace human sensory evaluation as a tool ta evaluate food texture (Peleg, 1983). The following statement by Lord Kelvin is frequently seen in discussions dealing with measurement "1 often say that when you can measure what you are speaking about and express it in numbers you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is ofa meagre and unsatisfactory kind; it May be the beginning of knowledge, but you have scarcely, in your thoughts, advanced ta the stage of science, whatever the matter May be."C Mohsenin, 1986). Many early instruments were used to aid in the texture evaluation of foods. One early texturometer, called the denture tenderometer, was built and used by the Food Technology Laboratory of the Masssachusetts Institute ofTechnology. This instrumentemployed strain gauges connected to the jaws of a dental articulator. Another versatile and well-known

38 instrument that has undergone severaI name changes is texture shear .' press. This is manufactured by several companies and is also adaptable ta the Instron universal testing machine. The Warner-Bratzler shear is probably the most widely used instrument in the United States for measuring toughness of meat. The test cell can be mounted on a universal testing machine capable of recording force and motion of the crosshead. AlI texture measuring devices have five essential elements: 1) the driving mechanism, 2) a probe element in contact with food, 3) a system ta sense force-direction, 4) a sensor for type and rate of application, and 5) a read-out system. The probe element in contact with the food can be a flat plunger, ~hearingjaws, a tooth shaped attachment, a piercingrod, a spindle or a cutting blade. The force may be applied in vertical, horizontal or levered manner, and may he of the cutting , piercing, puncturing, compressing, grinding, shearing or pulling type. The sensing eIements May be a simple spring or a more sophisticated strain gauge. The results can • he expressed as force and deformation or stress and strain. 2.6.3. Rehydration Many dried foods are rehydrated before consumption. The structure, density, and particle size of the food plays an important raIe in reconstitution. The degree ta which a dehydrated sample will rehydrate is influenced by structural and chemical changes caused by dehydration, processing conditions, sample preparation, and sample consumption. Rehydration is maximised when cellular and structural disrnptions snch as shrinkage are minimised (Okas et al. 1992). Several researchers have found that freeze-drying causes fewer structural changes andfewer changes to the product's hydrophillic properties than do other drying processes (Hamm, 1960; MclIrath et al. 1962). Most of the shrinkage occurs in the early drying stages, where 40 to 50% shrinkage may occur. Hence the • 39 rehydration characteristics of a dehydrated product are important to • consider in choosing a method of drying. 2.6.4.Sensory Evaluation Sensory quality is of great importance to the processor and ta the consumer. To the processor, quality may represent higher value-added product which could fetch a better selling price or simply lead to a greater market share. Ta the consumer, aesthetic and gustatory satisfaction can guide choice of product. A comhination of sensory perceptions are used to assess sensory quality CRanganna, 1986). Sensory qualities, particularly the flavour attributes, are essentially usually measured subjectively. From early times, judging of foods and beverages has been the domain of experts who had been trained to remember and distinguish small differences in odour and taste of specifie products like tea, coffee, wine , etc. With the development of sensory evaluation techniques on scientific tines, the experts are being replaced by • panels whose sensitivity and consistency have been estahlished by training and repeated tests. Sometimes untrained and semitrained panels are constituted. Several methods May he adopted to evaluate the product. These are ranking, single sample, two sample difference, multiple sample. Quality differences can he based on the Hedonic scale, or Numerical scoring methods.

• 40 CHAPTERTII

MATERIALS AND METHOnS

This chapter presents materials and methods used that were common to severa! phases of the experimental work. This is done in order to avoid repetition in the rest of the thesis.

3.1. MICROWAVE DRYING SETUPS 3.1.1 Setup used in Preliminary Studies A programmable microwave oven Œaton Viking) was modified and used in the preliminary experiments on whale, sliced and pureed strawberries (Chapter 4). A schematic diagram is presented in Fig 3.1. The oven had a nominal power of 600 W, and operated at 2450 l\1Hz. The 3 cavity volume was 0.4 m • Duty cycles from 0 to 100% could be chosen, in increments of 10%. However, the applied power was always 100% of the rating during ON portions of dutY cycles. The commercial unit was modified for the experimental work as follows. A 0.2 m diameter hale was drilled inta the bottom of the microwave oven. An acrylic pipe of equal diameter was firmly attached to the hole, forming a duct for the delivery oftemperature-modulated air. A fine mesh metallic screen was fitted ta the top of the pipe at the cavity floor level, to prevent microwave leakage. A small hale drilled on the back wall of the oven and fitted with a metal wire mesh served as the air outlet. Air was introduced inta the chamber through the tube using a 0.25 kW blower from below the oven. Heating elements (2 kW) were positioned along the air supply pipe length ta heat the incoming air, the temperature being controlled using a power regulator. A mercury in glass thermometer fixed

41 at the bottom of the oven was used to monitor the inlet air temperature. A second piece of the sarne acrylic pipe with a perforated teflon bottom served as a sample holder for the fruit. During the test runs the sample holder was placed in the microwave oven just above the hole to force the conditioned air through an evenly placed thin layer of whole or sliced strawberries. A mechanically driven fan type mode stirrer was fixed at the top ofthe microwave aven ta promote better uniformity of microwave distribution.

1 *2

1. Mode Stirrer 2. Air outlet 3. Sample Holder 4. Serry Slice. 5. H.aUng elementa Air inlet

Figure 3.1. Microwave drying setup used in the preliminary experiments.

A hot wire anemometer (Dwyer 5106, Thermo System Inc., St Paul, MN) was used to measure the air velocity. The air velocity was set to 2 mis

% 3% sinee higher air flow rates were found to result in fluidization of

42 Because this setup could not accommodate real-time measurements of mass, the microwave was turned off every twenty minutes and samples • were taken out and weighed on a digital electronic balance (Acculab Madel 121, Canadawide Scientific, Ottawa, ON), then put back in the oyen.

3.1.2 Experimental Microwave Drying Unit Due to inadequacies in the previously described setup, a more elaborate unit was put together and used for the studies described in Chapters 5, 6, and 7). The improved setup is shown schematically in Figs. 3.2 and 3.3. It consisted of a 750 W microwave generator operating at a frequency of2450 MHz frequency. The microwaves were conveyed through a series of rectangular (7.5 x 4.0 cm) wave guides ta a metallic cavity of 0.035 m3 (40 x 35 x 25 cm) connected by a circulator which absorbed the reflected power. Two power meters connected to the wave guide assembly permitted measurement of the incident and reflected power while 3 screws inserted at the top of the wave guide assembly permitted tuning and adjustment of the reflected power. The microwave drying cavity had an access door for sample insertion and removaI and a 20 cm diameter circular hole at the bottom of the cavity covered with a perforated metal sheet to admit air from the blower while preventing microwaves from leaking from the cavity. A strain gauge was fixed to the cavity. The circular (20 cm diameter) sample holder was suspended from the strain gauge assembly by a Teflon string through a 6 mm diameter hole, such that real-time mass measurements could be obtained. The holder had a plastic mesh bottom to allow air flow and was suspended exactly 1 cm above the air inlet. A 0.25 kW blower was used to force air through a pipe at the bottom of the cavity directly beneath the sampIe holder. Three 2 kW electrical heaters were used to heat the inlet air ta the required temperature. Air temperature and

43 velocity controis were provided near the blower. A lOxlO cm opening covered with a perforated metallic sheet was cut • into the back of the cavity to serve as an exhaust for air and moisture. A constant air stream of2m/sec was used in al1 the experiments as explained in the previous section. AlI sensors were connected to a data acquisition system. The following parameters were recorded: ambient temperature, inlet air temperature, outlet air temperature, relative humidity, incident power, reflected power, and sample mass. An essentiaI feature of this setup, which did not exist in the equipment used in the preliminary experiments, was a control dial for applied power output. Thus, rather than subjecting samples to full power at various duty cycles, it was possible to apply microwave energy to variable degrees of power in continuous mode. This also permitted adjustment of applied power in tenns of W/g. In a typical run, about 100 g of fruit were placed in the sample holder inside the cavity, and the power could he adjusted ta anywhere in the range of 0 ta 750W. Thus, it was possible to obtain low incident powers of the arder of 0.1 W/g where previously, the incident power for this size sample would be always 6 W/g during the ON period of any dutY cycle (rating offirst aven was 600W). Bince the initial moisture content and mass were determined before each ron, it was possible to input the final mass at which the desired moisture content would he reached and have the system shut down automatically. In all experiments, the final moisture content was set at 0.2 kg water/kg dry matter. The only human control required was to adjust the tuning screws in order to minimize the reflected power.

3.1.3 Initial Moisture Determinations The initial moisture contents of the samples were determined by drying in a hot air aven at 70Ge for 6 h, or by the vacuum dry method

44 (Boland, 1984; Ranganna, 1986; Canellas et al. 1993). These methods were found ta compare weIl for initial moisture determinations. Therefore, the aven method was used for initial moisture content since it was faster. The final moisture contents of the dried samples were determined by the vacuum dry method to avoid burning of the samples at low moisture contents.

MW Drying setup

1 ---. Computer Control :: Il Il 5 1 1 1 1 ~--..~----~-:::=:::==::::=~ 1 MW Generator o 2 Clrculator 6 3 Power Meter. ~==~~aa~------4 Tunlng Screw. 5 Straln gauge 6 MW Cavity 7 Sam pie Holder 8 Blower 8

Figure 3.2 Experimental microwave drying setup.

45 •

• Figure 3.3 Photograph of the experimental microwave drying setup.

(1 ~ IF') ~ .iOl1 .W3J~~"9 .afture .aril".

-- ---~.~-~. Figure 3.4 Data acquisition system connected ta the drying setup.

• 46 3.1.4 Data Acquisition System The data acquisition system (3497 A- Data Acquisition/Control Unit, Hewlett-Packard, USA) Fig 3.4 was used to monitor and record the data. A computer programme written in HP-QBASIC monitored the process. The programme used the alogorithms for sensors in case of mass, relative humidity, incident power and reflected power. Inlet air temperatures were monitored by T-type thermocouples located in the air streams outside the applicator. At each location, the temperature was the average of three measurements. Air velocity was measured by a thermal amenometer (Dwyer 5106, Dwyer Instruments Inc., Michigan, IN) positioned in the air stream. The electrlcal output from the strain gauge indicated the mass measurements. The two power detectors located on the wave guide read the incident power and the reflected power in watts. The data from various sensors were received and recorded by the Data acquisition system. The calibration equations were provided in the program ta convert various electrical signals into appropriate quantities. In the case of power measurements the program recorded a mean value of power over the measurement time interva1. The program had the capability to turn the MW power ON or OFF at any required time interval and also to tum the system OFF when the desired final mass (required moisture content) was reached. To maintain the safety parameters of the system, the program would automatically turn OFF the system when the safety settings of parameters such as power, air temperature, mass, and reflected power were exceeded. The programme recorded the data st the required time intervals. Provision was also made in the applicator to iDsert fibre optic temperature sensors for measuring the centre and surface temperatures of the fruit at different power levels. The temperatures were measured using • 47 •

•

Figure 3.3a Lyo Tech Canada, Freeze drier (Used in the experiments for drying strawberries and blueberries).

• 48 a digital fiberoptic thermometer (Norteeh Fibronie Inc. Canada) specially used in microwave atmosphere. AlI the data was stored and was printed continuously at the programmed time intervals.

3.2 FREEZE DRYlNG Fnrit samples were freeze-dried in a 20 kg capacity freeze drier (Lyo Tech Canada) Fig 3.3a. The unit consisted of a circular chamber with 9 steel trays to hold the samples. Samples were dried to approximately 0.2 kglkg (DB). This required a drying time of the arder of 24 h or longer depending on the type of sample. After freeze drying, samples were stored in an air tight container so that they would not absorb moisture from the aIr.

3.3 OSMOTIC DEHYDRATION

Osmotic dehydratian was performed by mixing fruit samples with granular sucrose. Fruit to sugar mass ratios of 3:1 or 4:1 were used, fruit samples consisting of100 g lots. The extent ofdehydration was determined for various periods (12, 24, 36 or 48 h). Mass 10ss due to osmosis was determined after rinsing the syrup from the surface with tap water and blotting to remove surface water.

3.4 QUALlTYEVALUATION OF THE DRIED PRODUCT 3.4.1 Rehydration tests Rehydration tests of dried samples were performed by the method recommended by the USDA (Anon, 1944). A 5 g sample ofthe dry material was weighed into a 500 ml beakercontaining 150 ml ofdistilled water. The beaker was placed on a hot plate and covered with a watch glass, the water

49 •

Figure 3.5 The Minolta chroma meter. (used in colour measurements). •

Figure 3.6 Instron machine used in measuring the texutre. • 50 was brought ta the boiling point in 3 min and sample was added ta the boiling water and boiled for an additional 5 min. The sample was transferred to a 7.5 cm Buchner funnel covered with Whatman no.4 filter paper. Water was drained out by applying a gentle suction until there were no more drops from the funnel. The sample was then removed and weighed. Rehydration ratio was calculated as the ratio of mass of rehydrated sample to that of the dehydrated sample. The coefficient of rehydration, calculated by the equation 3.1, is used by sorne authors, but yields exactly 10 times the rehydration ratio.

COR=_m_rA_(l_OO_-_M_~_ (3.1) m.(lOO-M.,)

COR= Coefficient of Rehydration Inrh=Mass of rehydrated sample mdh=Mass of dehydrated sampIe Mu,=Initial MC % (Wet basis) of the sample before drying Mdh=MC% of the dry sample (wet basis) 3.4.2 Colour determinatioD The chromacities offresh and dried samples were measured using a chroma meter (Minolta chromameter CR·300, Minolta camera Co. Ltd. Azuchi-Machi, Chuo·ku, Osaka 541, Japan ) Fig 3.5. This meter was calibrated against a standard white surface plate. The L, a and b coordinates were measured. The measurements were repeated thrice for each sample. L is the lightness variable and a and b are the chromaticity coordinates.

51 In the experiments on drying of strawberry slices and puree, colour was expressed as the ratio aIb, which is convenient way of reducing two colour parameters ta one (Francis and Clydesdale, 1975). A higher a/b ratio indicates a darker (more red) product. In the other experiments colour difference values ÂL, âa and âb are calculated according ta the following formulas.

dL= L-Lt , Âa= a-ëlt, Âb= b-bt (3.2) ~ Where L, a, b are the measured values of the specimen and Lt , ,bt are values of the target colour. The target colours in this experiment are the L, a and b values of the fresh (strawberry or blueberry) fruit. The total colour difference dE.b is determined using the L, a, b colour coordinates and as defined by the equation below (Minolta, 1991).

(3.3)

The colour differences between the Cresh berry colour and the sample colour indicates the variation in colour.

3.4.3.Texture Texture is the property ofthe fruit (food) which is associated with the sense of fee! or touch experienced by fingers or mouth. Objective methods involving different types of instruments have been made use of to provide efficient and precise quantitative prescriptions (Ranganna, 1986). In practice texture measurements are expressed in terms of force. In these experiments texture measurements were made by a puncture test using a 6mm probe on Instron Corporation Series IX Automated Material Testing System 1.16, Fig 3.6. The puncture tests were conducted on the dried berries and the measurements were taken thrice on each microwave or freeze dried sample and the Mean of the three measurements were taken 52 as the sample toughness. Toughness is the force required for unit volume, it is expressed in these measurements in l\IIPa. The comparison of toughness between the various treatments gives an impression to decide the hest treatment.

3.4.4 Sensory evaluation Sensory evaluation ofconvection, microwave and freeze dried berries under different treatments was done by a panel of ten or more untrained judges . The various products were displayed on a desk and random numbers were assigned to them so that the methods of drying were unknown ta the judges. The judges were asked ta observe the samples carefully for total appearance, aroma, taste and colour and then give them a rating using the Hedonic scale. Here, different ratings, ranging from "Like extremely" ta "dislike extremely" were given by the judges. These were later converted to numerical values from 9 (1ike extremely) to 1 (dislike extremely), respectively. These scores were then averaged for each • treatment. Treatments which obtained a Mean score of 5 and above are acceptable and the one's which score below 5 points were rejected (Watts et al. 1989).

3.5 EXPERIMENTAL DESIGNANDANALYSES

AlI experiments were designed as full factorial experiments, with extra samples being prepared for freeze-drying, the freeze-dried products being used as a basis for quality evaluation. The statistical analyses of all quantitative measurements were based on the main effects andinteractions models. Duncan's new multiple range test was used to rank the means at the 0.05 levei in all cases.

