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COMBINED OSMOTIC AND MICROWAVE DRYING OF STRAW'BERRJESAND BLUEBERRIES
A Thesis submitted to The Faculty of Graduate Studies and Research of McGill University
by Kamadenahally Venkatachalapathy
In Partial FuIflment 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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Kamadenahally Venkatachalapathy Ph.D. (Agr. and Biosystems Eng.)
Combined osmotic and microwave drging of strawberries and b1ueberries
This work was aimed at obtaining high quality dried strawbemes using microwaves to assist convection air of 2 m/s at 30-45OC. Preliminary trials with whole strawberries were unsuccessful. Fruits 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 dipping the berries in a solution of ethyl oleate and sodium hydroxide was studied. Such treatments are used in industry to reduce the skin resistance to 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 be used for strawberries. Rehydration was similar to that of the dipped and fieeze-dried samples, but the microwaved samples were a bit softer, and had less aroma, colour and flavour. Osmotic dehydration was then studied as a technique of binding flavours and aromas and of reducing the time required for finish drymg with microwaves. These studies were performed on strawberries and bluebemes. Results showed bemes that were dipped and then osmotically dehydrated for 24 h in sucrose, yielded a microwave-dried final product that was equal to the freeze-dried one in terms of quality, and this, with a much lower time for finish drying. The shridtage ratio of strawberries has a straight line relation to 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 to freeze- drying when berries are first subjected to a pretreatement of ethyl oleate and partial dehydration by osmosis. It was also found that if microwave energy is applied in continuous mode, the initial applied power should not exceed 0.2 W g-', otherwise burning may occur. It 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 m/s.
iii Kamadenahally Venkatachalapathy Ph.D. (GBnie agricole et biosystèrnes.) Séchage par osmose et par micro-ondes de la &aise et du bleuet
Un séchoir hybride combinant l'énergie micro-onde (2 450 MHz) et l'air chaud fut utilisé pour la production de fraises séchées à partir de fmit entiers, tranch6s ou réduits en puree. Il n'a pas été possible de sécher les fruits entiers avec ce systèmes puisque les fitscuisaient et se fendillaient. Les hits tranchés ou réduits en purée ont s6ché mais les produits finis avaient perdu leur arôme, leur savecr et leur couleur caractéristiques lorsque compares des fmits lyophilisés. Pour remédier A ce problème, un traitement par trempage dans une solution d'éthyle d'oléate et d'hydroxyde de sodium fut utilisée. Ce traitement utilise par l'industrie pour facilité le passage de l'eau à travers l'enveloppe des kits a permis d'accroître considérablement le tau 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'ol6ate a 6té 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 lyophilisbs. Par contre, la qualité des échantillons séch6s dans le systéme hybride était inférieure A la lyophilisation tant au niveau de la texture, que de l'arôme, de la saveur et de la couleur. Par la suite, la dbshydratation partielle par osmose a &téemployé pour favoriser la rbtention de l'arôme et de la saveur, et pour réduire les temps de dchage. Des essais furent effectués sur des fraises et des bleuets. Les résultats ont démontre qu'un traitement par trempage dans une solution d'éthyle d'ol6ate et d'hydroxyde de sodium suivi d'un traitement de 24 heures de déshydratation partielle par osmose permettait d'accroître I'efficacitb 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 & celle d'bchantillons 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 qualit6 des produits séchés. Afin d'6viter 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 (resultant du sechage) &ait relié linéaire au rapport de la teneur en eau du produit, tandis que la réduction du diamètre équivalant du fmit pouvait être représentbe par un relation logarithmique réciproque. Cette Btude 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 systhme hybride combinant l'air chaud et les micro-ondes est une alternative viable et plus rapide que la lyophilisation pour la production de petits fmits séchds. This work is dedicated to my father late Mr. & Venkaturamaiah as a token of my gratitude for his great respect for education and educated people. 1 wish to express my deep gratitude to Professor Dr. G.S.V. Raghavan, Chair, Department of Agricultural 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 acadernically, morally and in dl walks of rny life during my studies at McGill, extremely influenced me to accomplish this work. He inspired me in many ways in my Me, his devotion to work and academic excellence, fiiendliness with his students are emulative. 1 personally benefitted a lot from him but for his initiation 1 would have not accomplished this work. 1 thank and respect Mrs. Subhadra Raghavan for her support in encouraging him in al1 his pursuits and for her great simplicity. May their tnbe grow. 1 am grateful to the following faculty members, Prof Dr. Suzelle Bamngton, 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 helpfid in many aspects, which made my life cornfortable, 1 am very grateful to them. Many people contributed directly and indirectly in realizing this work, I acknowledge with gratitude the continuous support given to me by Mr. Yvan Griepy and Ms. Valerie 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 of texture. Mr. Peter Alvo a scientûic
vii wizard was responsible in cntically analyzing the results and discussing the possibilities and criticizing for the improvement of presenting the results in a scientific way, he is a gentleman to work with. 1 thank him immensely for his editorial comments and help and for his friendship. Mr. E. Noroozi of the Dept of Food Science was helpful in providing al1 the necessary equipments for conducting rehydration studies, 1 thank him for al1 that help. Mr. Mark McBratney is a very interesting and a meticulous person to discuss on various experiences of Me and his help in conducting the studies on strawberry temperature and shrinkage measurements dunng microwave drying was gratefully acknowledged. Prof Dr. Chandra Madramoottoo was always enquiring about my progress of work and helped in providing the chromameter for measuring the color, 1 sincerely thank him. 1 sincerely thank Dr. Samson Sotocinal for his friendship and help in conducting freeze drying studies. 1 express my gratitude to Dr. Zaman Alikhani, very honest humble and a tme &end for his advise and encouragement during my studies. 1 am very much indebted to 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 Economics and Mrs. Sharmila Gunjal for their help friendship and continuous support and encouragement. 1 sincerely 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 to al1 my colleagues from the Department of Agricultural Engineering UAS Bangalore for their help and encouragement. I specially thank for the support given to 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 Sosle, Mr. Venkatesh Meda, and Mr. K. Harish 1 sincerely thank the Natural Science and Engineering Research Council of Canada RJSERC) and Canadian International Development Agency (CIDA) for their financial help during my studies and the University of Agricultural Sciences for the study leave. I am very grateful to my late father K. Venkataramaiah, whom 1 missed during his last days, who had high regard and respect for education and educated people, his interest in education brought me to this stage. I thank my mother for her benediciions and my wife Dr. M. Lalitha, my daughters V. Gayathri, V. Suvarna and V. Supama for their support and encouragement during my studies. Finally my contribution in this work is very trivial compared to many people who directly and indirectly responsible and contributed and supported me in carrying out this work, 1 salute and thank al1 of them very sincerely. TABLE OF CONTENTS
Page C-ER I BVTRODUCTION 1 1.1 Hypothesis 1.2 Objectives 1.3 Scope
C&APTER II REVIEW OF LITERATCIRE 2.1 Introduction 2.1.1 Strawbemes 2.1.1.1 Some physical properties of strawberries 2.1.1.2 Nutritional content of strawbemes 2.1.1.3 Strawberry flavour and aroma 2.1.2 Bluebenies 2.1.2.1 Properties of Lowbush bluebemes 2.1.2.2 Bluebemy colour 2.2 Dehydration and drying 2.2.1 Drying of fruit 2.2.1.1 Specific studies 2.2.2 Pretreatments for drying hit 2.2.3 Osmotic dehydration 2.2.3.1 Energy consumption in osmotic dehydration 24 2.3 Shrinkage 24 2.4 Mlicrowaves 26 2.4.1 Introduction 26 2.4.2 Advantages of Microwave Heating 27 2.4.3 Advantages of Microwave Drying 28 2.4.4 Interactions of Microwaves and Biologicd Materials 2.4.5 Microwave Drying of Agricultural Froducts 2.5 Drying Models 2.6 Quality Assessrnent 2.6.1 Colour Measurement 2.6.2 Texture Measurement 2.6.3 Rehydration 2.6.4 Sensory Evaluation
CIIAPTER III UTERLALS AND METHODS Microwave Drying Setups 3.1.1 Setup used in Preliminary Studies 3.1.2 Expenmental Microwave Drying Unit 3.1.3 Initial Moisture Determinations 3.1.4 Data Acquisition System Freeze drymg Osmotic Dehydration Quality Evaluation of the Dried Product 3.4.1 Rehydration tests 3.4.2 Colour determination 3.4.3 Texture 3.4.4 Sensory evaluation Experimental Design and Analyses
CHAPTER N PRELnfINiWY STWDmS ON M'CROWAVE DRYING OF WHOLE, SLICED AND PUREED STRAWBERRIES 4.1 Introduction 4.2 Materials and Methods 4.2.1 Initial Investigations 4.2.2 Microwave Drying of Whole Strawbemes 4.2.3 Drying of Sliced Strawbemes 4.2.4 Drying of Strawberry Puree 4.2.5 Freeze-Drying 4.2.6 Quality Evaluations 4.3 Results and Discussion 4.3.1 Drying of Slices 4.3.2 Drying Puree 4.3.3 Rehydration of Strawberry Slices 4.3.4 Quality and Colour Analysis 4.3.5 Chromacity 4.4 Conclusions 4.5 Connecting Statement to Chapter 5
CZIAPTER V MICROWAW DRY12VG AND SHRLAEAGE OF PRETREATED WHOLE STRAWBERRIES 5.1 Introduction 5.2 Materials and Methods 5.2.1 Microwave Drying 5.2.2 Freeze Drying 5.2.3 Shrinkage of Strawbemes 5.2.4 Relative Drying Rate 5.3 Results and Discussion 5.3.1 Drying Kinetics 5.3.2 Rehydration Ratio 5.3.3 Toughness 5.3.4 Colour DSerence With Respect to Fresh Berry 5.3.5 Shrinkage
xii 5.4 Conclusions 5.5 Connecting Statement to Chapter 6
CHAPTER VI OSMOTIC AND MICROWAW DRYZNG OF STRAWBERRIES Introduction Materials and Methods 6.2.1 Dipping Treatment 6.2.2 Osmotic Dehydration 6.2.3 Drying Experiments 6.2.4 Quality Assessrnent Results and Discussion 6.3.1 Osmotic Dehydration 6.3.2 Drying Times 6.3.3 Empirical Model of Finish Drying With Microwaves 6.3.4 Quality analyses Conclusions
CHAPTER VI1 OSMOTIC AND MICROWAVE DRYEVG OF BLrnBERRIES Introduction Materials and Methods Results and Discussion 7.3.1 Osmotic Dehydration 7.3.2 Drying Kinetics 7.3.3 Empirical Model of Finish Drying With Microwaves 7.3.4 Quality Conclusions CHAPTER Wr CONCLUSIONS, COnrnUSuTIONS TO RîVOWLEDCE AND RECOMIM%NDATIONS FOR FURTmR WORg 8.1 Summary and Conclusions 123 8.2 Contributions to Xnowledge 126 8.3 Recommendationa for Further Studies 127
REFERENCES 129
APE1VDICES Appendix A Appendix B Appendix C Appendix D Appendix E Appendix F
xiv Figure Page
Microwave drymg setup used in the prelirninary experiment S. Experimental microwave drying setup. Photograph of the experimental microwave drying setup. Lyo Tech Canada, Freeze drier (used in the experiments for drying strawbemes and bluebemes). Data acquisition system comected to the drying setup. The Minolta chroma meter (used in colour measurements). Instron machine used in measuring the texture. Dehydration of strawberry slices under microwave power levels 0,2,3and 4 at inlet air temperatures 35T. Dehydration of strawbemy puree under microwave power levels O, 1, 2, and 3 at inlet air temperature 35°C. Dehydration of strawberry slices and puree under microwave power level 2 and inlet air temperature 35°C. Convective and microwave drying of strawbemes treated with 2% ethyl oleate and 0.5% sodium hydroxide, at different power levels. Microwave drying of strawbemes at 0.2 Wlg power with different pretreatment levels. Microwave drying of strawberries at power level0.1 Wlg correlated between shrinkage ratio and moisture ratio. Microwave drying of strawbemies at power leve10.2 Wlg comelated between shrinkage ratio and moisture ratio. Microwave drymg of strawbemes at power level0.1 W/g correlated between equivalent diameter and moisture ratio. 83 Microwave drymg of strawberries at power leve10.2 W/g correlated between equivdent diameter and moisture ratio. 83 Surface (Tl) and centre (T2)temperatures of strawberry fruit during microwave drymg at power levelO.l W/g. 85 Surface (Tl)and centre (T2)temperatures of strawbeny fruit during microwave drying at power level0.2 W/g. 85 Osmotic dehydration of treated and untreated strawbemes. 96 Microwave drying of osmotically dehydrated strawberries at different power levels. 96 Microwave drying of strawberries at two power levels dehydrated at two osmotic levels (F:S). 98 Microwave drying rate of osmotically dehydrated strawberries at different power levels and inlet air temperatures of 35°C and 45°C. Predicted moisture content of strawberries by the exponential model compared with the experimental values (PL= O W/g). Cornparison of the moisture content of strawbemes predicted by the exponential model and the experimental values (PL= 0.1 W/g). Predicted moisture content of strawberries by the exponential model compared with the experimental values (PL- 0.2 W/g). Osmotic dehydration of untreated and treated bluebemes at different durations of time. Microwave drying of osmotically dehydrated bluebemes at different power levels. 7.3 Microwave drying of blueberries osmotically dehydrated with F:S, 3:l and 4:l and dried at power levels 0.1 and 0.2 W/g. 7.4 Microwave drying rate of osmotically dehydrated bluebemes at dflerent power levels. 7.5 Predicted moisture content of bluebemes by the exponential model compared with the experimental values (PL= O Wfg). 7.6 Cornparison of the moisture content of blueberries predicted by the exponential model and the experimental values (PL= 0.1 W/g). 7.7 Predicted moisture content of blueberries by the exponential model compared with the experimental values (PL= 0.2 W/g). LIST OF TABLES Table Page
Cost cornparison of drying methods used in the food industry.