53 CHAPTERIV

PRELIMINARY STUDIES ON MICROWAVE DRYING OF WHOLE, SUCED AND PUREED STRAWBERRIES

4.1. INTRODUCTION The industrial application of freeze drying to a wide range of fruits has been motivated by the good quality and rehydration characteristics attainable, but has been limited primarily by high capital and operating costs and long drying time (Salunkhe et al. 1991; Somogyi and Luh, 1986). As mentioned in the literature review, freeze·dried blueberries and l strawberries are marketed by processors at $16·35 kg· . An alternative that has shown promise for grapes, also a high moisture commodity, is microwave drying. This method has proven successful for obtaining high • quality raisins from grapes in a short time and with a very low specifie energy consumption compared to convective drying (Tulasidas et al, 1994). It was therefore decided to investigate this technique and compare it to freeze and convection drying. The preliminary work focused solely on strawberries.

4.2. MATERlALS ANn METROnS

4.2.1 Initial investigations Strawberriesofunknown cultivar were procuredfrom the market and stored in the cold room at IOC. The fruits were removed from the cold room about 2 h before the experiments ta attain room temperature. Samples were taken for the initial moisture content determinatioD. The initial moisture content of strawbenies varied from 89% to 92% across all the

54 experiments. About 100 g of the fruits were weighed and taken for microwave drying in the modified microwave drying equipment shown in • Fig 3.1 and explained in Chapter Ill. A hot wire anemometer (Dwyer 5106, Thermo System Inc., St Paul, MN) was used ta measure the air velocity. The air velocity was set to 2 mis ± 3% and air temperature was kept at the required level (35°C or 45°C etc.) by adjusting the temperature controls. Higher air flow rates were not feasible sinee they result in fluidization of samples towards the end ofthe drying periode Test samples from the mierowave oven were weighed at 20 minute intervals using a digital electronic balance (Acculab Madel 121, Canadawide Scientific, Ottawa, ON).

4.2.2 Microwave drying of whole strawbenies The tirst trials were conducted to evaluate the possibility of drying whole strawberries in a microwave field without preliminary treatments. These trials were performed on under purely convective conditions with air heated ta 35°C, and at 5 microwave duty cycles 10%, 20%, 30%, 40% and 50% [Power Level {PL} 1, 2, 3,4 and 5, respeetively]. It was found that at 40% and 50% duty cycles, there was buming of the product. At lower power levels (1, 2 and 3) the fruit faHed ta dry. Rather, they swelled and burst open, releasing juiee (bleeding). Since this behaviour was thought ta he related to high resistance to diffusion at the skin, a physical method to circumvent this problem was attempted. This involved puneturing the surface ofthe fruit with a pin at 15 locations. Nevertheless, the vapour pressure inside the Croit was still too high and the fruit burst open and started bleeding even at the low power levels. Even the berries dried in the convective conditions at reasonably low temperatures failed to dry. Rather, they 10st their colour and became very 55 soft. Under these conditions however, they did Dot burst. Sînce the products were sa poor, no quality assessments were made. These initial experiments led ta the idea of slicing and pureeing, ta overcome the problem ofskin resistance. Moreover, the fruit freeze-drying industry does market strawberry pieces and slices, whereas fmit leathers (dried puree strips) are aIso sold ta hikers and campers for sustenance.

4.2.3 DryiDg of sliced strawbenies A parallel blade knife, with three blades in parallel position and 1 cm apart was fabricated and used for producing strawberry slices. In the experiments reported here, about 100g of strawberry slices were spread on a sample holder evenly in a single layer, for each drying experiment. Convective drying and microwave drying at duty cycles of 20%, 30% and 40% (Power leveis 2,3 and 4) were each studied at inlet air temperatures of 30°C, 35°C and 40°C with three replicates. A dutYcycle of 10% was not used because initial trials had shown that there was no advantage over convective drying. Drying continued until the weighings indicated that a target moisture content of about 0.2 kglkg (dry basis) had been reached, which is the moisture level of commercially dried fruits samples (Bains et aL, 1989).

4.2.4 DryiDg of strawberry puree For the rons with the pureed strawberries, good quality strawberries were selected from the cold storage and left to reach ambient temperature (2h). About 100 g of strawberries were pureed using a domestic blender (Philips Eleetronies Ltd, model KB 5440, 840 W, LR 467, USA) at a constant speed for 5 minutes. Fibreglass pads of 10 x 10 cm were used to hold the puree sinee these pads allow the air ta pass through them while retaining the puree for dehydration. The same inlet air conditions were

56 used as for the slices and the target moisture content was also 0.2 kglkg (dry basis). However, the duty cycles here were 10, 20 and 30% since it was expected that the purees would dry much Caster than the slices. The dry basis moisture content was calculated every 20 minutes time in all trials.

4.2.5 Freeze.drying About 2 kg of strawberries were selected from the storage and allowed ta attain the ambient temperature. Half of the strawberries were cut into slices and the other half were pureed in the blender as explained above. The sliced and pureed samples were transferred inta trays after obtaining their initial masse These trays were kept in the tunnel freeze dryer and dried for 24 h. The mass loss was recorded and percent moisture content was calculated.

4.2.6 Quality evaluations The freeze-dried berry slices and puree were compared with the microwave dried slices and puree for colour, texture, rehydration, according to the methods described in Chapter 3. In these preliminary experiments, the sensory evaluation was different than that descrihed in Chapter 3. Here, the product was evaluated by a panel of five judges only. Assessments were based on the presence or absence of serious visual and flavour defects (colour uniformity, bumt colour/flavour). The product was considered "good" when it was unifonn in calour with no serious defects snch as stickiness, bumt colour/flavour or cracks; it was considered "poor" when it had clearly formed colour and flavour defects. When the defects were only marginal, it was considered "satisfactory".

57 4.3 RE8ULT8 AND DISCUSSION The initial moisture content ofthe strawberryslices and puree varied from 86 to 92% (wet basis, kg water/ 100 kg slices or puree), the average being 89%, which corresponds ta an initial moisture content of 8.09 (Mm,

kg water/kg dry matter). The temperature ofthe inlet air varied :t 1°C and the relative humidity by:t 1.5%. The inlet air velocity varied within 2% of

the set inlet velocity of 2 ID S·l.

4.3.1 Drying of süces Table 4.1 shows that the influence on drying times due ta iniet air temperature (30, 35 and 40°C) was slight compared ta power leveI, although statisticallysignificant (Table A.4.1). The Duncan's groupings show that the drying times were significantly shorter for inlet air temperatures of 40°C (Table 4.2). The influence of microwave energy on drying time is significant (Tables A.4.1, 4.3). At power level 2, the average drying time acrass the • iolet air temperatures was about 50% of that for convection. Although drying times were significantly shorter at power leveis 3 and 4 than at power level 2, the slices had burnt areas (black spots) because of concentration of microwave power, in spite of having provided the mode stirrer in the mierowave equipment. Even had the quality been good, there would have been little value in increasing the applied microwave energy from 30 ta 40% sinee the time savings was but 1 minute on averages of the arder of 40 min, as evidenced by the absence of significant difference between these two power levels (Table 4.3). The reduction in drying time due to an increase in dutYcycle decays rapidly because the drying efficiency of the applied energy decreases. This is likely due ta an over-production of heat relative ta the rates ofmoisture removal from the surface and diffusion from the interior to the surface.

58 Table 4.1. Drying times required for slices ta obtain a moisture content 0.2 kg/kg (db) at different power levels and inlet air temperatures. Drying time in minutes Power level aooe 35°C o 120 120 100 1 120 120 90 2 60 60 55 3 45 45 40 4 40 40 40

The dry basis moisture contents ofslices were plotted against time for inlet air temperatures of 35°e (Fig 4.1) for power levels 0, 2, 3, and 4.

Table 4.2 Mean drying time of sUces at different air temperatures Air Temp OC Mean (min) Duncan Grouping 35 67.917 A 30 67.500 A 40 59.583 B

Means with the same letter are not significantly different at the 0.05 level

Table 4.3 Mean drying time of sUces at different power levels Power Mean (min) Duncan Grouping o 115.000 A 2 59.444 B 3 43.333 C 4 42.222 C

Means with the same letter are not significantly different at the O.05level

59 4.3.2 Drying puree Table 4.4 shows the drying times for puree at the various experimental conditions. Again, the influence of inlet air temperature on drying times was significant, but much sma11er than that of power level (Table A4.2); moreover, it was the 40°C inlet air temperature that improved the drying rate, with no significant improvement from 30 ta 35°C (Table 4.5). A 10% duty cycle did Dot yield much improvement over convection, aIthough the improvement was significant (Table 4.6). The improvements at 20% and 30% dutY cycles were ta reduce drying time by 5/6 and 9/10 of the time required for convective air drying. Again, at power level 3 and above, the product had burnt spots. Therefore, power level 4 was not attempted.

Table 4.4. Drying time required for puree to obtain a moisture content of 0.2 kg/kg (db) at different power levels and inlet air temperatures.

Drying time in minutes Power level 300 e 35°C 40°C

0 180 180 180

1 180 180 170

2 35 30 30

3 18 18 17

60 Table 4.5 Mean drying time of puree at different air temperatures

Air Temp oC Mean (min) Duncans Grouping

35 118.083 A

30 116.500 A

40 107.917 B

Means with the sarne letter are not significantly different at the 0.05 level

Table 4.6 Mean drying time of puree at different power levels Power Mean (min) Duncans Grouping o 205.556 A 1 190.000 B 2 42.778 C 3 18.333 C

Means with the same letter are not signmcantly different at the 0.05 levei

The drying kinetics ofthe puree are presented in Fig. 4.2. Although fibreglass pads were used as a support for the puree in the microwave oyen, it was difficult to separate the dried puree from the pads. Hence, it is recommended to use sorne other type of puree holder which could he more easily separated from the product. A comparison of the drying kinetics of slices and puree is given in Fig. 4.3 for power level2 and an inlet air temperature of35°C. It is clear from this figure that the puree clries faster than slices. This might he due to the thickness of the drying product. The slices were 1 cm in thickness,

61 10 -r------...,.....--..., -PLO +PL2 8 . .a *PL3 -~ Q "PL4 -~ 6 .--...... -1 Q ~

i 4 ';;= "0 :2

o 1 (:) fl,(:) tlaf:::l rct::i fbt::i,,~ "fl,ti "tlat::i "rct::i ,,~ti fl,r§J fl,fl,f:::I ~ti fl,roti fl,fb(:)

lime (Minutes)

Figure 4.1 Dehydration of strawberry slices under microwave power levels 0,2,3 and 4 at inlet air temperatures 35°C. 10....,...... ------r-----, -PLO +PL 1 8 ••...••...... •....•••...•.•.••...... •••••....•••.•• *PL2 .... PL3 Fa ...... _ -.~~~ en ~

Q) ~ 4 ën ·0 :E

O+-.....-....~...--~ -...... ,.-,...... ,....-,-...... ,-;--...;;::-=-t f:::I ~ ~t::i rof:::l riJ "tit::i fl,ti ....~ti ....tJti ....fbti r&(:) ",t),ti ~(:) ~ti ~(:)

Time (Minutes)

Figure 4.2 Dehydration of strawberry puree under microwave power levels 0, 1, 2, and 3 st inlet air temperature 35°C.

62 • 10~------,---..., -Slices +Puree 8 -..•...•.•.•...... •...... •...... •.- -~ ~

~ El •••...•••.•.•••••••••••••••...... •••••••••••••••••••••••••••••

C) .JI::-

o-+--_-....-...,...-...,...-...... - ...... -_-~--t ()

Time (Minutes)

Figure 4.3 Dehydration of strawberry alices and puree under microwave power level 2 and inlet air temperature 35°C.

63 but the puree tended to spread on the pad and got thinner. For a fairer comparison, the pad should have been inserted into a glass holder with no bottom and raised borders sa that a 1 cm thickness sample could have been dried.

4.3.3 Rehydration of strawberry slices Table 4.7 gives the rehydration ratio and the coefficient of rehydration for microwave-dried, convection-dried and freeze-dried strawberry slices. There were significant differences in rehydration ratio due to the different drying methods (Table A.4.3). The means separation test (Duncan's) showed that there were no signifieant differenees between rehydration ratios of the slices due to the applied microwave power. However, bath the freeze-dried and conveetion-dried products had significantly higher rehydration ratios than the microwave-dried ones, the freeze-dried product having the highest. This may be due to the freeze dried product having a higher porosity (sinee it doesn't shrink).

Table 4.7. Rehydration ratio and rehydration coefficient of microwave dried and freeze dried strawberry slices. Treatment RH ratio COR Convection dried 2.91 B 0.29 Mierowave dried at 10% duty cycle 2.82 Be 0.28 Microwave dried 20% duty cycle 2.72 C 0.27 Microwave dried at 30% duty cycle 2.71 C 0.27 Freeze dried 3.80 A 0.38

Means with the same letter are not significantly different at the 0.051evel.

64 The rehydration characteristics ofthe microwaved and freeze-dried purees could not he determined because the product would come apart in boiling water and could not he separated well even by filtering.

4.3.4 Quality and colour analysis

The qualityjudgement of the microwave and freeze dried strawberry slices and puree from the five member judges panel is given in Table 4.8. These results show that the acceptability of the product goes down as the power level increases, even though the drying time is lower. This may be due to higher product temperatures as the power level increases. The microwave dried product is clearly inferior to the freeze-dried product. The microwave-dried slices at a 20% dutY cycle are about the sarne quality as the convection-dried product at 35°C, but the microwave-dried puree is of lower quality.

Table 4.8 Means separation by Duncan's new multiple range test for the quality assessment of microwave (MW) and freeze-dried (FD) slices and puree by the judges. Slices Puree Treatment Mean Score Grouping Mean Score Grouping Convection 2.86 B 4.19 B MW 10% nia nia 2.86 BC MW 20% 2.60 B 2.86 BC MW 30% 1.26 C 2.33 C MW 40% 1.26 C nia nia Freeze- 8.60 A 8.60 A dried

Duncan groupings: Means with the same letter are not significantly different

65 4.3.5 Chromacity • The results of the chromacity analysis are presented in Table 4.9 below. It is evident that both the sliced and pureed freeze-dried berries

Table 4.9 Chromacity measurements (aIb) for microwave (MW) and freeze dried (FD) strawberry slices and puree. Treatment Means of (a/b) values Grouping 1. Fresh Cslices) 1.21 B 2. FD (slices) 2.15 A 3. FD (puree) 2.13 A 4. MW 20%(slices) 1.08 B 5. MW 20% (puree) 1.21 B

Means with the same letter are not significantly different at the 0.051evel.

• were brighter red (higher a/b ratios) than both the microwave dried berries and the fresh bernes. Freeze-drying aHows the retention or prevents the modification of compounds involved in pigmentation while at the same time concentrating them due ta dehydration. This leads ta brighter colour. Because microwave-drying involves heating, transformations and volatilization losses May occur to an extent not counterbalanced by concentration. Thus, the microwaved product is of about the same brightness and colour as the fresh product. Chemical analyses would be needed to verify these hypotheses.

4.4 CONCLUSIONS Whole strawberries cannat be properly dried in a microwave field sinee they swell, burst and bleed. Convection drying at low temperatures

66 is very slow, and leaves the berries vulnerable to spoilage. Puncturing the strawberries did not help in reducing bursting and bleeding. The drying rate was not improved by raising the inlet air temperature from 30 to 35°C, but was significantly improved with a further increase to 40°C. The results ofthese preliminary experiments indicated that although there is a great potential time savings in drying strawberry sliees and puree with mierowaves eompared to either convection or freeze drying, there is no advantage in tenns of quality retention or rehydration charaeteristics. Furthermore, it is important ta restriet the rate of excitation by microwaves sinee buming May occur. This restriets the time.. savings possible. Freeze-dried strawberries were rated ofhigher quality for aH the eharacteristies tested.

67 4.5 Connecting statement to chapter 5 From these experiments it can he concluded that it is not possible to obtain a good quality dried product using microwaves to dry either whole, sliced or pureed strawberries. It was therefore decided to investigate the possibility ofobtaining an acceptable product by microwave drying after an initial pretreatment of dipping in a solution of ethyl oleate and sodium hydroxide. Such treatments are used in industry to promote drying.

68 CHAPTERV • MICROWAVE DRYING AND SHRINKAGE OF PRETREATED WHOLE STRAWBERRIES

5.1 INTRODUCTION In the previous chapter, it was found that microwave drying was quite advantageous ifberries were not whole. However, the product quality of slices dried using microwaves was still below standards. Whole berries would simply cook rather than dry, and would aIso swell, burst and bleed. High heating rates of microwave drying, the development of internaI pressure by evaporation of moisture and resistance to moisture transfer at the skin were the related causes of the bursting and cooking. In commercial operations, drying is usually preceded by dipping in fluids that help to break down the waxy cuticle and significantly enhance the drying process. Treatments with either alkali emulsions ofethyl oleate

CH3(CH2),CH, (EO), olive oil or potassium carbonate are all effective in increasing the drying rates of fruits having a waxy cuticle (Saravacos et al. 1988, Ponting and McBean, 1970; Raouzeos and Saravacos, 1986; Rahman and Perera, 1996; Harrington et al. 1978). Stress cracking can aIso be reduced by decreasing the resistance ta moisture transfer, and such treatments have aIso been tested on corn (Suarez et al. 1984; Williams, 1989). Thus, it was proposed to study the possibility that pretreatments with chemical solutions used in industry for drying (ethyl oleate and sodium hydroxide) could enhance the drying of whole strawberries in a microwave field, possibly leading ta a higher quality product than possible without this treatment. Convection drying was used for comparison of drying rates and freeze-dried strawberries were used as a standard for

69 quality assessment based on colour, texture and rehydration. At this point in the work, it had not been decided to experiment with • blueberries, and shrinkage studies were also executed on strawberries since the possibility of working with the model developed by Tulasidas et al. (1997) had been entertained. Although the research was later oriented ta osmotic dehydration of strawberries and blueberries, rather than ta extensive simulations, it was feIt that the results of the shrinkage work should be included in the thesis.

5.2 MATER1ALS AND METHOnS Solutions of the following compositions were prepared and warmed to 40°C before dipping the fruits: a) 1% EO + 0.5% NaOH, b) 2% EO + 0.5% NaOH, c) 3% EO + 0.5% NaOH. About 100 g ofwhole strawberries at room temperature were selected and dipped in the chemical solution for 1 minute (Suarez et aL, 1984). The treated fruits were removed form the solution and washed in tap water ta remove the chemicals from the surface of the • fruit. The treated fruits were transferred on to a mesh and surface-dried with a low velocity airstream. Samples were taken ta determine initial moisture content before drying experiments.

5.2.1 Microwave Drying The microwave drying setup used in this study is shawn in Fig 3.2. and explained in Chapter 3. The microwave settings used in combination with the dipping treatments were 0 W/g (convection), 0.1 W/g, 0.2 W/g or 0.3 W/g (ie 10 W, 20 W and 30 W for 100 g sample). A set of untreated contraIs was aIso run at eaeh of these settings. This mode of operation is substantially different than that used in the preliminaryexperiments sinee the sample is continuously exposed to microwave energy, rather than in duty cycle mode, and the field intensity is much lower. • 70 As the drying proceeded, the mass of the sample was recorded by the data acquisition system. The experiment was terminated when the sample attained the targetmoisture content(0.2 kg/kg). Each treatmentimicrowave combination was replicated thrice.

5.2.2 Freeze Drying About 2 kg of strawberries were selected from the storage and allowed to attain ambient temperature then their initial moisture content was determined. They were then pretreated with ethyl oleate and sodium hydroxide (NaOH) in different proportions, as described above. The treated fruits were transferred into trays and freeze-dried until the moisture reached 0.2 kglkg, which took 24 h or longer. The freeze-dried berries were compared with microwave dried berries on the basis ofrehydration, texture and colour according ta the methods described in Chapter 3.

5.2.3 Shrinkage of strawbernes About 5 or 6 berries around 100 g mass were selected and their exact weight was determined using a digital balance to an accuracy of 0.001 g (Mett1er PE 200, balance). The initial moisture content was determined in a vacuum oven at 70°C for 6 h. Their initial and final mass after drying was detennined. The initial volume of the fruits were measured using a displacement method in toluene (Tulasidas, 1994). The fruits were then dipped in a solution of2% ethyl oleate and 0.5% NaOH solution for one minute, washed with water and surface dried with a gentle air stream. Samples were then dried ta five moisture ratios (m/Illo = 0.62, 0.43, 0.31, 0.21, and 0.13) in microwave regimes with power levels of 0.1 W/g or 0.2 W/g assisted by 45°C air convected at an inlet velocity of 2 mis. Each set of conditions was replicated three times and the volumes at each stage were also determined by the toluene displacement method. 71 The surface (Tt) and centre (T2) temperatures ofthe strawberry fruit were monitored during drying at power levels 0.1 W/g and 0.2 W/g using digital fiberoptic thermometers (Nortech Fibronic Inc. Canada).

5.2.4 Relative Drying Rate A relative rate of drying was defined as (Weitz et al., 1989)

(5.1) where, ç is the relative drying rate, te is the drying time of the control sample (min), and t. is the drying time ofthe treated sample (min). Drying times are defined as the time ta reach a final moisture content of0.2 kg/kg (DB).

5.3 RE8ULTS AND DISCUSSION 5.3.1 Drying kinetics The data analysis Ïndicated that the dipping treatment significantly enhanced the drying rate, and that there was significant interaction between dipping treatment levels and the applied microwave power, which itself was also significant according to the analysis of variance (Table A.5.!). The means separations of the Duncan's New Multiple Range Test show that: 1) the lowest ethyl oleate concentration (1%) used enhanced the drying rate ta the same extent as the higher concentrations (Table 5.1), and 2) each increment of applied microwave power led to a significantly lower drying time (Table 5.2). However. as found in the preliminary studies, the reduction in drying time possible decreases as the dutY cycle is incremented. In addressing point 1), it is possible that the 1% EO solution was of sufficient strength to completely dissolve the waxy cuticle. The relative drying rates of microwave and convection-dried 72 strawberries are presented in Table 5.3. Convection drying after dipping

Table 5.1 Mean drying time ofstrawberries at different ethyl oleate (EO) concentrations. Treatment Mean (min) Duncans Grouping Untreated 117.778 A l%EO 96.111 B 3%EO 95.000 B 2%EO 93.333 B

Means with the same letter are not significantly different at the 0.05 level

Table 5.2 Mean drying time ofpretreated strawberries at different power levels. Power W/g Mean (min) Duncans Grouping 0.1 137.917 A 0.2 90.000 B 0.3 73.750 C

Means with the same letter are not significantly different at the 0.05 level in 1% EO and 0.5% NaOH was taken to be the control in calculating the drying rate (Eq. 5.1). The case of convection drying with no pretreatment was not presented since the target moisture level could not be reached even after 32 h. It should be stated that the strawberries that were not pretreated, dried, but also burst, even at these low incident microwave energies. An example ofthe drying kinetics ofconvection and microwave-dried berries is shown in Figure 5.1. Here, the case ofconvection drying with no pretreatment (uppermost curve) is presentedfor comparison to indicatejust

73 how slow drying can he under such conditions, which is why convection drying is not usually used in industrial drying of berry fruits. The relative drying rates (Table 5.3) varied from 1.09 to 9.50, the highest (9.50) being obtained with microwaves at 0.3 W/g microwave power and dipping in 2% EO and 0.5% NaOH; however, st this power level, the product had burnt spots and was not of marketable quality. Convection drying after dipping in 2% EO with 0.5% NaOH gave a rate of 1.09, onlya slight improvement over the control. No comparison could be made between drying rates under microwave and convective conditions of untreated berries because the convective

Table 5.3 Relative drying rate ofstrawberries under different treatments in reaching a MC of 0.2 kglkg (DB). Treatments Drying time. Relative min Drying Rate No Pretreatment Convection MW 0.1 W/g 164 3.48 MW 0.2 W/g 101 5.64 MW 0.3 W/g 81 7.04 1% EO + 0.5% NaOH Convection. 570 1 MW 0.1 W/g 122 4.67 MW 0.2 W/g 85 6.71 MW 0.3 W/g 61 9.34 2% EO+ 0.5% NaOH Convection 525 1.09 MW 0.1 W/g 122 4.67 MW 0.2 W/g 89 6.40 MW 0.3 W/g 60 9.50

* Control

74 conditions are at relatively low temperature and drying rate and the end moisture level could not be reached in convective drying as explained earlier. Nevertheless, the data show that the treatments did speed up drying in both types of regime and regardless of power level when microwaves were used (an example at 0.2 W/g is shown in Figure 5.2). The drying rate for microwaved strawberries increased with power level whether the berries were treated or not, as expected, with the usna! restriction that burning occurred at the higher power leveis (~Oo3 W go1), untreated fruits burst and bleeded.

5.3.2 Rehydration ratio Table 5.4 shows the rehydration ratio's of different treatmentso

Table 5.4 Rehydration ratios ofmicrowave and freeze-dried strawberries with different chemical pretreatments. Pretreatment Rehydration Ratio 1. 2% Ethyl Oleate and 0.5% NaOH Convection dried 1.68 Microwave dried with 0.1 W/g 2.09 Microwave dried with 0.2 W/g 2.57 Microwave dried with 0.3 W/g 2.20 2. Freeze dried Untreated 2.02 1% Ethyl Oleate and 0.5% NaOH 2.53 2% Ethyl Oleate and 0.5% NaOH 2.98 3% Ethyl Oleate and 0.5% NaOH 2.64

Statistical analysis (Table A.5.2) showed that these were related to bath the concentration of EO used and the drying regime - convective, freeze,

75 10~------....,

_8 e.CD ~ 6 Ci ~

~• 4 fi ë :E

10 180 270 360 450 540 630 720 lime min

- Convect + 0.1 W/g ..0.2 W/g ..0.3 W/g "* CanY-No Prt Figure 5.1 Convective and microwave drying of strawherries treated with 2% ethyl oleate and 0.5% sodium hydroxide, at different power levels.

10------~-----, -,....o.u O" +.,..0 011 • .., 0.... 011

a -L-....----..--:==::==*~=_ a 30 60 la 120 150 180 lime min Figure 5.2 Microwave drying of strawberries at 0.2 W/g power with different pretreatment levels. 76 microwave at different intensities. There was no interaction between the concentration of EO used and the drying regime. The means separations (Table 5.6) indicates that the influence of ethyl oleate concentration on rehydration was modest, with no particularly clear explanation for the arder shown.

Table 5.S. Rehydration ratios of strawberries dried by different methods. Type Mean Duncan Grouping Freeze Dried 2.71667 A MW 0.2 W/g 2.62222 A MW 0.3 W/g 2.37111 B MW 0.1 W/g 2.23444 B

Means with the same letter are not significantly different at the 0.05 level

Table 5.6. Rehydration ratios of strawberries at different ethyl oleate concentrations. Oleate % Mean Duncan Grouping 2 2.4060 A 3 2.32333 AB 1 2.25067 B

Means with the sarne letter are not significantly different at the 0.05level

The convection-dried product exhibited (Table 5.4, 5.5 and 5.6) the lowest rehydration ratio. The berries subjected ta 0.1 and 0.3 W gel exhibited significantly higherrehydration thanthe former, and significantly lower rehydration than the remaining classes; there was no significant difference between the freeze-dried berries and those dried with microwave energy st 0.2 W gel. Nevertheless, the rehydration of the freeze-dried

77 samples were slightly higher. Inspection of the original data showed that the rehydration ratios of the berries microwaved at 0.2 W g.l and treated with 2% EO were the highest, and that the three replicates were as consistent as the replicates of the other groups. Thus, tms quasi-maximum was not due ta an one extreme data point. It is interesting ta note that the rehydration ratios of sliced berries (Chapter IV) were higher than those of the whole pretreated berries, whether they were microwave or freeze-dried. This might be due ta the larger surface area of the dried slices, which were boiled for the same amount of time as the whole berries. It is possible that rehydration ratios May have been higher if the whole berries boiled for a longer time.

5.3.3 Toughness

Table 5.7 shows the toughness for different combinations of pretreatment and drying regime. There were significant differences in toughness due ta the concentration of EO, ta the drying regime, and ta the interaction ofthe two (Table A.S.3). The toughest were associated with the 2% Eû dip and with the berries microwaved at 0.1 W g-l (Table 5.8 and 5.9). The softest were those receiving the greater intensities of microwave power. Although all of the dried froits were soft and quite easy ta chew, one might conclude that the higher intensities led to sorne cooking of the product. Which might he responsible for lower toughness of the berries microwaved at 0.2 and 0.3 W gal, compared to berries dried by all the other regimes).

78 Table 5.7 Texture (Toughness) of microwave and freeze dried strawherries treated with different ethyl oleate CEO) concentrations. Pretreatment Toughness MPa 1. Microwave dried With 2% EO + 0.5% NaOH Power level 0 0.36 Power level 0.1 W/g 0.65 Power level 0.2 W/g 0.08 2. Microwave dried with 1% EO + 0.50/0 NaOH Power level 0 0.11 Power level 0.1 W/g 0.45 Power level 0.2 W/g 0.10 3. Freeze dried No pretreatment 0.14 1% EO + 0.5% NaOH 0.26 2% EO + 0.5% NaOH 0.31 3% EO + 0.5% NaOH 0.31

Table 5.8 Toughness of strawberries at different EO concentrations. Oleate % Mean Duncan Grouping 2 0.314000 A 3 0.284667 B 1 0.218000 C

Means with the same letter are not significantly different at the 0.05 level

79 Table 5.9 Toughness of strawberries according ta drying regime. Regime Mean (Toughness) Duncan Grouping MW 0.1 W/g 0.51000 A Freeze dried 0.27778 B Convection 0.23444 C MW 0.3 W/g 0.19333 D MW 0.2 W/g 0.14556 E

Means with the same letter are not significantly different at the 0.05 level

5.3.4 Colour difference with respect to fresh berry

Table 5.10 Colour measurements of microwave and freeze dried strawberries under different pretreatments. Treatment L a b ~ab 1. Fresh Fruit (Target Colour) 31.89 28.33 17.01 2. MW dried with 1% EO +0.5% NaOH Power level 0 34.77 17.50 7.73 14.54 Power level 0.1 W Ig 31.60 21.18 15.26 7.36 Power level 0.2 W/g 31.88 19.93 15.90 8.47 3. MW dried with 2% EO +0.5% NaOH Power level 0 28.79 26.32 15.23 4.10 Power level 0.1 W/g 29.92 29.26 19.17 3.06 Power level 0.2 W/g 30.20 19.45 18.40 9.14 4. Freeze dried No pretreatment 34.21 36.25 19.13 8.52 1% EO + 0.5% NaDH 33.19 37.93 19.85 10.09 2% EO + 0.5% NaOa 31.01 37.15 19.90 9.32 3% EO + 0.5% NaOH 32.28 33.29 20.93 6.33

MW (Microwave), EO (Ethyl Oleate), NaOH (Sodium Hydroxide).

80 Table 5.10 shows the colour differences between treatments. The convective dried strawberries had a higher difference âEab of 14.54 compared to the fresh fnrit colour and were darker red. When microwave dried and freeze-dried fnrits were compared, the freeze dried product was generally darker. This might be due to Iow pressure and low temperature in freeze drying which consolidates the pigments and iDcreases the colour. Unfortunately, only the averages for each treatment/regime combination are available. The original data including replicates were lost, thus not permitting a statistical analysis to he executed.

5.3.5 Shrinkage

The initial and fmal densities were calculated for different moisture levels using the respective mass and volumes. Shrinkage ratio was defined as the volume at a given moisture content divided by the initial volume:

(5.1)

Assuming that the ellipsoid shape of the berries can be approximated by a sphere ofequivalent volume V, an equivalent diameter De was calculated using the relation De= (6V/1t)1t.1 (Saravacos and Raouzeos, 1986). The mean equivalent diameter of the berries was detennined at different moisture contents, and correlated with the moisture ratio. Changes in volume and density ofthe strawberries during microwave drying at power levels of 0.1 and 0.2 W/g are shawn in Table 5.11. The results show that the volume reductions are quite similar for the two power levels. The density changes are quite suniIar, differences from one case to the other probably being due simply to experimental error.

81 .... ~ .... • o·,.. ] ••.••••••• ~ •..••.•..• ~ •••••••••• ~ ...••••... ; ...... ••.. ~ ...... '~ j i !-- <---.-.-...: _ ;.. - .

G"~ ~ •••••••••• : •••••••••••••••••••••• •••••••••••••••••••••••••••• ,• ..l .J . . 1 • • • • ~:: j~ ::::::.:::!.:~: ::::::i.:::::::::i::.:::::.::i:::.:_.:.. ·:::.:...:: ...... * ...... G.'': -:."'-:-~~-;--:-""""':"'""-r--:--"--:,--:--:~~-:--..,.--:---:--:--:---:---,.~_~ 0.\ Il.~ Il•• cu a.' 0.7

Moisture ratio MfMe

Figure 5.3 Microwave drying of strawberries at power level 0.1 W/g, correlation between shrinkage ratio and moisture ratio.

o~Q J J 1 Q ,Il':. ~ •••••••••• ~ •••••••••• : •••••••••• : •••••••••• : •••••••• •• • •••••••••

j, li ,':1') 1 . .: ...: : .