Major strawberry cultivars grown in North America in 1990.
Nutrients in frozen strawbemes per 100 g (Sweetened 4+1).
Volatile compounds (ng/g fresh masd80 litres) in strawberry fruit during ripening. Fruit quality measurernents on some important cultivars. Organic and Phenolic acids of lowbush blueberry cultivars. Physical and Chemical Characteristics of lowbush blueberry cultivars at three dflerent stages of maturity. Drying times required for slices to obtain a moisture content 0.2 kg/kg (db) at different power levels and inlet air temperatures. Mean drying time of slices at dserent air temperatures. Mean drying time of slices at different power levels. Drying time required for puree to obtain a moisture content of 0.2 kgkg (db) at different power levels and inlet air temperatures. Mean drying time of puree at different air temperatures. Mean drying time of puree at different power levels. Rehydration ratio and rehydration coefficient of microwave dried and freeze dried strawbemy slices. Means separation by Duncan's new multiple range test for the quality assessrnent of microwave (MW)and fkeeze-dried (FD) slices and puree by the judges.
xviii Chromacity measurements (ah)for microwave (MW) and freeze dried (FD)strawberry slices and puree. Mean drying time of strawbemes at different ethyl oleate concentrations. Mean drymg time of pretreated strawbemes at different power levels. Relative drying rate of strawbemes under different treatments in reaching a MC of 0.2 kgkg (DB). Rehydration ratios of microwave and freeze-dried strawbemes with different chernical pretreatrnents. Rehydration ratios of strawbemes dried by different methods. Rehydration ratios of strawbemes at difl'erent ethyl oleate concentrations. Texture (Toughness) of microwave and freeze dried strawbemes treated with different ethyl oleate concentrations. Toughness of strawbemes at difTerent EO concentrations. Toughness of strawbemes according to drying regime. Colour measurements of microwave and freeze dried strawbemes under different pretreatments. Shrinkage ratio, equivalent diameter and change in volume dunng microwave drying of strawberries at 0.1 W/g (top) and 0.2 Wlg (bottom). Constants for linear equations describing shrinkage ratio of strawbemes, and for reciprocal logarithmic equations (RL)describing equivalent diameter, as functions of the moisture ratio under microwave drying at power levels 0.1 and 0.2 W/g. Means separations by Duncan's test for moisture removal at the two hit to sugar ratios, treatment with EO/NaOH and time of dehydration. Microwave drying time and relative drying rate of osmotically dehydrated strawbemes. Duncan groupings for mean drying times at the two temperatures, hit to sugar ratios and power levels. Rehydration ratio and texture measurements of osmoticaliy dehydrated and microwave dried strawberries. Means separation of rehydration ratios at the experimental temperatures, fkuit to sugar ratios and power levels. Duncan's groupings for mean toughness at two temperatures, fmit to sugar ratios and drying regimes. Color measurements of osmotically dehydrated and microwave and freeze dried strawbemes. 104 Means separations of colour dserences due to drying regime, fruit to sugar ratio and inlet air temperature. 105 Sensory evaluation of osmotically dehydrated and microwave and freeze dried strawbemes. 105 Duncan's groupings for MCEM (moisture content removed) from blueberries in osmotic dehydration under different F:S, pretreatments, and time. Duncan's groupings for mean drying time, at different temperatures, F:S, and microwave power levels. Drying time and relative drying rate of bluebemes osmotically dehydrated and microwave dried at Merent microwave power leveis. 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 dBerent 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 bluebemes osmotically dehydrated then microwave or freeze dried under different treatments. 119 7.9 Duncan's groupings for mean colour dserence, at different temperatures, F:S, and drying regimes. 120 7.10 Duncan's groupings for mean score by difYerent judges. 121 7.11 Duncan's groupings for mean score at dflerent regimes. 121 NOTATIONS
Chromacity coordinate (redness or greenness) Sugar concentration % in equation 2.3 Chromacity coordinate (Yellowness or blueness) regression prameter estimates equation 6.1 C Sugar concentration % in equation 2.1 CP Specfic heat D Depth of penetration in cm D Dfisivity of moisture cm2/sec in equation 2.11 DB Dry basis De Equivalent diamet er cm E Electric field strength Voltdm
Eab Color clifference form the target color F Mass % in equation 2.3 F:S Fmit to sugar ratio (mass / mass) FD Freeze dry f Frequency Hz k Parameter of the drymg regime in equation 2.12 kg kilo gram L Lightness or darkness (Chromacity coordinate) Mo Initial moisture content kgkg M Moisture content at any time kgkg MC Moisture content kgkg MHz Mega Hertz (frequency) MPa Mega Pascal MW Microwaves me Equilibrium misture content kgkg 111, initial moisture kgkg Mass of rehydrated sample g Mass of dehydrated sample g Initial MC % (Wet basis) of the sample before drymg MC% of the dry sample (wet basis) Parameter of the drymg regime in equation 2.12 Nano grams Microwave power level, Wattdg Radius, in m Shrinkage ratio
Temperalure OC
Surface temperature OC
Centre temperature OC time in h Drying time of control sample (min) Drying time of treated sample (min) Volume at a given mositure content cm3 Initial volume m3 Buk volume m3 at initial moisture content Bulk volume at moisture x m3 Dielectric constant F/cm Dielectric loss factor F/cm Complex dielectric constant Free space wavelength cm Density kg/m3 Relative drying rate Incremental change Subscripts b Bulk O Initial t target x at moisture x 1, 2 Surface, Centre c Control t treated, in equation 5.1 INTRODUCTION
The use of microwave energy for thermal treatment of agriculturd comrnodities has been studied since the 1945's. Where hydroelectric power is available, microwave energy can reduce fossil fuel needs in a wide variety of thermal 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 (Schiffmann., 1995). Rapid initial moisture loss has also been attributed to mechanical expulsion of moisture caused by the strong, yet short-lived vapour pressure gradient induced by volumetric heating (Decareau, 1985; Kostaropoulos and Saravacos; 1995). Microwaves have been shown to 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 specûic energy consumption for drying grapes into raisins by microwaves was at least 50% lower than by hot-air drying. Microwave drying has also been accepted in the food industry for processing applications including drying of pasta and snack foods such as potato chips and cookies (Decareau and Peterson, 1986). Although such experimental work and real-life applications are encouraging, it is clear that microwaves are not suitable under al1 circumstances. High moisture materials with surface layers that resist moisture diffusion may burst when heated with microwaves due to interna1 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 difEcult to obtain browning reactions when using microwaves to cook meat or bakery products. Thus, one cannot indiscriminately use microwaves to process all commodities. To date there has been little work on the microwave drying of berry fruits such as strawbemes (Fragaria ananassa) and blueberries (Vaccinium angustifolium). These high moisture commodities are important to local nual economies as cash crops, and are important sources of vitamins and minerals 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 to develop markets for the dned product, which could be facilitated if a less expensive drying technology could be developed. The industry quality standard for dried berries is presently the freeze-dned 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, colour and aroma. However, freeze-drying is energy-intensive and time-consuming. Thus, the possibility that microwave technology cm be applied to the production of high quality dried strawberries and blueberries should be considered.
The hypothesis entertained here is that it is possible to produce high quality dried strawberries and bluebemes using microwaves as the energy source. This implies that colour, aroma and flavour must be retained to a great an extent as possible, and that the rehydration characteristics are good. Therefore, it is anticipated that industry techniques to reduce skin resistance to diffusion and to stabilize aromas, flavours and pigments may be needed.
The main objective of this study was to develop a microwave-based process to produce dehydrated strawbemes and bluebemes having quality attributes equivalent to those of the freeze-dried product.
The specific objectives pursued as the work evolved, were: Evaluate microwave-drymg of whole and pureed hitin terms of drying kinetics and product quality.
ii) Study possible improvements due to sodium hydroxiddethyl oleate and determine optimum conditions of microwave operation with this pretreatment.
iii) Determine the osmotic dehydration rates of untreated and pretreated strawbemes and bluebemes in sucrose and the microwave-drying kinetics of osmotical?y dehydrated bemes.
Perform quality evaluations of the bemes from (iii) and compare the products to pretreated fkeeze-dried samples according to texture, colour, taste and rehydration.
Determine and model shrinkage of pretreated microwave-dried strawberries in terms of power level and moisture ratio.
vi) Generate empirical models to predict the drying kinetics of osmotically dehydrated strawbemes and bluebemes in terms of the microwave power applied.
1.3 SCOPE
This study is limited to bluebemes and strawbemes. The models were purely empirical, although the mode1 described by Tulasidas et al. (19S4),which also accounts for shrinkage, could be tested for strawbemes, since shrinkage/moisture relationships were elaborated in this study. This study did not include the evaluation of storage time or keeping quality of the microwave-dried fit. Although it was recognized that infusion of sugars could affect the drying behaviour and perhaps quality attributes to a certain extent, no attempt was made to conduct the relevant chernical analyses. Microscopic 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 symps recovered from the osmotic dehydration step. Although microwaves were applied in duty cycle mode in the first experiments, no attempt to study the drymg kinetics in pulsed mode was made in the subsequent experiments, even though this option could have been executed with the new micmwave apparatus. REWEW OF LITERATURE This literature review will briefiy cover some basic properties of strawberries and bluebemes, principles of microwaves and microwave drying of agricultural materials, as well as osmotic dehydration. A short section on modelling is also included since 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 previously investigated for grain by Shivhare (1991) and papes by Tulasidas ( 1997).
2.1 INTRODUCTION Strawbenies and bluebemes are cash crops which are produced mainly in Northern Temperate Regions. They are excellent sources of vitamins and minerals and are consumed to a large extent in fresh condition. However, the fresh product is usually locally available over a very limited time and production far outweighs fresh demand. Blueberries may be cold stored for up to six weeks, whereas fiesh strawbemes have a storage life of only 5-7 days. Of the 2.1 million tons of strawbemes produced annually, 70% are marketed fresh. Bluebemy production is only about 85,000 tonnes, 90% being produced in North America. Roughly 50% are marketed fresh, whereas about 33% are presemed frozen. Both of these fniits are used in a wide vanety of food products including yogurts, pastnes, muffins, snack foods, ice creams, cereals, baby food, concentrate, and juice drinks. Dried bemes are consumed mainly in products where they wiU be rehydrated either prior to consumption, such as breakfast cereals, or during preparation, such as pancake and muffin mixes. The main advantage of using the dried bemes in baked goods is that they do not "bleed"juice, and thus give a more aesthetic overall product; however, the drying method used must yield a product with good rehydration characteristics, as weli as retention of colour, flavour, aroma and nutrients. These and other bemes are usually fieeze-dried since 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 bluebemes are available in several forms - whole, sliced, diced, leathers - and Vary widely in price according to size, availability and source. For example, freeze-dried strawberries currently are sold in the range of 30-60US$ per kg, whereas freeze-dried cuitivated bluebemes sel1 for about 35 US$ and wild bluebemes sel1 for about 12 $US per kg (pers. comm.- Oregon Freeze Dry).
Table 2.1. Cost cornparison of drying methods used in the food industry.