...... o." ......

...... ,

......

o.... ~,---:~~~--:---~~~....,...~~, --:'----:~:--":"", --:---:"'--:-~.-:--O:--~I 0.1 002 U o.. lU 0" 0.1'

Moistu,. ratio MlMo

Figure 5.4 Microwave drying ofstrawberries at power level 0.2 W/g correlation between shrinkage ratio and moistue ratio.

82 ~:~) 1 ~ .. . . . • 'J....~G ~ • . 1 • • • • ~ -E . . . . (,J ~~~ } ...... : : : . c C j . . co: -.. .," _~I •••••••••• : •••••••••• : ••• •. • ••• : •••••••••• : •••••••••• : •.•••••••• .- . . . . 1 : : • • . .. '\~ --: ... .. : : : : .. •. ~ ~ ~ .. ~ ~ i /; . ...",1 ï ...... : ~ ~ ~ : .. j : : : : : ~ /e

\'~\ 1 a.: C.: Q.~ G•.a C.! C.! O.i"

Meis::..t"e r.1tio MIMe

Figure 5.5 Microwave drying of strawberries at power level 0.1 W/g, correlation between equivalent diameter and moisture ratio.

~~. j

1 J 1 • • • • ~::"" 1 :. i i - i . . . ].= ... ':~ ~ : : ~ . Q C J 1 • • • • • . .,,4-··········:··········~··· : : ~ . .. j . . i1 ••. . • ..,j ~ ~ ~ : : . ~.. ,~ .: ~. j ...... ~ ...... : ...... 1•••••••••• 1 ••••••••• • : • • •••••• •• ~ ~ .....

1 \.tfi1 1 G.1 0': '.2 cl.! o•• I.r

Moistunt ï.ltia MIMa Figure 5.6 Microwave drying of strawberries at power level 0.2 W/g, correlation between equivalent diameter and moisture ratio.

83 Figures 5.3 and 5.4 show the relationships between shrinkage ratio and moisture ratios for the two power levels. The relationships are lînear. • The relationships between equivalent diameter De and the moisture ratio (Figs. 5.5, 5.6) were hest fit by reciprocallogarithmic equations of the form:

D = l (5.2) fJ A +sln.l!.. Mo

Table 5.11. Shrinkage ratio, equivalent diameter and change in volume during microwave drying of strawberries at 0.1 W/g (ahove) and 0.2 W/g (below). Initial Final Change MlMo Sb De, cm Density Density in Vol. g/ml g/ml % 0.13 0.21 1.90 1.00 0.96 77.10 0.23 1.98 0.95 0.96 76.92 0.21 0.29 2.12 0.98 0.96 69.95 0.30 2.17 0.95 0.90 70.87 • 0.31 0.36 2.30 0.98 0.98 63.43 0.37 2.32 0.97 0.96 63.50 0.43 0.47 2.53 0.97 0.97 47.66 0.52 2.61 0.96 0.94 52.77 0.62 0.65 2.79 0.98 0.99 33.97 0.66 2.80 0.97 0.97 35.10

The constants for the equations obtained for both types of relationship are given in Table 5.12, along with the coefficient of detennination (R2). The variations in temperature at the surface and the centre (Tt and

T2°C) during microwave drying st power levels 0.1 W/g and 0.2 W/g while the strawberry was shrinking is presented in Figures 5.7 and 5.8. In the f!rst 30 min, at both the power levels, the temperature

84 12 -.---.------,...----140

10 120

100

80 U G- E 60 ~

0-+-r~r_T""T_r_T""_r_r_r_T'"T_r_T""T""T""'1""T"""I'"'"T""T""T"""T""'1r_T""'T".,....,...,.....,....,~0 o 30 60 90 120 150165 lime min Figure 5.7 Surface (Tl) and centre (T2) temperatures of strawberry fruit during microwave drying at power level 0.1 W/g

12~---.....------~--'T-140

la 120 100 Co)

80 •~ ii.. 60 l E ~ 40 2 ...... 20

0+T"'T""r~~.,..,...~r-r-r...,..,.""T""'T''''r"'T'~T""T'""~r+0 o 30 60 10 120 140 nme (Min) Figure 5.8 Surface (Tl) and centre (T2) temperatures of strawberry fruit during microwave drying at power levei 0.2 W/g.

85 rise is sunHar for Tl and T 2. Thereafter, the temperatures at the centre (T2) tend to be higher than at the surface (Tl)' This is probably due to the difference in moisture content between the centre and the surface,

Table 5.12 Constants for linear equations descrihing shrinkage ratio ofstrawherries, and for reciprocallogarithmic equations (RLl describing equivalent diameter, as functions ofthe moisture ratio under microwave drying at power levels 0.1 and 0.2 W/g.

Power Correlation Model Constant Coefficient R2 W/g A B Value 0.1 Sb vs. MIMa Linear 0.0943 0.8870 0.9989 0.2 Sb vs. MIMa Linear 0.1329 0.8325 0.9980 0.1 De vs. MIMa RL 0.3067 -0.1071 0.9995 0.2 De vs. MIMa RL 0.3157 -0.0899 0.9968

URL" Reciprocal logarithmic model which leads to a gradient in heat generation and a resulting temperature gradient. Furthermore, moisture at the surface is continuously being convected away.

It is interesting to note that the temperature differences (T2 .. Tl) are usually greater at power level 0.1 W/g than at 0.2 W/g. This might be due ta higher moisture content at the centre in the case of 0.1 W g.l, due to slower rate of drying. After the rapid initial temperature rise in the first 30 min, the temperatures at the centre tend to level off for about one hour, and then rise again. The level period might he associated with a relatively constant removal of free moisture, whereas the oscil1ations and increases apparent later on May he associated with the interplay of different heating

86 mechanisms and perhaps the inhibition ofmoisture movement by osmotic forces connected with dissolved material. This temporary inhibition of moisture migration leads to a temperature rise, which then drops off as more moisture is vaporized and diffuses outward. It should also be noted that the centre and surface temperatures of the strawberries are very high (BO - 100°C) compared to what one expect in the convective situation, where the surface and centre temperatures tend to equilibrate with the temperature of the drying air. It is aimast difticult ta imagine that the berries would dry rather than cook in the high temperatures resulting from the microwave regimes. On the other hand, since the berries have been treated, there is a rapid moisture 10ss at the same time as the temperature rises, and the final product dries. However, the quality of microwaved, dipped berries is not totally like freeze-dried berry.

5.4 CONCLUSIONS

From this experiment, it can he concluded that dipping in an EO/NaOH solution increases the drying rate, and does have sorne effect on quality. The microwaved product is similar in quality 10 the freeze-dried product if low power levels are used, which constrains the rate advantage to an upper limite The rate advantage is best enhanced by treating the berries with EO/NaOH solution of 1% EO with 0.5% NaOH prior 10 drying at a power level of 0.2 W/g. However, this leads to a product that is softer than the freeze-dried product and May have some internaI damages that could affect the keeping quality. The drying rate at this combination of power and pretreatment was 6.4. Higher rates are possible with higher power application, but, fruit quality deteriorates. There May be room for 87 fine-tuning at rates intermediate 10 0.1 and 0.2 W/g power or devising an apparatus that would permit containment of the samples with the use of • higher air velocities. Concentrations ofEO higher than 1%, combined with 0.5% NaOH do not help in accelerating the drying rates of whole strawberries. This may he due to the chemical kinetics under the conditions ofdipping, which were restricted to 1 min at room temperature. The highest difference of colour of 14.54 was associated with the convectionally dried berries with 1% EO + 0.5% NaOH pretreatment. The arama of the microwave..dried strawberries was 10st. More detailed work focusing on porosity and possibly the rehydration adsorption or absorption of EO, would be needed to determine why the rehydration ratios are Unked ta the dipping treatments and the type of drying regime. The shrinkage experiments led ta empirical relations describing shrinkage ratio and equivalent diameter in terms ofthe moisture ratio. The equations were excellent fits ta the data in aIl cases and could he used in comprehensive drying models

88 5.5 Connecting statement to chapter 6 This part of the study (Chapter 5) had shown that pre-treating the strawberries with ethyl oleate and sodium hydroxide helps in drying them whole with microwave power. The toughness, colour and rehydration rate of the product can he very close to those of the freeze dried product. However, the aroma of strawberries in the dried fruit was lost. The literature suggests that the aroma can be retained by osmotically treating the fruits before subjecting the fruits for heat treatment. Hence it was decided to pretreat with EO and NaOH, osmotically dehydrate the strawherries for a certain period and use microwaves for the finish drying rather than continue with the conventional freeze-drying. Osmotic dehydration should also reduce the time of exposure ta high internaI temperatures witnessed in the data, possibly leading to an equivalent..ta.. freeze..dried product.

89 CHAPTERVI

OSMOTIC AND MICROWAVE DRYlNG OF STRAWBERRIES

6.1 INTRODUCTION The preliminarystudies on microwave drying ofstrawberries and the experiments on microwave drying after dipping in a solution of ethyl oleate and sodium hydroxide led to the conclusion that marketable dried strawberries cannot be obtained by the methods tried. Since partial dehydration in an osmotic solution has been reported ta improve flavour retention in convection-dried fruits (Jackson and Mohammed, 1971; Ponting, 1973; Dixon et al, 1976; Flink, 1979; Voilley and Simatos, 1979), it was decided to investigate the possibility that this technique could he used as a pretreatment for microwave drying ta improve the quality of the finished product. The objectives of this study were to determine the osmotic dehydration rate in sucrase ofstrawbenies pretreated with ethyl oleate and sodium hydroxide, ta compare the microwave, convection and freeze drying rates of the osmotically dehydrated berries, and to evaluate the quality of the dried product.

6.2 MATERIALS AND METHODS

6.2.1 Dipping treatment In Chapter 5, it was round that the concentration ofEO used in the dipping treatment did not significantly affect the drying time. However, the highest rehydration ratio, on the average, was that associated with a 2% EO concentration and resulted in a toughness similar to that of the

90 freeze.dried product. It was therefore decided ta use only 2% EO in 0.5% NaOH as a pretreatment.

6.2.2 Osmotic dehydration Four 100 g samples of treated berries were taken and mixed with granular sucrose in mass ratios of 3:1 or 4:1 froit ta sucrase (F:S) at roam temperature and mixed periodically. The samples were taken out at 12 h, 24 h, 36h h and 48 h respectively and rinsed in water at the same raom temperature to remove the syrup fram the fruit surface. Surface water was removed as before and 10ss in mass due ta osmosis was determined.

6.2.3 Drying experiments The strawberries that were osmotically dehydrated for 24 h were used for microwave (MW) drying experiments, convective drying and freeze drying trials. Microwave, convective and freeze-drying trials were conducted in the sarne equipment as described in Chapter 5. There were three replicates of the experimental conditions summarized in Table 5.1. The experiment was a 2x3x3 factorial, with 2 froit to sugar ratios, three inlet air temperatures and three drying regimes (convective, microwave at 0.1 or 0.2 W gol). Three samples from each fruit to sugar ratio were freeze ... dried for comparison in the quality assessments. The dried sampIes were cooled ta ambient temperature, packed in polyethylene covers and stored in the cold room 8t 1°C for rehydration, texture, color and sensory evaluation studies.

6.2.4 Quality assessment Assessment of the quality of the dried products was based on chromacity, rehydration ratio, toughness, and sensory evaluation including flavour and arama by a panel ofjudges, all as described in the appropriate 91 sections of earlier chapters (3 and 5).

• 6.3 RESULTS AND DISCUSSION 6.3.1 Osmotic dehydration The data analysis (Table A.6.1) was as expected, showing that there were significant differences in amount ofmoisture removed due to the fmit ta sugar ratio, to the EO/NaOH dip and ta the time ofosmotic dehydration. The means separations by Duncan's new multiple range test (Table 6.1) are also consistent with expectations that more moisture is removed when more sugar is applied, when the berries have been dipped, and when the dehydration is continued for a longer time. However, the increase due ta a greater amount of sugar is small and doesn't justify its use.

Table 6.1 Means separations by Duncan's test for moisture removal at the two fruit to sugar ratios, treatment with EO/NaOH and time of dehydration.

F:S Ratio Means Dipping Means Duration Means Treatment (h) 2%EO 3:1 22.62 A 0.5% NaOH 29.95 A 0 D.DA 4:1 21.59 B None 14.26 B 12 16.2 B 24 25.1 C 36 30.6 D 48 38.7 E

Means with the sarne letter are not significantly different at the 0.051evel.

The moisture removals due to osmotic dehydration of untreated strawberries and ofstrawberries pretreated with 2% ethyl oleate ŒO) and 0.5% sodium hydroxide (NaOH) and osmotically dehydrated in a fruit to

92 sugar ratio of3:1, are shown in Figure 6.1. The time advantage due to the pretreatment is clear, dipped strawberries losing far more moisture than UDdipped strawberries. This is due ta dissolution of the waxy cuticle by ethyl oleate and the creation ofmicropores in the skin by sodium hydroxide. Treated berries reached a wet basis moisture content of about 62% in the frrst 24 h, and 53% after 48 hours. In contrast, the untreated berries dropped only ta 85% m.c. in the tirst 24 h. However, there was sorne discoloration ofthe strawberries duringosmotic dehydration. Furthermore, off-odours developed by 36 hours, which is the reason why only berries dehydrated for 24 h were used to compare drying rates and quality in the various regimes tested.

6.3.2 Drying times The drying times and relative drying rates of the 24 h osmotically dehydrated berries which were then dried under various regimes are shawn in Table 6.2. A preliminary analysis (Table A.6.2) indicated that the inlet air temperature, and the power level (including convection, or 0 W g-l), significantly influenced the drying times, with increases in bath reducing the drying time. There were also significant interactions between fruit ta sugar ratio, power level and temperature, as weIl as between temperature and power level. However, inspection of Table 6.2 shows that the microwaved samples are inconsistent with respect to the effect of iniet air temperature. For sorne settings ofpower level and fruit ta sugar ratio, the drying time is longer at 45°C than for the 35°C counterpart. The data were therefore reanalyzed, excluding the convection situation, ta see whether the inlet air temperature significantlyaifected drying time in the microwave regimes. There was no such effect, contrary to the findings in the preliminary studies which had shawn a significant temperature effect when air inlet temperature was increased ta 4DoC from

93 35°C. One might hypothesize that the waxy layer somehow was affected by the temperature change in the preliminary studies, whereas in the experiments presented here, the waxy layer had been partially removed by the dipping. The influence oftemperature should have been verified in the experiments on EO concentration. Onlypower level and fruit to sugarratio were significant main effects when only the microwaved samples were considered. There were also significant interactions between inlet air temperature and power leveI, as weIl as between fruit to sugar ratio and power. Nevertheless, inlet air temperature does increase the relative drying rate in the convection situation by about 50%.

Table 6.2 Microwave drying time and relative drying rate ofosmotically dehydrated strawberries. Treatment Drying Time Relative Air F:S PL, W/g Min Drying Rate 35°C 3:1 0 810 1 0.1 100 8.1 0.2 75 10.8 35°C 4:1 0 810 1 0.1 85 9.53 0.2 60 13.5 45°C 3:1 0 510 1.59 0.1 120 6.75 0.2 60 13.5 45°C 4:1 0 570 1.42 0.1 90 9 0.2 60 13.5

94 Table 6.3 shows that the 3:1 fnrit 10 sugar ratio led to significantly longer drying times than the 4:1 ratio, whieh one would not necessarily expect, sinee after 24 h ofosmotic dehydration, the moisture content ofthe strawberries infused at a 3:1 ratio was lower (53% compared to 58%) and should therefore have reached the final moisture content more rapidly. One possibility for the opposite result is that more sugar entered the berries infused at a 3:1 ratio, resulting in a higher osmotic tension in the froit which inhibited the outward diffusion ofmoisture, as suggested by Rahman et al. (1991) and Sankat et al. (1996) in air drying studies of osmotically dehydrated fruit. Unfortunately, this could not be verified in this study because samples of the osmotically dehydrated samples (24 h) were not taken for dry weight measurements. This also means that the moisture determinations are slightly off sinee the smaIl dry matter increase was not taken into account.

Table 6.3 Duncan groupings for mean drying times at the two temperatures, froit ta sugar ratios and power levels.

Temp Mean F:S Ratio Mean Power Mean oC Level W go1

45 83.333 A 3:1 88.750 A 0.1 98.750 A 35 80.000 A 4:1 74.583 B 0.2 64.583 B

Means with the same letter are Dot significantly difTerent at the 0.05 level.

Figure 6.2 clearly indicates the rate advantage due to microwaves. The fruit-to-sugar ratio has sorne influence on microwave drying, the 3:1 ratio leading to somewhat lower drying rates. However, these calculated difJerences are largely insignificant based on the kinetics shown in Figure

95 • 50

'a 40 oJ E .! 30 iD !.20 fi. u :lE 10

12 24 36 48 Tlme ln Hour.

Figure 6.1 Osmotic dehydration of treated and untreated strawberries

7------r------, • - PL 0 , + PL 0.1 w/g .. PL 0.2 w/g 6

O-+-O-...... ,~ ...... ~~~..,..,.., ...... "'P"9""f'...... ~~T"T""I""'I"""I'""'IT"T"'r"I o 60 120 180 240 300 360 420 480 540 Minute.

Figure 6.2 Microwave drying ofosmoticalIy dehydrated strawberries at different power levels.

96 6.3. In this figure, the curves at different F:S ratios for the sarne power level are essentially coincident. Figure 6.4 shows the drying rates versus moisture content for the 4:1 treated berries at the two MW power levels and for convective drying, at bath inlet air temperatures, aIso confmning the general absence of influence ofinlet air temperature on microwave drying. As the power level goes up the drying rate goes up. These drying experiments clearly indicated that there is a significant advantage to pretreating the berries for the osmotic dehydration stage and that microwaves tremendously enhance the drying rate compared ta convection. However, the quality of the final product must be considered. In this study, we used the freeze-dried product as a standard of quality.

6.3.3 Empirical mode} of finish drying with microwaves The following approach was taken to generate an empirical model to describe microwave and convective finish dryingofstrawberries dehydrated in a 4:1 fruit to sugar ratio. For each replicate, ofconvection or microwave finish drying, an exponential curve of the forro of M = Mo e·kl was fit to each set of dry basis moisture contents observed in the trials (M is the dry basis moisture content; Mo is the initial moisture, k is a rate constant and t is time). The rate constants thus obtained (R2~.99 in aIl cases), were then expressed in tenns of the power level using a best-fit (using Curvexpert™ software version 1.3) second-order linear equationoftheform:

(6.1)

where, PL is the microwave power level (0, 0.1 or 0.2 W gel) and the bi are regression parameter estimates. This procedure led to the following equation to predict moisture content as a function of time and power level:

97 7-r------r------, - PL 0.1, 3:1 + PL 0.1, 4:1

6 ...... PL 0.2, 3:1 ~ PL 0.2, 4:1 ms -Q

O-+--r---"T"--r----.~-...... -~---,-.....,--...,..-----.---l o la 20 30 40 50 60 70 80 90 100 110 Minut••

Figure 6.3 Microwave drying of strawberries at two power levels dehydrated at two osmotic levels (F:S). • 45·C -PL:O W/g +PL=0.1 W/g "PL:O.2 Will PL=O W/g *PL:0.1 Will +PI.:0.2 Will 8------, ...... f:S .. 4:1 .

Figure 6.4 Microwave drying rate of osmotically dehydrated strawberries at different power levels and inlet air temperatures of 35°C and 45°C.

98 7.0 .,..------.,

&.0 ~------~

m 5.0 +---t....------{ Q -en ~ 4.0 +- ....-.;:l~------...... ; C) .:..: ~ 3.0 +------~:------~ :s ~- ~ 2.0 4------;:~------____l

1.0 4------=--...... ;

0.0 4----_----,...---..,...------.-----r------; o 120 240 360 480 600 720 Time, min

Figure 6.5 Predicted moisture content of strawberries by the exponential model compared with the experimental values (PL: oWle:). 7.0,..------.

6.0 -r------~ --Model m • Expt -c -C) 5.0 ~ C) -~ 4.0

=en 3.0 "0- ::i 2.0

1.0 • 0.0 0 20 40 60 80 1QO 120 140 Time, min Figure 6.6 Comparison of the moisture content of strawberries predicted by the exponential model and the experimental values (PL= 0.1 W/g). 99 7.0 .,..------. • 6.0 ,------~ ~ 5.0 T-~~------~ -o ~ 4.0 1---~:_e------I C1 -~ (JJ- 3.0 +------:l~-~------~ .3- .~ 2.0 "!------.;~------...; :E 1.0 +------~~-----~

0.0 +.----~--- __---_,_----~-----i• o 20 40 60 80 100 Time, min Figure 6.7 Predicted moisture content of strawberries by the exponential model compared with the experimental values (PL: 0.2 W/g) . •

100 MC(e,PL) =5.98 exp «-O.0025-0.0413*PL+O.078S*PL2 )*t)

(6.2) The above equation was then used to generate predicted moisture contents for each power leveI, as shown in Figures 6.5 to 6.7. This approach gave a very good fit to the convection data and to the data obtained at a microwave power level of 0.1 W g-t over most of the drying periode In the case of0.2 W g-t moisture content was grossly underestimated for the fll"st 45 minutes of drying, but tended towards the experimental data near the end of the drying periode As the power levei goes up the estimation of the moisture by the modei slightly faIls, because of the high rate of moisture removal by the microwaves.

6.3.4 Quality analyses The rehydration ratios and toughness measurements are presented in Table 6.4a, the chromacities and colour differences in Table 6.5a and the sensory evaluations in Table 6.6. The analysis of variance on rehydration ratio (Table A.6.3) showed that there were significant differences due ta air inlet temperature or froit to sugar ratio among the microwaved strawherries. The range of rehydration ratios (Table 6.4a), for the experimental berries (1.64 to 2.12) includes that ofthe freeze-dried product (1.88 ta 1.90). The highest rehydration ratios were obtained in the 0.2 W g.l regime (Table 6.4b), and both microwave regimes led to better rehydration than the convection-dried osmotically dehydrated berries. The rehydration ratio offreeze-dried berries was not significantly differentfrom those dried in the 0.1 W g-l regime. The low rehydration potential of the convection-dried berries could be due to case..hardening. These results

101 indicate that microwave dried bernes are equal or better than freeze dried berries in rehydration, thus it should be possible to determine the precise microwave-drying conditions that yield the same value if that is an essential product parameter.

Table 6.48 Rehydration ratio and texture measurements of osmotically dehydrated and microwave dried strawberries. Treatment Rehydration Texture Air F:S PL, W/g Ratio (Toughness) l\1Pa 35°C 3:1 0 1.64 0.56 0.1 1.82 0.45 0.2 2.06 0.26 35°C 4:1 0 1.62 0.65 0.1 1.84 0.55 0.2 2.12 0.46 45°C 3:1 0 1.70 0.36 0.1 1.68 0.35 0.2 2.05 0.48 45°C 4:1 0 1.69 0.41 0.1 1.93 0.38 0.2 1.79 0.42 Freeze 3:1 1.90 0.78 Dried 4:1 1.88 0.78

102 Table 6.4b Means separation of rehydration ratios at the experimental temperatures, fruit to sugar ratios and power levels.

Temp Mean F:S Mean Power Level Mean oC Ratio W g-l 35 1.85 A 4:1 1.83 A 0.2 2.01 A 45 1.80 A 3:1 1.82 A 0.1 1.82 B 0.0 1.66 C

Means with the same letter are not significantly different at the 0.05 level.

Table 6.4c shows that the toughness was influenced by inlet air temperature and drying regime, but not by fruit ta sugar ratio. Application of microwave energy seemed ta soften the berries, probably due to the greater internal heating compared to the convection case. The freeze-dried berries were significantly tougher than the thermally dried ones.

Table 6.4c: Duncan's groupings for Mean toughness at two temperatures, froit ta sugar ratios and drying regimes.

Temp Mean F:S, Mean Power Mean oC Ratio W/g 35 0.50000 A 4:1 0.48500 B 0 0.49833 B 45 0.40667 B 3:1 0.42167 B 0.1 0.45083 B FD 0.80500 A FD 0.80500 A 0.2 0.41083 B FD 0.80500 A

Means with the same letter are not significantly different at the 0.05 level.

The analysis of colour difference (Table A.6.5) showed that air inlet temperature was not influential, but that both fruit to sugar ratio and power level significantly affected this parameter. Table 6.5b shows that the

103 greatest colour difference with thefresh bernes was obtained at the highest power level and at the higher fnrit to sugar ratio. The greater heating should lead to faster darkening, with sorne possibility of occurrence of imperceptible burnt spots. It is difficult to explain why the berries infused with less sugar should exhibit a significantly larger colour difference, particularly since this group dried more quickly than those dehydrated at a 3:1 ratio. It is interesting to note that the freeze-dried berries were the lightest, and closest to fresh berry colour.

Table 6.5a Color measurements ofosmotically dehydrated and microwave and freeze dried strawberries. Treatment L a b ~.b Air F:S PL, W/g Fresh fruit (target calor) 35.19 33.01 23.20 35°C 3:1 0 27.76 23.40 12.50 16.22 0.1 27.34 20.63 10.71 19.27 0.2 27.57 19.24 Il.96 19.46 35°C 4:1 0 28.27 23.59 Il.66 16.43 0.1 26.98 19.67 Il.56 19.64 0.2 23.98 17.62 Il.31 22.46 45°C 3:1 0 27.30 24.31 14.03 14.91 0.1 26.62 20.37 Il.72 19.21 0.2 27.26 23.47 14.18 15.34 45°C 4:1 0 26.46 20.40 14.63 17.60 0.1 27.77 21.88 14.64 16.07 0.2 25.08 14.62 9.74 24.96 Freeze 3:1 41.69 34.73 20.48 7.25 Dried 4:1 46.19 33.81 20.39 Il.39

104 Table 6.5b Means separations of colour differences due to drying regime, fruit to sugar ratio and inlet air temperature.

Drying Mean F:S Mean Temp Mean Regime 0.2 W g.l 21.243 A 4:1 19.18 A 45 19.31 A 0.1 W g.l 18.731 B 3:1 16.74 B 35 19.18 A Convection 17.768 B Freeze-dried 10.231 C

Means with the sarne letter are not significantly different at the 0.05level.

Table 6.6 Sensory evaluation of osmotically dehydrated and microwave and freeze dried strawberries.

Treatment Sensory Duncan Air F:S PL, W/g Score Groupings (Mean)

45 DC 3:1 0 6.50 A 0.1 6.12 A 0.2 5.75 A

45DC 4:1 0 5.75 A 0.1 5.12 A 0.2 5.00 A Freeze 3:1 5.75 A Dried Freeze 4:1 5.62 A Dried

Means with the same letter are not significantly different at the 0.05 level.

105 The results of the sensory evaluation (Table 6.6) indicate that the panel ofjudges had difficulty in distinguishing the drying conditions used, sinee there were no significant differenees in rating over the various treatment combinations. Interestingly, the eonveetively dried product scored highest. The fact that sorne of the microwave-dried berries scored as high as the freeze-dried product is encouraging.

6.4 CONCLUSIONS

The results of this study indicate that dipping in an alkaline ethyl oleate solution promotes rapid dehydration by osmosis. The osmotic dehydration step has been shown to he necessary to obtaining a microwave dried strawberry ofquality close to that ofthe freeze-dried product in terms of rehydration characteristics and overall sensory evaluation. The microwave·dried product is nevertheless darker and softer than the freeze· dried product. A simple exponential decay model with the rate constant expressed in tenns of time and power levels

106 • CHAPTER VII OSMonc AND MICROWAVE DRYING OF BLUEBERRIES

7.1 INTRODUCTION Since the experiments on microwave-drying ofosmotically dehydrated strawberries were encouraging, it was deeided ta foeus attention on blueberries, a berry fruit whose production and popularity are inereasing. North America is the world's major supplier of blueberries, aceounting for about 90% of production. The objective of this study was ta rmd the osmotic dehydration rate of blueberries pretreated with ethyl oleate and sodium hydroxide. The osmotically dehydrated blueberries were dried and studied under convective, convective-microwave and freeze drying. Their drying rates, rehydration ratios, colour, texture and sensory evaluation were compared.

• 7.2 MATERIALS AND METROnS The materials and methods used in the blueberry experiments were the sarne as those used in the experiments on osmotic dehydration and finish drying of strawherries with microwaves. The levels of pretreatment with ethyl oleate and sodium hydroxide, fruit ta sugar ratio in osmotic dehydration and air temperatures and the power levels used in the microwave drying were same as that ofosmotic dehydration ofstrawberries (chapter 6.).

7.3 RESULT8 AND DISCUSSION 7.3.1 Osmotic dehydratioD The details of osmotic dehydration at different periods of time are

107 illustrated in Fig 7.1. It is obvions that the pretreatment is beneficial in osmotic dehydration ofblueberries, and this was borne out in the statistical • analysis (Table A.7.1 and Table 7.1).

Table 7.1. Duncan's groupings for MCEM (moisture content removed) from blueberries in osmotic dehydration under different F:S, pretreatments, and time.

F:S Mean TRT Mean Time Mean

3:1 9.54 A 2%EO 15.11 A 0 0 E

4:1 8.94 B No, EO 3.36 B 12 6.77 D

24 11.24 C

36 13.01 B

48 15.17 A

• Means with the sarne letter are not significantly different at 0.05 levei

The osmotically dehydrated in a 3:1 ratio blueberries treated with 2% ethyl oleate (EO) and 0.5% sodium hydroxide (NaOH) solution last 20 % moisture (WB) in 24 h compared ta untreated berries which lose about 4 %. In 48 hours, treated berries give up 24.62 % moisture compared ta untreated 7.36 %. These figures are substantially lower than the moisture lasses over time of the strawberries. Table 7.1 indicates that the moisture 10ss in blueberries was significantly higher when more sugar was used (3:1 fruit ta sugar ratio); however, this significant diff'erence is very small (0.6% m.c.) and does not justify the additional quantity of sugar needed.

108 7.3.2 Drying kinetics The drying kinetics ofblueberries at microwave powers ofO.l W/g, 0.2 W/g and convective drying are shawn in Fig 7.2. for the case of 45°C inlet air temperature, whereas the drying rates are plotted in Fig. 7.4. The statistical analysis indicated (Table A.7.2 and Table 7.2) that drying was significantly faster at the highertemperature and at the higher power level. As in the case of strawberries, the blueberries that had been osmatically dehydrated with the greater amaunt of sugar dried more slowly.

Table 7.2. Duncan's groupings for mean drying time, at different temperatures, F:S, and microwave power leveis. Air Mean F:S Mean MW Mean TempoC W/g 35 444.89 A 3:1 383.222 A 0 592.33 A 45 308.33 B 4:1 370.000 B 0.1 297.50 B 0.2 240.00 C

Means with the same letter are not significantly different at 0.05 level

Microwaves reduced the drying time by 50% (4.5 h) and 67% (6 h) over the convection case which took 9 h ta dry, at power levels 0.1 and 0.2 W gel, respectively and inlet air temperature of 45°C. Yang et al. (1987) aIso found that the drying time increases as the fruit to sugar ratio goes up, in studies offreeze-drying. Fruit to sugar ratios of 4:1, 3:1 and 2:1 resulted in 15 h, 18 h and 24 h drying times ta reach a moisture content of 16 %. Relative drying rates for different treatments shawn in Table 7.3 varied fram 1 to 3.83, these being in a lower and much smaller range than the relative drying rates of strawberrles under the same conditions.

109 Table 7.3 Drying time and relative dryingrate ofblueberries osmotically dehydrated and microwave dried at different microwave power levels.

Treatment Drying Time Relative to MC 1 Drying Rate kg/kg (DB), min

Air temp F:S Power level, oC W/g

·35°C 3:1 0 690 1

0.1 360 1.92

0.2 300 2.09

35°C 4:1 0 690 1

0.1 330 2.09

0.2 300 2.30

45°C 3:1 0 510 1.35

0.1 260 2.65

0.2 180 3.83

45°C 4:1 0 480 1.44

0.1 240 2.87

0.2 180 3.83

• Control

110 30 i" 25 . . !. .. 120·'· . o E .. ! 15 . ! =10 .' Ci• ~ 5· ~

o"""'---__--ol~__---oIt. --L.. _' 12 24 36 48 Houra of oamo.la

Figure 7.1. Osmotic dehydration ofuntreated and treated blueberries at different durations of time.

6...,...... ----r------., - PL:O + PL:O.1 W/g .. PL:O.2 W/g 5 .

O-+--,--r--r--r--r--r-..,..----r-..,..-__...--~~r--r--r--,...... --4 <)

nme min Figure 7.2. Microwave drying of osmotically dehydrated blueberries at düFerent power levels.