Drying Method Approximate cost
U.S cents / kg
Drum
Air
spray Foam-mat Vacuum PUE
Freeze
Source: (Heldman and Lund, 1992)
6 Although fieeze-dried quality is considered to be the best, fkeeze-dried products have some disadvantages. These include high cost of the freeze- dned 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 fieeze dned bemes is more expensive.
2.1.1 Strawberries World production of strawberries is estimated to be 2.1 million tonnes, the main contributors, (descending order of prodiiction) being the USA, Spain, Japan, Italy, Korea, Poland, Russia, France, Turkey, the United Kingdom and Germany (FAO, 1995). Of the 750,000 tonnes of strawbemes produced in the United States every year, about 450,000 tomes are grown in California (Pszczola, and Donald., 1995; FAO, 1995). Yields among the many cultivars range from 20 to 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 strawberries At 0°C and 90-95%RH, strawbemes consist of 89.9% water and 8.3% solids. Their density is 1033 kg/m3,specific heat is 3.849 kJ.kgm',thermal conductivity is 1.3450 W.m/m2 K and the thermal diffisivity (27" to -18OC) is 1.47 X 10' m2/s(Hardenbirg et al., 1990; Rahman, 1995; Arthey and Ashurst, 1996). California 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, Blornidon (Ont,Quebec, MP) Northeastern US Earliglow, Honeoye, Kent, AlIstar (WV,MD, NJ, VT, NH, MA, ME) Lower Midwest Earliglow, Wtan, Redchief, Allstar, (NE IAyMO,IL,IN ML, OH) Surecrop, Delite Southern U.S. Chandler, Earliglow, Cardinal, Apollo Canadian cultivars (recent) Acadia, Annapolis, Blomidon, Bounty, Coronwalis, Goosecap, Governor Simco, Kent, Mic Mac, Mira
2.1.1.2 Nutritional content of strawberries Even in frozen form, strawberries are important sources of several dietary components, as show in Table 2.3. Table 2.3. Nutrients in frozen strawbemes per 100 g (Sweetened 4+1).
Nutrients Unit mole Whole Sliced Unsweetened Sweetened Sweetened
Water Ash
Calories
Sodium
Potassium
Carbohydrates
Dietary Fibre
Sugars
Fructose
Glucose
Sucrose
Protein
Vitamin C
Calcium Phosphorus
Magnesium
Source: California Strawberry Commission, Watsonuille Ca. According ta FDA definitions, processed strawbemes are sodium free, 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% hctose, 3% glucose, and 0.5% sucrose). The typical aroma of strawberries resides in the oil fraction. The colour pigment (cyanidin 3-glucoside) is said to 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 of processed bemes ranges from 3.3-3.6. Citric acid LE the primary acid (Pszczola, 1995).
2.1.1.3 Strawberry tlavov and amma One of the most challenging areas in strawberry research is the assessrnent of fruit aroma and flavour, the important flavour impact compound is furaneol. Strawberry aroma is extremely cornplex, consisting of 35 to 200 volatile compounds. Most of the ripe strawberry aroma originates from methyl esters of methyl alcohol in the fruit. Enzymes convert the esters into many volatile cornpounds. Of these, seven volatiles whose production depend on stage of ripeness, appear to be correlated with strawberry aroma (Table 2.4). The relative concentrations of these volatiles also var-widely among cultivars (Perkins-Veazie and Collins., 1995).
Good strawberry flavour is a combination of aroma, 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 80990% of the SSC is due to the sugars: glucose, mictose and sucrose. The SSC in strawbemes increases continuously during fitgrowth and ripening, fkom 5% in small green Table 2.4. Volatile compounds (ng/g fiesh masd80 litres) in strawberry fruit during ripening.
Compound Ripeness Stage
.- - --- Green White Pink Red
Ethyl Hexanoate 18.7 82.2 110.2 392.3
E thyl Butanoate 18.1 81.9 88.4 317.2
Methyi Butanoate 2.3 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) fmit to 6-9% in red bemes, and is dependent upon cultivar and environmental conditions (Perkins et al.. 1995; Kader, 1991). Some flavour characteristics of various cultivars are given in Table 2.5. Table 2.5. Fruit quality measurements on some important cultivars
Cultivar Ascorbic acid Soluble Titratable SS/acid
(mg/100g) Solids % Acid 9% Ratio
Chandler 51
Seascape 46
Douglas 43
Irvin 41
Oso Grande 40
Capitola 39
Selva 33
(Source: Perkins, et al., 1995; Kader, 1991).
2.1.2 Blueberries Lowbush bluebemes (Vacciniumangustifolium) are native to Eastern Canada and the Northeast United States, whereas highbush blueberries (Vaccinium coryrnbosum) are native to Europe. North America is the world's largest producer of bluebemes, accounting for nearly 90% of world production. The lowbush fniit is smdl compared to highbush or rabbitteye bluebemes (Vacciniurn ushei). The total area of cultivated bluebemes planted in North America is 18,088 ha, 80% of which are cumently bearing. Approximately 19,800 ha of blueberries are commercially cultivated worldwide (Eck,1988), yields averaging 4-5 tonnes per hectare. 2.1.2.1 Pioperties of Lowbush blueberries The lowbush blueberry averages about 0.414 gherry with a dry matter content of 15.14%and soluble solids of 10.74%. It can be stored at a temperature of 0-5°C and responds well at oxygen and carbon dioxide concentrations of 5-10% and 15920% respectively. The shelf Me is limited by fungal spoilage. For long tem storage, the fniit is frozen or mixed with syrup and flash frozen. The specific heat of the lowbush blueberry is 3.60 kJ/kg°K (Somogyi and Ramaswamy, 1996).
Table 2.6. Organic and Phenolic acids of lowbush blueberry cultivars
Acids (%) Cultivar Mean
Maturity (Ripe) Blomidon Cumberland Fundy % Chlorogenic Citric Malic Quinic Acetic Caf5eic p-Coumaric Ferulic Shikimic
(Source: Kalt and McDonald. 1996.) 2.1.2.2 Blueberry colour Blueberry colour is an important quaIity factor infîuencing fresh market value and the suitability of the bemes for processing. Theh intense
Table 2.7. Physical and Chernical Characteristics of lowbush blueberry cultivars at three different stages of maturity.
- - BeTY Dry - Solu- Gloc- Total mnes able ouse, Fresh Matter Solids Fruct acid -ose
Unnpe Blomidon Cumberland Fundy Mean Ripe Blomidon Cumberland Fundy Mean Overripe Blomidon Cumberland Fundy Mean Grand Mean
(Source: Knlt and McDonald 1996.) blue to red colour, and high pigment content, make them valuable as food colorants ingredients for foods. Bluebemes derive their bold colouring from the high content of anthocyanin. Anthocyanin is a soluble pigment that imparts colours ranging from blue to shades of red. 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 chernical characteristics (Table 2.7) of some cultivars of blueberry are presented.
2.2. DErnRATION AND DRYXNG Drying and dehydration are the removal of moisture from a given material. Although these terms are often interchanged, it is perhaps necessary to make a distinction since "drying" implies the process of moisture removal due to simultaneous heat and mass transfer (ie. thermal drying). Drying as such, therefore refers primarily to the removal of moisture in the vapour phase, whereas dehydration is a more encompassing term and includes methods of moisture removal that can be done without addition of heat (eg. compression, reverse osmosis, filtration, etc.). Since drying is based on phase change, the theoretical minimum energy required to remove moisture by dryng is about 2.5 MJIkg moisture, equivalent to the latent heat of vaporization of water. However, heat recovery from the released vapour, as well as initial steps of mechanical or osmotic dehydration can reduce the specific energy required substantially. In the food processing industry, specific energy requirements for dehydration are usually in the range of 1 to 2 MJkg moisture removed (Rizvi 1995) In drying, heat supplied fiom the environment conducts into the material and increases the vapour pressure within the material. As long as the vapour can diffuse to the surface of the material and the surrounding air is not saturated, this moisture is taken up and carried away convectively. To some extent, the vapour-pressure difference between the air and the surface of the material cm 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 of penetrating the surface and interacting volumetrically with water molecules to generate heat internally, thus eliminating the surface-to-centre conduction stage. This leads to much faster heating rates, and can create a pressure-driven liquid flow in some circumstances (Ratti and Mujumdar, 1996).
2.2.1 Drying of hit Three basic techniques are used to dry fhts: 1) Sun drying; 2) Atmospheric drying in batch (kiln, tower, and cabinet dryers) or continuous (tunnel, belt, belt-trough, fluidized bed, explosion p&, foam-mat spray, drum and microwave heated) mode, 3) Subatmospheric dehydration (vacuum, shewdrum and fieeze dryers). Low temperatureflow energy processes such as osmotic dehydration and microwave drymg have recently received more attention (Jayaraman and Das Gupta, 1992). The main problems in hit drying are damage to sensory characteristics and loss of nutritional components due to long exposure to high temperatures (Van Arsdel et al. 1973; Fellows, 1988). These include loss of aroma volatiles, oxidation of pigments and vitamins, due to and case- hardening in certain products. Case-hardeningis a common defect of dried fruits and is caused by drying that is too rapid compared to 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 strawbemes and bluebemes are high moisture Wts, are very fiagrle, and are grown extensively in temperate and cool climates, sun drying is not usudy possible. These fniits 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 small fimit in North America. A variety of novel methods have recently been investigated for drying these fruits. Among them are vacuum drying, microwave drying, and combinations of fkeeze 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-drymg or to reducing the costs and energy requirements for freeze-dryuig, under the constraint of quality maintenance.
2.2.1.1. Specific studies Surprisingly, there is very little published work on drying of strawberries although there is a fair amount on the drying of bluebemes. Lowbush bluebemes were dehydrated by Yang and Atallah (1985) to moisture contents of 16-25% using four different methods: forced air, vacuum oven, freeze dry and micro-convection methods. The quality attributes of dried bluebemes were determined. The levels of vitamin A, C and niacin found in the dned bemes were low compared to fresh berries. The forced air drymg 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 tirne. However, the quality of the dried bemes was poor in cornparison with bemes dned by other methods. Freeze drying gave the highest retention of the other important components such as soluble solids and colour and led to the highest rehydration ratio and lowest bulk density. Berries dried by the vacuum oven method were also high in soluble solids and colour retention. The authors suggested the combination of vacuum oven and freeze drying methods to bring down the tirne and cost, since the these methods led to a good quality product. They also suggested that the results of this study could be applied to small fruits like raspberries, cranbemes, strawbemes and blackberries. Rabiteye bluebemes were dried using a high temperature fluidized bed drier (HTFB)by Kim and Toledo (1987). A 15 m/sec air velocity was required for fluidization and at 170°C the moisture content was reduced from 5.8 kgkg to 0.7 kgkg after 8 min. Mer osmotic dehydration in sucrose, the moisture content was 1.3 kgkg. With a 4 min treatment in the HTFB at 150°C, the rnoisture content reduced to 0.28 kgkg. The IITFB simultaneously dried and paed the berries, resulting in reduced bulk density compared to bemes produced using conventional drying. Osmotic dehydration pnor to HTFB imparted a raisin-like texture to the product. A hybrid process of osmotic dehydration and fkeeze drying was investigated by Yang et al. (1987) to produce a raisin-type lowbush blueberry product. Using a berry:sugar ratio of 3:l or 49 for osmotic dehydration, followed by a sequence of thorough rinsing, freeze drying with abrupt release of vacuum, and a thermal conditioning, it was possible to produce a raisin-type blueberry product. The final product had good texture, flavour, overall acceptability, and predicted shelf lives of 16 and 64 months at 25OC and 5°C storage respectively.