111 6------.------, - PL=0.1, 3:1 + PL=0. 1, 4:1 5 PL=0.2, 3:1 PL=0.2, 4:1

O-+---,.--...,--...... --....--.,...... -r---,...--,...-----i o 30 60 90 120 150 180 210 240 270 Tlme min

Figure 7.3. Microwave drying ofblueberries osmotically dehydrated with F:S, 3:1 and 4:1 and dried at power levels 0.1 and 0.2 W/g.

2.5...,--...,------, - PL=O W/g + PL:O.1 W/g .... PL:O.2 W/g

.: 2 ::::::.. ID Q '5 ~1.5 ~.- a:• CD c:: ~ PL:O Q 0.5

o -.=:;~"""'T"'""--r-__r_---r__r__r~___r___.,r__,..._r__...._...... __ __f o 1 2 3 4 Mol.ure Content kg/kg (DB)

Figure 7.4. Microwave drying rate of osmotically dehydrated blueberries at different power levels.

112 Table 7.4. Rehydration and texture measurements of blueberries osmotically dehydrated followed by microwave or freeze clried, under different treatments. Treatment Rehydration Texture Ratio (Toughness) MPa Air temp F:S Power level W/g 35°C 3:1 0 1.38 0.350 0.1 1.42 0.375 0.2 1.36 0.325 35°C 4:1 0 1.28 0.285 0.1 1.28 0.245 0.2 1.40 0.350 45°C 3:1 0 1.36 0.235 0.1 1.39 0.212 0.2 1.36 0.265 45°C 4:1 0 1.20 0.330 0.1 1.30 0.197 0.2 1.26 0.187 Freeze 3:1 1.43 0.296 dried Freeze 4:1 1.30 0.251 dried

113 7.3.3 Empirîcal model offinish drying with microwaves

The same procedure as used for modelling the finish drying of strawhenies was used here. The resulting equation was:

MC = 4.843 exp «-0.003-0.041 PL + 0.078 PL2 ) *t) (7.1)

The fits ta the experimental data are shown in Figures 7.5 ta 7.7 for convective drying and drying at the two power levels. Again, the fit was very good for the convection regime, and reasonably accurate for 0.1 W g.l and 0.2 W gol. In general, the fit for blueberries is better than that for strawberries. This May he due to the longer duration of drying for the blueberries and perhaps to a greater homogeneity in the case ofblueberries, which are more spherical and have no central air gap.

7.3.4 Quality The rehydration ratios of blueberries (Table 7.4) were significantly influenced by the air inlet temperature used in microwave and convective drying, and by the fruit ta sugar ratio used during osmotic dehydration (Table A.7.3 and Table 7.5). However, there were no significant differences between the microwaved, convective and freeze-dried samples (Table 7.6). The range ofrehydration ratios for bluebenies was lower (1.26 to 1.43) and quite narrow compared ta that of strawberries (1.62 to 2.12).

114 &.0

S.O

-al e. 4.0 CS ~ ~ 3.0 ...GJ- :2 en 2.0 "0 == 1.0

0.0 0 '20 240 360 480 600 Time, min

Figure 7.5 Predicted moisture content of blueberries by the exponential model compared with the experimental values (PL: oW/g).

-Model • Expt

4.0 ai c -Ct ~ 3.0 Ct ~ ...ai :2 2.0 rit "0- == 1.0

0.0 0 60 120 180 240 300 360 420 lime, min

Figure 7.6 Comparison of the moisture content of blueberries predicted by the exponential model and the experimental values (PL: 0.1 W/g). 115. 6.0 ,------,

5.0 m a 4.0 +-~------l .xCI 01 .x- 3.0 cr -::2 ëii ë5 2.0 +-----~~------l :E

1.0 • • 0.0 o 60 120 180 240 300 360 420 Time, min Figure 7.7 Predicted moisture content of blueberries by the exponential model compared with the experimental values (PL: • 0.2 W/g).

116 Table 7.5. Duncan's groupings for mean Rehydration, at different temperatures, F:S, and microwave power levels. Air Mean F:S Mean MW Mean TempoC Power W/g 35 1.3577 A 3:1 1.3822 A 0 1.3500 A 45 1.3167 B 4:1 1.2922 B 0.1 1.3425 A 0.2 1.3192 A

Means with the same letter are not significantly different at 0.05 level

Table 7.6. Duncan's groupings for mean Rehydration, at different drying regimes and microwave power levels. Regime Rehydration Duncan Grouping Freeze dried 1.3650 A 0.2 W/g 1.3500 A 0.1 W/g 1.3425 A o(Convective) 1.3192 A

Means with the sarne letter are not significantly different at 0.051evel

The toughness of dried blueberries (Table 7.4) was significantly influenced only by the inlet air temperature (Table A.7.4), the berries dried at 35°C being tougher than the others. There were no significant differences in toughness between freeze-dried blueberries and those dried in other regimes (Table 7.7). The blueberries were, in general, quite a bit softer thanthe strawberries, toughness beinginthe range 0.19 ta 0.37 MPa,

117 compared ta 0.26 ta 0.78 MPa for strawberries. It is

Table 7.7. Duncan's groupings for Mean toughness, at different drying regimes. Regime Regime Duncan Grouping Conv Air 0.3092 A 0.2W/gMW 0.2875 A Freeze dried 0.2767 A 0.1 W/gMW 0.2642 A

Means with the same letter are not significantly different at 0.05 levei

interesting ta note that the freeze-dried blueberries were intermediate with respect ta toughness, whereas in the case of strawberries, the freeze-dried samples were substantially more resistant ta penetration than the others. Proximate analysis and possibly structural studies could lead ta explanations for these differences. The difTerence in colour (Table 7.8) between the fresh blueberries and those dried convectively or in microwave regimes were influenced only by the fruit ta sugar ratio used during the osmotic dehydration (Table A.7.5); however, the measurements were very precise, and differences as small as 0.8 colour difference units were considered

118 Table 7.8 Colour measurements of blueberries osmotically dehydrated then microwave or freeze dried under different treatments

Treatment L a b Eab Air temp F:S Power levei W/g 35°C 3:1 0 15.24 1.08 1.37 10.65 0.1 16.56 0.73 0.45 7.00 0.2 20.33 0.57 0.09 12.01 35°C 4:1 0 17.03 1.43 0.85 10.17 0.1 17.29 0.84 0.47 9.92 0.2 15.17 0.46 0.67 12.06 45°C 3:1 0 16.06 0.96 0.87 11.15 0.1 18.31 2.61 2.42 9.23 0.2 17.43 1.56 1.33 9.82 45°C 4:1 0 14.07 0.43 0.65 13.16 0.1 16.71 0.17 0.65 10.57 0.2 16.25 1.18 0.41 10.94 Freeze 3:1 12.69 0.55 1.49 14.59 dried 4:1 14.02 0.89 1.56 13.25 Fresh fruit (Target colour) 27.17 1.52 0.86 statistica1ly significant, even though the human observer would not be able to make a distinction. The freeze-dried berries gave a significantly higher colour difference (44.6), but again, this was ooly 1.8 units greater than the 10west (42.8), associated with the blueberries dried at 0.1 W g-l microwave power (Table 7.9).

119 Table 7.9. Duncan's groupings for Mean colour difference, at different temperatures, F:S, and drying regimes. • Air Mean F:S Mean Regime Mean TempoC

35 43.36 B 3:1 43.03 B 0 43.68 B

45 43.12 B 4:1 43.85 A 0.1 42.88 C FD 44.63 A 0.2 43.18 CB FD 44.63 A

Means with the sarne letter are not significantly different at 0.05 level

It was observed that the sugars used for osmotic dehydration did not change to blueberry colour as much as that used during dehydration of strawberries. This might explain why the colour differences associated with the blueberries are substantially greater than those associated with the strawberries. The dried blueberries resembled dark raisins. The assessments of the panel of 10 judges were analyzed, and it was found that there was a significant difference only between the two judges giving the highest rankings and the two judges giving the lowest rankings (Table 7.10). The freeze-dried product was rated slightly higher than the others, but was only considered significantly more acceptable than the convection-dried product (Table 7.11). Overall, the microwave-dried blueberries were quite similar to the freeze-dried blueberries; however, they did dry much more rapidly.

120 Table 7.10 DuncanJs groupings for Mean score by different judges. Judge Mean score Grouping 10 6.6429 A 6 6.5000 A 5 6.3571 A B 8 6.1429 A B 3 6.0714 A B 2 6.0714 A B 9 5.6429 A B 4 5.6429 A B 7 5.2143 B 1 5.2143 B

Means with the same letter are not significantly different at 0.05 level

Table 7.11. Duncan's groupings for Mean score at different regimes

Regime Mean score Grouping

FD 6.3500 A 0.1 W/g 6.2000 A B 0.2 W/g 5.9000 A B o (Conv) 5.5500 B

Means with the same letter are not significantly different at 0.05 levei

121 7.4 CONCLUSIONS The blueberries osmotically dehydrated and microwave dried with convective air at a power level of 0.1 W g.l were of equal quality to the freeze-dried product. Sînce they dried far more quickly, it remains ta determine the energy requirements and economics for the process and compare them to those for freeze-drying. The only drawback expected is the capital investment for appropriate microwave equipment.

122 • CHAPTER Vin CONCLUSIONS, CONTRIBUTIONS TO KNOWLEDGE AND RECOMMENDATIONS FOR FURTHER WORK

8.1. Summary and Conclusions The principal objective ofthis study was ta evaluate the feasibility of developing a microwave..based process for drying strawberries and blueberries. The drying experiments and quality evaluations led to the following conclusions.

1. Whole strawberry and blueberry Croit cannot he dried in the convective..microwave regimes used in this study without pretreatment. High skin resistance to moisture diffusion combined with rapid internaI heat generation resuit in bursting of the fruits. Buming aiso occurs at higher microwave power levels. Puncturing the surface ofstrawbenies at severaI locations is not an appropriate way of improving diffusion of moisture, as the berries bleed and sometimes burst at higher incident microwave energy levels.

2. Although sliced strawberries can be dried without bursting, there are substantiallosses in quality attributes, probably due to volatilization of aromas.

3. The pureed product can be dried into a fruit leather; however, flavour and aroma are 10st and the product is difficult to separate from the screen support.

123 4. The prohlem ofskin resistance to diffusion can he overcome, as shawn by other researchers, by dipping in alkaline solutions of • ethyl oleate for one minute at room temperature. A concentration of 1% ethyl oleate is sufficient ta dissolve the wax, since no further improvement in the drying rate is obtained at higher concentrations.

5. Dipping significantly improves both the convection and microwave drying rates of strawberries and blueberries.

6. Although dippingin the EO/NaOH solution led to a reasonable quality microwave-dried product, it was not as good as the freeze-dried product, primarily due to loss of aroma and flavour.

7. Osmotic dehydration of strawberries prior to drying reduces 10ss of aroma, flavour and colour, leading to a dried product that is equivalent in sensory quality to freeze-dried strawberries.

8. Osmotic dehydration removed significantly larger quantities of moisture from strawberries than from blueberries; however, if the osmotic step was carried out for longer than 24 hours, off odours developed in both types of fruit.

9. Strawberries pretreated and osmotically dehydrated for 24 hours, dried in a 0.2 W g-1 regime in approximately 1/10 ofthe time taken by convection drying (45°C / 2 m S-l). Under the same conditions, blueberries dry in 1/3 of the time needed for

124 convection. Berries microwave dried at power level 0.1 or 0.2 W g-t were equal in quality to dipped, osmotically dehydrated and then freeze-dried berries.

10. Blueberries pretreated with 2% ethyl oleate and 0.5% sodium hydroxide, 10st significantly more moisture (WB) during osmotic dehydration than untreated blueberries.

Il. Blueberries and strawberries that have been osmotically dehydrated with greater amount of sugar (3:1 fruit..to-sugar) lose more moisture in the sarne dehydration time, yet dry to the final state less quickly thereafter.

12. Power levels greater than 0.2 W g-l lead to burnt spots or general burning of the berries, whether or not they have been pretreated.

13. The shrinkage ratio ofstrawberries has a straight line relation to the moisture ratio. The reduction in equivalent diameter is well..described by a reciprocallogarithmic function.

Finally, one can conclude that it is possible ta obtain high quality blueberries and strawberries in a microwave drying regime if skin resistance ta diffusion is reduced by an appropriate dipping treatment and ifpigments and aromas are bound by infusion in sucrase. Since the freeze drying industry uses infusion prior to freeze-drying, there is a potential time savings due ta replacement offreeze-drying with microwave-drying for the finish drying. Detailed economic and energy analyses could he performed to evaluate the overall cost-benefit ratio associated with this 125 technological alternative.

8.2. Contributions to knowledge This thesis has made original contribution to knowledge by providing basic and applied information on microwave-drying of strawberries and hlueberries. The main contributions are as follows:

1. This study demonstrates that it is possible to obtain good quality dried strawberries by using microwaves to assist convection, only if they have heen previously pretreated to reduce skin resistance to moisture diffusion.

2. Strawberries and blueberries that were dipped in ethyl oleate and sodium hydroxide and osmotically dehydrated and then microwave..dried were considered to he equally acceptable as those that were freeze-dried after dipping and osmotic dehydration.

3. It was established that 1% ethyl oleate and 0.5% sodium hydroxide reduces the skin resistance ta moisture diffusion in blueberries and strawberries. This is a lower concentration than reported in the literature for other products.

4. In the type of equipment used, and given the limitation of

maximum air velocity of 2 m S·l, continuously applied

microwave power levels should not exceed 0.2 W g ·1 for a frequency of 2450 MHz.

5. It has been established that microwave drying at power level 126 the time for strawberries and 113 ofthe time for bluehenies of the time required for convective drying at the same inlet air temperature.

6. The relationship hetween shrinkage ratio and moisture ratio of strawberries is a straight line, whereas that hetween moisture ratio and equivalent diameter is a reciprocal logarithmic curve.

8.3. Recommendations for further studies The main points that are yet ta be addressed are 'lIhether there are economic and/or energy henefits to substituting freeze-drying hy microwave drying after initial treatment of dipping in ethyl oleate/sodium hydroxide, followed by osmotic dehydration. Work by Shivhare (1991) aIso indicated that energy consumption in microwave-drying might he reduced by using a pulsed mode rather than a continuous mode of application of microwave energy. Along these lines, there May be reason to conduct a time/energy optimization combining different air inlet temperatures, inlet air velocities and pulsing modes of application of microwave energy. The fruit ta sugar ratio should also be included in such a study sinee it influences the rate of finish drying. Bince there is sorne absorption ofsugars hy the fruits during osmotic dehydration, studies could he conducted ta determine how these influence the drying rate ta reduce it, as observed. The slower drying when more sugars are absorbed may he due to higher osmotic tension or to a change in dielectric properties associated with a different solids to liquids ratio, or ta a combination of the two. This study showed that 1% ethyl oleate was sufficient to improve the drying rate, whereas higher concentrations did not fare better. The 127 possibility that lower concentrations yield the same results as 1% EO could be investigated. Also, the concentration of NaOH May he adjusted. One aspect that was not covered in the work presented here was an investigation into the storability of the final product. Sïnce this study showed that the interna! temperature of the microwave-dried product exceeded 100°C during drying, there is reason ta suspect that storage life could be affected. On the one hand, this effect might be positive since the product appears ta have been pasteurized; on the other, there May be a negative effective due to breakdown of cell structure due to the high temperatures.

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144 APPENDICES

Page

Appendix A

Table A.4.1 Analysis of MW drying of slices 147

Table A.4.2 Analysis of MW drying of Puree 147

Table A.4.3 Analysis rehydration ratio of slices 148

Appendix B

Table A.5.1 Analysis of MW drying of whole strawberries with pretreatment (Time) 149

Table A.5.2 Analysis of MW drying of whole strawberries with pretreatment (Rehydration) 149

Table A.5.3 Analysis of MW drying of whole strawberries with pretreatment (Toughness) 150

Appendix C

Table A 6.1 Analysis of osmotic and MW drying of whole strawberries (Time) 151

Table A 6.2 Analysis of osmotic and MW drying of whole strawberries (Rehydration) 151

Table A 6.3 Analysis of osmotic and MW drying of whole strawbenies (Not including freeze drying) for toughness 152

Table A 6.4 Analysis of osmotic and MW drying of whole strawbenies (this analysis ineludes the freeze drying numbers) toughness 152

Table A 6.5 Analysis of osmotic and MW drying of whole strawbenies (this analysis does Dot include the freeze drying numbers). 153

Table A 6.6 Analysis of osmotic and MW drying of whole strawbenies (this analysis includes the freeze drying numbers) for color difference. 153

145 APPENDIX D

Table A 7.1 Analysis of osmotic and MW drying of whole Blueberries Dependent Variable: MCREM (Moisture content removed). 154

Table A 7.2 Analysis of osmotic and MW drying of whole Blueberries Dependent Variable: DRYING TIME (Freeze dried not included). 154

Table A 7.3 Analysis of osmotic and MW drying of whole Blueberries Dependent Variable: REHY (Freeze dried not included). 155

Table A 7.4 Analysis of osmotic and MW drying of whole Blueberries Dependent Variable: TOUGHNESS (Freeze dried not included). 155

Table A 7.5 Analysis of osmotic and MW drying of whole Blueberries Dependent Variable: DIFF (Freeze dried not included). 156

Table A 7.6 AnaJysis of osmotic and MW drying of whole Blueberries Dependent Variable : SENSORY analyssis including freeze-dried and ta see influence of Judges. 156

APPENDIX E Computer programme in HP-QBASIC to monitor and record the data used in the Data Acquisition System. 157

APPENDIX F Evaluation forms used in the sensory evaluation of strawberries and blueberries. 169

146 APPENDIX A

Analysis ofMW drying of Slices and Puree

• Table A.4.1 Analysis of MW drying of slices Dependent Variable : Time DF Sum of Squares Mean of Squares F Value Pr> F Il 32616.6666 2965.1515 388.17 0.0001 24 183.3333 7.63888 35 32800 R·Square C.V. Root MSE Time Mean 0.99441 4.252083 2.763853 65.0000 Source DF TypeISS Mean Square F Value Pr> F AIR 2 718.16666 359.08333 8.53 0.0016 POWER 3 255447.222 85149.0740 2023.34 0.0001 AIR*POWER 6 2115.61111 352.60185J. 8.38 0.0001

Analysis of time to dry Puree

Table A.4.2 Analysis of MW drying of Puree Dependent Variable : Time DF Sum of Squares Mean of Squares F Value Pr> F Il 258281.00000 23480.09090 557.94 0.0001 24 1010.00000 42.08333330 35 259291.000 259291.000 R·Square C.V. Root MSE Time Mean 0.996105 5.682190 6.48716682 114.166666 Source DF Type ISS Mean Square F Value Pr> F AIR 2 718.16666 359.08333 8.53 0.0016 POWER 3 255447.222 85149.0740 2023.34 0.0001 AIR-POWER 6 2115.61111 352.601851 8.38 0.0001

147 Analysisofrehydrationratiosofstrawberryslicesforconvection, microwave and freeze drying methods.

Table A.4.3 Analysis rehydration ratio of slices. • Dependent Variable : Rehydration ratio DF Sum of Squares Mean of Squares F Value Pr> F 4 2.5604667 0.64011667 125.84 0.0001 10 0.05086667 0.00508667 14 2.61133333 R-Square C.V. Reot MSE Time Mean 0.980521 2.382657 0.07132087 2.9933333 Source DF TypeISS Mean Square F Value Pr> F TRT 4 2.56046667 0.64011667 125.84 0.0016 Source DF Type II1SS Mean Square F Value Pr> F TRT 4 2.56046667 0.64011667 125.84 0.0001 •

148 APPENDIX B

MW drying of whole strawberries with pretreatment

Table A.5.1 Analysis ofMW drying of whole strawberries with pretreatment (Time) Dependent Variable: Time Source DF Sum of Squares Mean of Squares F Value Pr> F Model 11 32855.55555 2986.86868 66.17 0.0001 Error 24 1083.3333 45.138888 Corr.Total 35 33938.8888 R..Square C.V. Root MSE Time Mean 0.968080 6.681429 6.71854812 100.5555555 Source DF TypeISS Mean Square F Value Pr> F TRT 3 3594.4444 1198.148148 26.54 0.0001 POWER 2 26709.7222 13354.861111 295.86 0.0001 TRT*POWER 6 2551.38888 425.2314814 9.42 0.0001

Analysis of Rehydration

Table A.5.2 Analysis of MW drying of whale strawberries with pretreatment (Rehydration) Dependent varialble: Rehydratian

Source D Sum of squares Mean of squares F Value Pr> F F Model 14 6.31440 0.451028 17.66 0.0001 Errar 30 0.76640 0.0255466 Corrected total 44 7.08080 R..Square C.V. Root MSE Rehy Mean 0.891764 6.869624 0.15983325 2.32666 Source DF Type 1 SS Mean Square F Value Pr> F OLEATE 2 0.1812133 0.09060667 3.55 0.0414 TYPE 4 5.9102444 1.47756111 57.84 0.0001 OLEATE*TYP 8 0.2229422 0.2786778 1.09 0.3963 E

149 Analysis ofToughness

Table A.5.3 Analysis of MW drying of whole strawberries with pretreatment Dependent varialble: Toughness

Source D Sum of squares Mean of squares F Value Pr> F F Model 14 0.96997778 0.0692841 115.47 0.0001 Error 30 0.01800 0.00060 Corrected total 44 0.987977 R-Square C.V. Root MSE Tough Mean 0.981784 8.998126 0.02449490 0.27222 Source DF Type 1 SS Mean Square F Value Pr> F OLEATE 2 0.0726044 0.03630222 60.50 0.0001 TYPE 4 0.72237778 0.18059444 300.99 0.0001 OLEATE*TYP 8 0.17499556 0.02187444 36.46 0.0001 E

150 APPENDIX C MW drying of whole strawberries osmotically dehydrated Table A 6.1 Analysis of osmotic and MW drying of whole strawberries Dependent Variable : Time Source DF Sum of Squares Mean of Squares F Value Pr> F Model 9 3025006.8333 336111.870370 3014.75 0.0001 Error 26 2898.72222 111.489316 Corre.Total 35 3027905.5555 R·Square C.V. RootMSE Time Mean 0.999043 3.783028 10.555885 279.11111 Source DF Type 1 SS Mean Square F Value Pr> F TE:MP 1 65195.1111 65195.11111 584.77 0.0001 SUGAR 1 1.777777 1.777777 0.02 0.9005 POWER 2 2813874.38 1406937.1944 12619.48 0.0001 TEMP*SUGAR 1 281.77777 821.77777 7.3773 0.0116 TEMP*POWER 2 141307.7222 70653.8611 633.73 0.0001 SUGAR*POWER 2 3806.05555 1903.0277 17.07 0.0001

Table A 6.2 Analysis of osmotic and MW drying of whole strawberries Dependent Variable : Rehydration Source DF Sum of Squares Mean of Squares F Value Pr> F Model 9 0.90813889 0.10090432 7.77 0.0001 Errar 26 0.33768332 0.01298782 Corre.Total 35 1.245822222 R·Square C.V. Root MSE Rehy Mean 0.728947 6.235119 0.11396412 1.8277778 Source DF Type 1 SS Mean Square F Value Pr> F TEMP 1 0.01690000 0.01690000 1.30 0.2644 SUGAR 1 0.00054444 0.0005444 0.04 0.8394 POWER 2 0.71637222 0.35818611 27.58 0.0001 TEl\fP*SUGAR 1 0.0016000 0.0016000 0.12 0.7284 TEl\fP*POWER 2 0.08581667 0.42908333 3.30 0.0527 SUGAR*POWER 2 0.08690556 0.0345278 3.35 0.0509

151 Table A 6.3 Analysis of osmotic and MW drying ofwhole strawberries (Not including freeze drying. Dependent Variable: Toughness Source DF Sum of Squares Mean of Squares F Value Pr> F Model 9 0.32375000 0.03597222 4.09 0.0023 Error 26 0.22865000 0.00879423 Corre.Total 35 0.55240000 R-Square C.V. Root MSE Tough Mean 0.586079 20.686223 0.09377756 1.8277778 Source DF Type 1 SS Mean Square F Value Pr> F TEMP 1 0.0784000 0.0784000 8.91 0.0061 SUGAR 1 0.03610000 0.0361000 4.10 0.0531 POWER 2 0.04605000 0.02302500 2.62 0.0921 TEMP*SUGAR 1 0.0016900 0.0016900 1.92 0.1774 TEMP*POWER 2 0.14615000 0.07075000 8.31 0.0016 SUGAR*POWER 2 0.00015000 0.0000750 0.01 0.9915

Table A 6.4 Analysis of osmotic and MW drying of whole strawherries (this analysis includes the freeze drying numbers). Dependent Variable : Toughness Source DF Sum of Squares Mean of Squares F Value Pr > F Madel 10 0.95976429 0.09597643 11.67 0.0001 Error 31 0.25500000 0.00822581 Corre.Total 41 1.21476429 R-Square C.V. Root MSE TOUGH Mean 0.790083 18.01060 0.09069623 1.50357143 Source DF Type 1 SS Mean Square F Value Pr> F TEMP 2 0.71441429 0.35720714 43.43 0.0001 SUGAR 1 0.03610000 0.0361000 4.39 0.0444 POWER 2 0.04605000 0.02302500 2.80 0.0763 TEMP*SUGAR 1 0.0016900 0.0016900 2.05 0.1618 TEMP*POWER 2 0.14615000 0.07307500 8.88 0.0009 SUGAR*POWER 2 0.00015000 0.0000750 0.01 0.9909

152 Table A 6.5 Analysis ofosmotic and MW drying of whole strawberries (this analysis does not include the freeze drying numbers). Dependent Variable: DIFF Source DF Sum of Squares Mean of Squares F Value Pr>F Madel 9 868.646425 96.5162694 4.197 0.0001 Error 98 2256.186459 23.02231081 Corre.Total 107 3124.8328849 R-Square C.V. Reot MSE DIFF Mean 0.277982 24.92874 4.78915702 19.24749125 Source DF Type 1 SS Mean Square F Value Pr> F TEMP 1 0.44913273 0.44132734 0.023 0.8892 SUGAR 1 108.200023 108.20002 44.70 0.0326 POWER 2 231.831437 115.91571876 5.03 0.0083 TEMP*SUGAR 1 1.1268481 1.12684810 20.05 0.8254 TEMP*POWER 2 206.407378 103.2036890 4.48 0.0137 SUGAR*POWER 2 320.631606 160.3158032 6.96 0.0015

Table A 6.6 Analysis of osmotic and MW drying of whole strawberries (this analysis includes the freeze drying numbers) for color difference. Dependent Variable: DIFF Source DF Sum of Squares Mean of Squares F Value Pr> F Madel Il 2239.81861 203.619874 9.867 0.0001 Error 114 2253.790739 20.64728719 Corre.Total 125 4593.609359 R-Square C.V. Reot MSE DrFF Mean 0.487594 2530112 4.54392861 17.95939670 Source DF Type 1 S8 Mean Square F Value Pr> F TEMP 2 1254.79951 627.397475 30.39 0.0001 SUGAR 1 188.117315 188.117315 9.11 0.0031 POWER 2 231.831437 115.91571876 5.61 0.0047 TEMP*SUGAR 2 38.035930 19.01796549 0.92 0.4010 TEMP*POWER 2 206.407378 103.2036890 5.00 0.0083 SUGAR*POWER 2 320.631606 160.3158032 7.76 0.0007

153 APPENDIX D MW drying of whole Blueberries osmotically dehydrated Table A 7.1 Analysis of osmotic and MW drying of whole Blueberries • Dependent Variable : MCREM (Moisture content removed). Source DF Sum of Squares Mean of Squares F Value Pr> F Model 15 4441.58236833 296.10549122 433.54 0.0001 Error 44 29.84535667 0.67830356 Corre.Total 59 4471.42772500 R-Square C.V. Reot MSE MCREMMean 0.993325 8.915744 0.82359187 9.2375000 Source DF Type 1 SS Mean Square F Value Pr>F SUGAR 1 5.41801500 5.418015000 7.99 0.0071 TRT 1 2067.53140 2067.53140167 3048.09 0.0001 TIME 4 1738.602750 434.65068750 640.7948 0.0001 SUGAR*TIME 4 1.95657667 0.48914417 0.7273 0.5821 SUGAR*TRT 1 2.208001670 2.20800167 3.2673 0.0780 TRT*TIME 4 625.8656233 156.46640585 230.67 0.0001

Table A 7.2 Analysis of osmotic and MW drying of whole Blueberries Dependent Variable : DRYING TIME (Freeze dried not included). Source DF Sum of Squares Mean of Squares F Value Pr> F Model 9 1044074.33333 116008.25925926 932.59 0.0001 Error 26 3234.222222 124.39316239 Corre.Total 35 1047308.55555 R-Square C.V. Root MSE TIME Mean 0.996912 2.961455 11.15316827 376.61111 Source DF Type 1 S8 Mean Square F Value Pr> F TEMP 1 167826.777 167826.7777 1349.16 0.0001 SUGAR 1 1573.44444 1573.4444444 12.6509 0.0015 POWER 2 857486.8888 428743.44444 3446.688 0.0001 TEMP*8UGAR 1 106.777777 106.777777 0.86 0.3627 TEMP*POWER 2 16133.55555 8066.77778 64.85 0.0001 SUGAR*POWER 2 946.8888889 473.4444444 3.81 0.0355

154 Table A 7.3 Analysis of osmotie and MW drying of whole Blueberries Dependent Variable: REHY (Freeze dried not included). Source DF Sum of Squares Mean of Squares F Value Pr> F Model 9 0.129050000 0.01433889 5.17 0.0005 Error 26 0.072072222 0.00277201 Corre.Total 35 0.201122225 R-Square C.V. Root MSE REHYMean 0.641650 3.937256 0.05264987 1.3372222 Source DF Type 1 SS Mean Square F Value Pr> F TE:MP 1 0.01521111 0.015211111 5.49 0.0271 SUGAR 1 0.07290000 0.072900000 26.30 0.0001 POWER 2 0.00620556 0.00310278 1.12 0.3417 TE:MP*SUGAR 1 0.00934444 0.00934444 3.37 0.0778 TE:MP*POWER 2 0.00493889 0.00246944 0.89 0.4225 SUGAR*POWER 2 0.02045000 0.01022500 3.69 0.0389

Table A 7.4 Analysis of osmotic and MW drying of whole Blueberries Dependent Variable: TOUGH1'1ESS (Freeze dried not included). Source DF Sum of Squares Mean of Squares F Value Pr> F Model 9 0.11016389 0.01224043 3.85 0.0033 Error 26 0.08260000 0.00317692 Corre.Total 35 0.19276389 R-Square C.V. Root MSE TOUGH Mean 0.571497 19.64290 0.05636420 0.2694444 Source DF Type 1 SS Mean Square F Value Pr> F TEMP 1 0.07380278 0.07380278 23.23 0.0001 SUGAR 1 0.00122500 0.00122500 0.39 0.5400 POWER 2 0.01215556 0.00607778 1.91 0.1678 TEMP*SUGAR 1 0.00202500 0.00202500 0.64 0.4319 TEMP*POWER 2 0.00575556 0.00287778 0.91 0.4166 SUGAR*POWER 2 0.01520000 0.00760000 2.39 0.1113

155 Table A 7.5 Analysis of osmotic and MW drying of whole Blueberries Dependent Variable: DIFF (Freeze dried not included). • Source DF Sum of Squares Mean of Squares F Value Pr:> F Madel 9 68.27610705 7.58623412 5.99 0.0001 Error 62 78.51864520 1.26642976 Corre.Total 71 146.7947522 R-Square C.V. Root MSE DIFF Mean 0.465113 2.602339 1.12535761 43.24407434 Source DF Type 1 SS Mean Square F Value Pr:> F TEMP 1 1.03347122 1.03347122 0.82 0.3698 SUGAR 1 22.03100705 22.03100705 17.40 0.0001 POWER 2 7.88783111 3.94391556 3.11 0.0514 TEMP*SUGAR 1 14.9927886 14.99278861 11.84 0.0010 TEMP*POWER 2 16.9326971 8.46634855 6.69 0.0023 SUGAR*POWER 2 5.39831197 2.69915599 2.13 0.1273

Table A 7.6 Analysis of osmotic and MW drying of whole Bluebenies Dependent Variable: SENSORY analyssis including freeze-dried and 10 see influence of Judges. Source DF Sum of Squares Mean of Squares F Value Pr:> F Madel 20 97.49047619 4.87452381 2.35 0.0024 Error 119 247.1595238 2.07697079 Corre.Total 139 344.6500000 R-Square C.V. RootMSE SENS Mean 0.2828680 24.22134 1.44116994 5.950000 Source DF Type 1 SS Mean Square F Value Pr> F Judge 9 32.0071428 3.55634921 1.71 0.0934 TEMP 2 3.86666666 1.93333333 0.93 0.3971 SUGAR 1 2.06428571 2.06428571 0.99 0.3208 POWER 2 8.46666667 4.23333333 2.044 0.1348 TEMP*SUGAR 2 12.15238095 6.07619048 2.93 0.0575 TEMP*POWER 2 1.86666666 0.93333333 0.45 0.6391 SUGAR*POWER 2 37.0666667 18.5333333 8.92 0.0002