2.2.2 Pretreatrnents for drying fruit Fruits and vegetables are subjected to certain pretreatments in order to facilitate drying and minimize adverse changes during drymg and subsequent storage of the products. Alkaline dips are used particularly to 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 @or to drying and is used primarîly-for miits that are dried whole, especially prunes and grapes. A sodium carbonate or lye solution (0.5% or less) is usually used at a temperature ranging from 93.3"C to 100°C (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 to be not effective in reducing drying time. Oleate esters constitute the active ingredients of commercial dip solutions used for grapes. They accelerate moisture loss by causing the wax platelets on the grape skin to dissociate, thus facilitating moisture diffusion. Ponting and McBean (1970) studied the pretreatment of waxy fmits like chemes, blueberries, prunes and grapes. The rnost effective dipping materials for increasing drying rate were found to be the ethyl esters of fatty acids. Ethyl oleate was the most convenient to handie and was effective. Dipping the waxy fmits for a few seconds in a cold aqueous emulsion of ethyl 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 dned 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 kemels. This was attnbuted to the action of ethyl oleate on the waxy cuticle of corn kernel which resulted in reduced cuticular resistance to water loss. Shelled corn 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 increases in dryhg 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 atarch significantly increased the drying rate dunng initial drying but had little effect on granular and gelatinized starches. Dipping grapes in ethyi oleate emulsions increased significantly the drying rate throughout the drying period. Rahman and Perera (1996)pretreated chemes by dipping in sodium metabisulphate, sodium hydroxide, citnc acid, tartaric acid, ethyl oleate, potassium carbonate solution each at 2% (w/w)concentration. Ethyl oleate was found to be the best pretreatment for drying. Tulasidas et al. (1994) also 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 of the 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 fmit 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 be a safe additive to incorporate into fruit and vegetable products up to certain limits. However, recently there are reports on the hypersensitivity of a few inidividuals to the ingested sulfite, (Jayaraman and Das Gupta, 1992).
2.28 Osmotic dehydration The concentration of food products by means of product immersion in a hypertonie solution (i.e., sugar, salt, sorbitol, or glycerol) is known as osmotic dehydration (Raoult-Wack et al. 1989; Raoult-Wack et al. 1991a). For fruits it is defined as the partial dehydration of fruits through 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 usefid means of reducing the processing time and energy consumption of drying. Water loss to the extent of 30-50% of the hits is attainable and this is dependent on the strength of the sugar solution. Although it adds to the overall drymg time, there is the advantage of binding flavour compounds and colour which would otherwise be lost on heating (Yang et al. 1987) and can improve other sensory characteristics (Jayaraman and Das Gupta, 1992). Strawbemes were osmoticdy dehydrated in a batch recirculation system by Yang and Le Maguer (1992). Physical and chernical 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 25OC and 50°C. Sugar accumulation occmed steadily in strawbemes dehydrated at 50°C, while the increase occurred only during the first 2 h of dehydration at 25OC. No simcant difference was found in the weight loss and moisture content between cultivars during osmotic dehydration. Colour tended to decrease at higher immersion temperatures. In practical applications, 63% of sucrose at 25OC for 2 h was found to be the best processing combination; it removed over 40% of moisture kom strawberries but accumulated less than 1 mg/g of sucrose for both cultivars. Alvarez et al. (1995) studied the effects of blanching and osmotic pretreatment of strawbemes on kinetics of moisture transport during air dehydration. It was found that the effective diffusion coemcient of water in strawbemes was strongly affected by heat pretreatment, but glucose dipping &er blanching caused no additional effect. The authors obtained an effective diffisivity coefficient of 5.7621.35 x lu6cm2/s for raw fkuit and of 5.4320.65 x 1W6 cm2/s when the fdtwas 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 of the 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 pior to 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 of osmotic dehydration. It effects not only the rate of the process but also influences the chemical composition and properties of the product. Increased temperature increases the rate of chemical 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 into the tissue (Pavasovie et al, 1986) Crystalline osmoactive substances are used at fmit to substance ratios of 1:l to 6:l for hits. For substances investigations were done at weight ratios 1:l to 6:6. It is recommended that osmotic dehydration of fmits and vegetables be done with weight ratios of 4:l to 5:l of product to osmoactive substance (Lewicki and Lenart, 1995). Magee et al. (1983) developed a mode1 for solute diffusion during osmotic dehydration of apples. It was based on solids gain divided by water content m. m was expressed as: where k is a rate constant, t is tirne and m, is the initial mass. The rate constant k was expressed in terms of temperature and sucrose concentration, C:
(2.2)
The average activation energy of the process was 28.2 kJ mole".
Videv et al. (1990) developed the following empirical equation to predict the rate of osmosis F during dehydration of apple slices:
Where F = decrease in mass %, B is the sugar concentration, T is the temperature and t is tirne. This expression was valid for B=60-75%,T=40- 80°C and t= 0.5-4.5 h. The increased interest in like-fresh food products makes osmotic dehydration a good option for food presewation. The quality of dried products treated by osmotic dehydration followed 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. 2.2.4,l Energg 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 kJ/kg of water removed at temperatures 20°C, 30°C and 40°C respectively. To keep the process running at a desired temperature, a supply of heat is necessary. Depending on the amount of water removed fkom the material, the heat supply amounts to 180-240 kJ/kg at 30°C and 380-600kJ/kg of water removed at 40°C (Lewicki and Lenart 1995).
An important physical change that occurs during thermal drying is shrinkage of the product, which is caused by microstnictural changes due to moisture gradients. Shrinkage influences the transport properties of individual particles as well as the porosity and thickness of the packed beds in dryers. Thus, shrinkage must be accounted for in process modelling, design and control. High moisture materials such as small fruit, shnnk to about 25% of their original size. Experimental data has shown that shrinkage is mainly a function of rnoisture content for a wide variety of food 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: Where S,,=Bulk shrinkage coefficient (Lozano et al. 1983) Vb=Bulk volume m3 at moisture content x Vb=Bulk volume m3 at initial moisture
Bulk volume at each moisture content was calculated from the bulk density and the corresponding sample weight W.
V, = Bulk volume, W = Sample mass kg, p, = Btilk density kg/m3 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 (MA&), where Mo is the initial moisture content. The same approach was used by Tulaaidas et al. (1994) to provide input parameters to a detailed mode1 describing the drying kinetics of grapes in a microwave environment, as they shrink to 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 (Van Arsdel, 1973; Ratti, 1991). Slow drying may lead to uniform shrinkage, whereas very rapid drying may result in lower shrinkage due to induction of a permanent tension that preserves the original dimensions to some extent; however, cracks and voids can develop internally. In freeze-drymg, the shape of the product is retained. These voids are also responsible for high rehydration rates. Several investigators have investigated the shrinkage of food grains (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 rnoisture content. Kilpatrick et al. (1955) studied volume shruikage of potatoes and other vegetables as drying proceeds. Charm (1978) reported on volumetric contractions of meat and potatoes. CWe(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 difTerent moisture contents were reported by Lozano et al. (1980). Tulasidas (1994)studied the shrinkage of grapes under convective and microwave drymg atmospheres and correlated with the moisture content. Shrinkage characteristics of individual particles and changes of fmed-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 Ratti (1994), a simple mode1 for correlating shrinkage of an individual particle with water content and air velocity was compared.
2.4. MICROWAVES 2.4.1.Introduction Microwaves are electromagnetic waves in the 1mm to 1m waveband, corresponding to fiequencies of 300 MHz to 300 GHz. Dielectric heating occurs between 1 MHz and 100 MHz, whereas microwave heating occurs between 300 and MHz and 300 GHz. Only certain frequencies are allowed for industrial, scientifîc and medical applications, since the microwave band has been exploited primarily for communication and military applications. The frequency allocations for different purposes are made by the International Telecommunications Union (ITU) and certain fiequencies are allocated to particular coutries. For example 915 MHz is allowed only in United States and North America. The fkequency allocations for various uses are listed in Decareau (1985). The use of microwave energy has been considered to be a suitable approach for coping with certain drawbacks of conventional methods of thermal treatment of foods (Decareau and Peterson, 1986). The following advantages are associated with microwaves in heating and drymg.
Advantages of Microwave Heating Since the transfer of energy is radiative, heating is instantaneous. Furthemore, heat is generated within the material, and not just conducted towards the centre, thus, interna1 temperature gradients tend to be smaller. ii) Microwave energy couples directly to the material being heated. The transfer of energy to the air, walls of the oven, conveyor or other parts, is minimal since their dielectric constants are very small. This can lead to significant energy savings. iii) Enicient and accurate control of heating rates can be achieved by controlling the output power of the generator. iv) Since the moisture fiow is partly pressure-dnven from the interior, there is no receding moisture front as in convection, which eliminates case hardening. This is favourable for some applications, but is considered a negative aspect in cooking or baking since cmst formation and surface browning do not occur unless special measures are taken. Many chemical and physical reactions are promoted by the heat generation by microwaves, leading to puffing, drying, meltine. protein denaturation. starch gelatinization. 2.4.3. Advantages of Microwave Dryhg 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, internal heat generation leads to an increase in internal vapour generation, which promotes liquid flow towards the surface, and also leads to higher internal temperatures, both of which increase the drying rate. ii) In microwave drymg, there is great potential for energy savings, due to the speed of drying and lower specific energy needs in the case of high-loss 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 handling tirne. v) Improves the product quality and in some cases eliminates case hardening, internal stresses and other problems like cracks. vil Cost savings may be realized through energy savings,increased throughput, labour reduction, reduced heat load in the plant, increased eficiency and reduced maintenance costs.
The following sections review the principles of microwave heating and of equations relevant to the understanding of microwave heating and drying kinetics.
2.4.4. Interactions of microwaves and biological materials The interactions of microwaves with target materials are usually described in terms of the dielectric properties of the material, the dielectric constant E' and the dielectric loss E", which are the real and imaginary components of the relative complex pemittivity E*. The complex dielectric constant E may be expressed as
were j =J-1, The loss tangent is defined by: tan S = E"/E'. These properties can be measured at various frequencies, and they are not constants: they are dependent on the temperature, moisture content, composition and particle density of the material. In a processing situation, the bulk dielectric properties, rather than the particle dielectric properties are of interest, since the microwaves will also interact with the air in the inter-particle space. Although the bulk dielectric properties cm be roughly estimated from the dielectric properties of the particles and air, the exchange of moisture between the particles and the air space make accurate prediction dificult and dependent on airflow characteristics and distribution which are scale dependent. The dielectric constant is analogous to a capacitance, since it is a measure of the material's ability to store microwave energy. The dielectric loss 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 loss to its dielectric constant is defined as its dissipation factor or loss tangent (tan 6). Materials can be classified as high loss or low loss. Among the substances associated with foods and agricultural products, water is the greatest absorber and dissipator of microwave energy k9=78,~"=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 cellulose. The dielectric properties are the basis for estimating important values such 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:
Where D = is the penetration depth in cm & = free space wavelength cm If'& is low then the above equation may be simplified to
This equation is reasonably accurate for most foods even though many have relatively higher c' values. From these equations it is obvious that materials with high dielectnc constants and loss factors will have smaller penetration depths than those with lower values. If the 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 of a material is also a function of the dielectric loss factor, and is given by:
where, fis the frequency of the applied microwave field, E is the electric field strength inside the sample (V rn-'), p is the density and C, is the specific 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 of Agricultural Products Chin et al. (1985) used a microwave oven to dehydrate tomato products to determine the total solids and compared the results with that of vacuum oven dried tomato products. The results obtained by the microwave oven drying procedure were equivalent to those obtained by the vacuum oven procedure. Because of the inherent speed and ease of use, they recommended that the microwave oven drying method be considered as an alternative to the officia1 vacuum oven method. Al-Duri and McIntyre (1991) used convection oven, microwave oven and combined microwave 1 convection oven for comparing drying kinetics of milk and milk products and fresh pasta. They concluded that for low moisture products convection oven is not advantageous as the diffusion process is slow. But in microwave operated oven 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 of hot air with microwave power increased the drymg rate but high temperatures are not recommended for obtaining good quality products. Hemphill and Martin (1992),used microwave oven drymg method for deterrnining total solids of strawberries, and compared the results wiih the freeze drying, and hot air oven drymg methods, since total solids are an important quality for processing. Low power microwave drying method values of total solids correlates well with those determined by either freeze- drying or oven drying. Bouraoui et al. (1994) employed convective and combined microwave and convective drying to dry potato slices. Effective moisture difisivity profiles were calculated using Fick's diffusion mode1 in one dimension. Statistical analysis showed diffusivity to increase with incrrasing interna1 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 dned product while considerably reducing drying duration. Microwave dried product had a better rehydration compared to convective dried product. Tulasidas et al. (1993),used convection and combination of convection 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 chernical 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 rehydration characteristics of the 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 the and the product quality was better when dried at the lower power level. 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 be of excellent quality. Yongsawatidigul and Gunasekaran (Part 1 and II, 1996) used microwave-vacuum drying method to dry cranberries and studied the continuous and pulsed application of microwave energy. They came to 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 colour, texture, water activity of microwave-vacuum dried berried in cornparison 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 method. 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 in foods is a subject of considerable importance. The mechanisms of moisture transport are numerous and often complex. Transport phenornena are usually classified as resulting hmpressure diffusion, thermal diffusion, forced diffusion, and ordinary difision (Van Arsdel, 1973). Often a difision transport mechanism is assumed and the rate of moisture movement is described by an effective dfisivity value, D,, 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 to 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 failing rate period); however, they can oRen predict moisture ratios to within 10% or better, and can therefore be useful for comparison purposes. The rate constants can be expressed in terms of operating conditions if desired, and should perform reasonably well 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 matenals, are those associated with the nature of the material to be dned. Biological materials oRen undergo chemical and structural changes during application of heat, and these can alter properties such as thermal conductivity, moisture diffusivity and other characteristics which are oRen assumed to be constants in drying models. Moisture itself may be free or bound, and the movement of free water may also 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: where, m is the moisture content (%), t is time (h) and D is the moisture
Crank (1975) gave the analytical solution to this equation for a homogeneous, isotropic sphere with constant dfision coefficient, as:
where, meis the equilibrium moisture content, m, 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 such 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 (m, - me). The modified model certainly gave reasonable fits to drying curves for corn at 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 difisivity, D was estimated from actual microwave drying trials. Later, Tulasidas et ai. (1997) developed a comprehensive semi- theoretical model of drylng 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 also essential components of the model. This model was quite accurate at predicting moisture loss with time under various conditions and performed better than the modified logarithmic model (Page, 1949), or Page's equation, which is oRen used to estimate drying:
where k, n are parameters associated with a particular drying regime and are estimated by non-linear regression fmm experimental data. The error of prediction of the 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 to 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. 2.6. QUALITYASSESSmNT 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 object, it is possible to define a foods colour in a purely physical sense in tems of the physical attributes of the food. A more satisfactory approach is to define colour as objectively as possible and interpret the output in tems of how 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 to the development of tristirnuius colorimeters. The concept is simple, one needs a light source and three glass filters with trammittance 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 well as dxerent axes in space diverse types of colorimeters are being produced and used. This system has some drawbacks in that it was not visually uniform, one unit of colour measurement in one area of the solid was not visually different from the same unit in another area. A number of attempts were made to calculate a colour solid that was visually uniform in al1 areas, but it was concluded that the task was impossible. However, some came close and three systems seem to be gaining prioity. One is the CIE XYZ system, and the second is the Judd-Hunter Lab solid. The later represents a colour solid in which L is lightness or darkness, +a is redness, -a is greenness, +b is yellowness, and -b is blueness. A third scale known as CIELAB, with parameters La,a', b' (Hunter and Harold, 1987) appears to be gaining in popularity. Based on the above principles many instruments were developed to measure the colour. Current instrumentation ranges from relatively simple designs to sophisticated colorimeter-computer combinations. Photovolt colorimeter is a simple colorimeter consisting of a measurement unit and eight exposure heads for dBerent 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 cornputer. With these instruments measurement of colour is a relatively mature science, but operator ingenuity is still required to measure samples such that meaningful data can be obtained.
2.6.2.Texture rneasurement Texture, appearance, and flavour are the three major components involved in food acceptability (Bourne, 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 to replace human sensory evaluation as a tool to evaluate food texture (Peleg, 1983). The following statement by Lord Kelvin is frequently seen in discussions dealing with measurement "1 oRen 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 of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be."( Mohsenin, 1986). Many early instruments were used to aid in the texture evaluation of foods. One early texturometer, called the denture tenderorneter, was built and used by the Food Technology Laboratory of the Masssachusetts Institute of Technoiogy. This instrument employed strain gauges connected to the jaws of a dental articulator. Another versatile and well-known instrument that has undergone several name changes is texture shear press. This is manufactured by several companies and is also adaptable to the Instron universal testing machine. The Wamer-Bratzler shear is probably the most widely used instrument in the United States for measuring toughness of meat. The test ce11 can be mounted on a universal testing machine capable of recording force and motion of the crosshead. Al1 texture measuring devices have five essential elements: 1) the driving mechanism, 2) a probe element in contact with food, 3) a system to 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 fiat plunger, ûhearing jaws, a tooth shaped attachment, a piercing ïod, a spindle or a cutting blade. The force may be applied in vertical, horizontal or levered manner, and rnay be of the cutting , piercing, puncturing, cornpressing, grinding, shearing or pulling type. The sensing elements may be a simple spring or a more sophisticated strain gauge. The results can be 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 role in reconstitution. The degree to which a dehydrated sample will rehydrate is influenced by structural and chemiçal changes caused by dehydration, processing conditions, sample preparation, and sample consurnption. Rehydration is maximised when cellular and structural disruptions such as shrinkage are minimised (Okas et al. 1992). Several researchers have found that fkeeze-drying causes fewer stnictural changes and fewer changes to the product's hydrophillic properties than do other drying processes (Hamm, 1960; Mcllrath et al. 1962). Most of the shrinkage occurs in the early drying stages, where 40 to 50% shrinkage may occur. Hence the 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 to 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. To the consumer, aesthetic and gustatory satisfaction can guide choice of product. A combination of sensory perceptions are used to assess sensory quality (Ranganna, 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 dserences in odour and taste of specific products like tea, coffee, wine , etc. With the development of sensory evaluation techniques on scientific lines, the experts are being replaced by panels whose sensitivity and consistency have been established by training and repeated tests. Sometimes untrained and semitrained panels are constituted. Several methods may be adopted to evaluate the product. These are ranking, single sample, two sample difference, multiple sample. Quality differences can be based on the Hedonic scale, or Numerical scoring methods. MATERIALS AND METHODS
This chapter presents materials and methods used that were common to several phases of the expenmental work. This is done in order to avoid repetition in the rest of the thesis.
3.1. MICROWAVE DRITVG SETUPS 3.1.1 Setup used in Preliminary Studies A programmable microwave oven maton Viking) was modified and used in the preliminary experiments on whole, sliced and pureed strawbemes (Chapter 4). A schematic diagram is presented in Fig 3.1. The oven had a nominal power of 600 W, and operated at 2450 MHz. The cavity volume was 0.4 m3 . Duty cycles from O 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 hole was drilleci into the bottom of the microwave oven. An acrylic pipe of equal diameter was firmly attached to the hole, forming a duct for the delivery of temperature-modulated air. A fine mesh metallic screen was fitted to the top of the pipe at the cavity floor level, to prevent microwave leakage. A small hole ddled on the back wall of the oven and fitted with a metal wire mesh served as the air outlet. Air was introduced into the chamber through the tube using a 0.25 kW blower from below the oven. Heating elements (2 kW) were positioned dong the air supply pipe length to heat the incoming air, the temperature being controlled using a power regulator. A mercury in glass thermometer fixed at the bottom of the oven was used to monitor the inlet air temperature. A second piece of the same acrylic pipe with a perforated tefion bottom served as a sample holder for the fruit. During the test nuis 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 fuced at the top of the microwave oven to promote better uniformity of microwave distribution.
1. Mode Stirrer 2. Air outlet 3. Simple Holder 4. Berry Slices t 5. Heating elements 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 m/s + 34 since higher air flow rates were found to result in fluidization of Because this setup could not accommodate real-the measurements of mass, the microwave was tumed off every twenty minutes and samples were taken out and weighed on a digital electronic balance (Acculab Mode1 121, Canadawide Scientific, Ottawa, ON), then put back in the oven.
3.1.2 Experimental Microwave Dryhg 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 of 2450 MHz frequency. The microwaves were conveyed through a series of rectangular (7.5 x 4.0 cm) wave guides to 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 rneasurement 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 removal 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 fiom 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 sample holder. Three 2 kW electrical heaters were used to heat the inlet air to the required temperature. Air temperature and velocity controls were provided near the blower. A 10x10 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 of Idsec was used in all the experiments as explained in the previous section. Al1 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 essential feature of this setup, which did not elest 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 terms of Wfg. In a typical run, about 100 g of hitwere placed in the sample holder inside the cavity, and the power could be adjusted to anywhere in the range of O to 750W. Thus, it was possible to obtain low incident powers of the order of 0.1 W/g where previously, the incident power for this size sample would be always 6 W/g during the ON penod of any duty cycle (rating of first oven was 600W). Since the initial moisture content and mass were determined before each run, it was possible to input the final mass at which the desired moisture content would be reached and have the system shut dom automatically. In al1 experiments, the final moisture content was set at 0.2 kg waterkg 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 oven at 70°C for 6 h, or by the vacuum dry method (Boland, 1984; Ranganna, 1986; Canellas et al. 1993). These methods were found to compare well for initial moisture determinations. Therefore, the oven 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
Figure 3.2 Experimental microwave drying setup. Figure 3.3 Photograph of the experimental microwave drying setup.
Figure 3.4 Data acquisition system connected to the dryuig setup. 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 cornputer programme written in HP-QBASICmonitored 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 thennocouples 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 electrical output from the strain gauge indicated the mass measurements. The two power detectors located on the wave guide read the incident power and the refiected 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 to 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 interval. 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 at the required time intervals. Provision was also made in the applicator to insert fibre optie temperature sensors for measuring the centre and surface temperatures of the fhit at different power levels. The temperatures were measured using Figure 3.3a Lyo Tech Canada, Freeze drier (Used in the experiments for dryng strawbemes and blueberries). a digital fiberoptic thermometer (Nortech Fibronic Inc. Canada) specially used in microwave atmosphere. AU the data was stored and was printed continuously at the pmgrammed time intervals.
3.2 FREEZE DRYXN% Fruit 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 kgkg (DB). This required a drying time of the order 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 rnoisture from the air.
3.3 OSMOTIC DEhlYDRATION
Osmotic dehydration was performed by mixing fruit samples with granular sucrose. Fruit to sugar mass ratios of 3:l or 4:l were used, fruit samples consisting of 100 g lots. The extent of dehydration was determined for vanous periods (12, 24, 36 or 48 hl. Mass loss due to osmosis was determined after rinsing the syrup from the surface with tap water and blotting to remove surface water.
3.4 QUA= EVALUATION OF THE DRlED PRODUCT 3.4.1 Rehydration tests Rehydration tests of dned samples were performed by the method recommended by the USDA (Anon, 1944). A 5 g sample of the dry material was weighed into a 500 ml beaker containing 150 ml of distilled water. The beaker was placed on a hot plate and covered with a watch glass, the water Figure 3.5 The Minolta chroma meter. (used in colour measurements).
Figure 3.6 Instmn machine used in measuring the texutre.
50 was brought ta the boiling point in 3 min and sample was added to the boiling water and boiled for an additional 5 min. The sample was transferred to a 7.5 cm Buchner helcovered 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, cdculated by the equation 3.1, is used by some authors, but yields exactly 10 times the rehydration ratio.
COR= Coefficient of Rehydration q,=Mass of rehydrated sample m,=Mass of dehydrated sample %=Initial MC 8 (Wet basis) of the sample before drying M,,=MC% of the dry sample (wet basis) 3.4.2 Colour determination The chromacities of fresh and dned 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. In the experiments on drymg of strawberry slices and puree, coiour was expressed as the ratio ah, which is convenient way of reducing two colour parameters to one (Francis and Clydesdale, 1975). A higher a/b ratio indicates a darker (more red) product. In the other experiments colour difference values AL, Aa and Ab are calculated according to the following formulas. &= LL, , Aa= a-a, , Ab= b-b, (3.2) Where L, a, b are the measured values of the specimen and L, , a, , b, 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 a,,is detemined using the L, a, b colour coordinates and as defined by the equation below (Minolta, 1991).
The colour dserences between the fresh berry colour and the sample colour indicates the variation in colour.
3.4.3.Tegture Texture is the property of the fruit (food) which is associated with the sense of feel 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 M Automated Materiai Testing System 1.16, Fig 3.6. The puncture tests were conducted on the dried bemes and the measurements were taken thrice on each microwave or freeze dned sample and the mean of the three measurements were taken as the sample toughness. Toughness is the force required for unit volume, it is expressed in these measurements in MPa. The cornparison of toughness between the various treatments gives an impression to decide the best treatment.