156 Computer programme in BP.QBASIC to monitor and record the data in the data acquisition system.

H~ I~:~;~al F~~!cr.a! C~Mpu:e~ le IOAT~ ~C~Ur5[T!OM SYSTEM FOR M[C~Q~AVE SEru~ 20 !FRaG?A~ FrLENAM:: MICF.OMULTIMûC: 3~ ILA;T Ë~!TION: 55-11-18 ~~ 1 Ylj~tJ Gn; rE~'( 50 IDA7,; F!L~ MAMES: CAïAI ET OMMJJMr.r.M 6~ 1 .EO FLO~FY crs~ 7'i) QPïIQtJ s,;:;:- 8~ ASSIGN 7 TO ·hpib" 5'a 1 I~~ 1. INIi!AL!ZE VARIA8LË3 Il~ û!M T5C 1~l,TCSC l~I ,ï:SC61,FiC91,FISE51,L(7) ,T4.5(::: ,TrMEOl'lsCSl ,TI~tE'JFFS(S! r2~ OIM T!ME5CANS(5!.TCHECK$CSl,TC(3).CQMME~TS(701.0N~;C3] 13~ SHORi P.{9,45) r~0 GOSUS 10~~0 IREMC TC COEFFICIENiS t5~ T:S-~Q~~1~0~ !OE~nULT rIME [NTE~VAL 01 MIN. 150 FI$·-ID~TA· 1 DATM F!LE ON ED 17~ 51-:50 !OË~MULï NUM=Ë~ OF SCMN le~ Yl-e IOU"MY UARIAELE Ta OETERM!NË IF PROG~AM WM5 RW~ 190 ;<-0 ze0 Jo:I-~ 210 11::-" 215 Nt-' ICOUNTE~ Ta COMPUTE FOWER 220 TrMEù~s··~0:30· 1 MU TIME ON • 3~ sec. :;~ i~~ECF~;·~2~:;~· 1 MU TIMË OF~ - ~0 !~C. 2:~ TIM~;CANS··01:00~ 1 TI~~ INTERVAL EËïWE~N SCAN - 01 ~~~. :5~ NSCAN.:Q 1 NUMeE~ OF SCAN ,50 T~AX·7= 1 MAx!nU~ aVEN rEMPERA7UP.: • 7S·C 270 FMAx-sa 1 MAX!MUM ~E~~ECTED PQWER • 50% CF !NC:~ENT :50 M~ssaUï·i~ 'OE~~ULT V~LUE ZSO E~E~GE~CY.O 1 NO F~AG 30~ 1 310 1. ALGC~!7HM FOR ~En5~RS 3:0 1 SC ..~L: 33~ OE~~·~ 'TARE SCALE 3~0 üE~ FN~CAL:

157 10'3Ç) 0 rsp • ) ... M M l l'l M t:. ri IJ ( t 100 OI5F l '10 OISF 11~0 QISP • )) FI • 34S7~ CLOCK" 1130 OIS? 11~0 DIS? • > ) F2 TIN: INTERVALS· 1 15~ OISP 1152 OISP • ) ) F3 SAFETY FA?A~ETEF.5" 1170 OIS? • F~· 1180 OIS? • > ) STA?T DATA ACQUrSITION" 1190 OISP 1:00 DISP • > } F5 - SET AIR TEMPERATU~E· 1:10 OISF 1:20 OISP • ) > F6 - TARE THE SCALE" 1:30 ors? 1210 DIS? • ) ) F7 PRINT CQMMSNTS" 1:50 DISF 1:50 OISP • )) Fa· END EXPERIMENT" 1:70 , lZ8~ ON ~EY: 1,"CLOCK" GOSUB :000 1~90 ON ~EY: 2,"TIMEr GOSU8 3000 130Q ON ~Ef: 3,"SAFETY" GOSUe 4000 1310 ON KEY: 4,"START" GOSUS 5J00 13:0 ON KEY: 5,"SET TEMp· GOTO 1400~ 1330 ON KEY: S,"TARE" GOTO 15000 13~0 ON KEY: 7,"COMNENTS" GOTO 16000 1350 ON KEY: a,"ENO· GOTO 13000 1360 ON KEY: 1:,"MAINMENU" GOTO 10a0 1370 KEY ~AeEL 1360 1 1390 IF K-I THEN 1380 ELSE 1000 14o" 1 1500 ,. QFï=KEYS 1510 CLEAR 15:0 OFF KEY: 1 1530 OF;: KEY: 2 1540 OFF KEY: :3 1550 OFF KEY: 4 • 1560 OFF KE'(: 5 1570 OFF K:Y: 6 Isa" OFF KEY: 7 1590 OFF K~Y: 8 1600 CRT r5 1 1610 PR!NTER rs 2 16Z0 RETURN 1630 !SUBENO 16ol0 1 2000 ,. SET 3497A CLOCK :010 K-0 Z~:0 GOSUB 1500 'OFFKEVS 20J0 ON KËV: 1.-SET TI~E· 60SUe 2500 Z040 ON KEY: 4.-~AINMENU· GOTO 2900 2050 CLEAR 2060 KEY LABEl. 2070 OUTPUT 709 r -TO" Z081 ENTER 709 t TIS 2090 OISP Z100 OISP "OATE: 1996/-tT01[1.2];"/-;T05[4,5] 2110 OISP "TIME: -,T0SC7.14] ZI:0 OISP 2130 WAIT 5000 Z140 GOTO :070 2150 1 2500 •• ENTER NEW TI~E & DATE 2510 C1.EAR 2521 ON ERROR 60TO 2611 IFOR"~T ERRO~ 158 2S3~ OISP 2S~0 OISP "E~liE? NEl.J TH1E AND DATE USING THE FOF.r1~T: Mi1:0~:HH:MM:SS" 2:53 INPUT T03 ZS=~ OUTPUT 70S i "TD"&T0:~-TO" 2570 OFF ERReR 2sa~ F.ETURI'l 2590 lSU5:NO 2500 ! 25!~ IERRQR ON TIME FORMAT 26Z~ OFF ERROR 2530 8E:F 26.10 CLEF:R 2650 DIS? "INVALID TIME, DATE OR BAD FORMAT" 2550 GOTO 2500 :;:670 1 29~~ ,. ~ETU~N TO MAIN MENU 2910 605ue 1500 IOFFr-EY 25:0 1'=0 2930 RETURN 2C;~0 ISU8ENO 2950 1 3Q0~ 1- SEi iIME INTERVALS 3010 K·~ 30:5 THEN 3740 3600 OISP -ENTER NE~ TIME INTERVAL 8ET~EEN SCAN (MM:SS)" 3610 INPUT TIMESCANS JS:0 IF LEN(TIMESCANSI<>S THEN 3740 3630 OIS? ·ENTE~ TOTAL NUM8ER OF SCANS (1 Ta 250)· 36~0 INPUT NSCAN 3650 NSCAN-A8S:S0 THEN 3630 3670 ONF-VAL(TIMEOFFICt ,Zl)+VAL(TIMEOFFSC4,51) ICHECY. TO see IF TIMEOFFS-0 3680 IF ONF-0 THEN TI~EONS·TIMESCANS !MICROWAVE IS ON ALL THE rIME 3690 IF ONF-0 THEN KZ-I 'SET FLA6 FOR CONTINUOUS OPERATION 3700 OFF ERROR 3710 RETURN 3720 lSUBENO 3730 1 3740 IERROR ON TI~E FOR~AT 3750 CLEAR 159 3750 OFF ERl-1 TH EN LCS).PISUM/NI !COMPUTE MEAN POWER INC. 5687 IF K2(>1 THEN L(7).PRSUM/NI ICOMPUTE MEAN POWER REF. 5690 MI-' 5700 IF KZ·' THEN 5740 ICONTINUOUS OPERATION 5710 OUTPUT 709 ; -004,t-&-OC4.Z- 1 S~ITCH OFF MICROWAVE 5720 WAIT 10e0 5730 OUTPUT 709 1 ·004.2 5740 OFF TIMERI 1 5750 P-SPOLL(709) 5760 STATUS 7.1 1 A 5770 NEXT L 5780 1 5791 CLEAR 719 5881 ABORTIO 7 161 5810 60SUe &~~~ IK~~U ~tN~U~ ~ ~~lNI U~I~ lU .eu nNU ~~lNI~~ 59~0 Garo 780~ Icur POWER rD MICROWAVE AND HEATER5 & SAVE DATA ON FLOFPY 5a3~ END 5a~~ ! 60~0 ,. REAO 5ENSOR5 6010 OUTPUT 709 ; ~rD~ 6~:0 ENTE~ 709 1 TI! !TIME OF READING 6030 ALPHA 15,55 ~ OIS? USING -14A- i Tl$ 6040 1 READ REFERENCE TEMPERATURE, CHANNEL 19 6050 07~6 ITHERMOCOUPLE TYPE-T 6060 02~19 ! CHnNNEL FOR TEMP. REF. 6070 Pl$·VALS(O~) 6080 IF LENCPIS)-I THEN PIS·-0-&PIS 6090 OUTPUT 709 1 ~VR5VNIVAIVF!VOSVC0VS0VW0" ! 3497 SETUP & TRG 6100 OUTPUT 709 ; -AC~&PIS&-VT3- 1 CLOSE CHANNEL 19 & REAO VOLTAGE 6110 ENTER 70S 1 P 6120 Q3-10·P 6130 1 6140 ITC AMBrENT AIR 61=0 P'-t2 6160 GOSUë 800~ ICLOSE CHANNEL PI & REAO VOLTAGE 617~ GOSUe 9~a0 ITEMPERATURE TRANSFORM 61 g0 L< 1 }.? 6190 ! 6200 FOR Jt-~ TO 3 !TC IN AND OUT 6:10 TC-0 6::0 FOR J2-1 TO 3 IMEAU OUER 3 TC 6230 P l-J 1+8 6~40 GOSU6 8000 'CLOSE CHANNEL PI & READ VOLTAGE 6250 GQS~8 900~ ITEMPERATURE TRAN5FDRM 62S~ rC-P/3... iC 6:70 NEXT J2 6:50 LU 1 )-p 6290 NEXT JI 6300 1 6310 FOR J-7 TC 9 1 1 RH & 1 SCALE 6~20 Pl-J ICHANNEL FOR TC 6330 GOSUS 8000 'CLOSE CHANNEL Pl AND REMO VOLTAGE 6340 IF J-7 TH EN GOSUB ~200 1 RH TRANSFORM 63S~ IF J-8 THEN GOSU8 ~300 1 MASS TRANSFQRM 636~ JC-J-3 6370 L( JC )-POUT 1 STORE CALCULATEO VALUES IN ARRAV L< ) 6380 NEXi J 6390 1 6400 IPRINT DATA TO .EO AND PRINTER 64t0 PRINT: 1 r TII,LC) IPRINT COLLECTEO DATA TO fD 64Z0 PRINT us ING - l'A ,xx ,8( '0.40 ,x )- ; TI S,Le 1) ,LC 2) ,l<:;) ,L< 4) ,L( 5) ,Le 6) ,L( 7) 6'30 ALPHA 12,55 lOIS? USIN6 -40.D,X,2A- , MASSOUT;- g- 6440 ALPHA 13,55 1 DISP USING -40.0,X,2A~ i LPMAX THEN E~ER6ENCY-Z !SET FLA6 ON PMAX EXCEEOEO 7700 MI-MI+I 7705 IPRINT NIIMIIPOUTIL(6)IPISU~ 7710 RETURN 7720 ISUBEND 7730 1 .7810 ,- E"ER6ENCY CLOSE 163 7810 OFF ERFWt-! 7820 OFF TIMER: 1 783~ MIal 76~0 OUTPUT 709 1 "S!~ IRE5ET I~TERFACE & 3&S7~ 7650 OUTPUT 709 "aC:,3,4'" ICUT OFF POWER TO MrCROwAIJE MlO H:~TERS .7860 u~rT 10'l0 7870 OUTPUT 709 -SI 7880 RESET 7 7850 CLEAR 7895 IF Yl(>1 TH EN 12000 7900 GOSUB 6000 1 REAO 5ENSORS AND PRINT DATA .7910 IF EMERGENCY-0 TH EN 12000 !TRANFER DATA TO FLOPPY 7920 PRINT "THE PROGRAM HAS INTERUPTEO THE FROGRESSION OF THE EXPERIMENT" 7930 PRINT "SINCE ONE OF THE SAFETY LIMITS WAS EXCEEOED" 794.0 PRINT 7950 IF EMERGENCY-l THEN PRINT "AIR TEMPERATURE IN THE OVEN WA5 ASOVE: "ITMAXI"· C" 7S50 IF E~ERS=NCY·2 THEN PRINT "REFLECTED PQWEP EXCEEDED: "IP~AX;·Watt" 7965 IF EMERGENCY-3 THEN PRINT "PROQUCT HAS REACHED rTS FINAL MASS OF ";MASSOUT; " g" 7970 GOTO 12000 ITRANSFER DATA TO FLOPPY 798Ç'l 1 8000 1. CLOSE CHANNEL PI 8010 PI$-VAL${Pll ~020 IF LEN(Pl$}-' THEN P'$-"0"~Pl$ 8030 OUTPUT 709 ; "VR5VN1VAtVF1V05VC0VS0VW0" ! 3497 SETUP 8040 OUTPUT 709 ; "AC"&P1S&"VT3" IClO:E CHANNEL Pl & READ VOLTAGE 8050 ENTER 709 ;P 8060 RETURN 8070 rsueENO 8060 1 9000 1. TRANSFORM VOLTAGE TO TEMPERATURE 9010 oe-p 9020 Pl~06 @ P:·Q3 9030 03-5 ITC TVPE-T 9'~40 0 I·P '+R< 03,41 )·P:.C R( 03,4: )+PZ·( R{ Q3 ,43 HPZ.R( 03 .4~) )) 9050 02-7 5060 IF Ol05) 9130 IF R3(>0 THEN 9150 1 Abo~t 9140 GOTO 9160 1 p.p 9150 P-9S9.0S 1 TEMPERATURE OUT OF RANGE 9160 RETURN 9t70 1 SU8ENO 9180 1 9200 ,. TRAN5FORM VOLTAGE INTO RH 9210 POUT-FNRHCP) 9220 RETURN 9230 ISUBENO 9~40 ! 9300 ,- TRANSFORM VOLTAGE INTO ~AS5 93t~ POUT-FNSCAlE-' AND POUT

168 Name . Date .

SENSORY EVALUATION OF STRAWBERRlES

Taste the Stra\vbeny samples and chedc:. ho\v much you like or dislike. Use the appropriate scale given to sho\v your attitude by checking at the point (v( Renlember you are the only one \vho can tell us \vhat you IBee.

97844 04776 49408 82840 Li ke E:

Like Very Much 1 Like Moderatelv

Like Slightlv 1

Neither Like Nor disIike 1 Dislike Slightlv

Dislike Moderatelv 1 1

Dislilce VerY l'vfuch 1 Dislike Extremeiv

1 65735 86494 159024 40748 Uke Extremelv

Like VeN Much 1 Like Moderatelv

Uke Sli~htlv 1 Neither Like Nor dislike Dislike Sli~htlv Dislike Moderatelv Dislike Verv Much Dislike E.xuemelv

Comments:

169 Name . Date .

SENSORY EVALUATION OF 8LUEBERRIES

Taste the Blueberry samples and check how much you like or disli~. Use the apprepriate scale given ta show your attitude by checking at the point (V). Remember yeu are the only one who can tell us what you Iike.

1 88758 1 356ô l 1 26335 1 95044 1

Like Extremely 1 1 1 1 !

Like Very f\J1uch 1 1 1 1 Like Moderately 1 1 1 1 1 Like Slightly 1 1 1 1 1

Neither Like Nor dislike 1 1 1 1 1

Dislike Slightly 1 1 1 1 : , , Dislike Moderately 1 1 1

Dislike Very Much 1 1 1 1 Dislike EX1remely 1 1 1 1 1

1 1 1 1841i6 1 1 83ï46 13094 38148 1

Lika E.'

Like Very l\Auch 1 1 1 1 1

Like Moderately 1 1 1 1 1

Like Slightly 1 1 1 1 1

1 Neither Like Nor dislike 1 1 1 1 1

Dislike SIightly 1 1 1 1 1

Dislike ModeratelY 1 1 1 1

Dislike Very Mueh 1 1 1 1 1

Dislike Extremely 1 1 1 J 1

Comments:

170