3.4.4 Sensory evaluation Sensory evaluation of convection, 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 to the judges. The judges were asked to observe the samples carefdy for total appearance, aroma, taste and colour and then give them a rating using the Hedonic scale. Here, different ratings, ranging from "Like extremely" to "dislike extremely" were given by the judges. These were later converted to numerical values from 9 (like 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 ZXPERMEhrTt DESIGN AND MALYSES
All experiments were designed as full factorial experiments, with extra samples being prepared for freeze-drymg, the freeze-dried products being used as a basis for quality evaluation. The statistical analyses of dl quantitative measurements were based on the main effects and interactions models. Duncan's new multiple range test was used to rank the means at the 0.05 level in all cases. PRELIMINARY STUDIES ON MICROWAVE DRYING OF WHOLE, SLICED AND PUREED STRAWBERRIES
4.1. 2NTRoDUcTION 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 pnmarily 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 bluebemes and strawbemes 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 specific 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. MATERUS AND METHODS
4.2.1 Initiai investigations Strawbemes of unknown cultivar were procured from the market and stored in the cold room at 1°C. The hitswere removed from the cold room about 2 h before the expenments to attain room temperature. Samples were taken for the initial moisture content determination. The initial moisture content of strawberries varied from 89% to 92% across al1 the experiments. About 100 g of the Mts were weighed and taken for microwave drying in the modifïed microwave drying equipment shown in Fig 3.1 and explained in Chapter III. 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 m/s
2 3% and air temperature was kept at the required level(35OC or 45°C etc.) by adjusting the temperature controls. Higher air flow rates were not feasible since they result in fluidization of samples towards the end of the drying period. Test samples fiom the microwave oven were weighed at 20 minute intervals using a digital electronic balance (Acculab Model 121, Canadawide Scientific, Ottawa, ON).
4.2.2 Microwave drying of whole strawberries The first trials were conducted to evaluate the possibility of drying whole strawbemes in a microwave field without preliminary treatments. These trials were performed on under purely convective conditions with air heated to 35OC, and at 5 microwave duty cycles IO%, 20%, 30%, 40% and 508 [Power Level (PL)1, 2, 3,4 and 5, respectively]. It was found that at 40% and 50% duty cycles, there was burning of the product. At lower power levels (1, 2 and 3) the fruit failed to dry. Rather, they swelled and burst open, releasing juice (bleeding). Since this behaviour was thought to be related to high resistance to diffusion at the skin, a physical method to circumvent this problem was attempted. This involved puncturing the surface of the kitwith a pin at 15 locations. Nevertheless, the vapour pressure inside the hit was still too high and the fkuit burst open and started bleeding even at the low power levels. Even the bemes dried in the convective conditions at reasonably low temperatures failed to dry. Rather, they lost their colour and became very soft. Under these conditions however, they did not burst. Since the products were so poor, no quality assessments were made. These initial experiments led to the idea of slicing and pureeing, to overcome the problem of skin resistance. Moreover, the fivit freeze-drying industry does market strawberry pieces and slices, whereas fmit leathers (dried puree strips) are also sold to hikers and campers for sustenance.
4.2.3 Drying of sliced strawberries 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 lOOg 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 208, 30% and 40% (Power levels 2, 3 and 4) were each studied at inlet air temperatures of 30°C, 35°C and 40°C with three replicates. A duty cycle 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 kgkg (dry basis) had been reached, which is the moisture level of commercially dried fmits samples (Bains et al., 1989).
4.2.4 Drying of strawberry puree For the runs with the pureed strawberries, good quality strawbemes were selected from the cold storage and lefk to reach ambient temperature (2h). About 100 g of strawbemes were pureed using a domestic blender (Philips Electrooics Ltd, mode1 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 since these pads allow the air to pass through them while retaining the puree for dehydration. The same inlet air conditions were used as for the slices and the target moisture content was also 0.2 kgkg (dry basis). However, the duty cycles here were 10, 20 and 30% since it was expected that the purees would dry much faster than the slices. The dry basis moisture content was calculated every 20 minutes time in al1 trials.
4.2.5 Freeze-drying About 2 kg of strawberries were selected from the storage and dlowed to attain the ambient temperature. Half of the strawbemes were cut into slices and the other half were pureed in the blender as explained above. The sliced and pureed samples were transferred into trays after obtaining their initial mass. 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 descnbed in Chapter 3. In these preliminary experiments, the sensory evaluation was dflerent than that described in Chapter 3. Here, the product was evaluated by a panel of five judges ody. 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 uniform in colour with no serious defects such 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". 4.3 RESUZTS AND DISCUSSION The initial moisture content of the strawberry slices and puree varied from 86 to 92% (wet basis, kg water/ 100 kg slices or puree), the average being 89%, which corresponds to an initial moisture content of 8.09 (K, kg waterkg dry matter). The temperature of the inlet air varied + 1°C and the relative humidity by 2 1.5%. The inlet air velocity varied within 2% of the set inlet velocity of 2 m s".
4.3.1 Drying of slices Table 4.1 shows that the influence on drying times due to inlet air temperature (30,35 and 40°C) was slight cornpared to power level, although statistically significant (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 across the inlet air temperatures was about 50% of that for convection. Although drying times were significantly shorter at power levels 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 microwave equipment. Even had the quality been good, there would have been little value in increasing the applied microwave energy from 30 to 40% since the time savings was but 1 minute on averages of the order 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 duty cycle decays rapidly because the drying efficiency of the applied energy decreases. This is likely due to an over-production of heat relative to the rates of moisture removal from the surface and difision fiom the interior to the surface. Table 4.1. Drymg tirnes required for slices to obtain a moisture content 0.2 kgkg (db) at different power levels and inlet air temperatures. Drying time in minutes Power level
The dry basis moisture contents of slices were plotted against time for inlet air temperatures of 35°C (Fig 4.1) for power levels 0, 2, 3, and 4.
Table 4.2 Mean drying time of slices at dflerent 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 dBerent at the 0.05 level
Table 4.3 Mean drying time of slices 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 difîerent at the 0.05 level
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 drymg times was significant, but much srnalier than that of power level (Table A.4.2); moreover, it was the 40°C inlet air temperature that improved the drymg rate, with no significant improvement from 30 to 35OC (Table 4.5). A 10% duty cycle did not yield much improvement over convection, dthough the irnpmvement was significant (Table 4.6). The improvements at 20% and 30% duty cycles were to 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 kgkg (db) at difl'erent power levels and inlet air temperatures.
Drying time in minutes Power level 30°C 35OC 40°C Table 4.5 Mean drymg time of puree at different air temperatures
-
Air Temp OC Mean (min) Duncans Grouping
Means with the same letter are not significantly Merent at the 0.05 level
Table 4.6 Mean drying time of puree at different power levels
Means with the same letter are not significantly different at the 0.05 level
The drying kinetics of the puree are presented in Fig. 4.2. Although fibreglass pads were used as a support for the puree in the rnicrowave oven, it was dficult to separate the dried puree from the pads. Hence, it is recommended to use some other type of puree holder which could be more easily separated from the product . A cornparison of the drying kinetics of slices and puree is given in Fig. 4.3 for power level2 and an inlet air temperature of 35°C. It is clear from this figure that the puree dries faster than slices. This might be due to the thickness of the drying product. The slices were 1 cm in thickness, Time (Minutes)
Figure 4.1 Dehydration of strawberry slices under microwave power levels 0,2,3 and 4 at inlet air temperatures 35OC.
Figure 4.2 Dehydration of strawberry puree under microwave power levels O, 1, 2, and 3 at inlet air temperature 35OC. Time (Minutes)
Figure 4.3 Dehydration of strawberry slices and puree under microwave power ievel2 and inlet air temperature 35OC. but the puree tended to spread on the pad and got thinner. For a fhirer cornparison, the pad should have been inserted into a glass holder with no bottom and raised borders so that a 1cm 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 dserences in rehydration ratio due to the different drying methods (Table A.4.3). The means separation test (Duncan's) showed that there were no significant differences between rehydration ratios of the slices due to the applied microwave power. However, both the fieeze-dried and convection-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 (since it doesn't shrink).
Table 4.7. Rehydration ratio and rehydration coefficient of microwave dried and freeze dried strawberry slices. Treatmen t RH ratio COR
- -- Convection dned 2.91 B 0.29 Microwave dned at 10% duty cycle 2.82 B C 0.28 Microwave dried 20% duty cycle 2.72 C 0.27 Microwave dned at 30% duty cycle 2.71 C 0.27 Freeze dried 3.80 A 0.38
Means with the same letter are not signüicantly different at the 0.05 level.
64 The rehydration characteristics of the microwaved and freeze-dried purees could not be determined because the product would corne apart in boiling water and could not be separated well even by filtering.
4.3.4 Quality and colour analysis
The quality judgement 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 tirne is lower. This may be due to higher product temperatures as the power level increases. The microwave dned product is clearly inferior to the freeze-dried product. The microwave-dried slices at a 20% duty cycle are about the same quality as the convection-dried product at 35OC, but the microwave-dried puree is of lower quality.
Table 4.8 Means separation by Duncan's new multiple range test for the quality assessrnent 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
Freeze- 8.60 A 8.60 A dried
Duncan groupings: Means with the same letter are not significantly different 63.5 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 (ah) for microwave (MW) and freeze dned (FD) strawberry slices and puree. Treatment Means of (ah) values Grouping 1. Fresh (slices) 1.21 B 2. FD (slices) 2.15 A 3. FD (puree) 2.13 A 4. MW 2O%(slices) 1.08 B 5. MW 20% (puree) 1.21 B
Means with the same letter are not signifïcantly different at the 0.05 level. were brighter red (higher ah ratios) than both the microwave dried bemes and the fresh bemes. Freeze-drying allows the retention or prevents the modification of compounds involved in pigmentation while at the same time concentrating them due to dehydration. This leads to 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 cannot be properly dried in a microwave field since they swell, burst and bleed. Convection drying at low temperatures is very slow, and leaves the bemes nilnerable to spoilage. Puncturing the strawbemes did not help in reducing bursting and bleeding. The drying rate was not improved by raising the inlet air temperature from 30 to 35%, but was significantly improved with a further increase to 40°C. The results of these preliminary experiments indicated that although there is a great potential time savings in drying strawbeny slices and puree with microwaves compared to either convection or freeze drying, there is no advantage in tems of quality retention or rehydration characteristics. Furthemore, it is important to restrict the rate of excitation by microwaves since buming may occur. This restricts the time- savings possible. Freeze-dried strawbemes were rated of higher quality for al1 the characteristics tested. 4.5 Connecting statement to chapter 5 From these experiments it can be 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 of obtaining 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. CWTER V MICROWAVE DRYING AND S-GE OF PRETREATED WHOLE STRAWBERRIES
5.1 INTRODUCT~N In the previous chapter, it was found that microwave drying was quite advantageous if bemes 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 also swell, burst and bleed. High heating rates of microwave drying, the development of interna1 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, drymg is usually preceded by dipping in fluids that help to break down the waxy cuticle and sigmîicantly enhance the drying process. Treatments with either alkali emulsions of ethyl oleate CH,(CH,),CH, (EO),olive oil or potassium carbonate are al1 effective in increasing the drying rates of fniits having a waxy cuticle (Saravacos et al. 1988, Ponting and McBean, 1970; Raouzeos and Saravacos, 1986; Rahman and Perera, 1996; Hamngton et al. 1978). Stress cracking can also be reduced by decreasing the resistance to moisture transfer, and such treatments have also been tested on corn (Suarez et al. 1984; Williams, 1989). Thus, it was proposed to study the possibility that pretreatments with chernical solutions used in industry for drying (ethyl oleate and sodium hydroxide) could enhance the drymg of whole strawbemes in a microwave field, possibly leading to a higher quality product than possible without this treatment. Convection drymg was used for cornparison of drymg rates and fieeze-dried strawbemes were used as a standard for quality assessrnent 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 mode1 developed by Tulasidas et al. (1997) had been entertained. Although the research was later oriented to osmotic dehydration of strawberries and blueberries, rather than to extensive simulations, it was felt that the results of the shrinkage work should be included in the thesis.
5.2 IMATERLALS AND METHODS 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 of whole strawberries at room temperature were selected and dipped in the chernical solution for 1minute (Suarez et al., 1984). The treated fniits were removed form the solution and washed in tap water to 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 to determine initial moisture content before drying experiments.
5.2.1 Microwave Drying The microwave drying setup used in this study is shown in Fig 3.2. and explained in Chapter 3. The microwave settings used in combination with the dipping treatments were O 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 controls was also nui at each of these settings. This mode of operation is substantially different than that used in the preliminary experiments since the sample is continuously exposed to microwave energy, rather than in duty cycle mode, and the field intensity is much lower. 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 target moisture content (0.2 kgkg). Each treatmenthicrowave combination was replicated thrice.
5.2.2 Freeze Dryiag About 2 kg of strawbemes were selecteci 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 difl'erent proportions, as described above. The treated fruits were transferred into trays and freeze-dried until the moisture reached 0.2 kgkg, which took 24 h or longer. The freeze-dried bemes were compared with microwave dried bemes on the basis of rehydration, texture and colour according to the methods described in Chapter 3.
5.2.3 Shrinkage of strawberries About 5 or 6 bemes around 100 g mass were selected and their exact weight was determined using a digital balance to an accuracy of 0.001 g (Mettler PE 200, balance). The initial moisture content was determined in a vacuum oven at 70°C for 6 h. Their initial and final mass afier drying was determined. The initial volume of the fkuits were measured using a displacement method in toluene (Tulasidas, 1994). The fniits were then dipped in a solution of 2% ethyl oleate and 0.545 NaOH solution for one minute, washed with water and surface dried with a gentle air stream. Sarnples were then dned to five moisture ratios (dm,, = 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. niesurface (Tl)and centre (T,) temperatures of the strawbeny fivit were monitored during drymg at power levels 0.1 W/g and 0.2 W/g using digital fiberoptic thermometers (Nortech Fibronic Inc. Canada).
5.2.4 Relative Drging Rate A relative rate of drying was defined as CWeitz et al., 1989)
where, 4 is the relative drying rate, t, is the drying time of the control sample (min),and t, is the drying time of the treated sample (min). Drying times are defined as the time to reach a final moisture content of 0.2 kg/kg (Dm.
5.3 RESULTS AND DISCUSSION 5.3.1 Drying kinetics The data analysis 'indicated 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.5.1). 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 to the same extent as the higher concentrations (Table 5.11, 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 l), it is possible that the 1%EO solution was of sufficient strength to completely dissolve the waxy cuticle. The relative drying rates of mimwave and convection-dried strawbemes are presented in Table 5.3. Convection drymg derdipping
Table 5.1 Mean drymg time of strawberries at Merent ethyl oleate (EO) concentrations. Treatment Mean (min) Duncans Grouping Untreated 117.778 1%EO 96.111 3% EO 95.000 2% EO 93.333 ------Means with the same letter are not significantly different at the 0.05 level
Table 5.2 Mean drymg time of pretreated strawberries at different power levels. Power W/g Mean (min) Duncans Grouping
. ------Means with the same letter are not signifïcantly 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 &ter 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 of the dryuig kinetics of convection and microwave-dried bemes is shown in Figure 5.1. Here, the case of convection drying with no pretreatment (uppermost cume) is presented for cornparison to indicate just how slow drying can be under such conditions, which is why convection drying is not usually used in industrial drying of berry fhits. 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, at this power level, the product had burnt spots and was not of marketable quality. Convection drying fier dipping in 2% EO with 0.5% NaOH gave a rate of 1 .O9, only a slight improvement over the control. No cornparison could be made between drying rates under microwave and convective conditions of untreated bemes because the convective
Table 5.3 Relative drying rate of strawterries under dflerent treatments in reaching a MC of 0.2 kgkg (DB). Treatments Drying time. Relative min Wing Rate No Pretreatment Convection MW 0.1 w/g MW 0.2 w/g MW 0.3 W/g 1% EO + 0.5% NaOH Convection' MW 0.1 w/g MW 0.2 w/g MW 0.3 W/g 2% EO+ 0.5% NaOH Convection MW 0.1 w/g MW 0.2 w/g MW 0.3 Wlg
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 drymg 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 drymg rate for microwaved strawberries increased with power level whether the berries were treated or not, as expected, with the usual restriction that buming occurred at the higher power levels (50.3 W g-'1, untreated fruits burst and bleeded.
5.3.2 Rehydration ratio Table 5.4 shows the rehydration ratio's of different treatments.
Table 5.4 Rehydration ratios of microwave and freeze-dried strawbemes with Merent chernical pretreatments. Pretreatment Rehydration Ratio ------1. 2% Ethyl Oleate and 0.5% NaOH Convection dried Microwave dried with 0.1 W/g Microwave dried with 0.2 W/g Microwave dried with 0.3 W/g 2. Freeze dried Untreated 1%Ethyl Oleate and 0.5% NaOH 2% Ethyl Oleate and 0.5% NaOH 3% Ethyl Oleate and 0.5% NaOH
Statistical analysis (Table A.5.2) showed that these were related to both the concentration of EO used and the drying regime - convective, freeze, lima min
"Convut + 0.1 Wlg * 0.2 Wlg * 0.3 Wlg * Conv-No Prt Figure 5.1 Convective and microwave drying of strawberries treated with 2% ethyl oleate and 0.5% sodium hydroxide, at different power levels.
O 30 60 90 120 150 180 tima min Figure 5.2 Microwave drying of strawberries at 0.2 W/g power with different pretreatment levels. microwave at different intensities. There was no interaction between the concentration of EO used and the drymg regime. The means separations (Table 5.6) indicates that the influence of ethyl oleate concentration on rehydration was modest, with no particularly dear explanation for the order shown.
Table 5.5. 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 difTerent at the 0.05 level
Table 5.6. Rehydration ratios of strawbemes at difTerent ethyl oleate concentrations.
-- pp Oleate % Mean Duncan Grouping 2 2.4060 A 3 2.32333 AB 1 2.25067 B
Means with the same letter are not significantly diEerent at the 0.05 level
The convection-dried product exhibited (Table 5.4, 5.5 and 5.6) the lowest rehydration ratio. The bemes subjected to 0.1 and 0.3 W g-' exhibited si@cantIy higher rehydration than the former, and significantly lower rehydration than the remaining classes; there was no ~i~cant difference between the fkeeze-dried bemes and those dried with microwave energy at 0.2 W go'. Nevertheless, the rehydration of the freeze-dried samples were slightly higher. Inspection of the original data showed that the rehydration ratios of the bemes microwaved at 0.2 W g-' and treated with 2% EO were the highest, and that the three replicates were as consistent as the replicates of the other groups. Thus, this quasi-maximum was not due to an one extreme data point. It is interesting to note that the rehydration ratios of sliced bemes (Chapter IV) were higher than those of the whole pretreated berries, whether they were microwave or freeze-dried. This might be due to the larger surface area of the dned slices, which were boiled for the same amount of time as the whole bemes. 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 dEerent combinations of pretreatment and drying regime. There were significant differences in toughness due to the concentration of EO, to the drying regime, and to the interaction of the two (Table A.5.3). The toughest were associated with the 2% EO dip and with the bemes microwaved at 0.1 W g-' (Table 5.8 and 5.9). The softest were those receiving the greater intensities of microwave power. Although al1 of the dned fniits were soR and quite easy to chew, one might conclude that the higher intensities led to some cooking of the product. Which might be responsible for lower toughness of the bemes microwaved at 0.2 and 0.3 W g-', compared to berries dried by dl the other regimes). Table 5.7 Texture (Toughness) of microwave and freeze dried strawbemes treated with different ethyl oleate (EO) concentrations. Pretreatment Toughness MPa 1. Microwave dried With 2% EO + 0.5% NaOH Power level0 Power level 0.1 W/g Power level 0.2 W/g 2. Microwave dried with 1%EO + 0.5% NaOH Power level0 Power level 0.1 W/g Power level 0.2 W/g 3. Freeze dried No pretreatment 1% EO + 0.5% NaOH 2% EO + 0.5%NaOH 3% EO + 0.5% NaOH
Table 5.8 Toughness of strawbemes at different EO concentrations. Oleate % Mean Duncan Grouping
Means with the same letter are not significantly Merent at the 0.05 level Table 5.9 Toughness of strawbemes according to 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 O. 14556 E
Means with the same letter are not significantly different at the 0.05 level
5.3.4 Colour Merence with respect to £resh berry
Table 5.10 Colour measurements of microwave and freeze dried strawbenies under different pretreatments.
Treatment L a b f%, 1. Fresh F~it(Target Colour) 2. MW dried with 18EO +0.5% NaOH Power level O Power level 0.1 W /g Power level 0.2 W/g 3. MW dried with 2% EO +0.5% NaOH Power level O Power level 0.1 W/g Power level 0.2 W/g 4. Freeze dRed No pretreatment i% EO + 0.5%NaOH 2% EO + 0.5% NaOH 3% EO + 0.5% NaOH
MW (Microwave), EO (Ethyl Oleat e), NaOH (Sodium Hydroxide).
80 Table 5.10 shows the colour differences between treatments. The convective dried strawbemes had a higher difference a,,of 14.54 compared to the fiesh kitcolour and were darker red. When microwave- dried and freeze-dried fiuits were compared, the freeze dned product was generally darker. This might be due to low pressure and low temperature in freeze dryuig which consolidates the pigments and increases the colour. Unfortunately, only the averages for each treatmenuregime combination are available. The original data including replicates were lost, thus not permitting a statistical analysis to be executed.
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:
Assuming that the ellipsoid shape of the berries can be approximated by a spheie of equivalent volume V, an equivalent diameter De was calculated using the relation De= (6Vln)'" (Saravacos and Raouzeos, 1986). The mean equivalent diameter of the bemes was determined at different moisture contents, and correlated with the moisture ratio. Changes in volume and density of the strawbemes during microwave drying at power levels of 0.1 and 0.2 W/g are shom in Table 5.11. The resdts show that the volume reductions are quite similar for the two power levels. The density changes are quite similar, diîferences fkom one case to the other probably being due simply to exparimental error. Maisture ratio M:Mo
Figure 5.3 Microwave drying of strawbemes at power levelO.l W/g, correlation between shrinkage ratio and moisture ratio.
Moistum ntio Mo
Figure 5.4 Microwave drying of strawbemes at power leve10.2 Wlg correlation between shrinkage ratio and moistue ratio. Figure 5.5 Microwave drying of strawberries at power level 0.1 W/g, correIation between equivalent diameter and moisture ratio.
Moistum ratio WMo Figure 5.6 Microwave drying of strawbemes at power level 0.2 W/,correlation between equivalent diameter and moisture ratio. Figures 5.3 and 5.4 show the relationships between shrinkage ratio and moisture ratios for the two power levels. The relationships are linear. The relationships between equivalent diameter De and the moisture ratio (Figs. 5.5,5.6) were best fit by reciprocal logarithmic equations of the fom:
Table 5.11. Shrinkage ratio, equivalent diameter and change in volume during microwave drying of strawbemes at 0.1 W/g (above) and 0.2 W/g (below). Initial Final Change M/Mo 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
-- -- The constants for the equations obtained for both types of relationship are given in Table 5.12, along with the coefficient of determination CR2). The variations in temperature at the surface and the centre (T,and T$) during microwave drying at power levels 0.1 Wlg and 0.2 W/g while the strawbeny was shrinking is presented in Figures 5.7 and 5.8. In the fvst 30 min, at both the power levels, the temperature Molature kglkg (08) Wrtun kglkg (DB) 4 d d P O I ONrnrnON
Temp C rise is similar for Tland T,. ThereaRer, the temperatures at the centre (T,) tend to be higher than at the surface (TJ. This is probably due to the difference in moisture content between the centre and the surface,
Table 5.12 Constants for linear equations describing shrinkage ratio of strawbemes, and for reciprocal logarithmic equations (RL) describing equivalent diameter, as functions of the moisture ratio under microwave drying at power levels 0.1 and 0.2 W/g.
Power Correlation Mode1 Constant Coefficient R2 wk A B Value 0.1 Sbvs. M/Mo Linear 0.0943 0.8870 0.9989 0.2 Sbvs. M/Mo Linear 0.1329 0.8325 0.9980 0.1 Devs. M/Mo RL 0.3067 -0.1071 0.9995 0.2 Devs. MIMo RL 0.3157 -0.0899 0.9968
"RL" Reciprocal loganthmic mode1 which leads to a gradient in heat generation and a resulting temperature gradient. Furthemore, moisture at the surface is continuously being convected away. It is interesting to note that the temperature differences (T, - T,)are usually greater at power levelO.l W/g than at 0.2 W/g. This might be due to higher moisture content at the centre in the case of 0.1 W g-', due to slower rate of drying. Mer the rapid initial temperature rise in the first 30 min, the temperatures at the centre tend to level off for about one hou, and then rise again. The level period might be associated with a relatively constant removal of free moisture, whereas the oscillations and increases apparent later on may be associated with the interplay of different heating mechanisms and perhaps the inhibition of moisture 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 strawbemes are very high (80 - 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 almost dficult to imagine that the bemes would dry rather than cook in the high temperatures resulting from the microwave regimes. On the other hand, since the bemes have been treated, there is a rapid moisture loss at the same time as the temperature rises, and the final product dries. However, the quality of microwaved, dipped bemes is not totally Iike fieeze-dried beny.
From this experiment, it can be concluded that dipping in an EO/NaOH solution increases the drying rate, and does have some effect on quality. The microwaved product is similar in quality to the fkeeze-dried product if low power levels are used, which constrains the rate advantage to an upper limit. The rate advantage is best enhanced by treating the berries with EOMaOH solution of 1%EO with 0.5% NaOH prior to 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 interna1 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, fhit quality detenorates. There may be room for fine-tuning at rates intermediate to 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 of EO higher than 1%,combined with 0.5% NaOH do not help in accelerating the drying rates of whole strawberries. This may be due to the chernical kinetics under the conditions of dipping, which were restncted to 1 min at roorn temperature. The highest difference of colour of 14.54 was associated with the convectionally dried bemes with 1%EO + 0.5% NaOH pretreatment. The aroma of the microwave-dried strawbemes was lost. 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 linked to the dipping treatments and the type of drying regime. The shrinkage experiments led to empirical relations describing shrinkage ratio and equivalent diameter in terms of the moisture ratio. The equations were excellent fits to the data in all cases and could be used in comprehensive drying models 5.5 Connecting statement to chapter 6 This part of the study (Chapter 5) had shown that pre-treating the strawbemes with ethyl oleate and sodium hydroxide helps in drying them whole with microwave power. The toughness, colour and rehydration rate of the product can be very close to those of the freeze dried product. However, the aroma of strawbemes in the dried hit was lost. The literature suggests that the aroma can be retained by osmotically treating the miits before subjecting the fruits for heat treatment. Hence it was decided to pretreat with EO and NaOH, osmotically dehydrate the strawbemes 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 to high interna1 temperatures witnessed in the data, possibly leading to an equivalent-to- freeze-dried product. OSMOTIC AND MICROWAVE DRYING OF STRAWBERRIES
The preliminary studies on microwave drying of strawberries 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 canot be obtained by the methods tned. Since partial dehydration in an osmotic solution has been reported to improve fiavour retention in convection-dried hits (Jackson and Mohammed, 1971; Ponting, 1973; Dixon et al, 1976; Flink, 1979; Voilley and Simatos, 19791, it was decided to investigate the possibility that this technique could be used as a pretreatment for microwave drymg to improve the quality of the finished product . The objectives of this study were to determine the osmotic dehydration rate in sucrose of strawbemes pretreated with ethyl oleate and sodium hydroxide, to compare the microwave, convection and freeze drying rates of the osmotically dehydrated bemes, and to evaluate the quality of the dried product.
6.2 MATERIALS AND METHODS
6.2.1 Dipping treatment In Chapter 5, it was found that the concentration of EO 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 24 EO concentration and resulted in a toughness sirnilar to that of the freeze-dried product. It was therefore decided to 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:l or 4:l fruit to sucrose (F:S) at room 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 room temperature to remove the syrup fkom the hitsurface. Surface water was removed as before and loss in mass due to osmosis was determined.
6.2.3 Drying experiments The strawbemes 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 same equipment as described in Chapter 5. There were three replicates of the experimental conditions summarized in Table 5.1. The experiment was a 2x3~3factorial, with 2 fruit to sugar ratios, three inlet air temperatures and three drying regimes (convective, microwave at 0.1 or 0.2 W g"). Three samples from each hitto sugar ratio were freeze- dried for cornparison in the quality assessments. The dried samples were cooled to ambient temperature, packed in polyethylene covers and stored in the cold room at 1°C for rehydration, texture, color and sensory evaluation studies.
6.2.4 Quality assessrnent Assessrnent of the quality of the dried products was based on chromacity, rehydration ratio, toughness, and sensory evduation including flavour and amma by a panel of judges, all as described in the appropriate sections of earlier chapters (3 and 5).
6.3 RESUZTS 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 of moisture removed due to the fruit to sugar ratio, to the EONaOH dip and to the time of osmotic 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 bemes have been dipped, and when the dehydration is continued for a longer time. However, the increase due to a greater amount of sugar is small and doesn't justify its use.
Table 6.1 Means separations by Duncan's test for rnoisture removal at the two fniit to sugar ratios, treatment with EO/NaOH and time of dehydration.
F:S Ratio Means Dipping Means Duration Means Treatment (hl 2% EO 0.5% NaOH 29.95 A
Means with the same letter are not significantly diff'erent at the 0.05 level.
The moisture removals due to osmotic dehydration of untreated strawberries and of strawbemes pretreated with 2% ethyl oleate (EO)and 0.5%sodium hydroxide (NaOH) and osmotically dehydrated in a fruit to sugar ratio of 3:1, are shown in Figure 6.1. The time advantage due to the pretreatment is clear, dipped strawberries losing far more moisture than undipped strawberries. This is due to dissolution of the waxy cuticle by ethyl oleate and the creation of micropores in the skin by sodium hydroxide. Treated bemes reached a wet basis moisture content of about 62% in the first 24 h, and 53% after 48 hours. In contrast, the untreated berries dropped only to 85% m.c. in the first 24 h. However, there was some discoloration of the strawbemes during osmotic dehydration. Furthemore, off-odours developed by 36 hours, which is the reason why only bemes dehydrated for 24 h were used to compare drymg 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 dned under various regimes are shown in Table 6.2. A preliminary analysis (Table A.6.2) indicated that the inlet air temperature, and the power level (including convection, or O W g-'1, significantly influenced the drying times, with increases in both reducing the drying time. There were also significant interactions between fruit to sugar ratio, power level and temperature, as well 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 inlet air temperature. For some settings of power level and fruit to sugar ratio, the drying time is longer at 45OC than for the 35°C counterpart. The data were therefore reanalyzed, excluding the convection situation, to see whether the inlet air temperature significantly dected drying time in the microwave regimes. There was no such effect, contrary to the findings in the preliminary studies which had shown a signûicant temperature effect when air inlet temperature was increased to 40°C from 35OC. 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 of temperature should have been verified in the experiments on EO concentration. Only power level and fitto sugar ratio were signuicant main effects when only the microwaved samples were considered. There were also significant interactions between inlet air temperature and power level, as well 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 of osmotically dehydrated strawbemes. Treatment Drying Time Relative Air F:S PL, Wlg Min Drying Rate Table 6.3 shows that the 3:1 miit to sugar ratio led to significantly longer drying times than the 4:1 ratio, which one would not necessarily expect, since after 24 h of osmotic dehydration, the moisture content of the strawbemes infused at a 3:l 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 bemes infused at a 3:l ratio, resulting in a higher osmotic tension in the fruit which inhibited the outward diffusion of moisture, as suggested by Rahman et al. (1991)and Sankat et al. (1996)in air drying studies of osmotically dehydrated hit. Unfortunately, this could not be verified in this study because samples of the osmotically dehydrated samples (24 h) were not taken for dry weight rneasurements. This also means that the moisture determinations are slightly off since the small dry matter increase was not taken into account.
Table 6.3 Duncan groupings for mean drying times at the two temperatures, fmit to sugar ratios and power levels.
Temp Mean F:S Ratio Mean Power Mean OC Level W g-'
Means with the same letter are not significantly difT'erent at the 0.05 level.
Figure 6.2 clearly indicates the rate advantage due to microwaves. The fruit-to-sugar ratio has some influence on microwave drying, the 3:l ratio leading to somewhat lower drying rates. However, these calculated dinerences are largely insignificant based on the kinetics shown in Figure / / I r 12 24 36 48 Time ln Houri Figure 6.1 Osmotic dehydration of treated and untreated strawberries
Minuter
Figure 6.2 Mïcrowave àrying of osmotically dehydrated strawberries at dinerent power levels. 6.3. In this figure, the curves at different F:S ratios for the same power level are essentially coincident. Figure 6.4 shows the drying rates versus moisture content for the 4:l treated berries at the two MW power levels and for convective drying, at both inlet air temperatures, also confjirming the general absence of influence of inlet 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 sigmficant advantage to pretreating the bemes for the osmotic dehydration stage and that microwaves tremendously enhance the drying rate compared to convection. However, the quality of the final product must be considered. In this study, we used the freeze-dried pmduct as a standard of quality.
6.3.3 Empirical model of finish drying with microwaves The following approach was taken to generate an empirical model to describe microwave and convective finish drymg of strawbemes dehydrated in a 4:l fitto sugar ratio. For each replicate, of convection or microwave finish drying, an exponential curve of the form of M = M,, e"' was fit to each set of dry basis moisture contents obsewed in the trials (Mis the dry basis moisture content; M,, is the initial moisture, k is a rate constant and t is time). The rate constants thus obtained (R2a.99in al1 cases), were then expressed in terms of the power level using a best-fit (using Curvexpertm software version 1.3) second-order linear equation of the form:
where, PL is the microwave power level (O, 0.1 or 0.2 W g-') and the bi are regression parameter estimates. This pmcedure led to the following equation to predict moisture content as a function of time and power level: Minutas
Figure 6.3 Microwave dryxng of strawbemes at two -power levels dehydrated at two osmotic levels (F:S). O 454 - PL=O wl.)p+ au1WIo il Pk~.2 wlp PL=O WIo *PL=O.l Wlg +PL=0.2 WIg
Moliture Content kghg (08) Figure 6.4 Microwave drying rate of osmotically dehydrated strawbemes at difFerent power levels and inlet air temperatures of 35°C and 45°C. 6.0 I -Model -a- Exp t
1 0.0 ! r a O 120 240 360 400 600 720 Time, min
Figure 6.5 Predicted moisture content of strawbemes by the exponential mode1 compared with the experimental values (PL=
L Model 6.0 1 - - a Expt L
O 20 40 60 80 1O0 120 140 Time, min Figure 6.6 Cornparison of the moisture content of strawbemes predicted by the exponential mode1 and the experimental values (PL=0.1 W/). Tirne, min Figure 6.7 Predicted moisture content of strawberries by the exponential mode1 compared with the experimental values (PL= 0.2 W/g). MC(t,PL)=5.98 exp ((-0.0025-0.0413*~~+0.0785*~~~)*t)
(6.2) The above equation was then used to generate predicted moisture contents for each power level, 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-' over most of the drying period. In the case of 0.2 W g-' moisture content was grossly underestimated for the fwst 45 minutes of drying, but tended towards the experimental data near the end of the drying period. As the power level goes up the estimation of the moisture by the mode1 slightly falls, 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 to air inlet temperature or fmit to sugar ratio among the microwaved strawberries. The range of rehydration ratios (Table 6.4a), for the experimental bemes (1.64 to 2.12) includes that of the freeze-dried product (1.88 to 1.90). The highest rehydration ratios were obtained in the 0.2 W g-' regime (Table 6.4b), and both microwave regimes led to better rehydration than the convection-dried osmotically dehydrated berries. The rehydration ratio of fkeeze-dried bemes was not significantly different from those dried in the 0.1 W g-' regime. The low rehydration potential of the convection-dried berries could be due to case-hardening. These results indicate that microwave dried bemes are equal or better than freeze dried bemes 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 6.4a Rehydration ratio and texture measurements of osmotically dehydrated and microwave dried strawberries. Treatment Rehydration Texture Air F:S PL,W/g Ratio (Toughness) MPa
Freeze 3:l .. Dried 4: 1 - Table 6.4b Means separation of rehydration ratios at the experimental temperatures, hitto sugar ratios and power levels.
Temp Mean F:S Mean Power Level Mean OC Ratio W g-' 35 1.85 A 4: 1 1.83 A 0.2 2.01 A
Means with the same letter are not significantly different at the 0.05 level.
Table 6.4~shows that the toughness was influenced by inlet air temperature and drying regime, but not by fruit to sugar ratio. Application of microwave energy seemed to soften the bemes, prgbably due to the greater interna1 heating cornpared to the convection case. The freeze-dried bemes were significantly tougher than the thermally dried ones.
Table 6.4~: Duncan's groupings for mean toughness at two temperatures, hit to sugar ratios and drying regimes.
Temp Mean ES, Mean OC 1 Ratio
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 influentid, but that both fhit to sugar ratio and power level significantly affected this parameter. Table 6.5b shows that the greatest colour clifference with the fresh bemes was obtained at the highest power level and at the higher Mt to sugar ratio. The greater heating should lead to faster darkening, with some possibility of occurrence of imperceptible bumt spots. It is Micult to explain why the bemes infused with less sugar should exhibit a significantly larger colour difference, particularly since this group dried more quickly than those dehydrated at a 3:l ratio. It is interesting to note that the freeze-dried bemes were the lightest, and closest to fresh berry colour.
Table 6.5a Color measurements of osmotically dehydrated and microwave and freeze dried strawberries. Treatment L a b AL Air F:S PL, Wk - Fresh fmit (target color)
Freeze 3:l - Dried Table 6.W Means separations of colour ciifferences due to drymg regime, miit to sugar ratio and inlet air temperature.
0.2 W g-' 21.243 A 0.1 W g-l 18.731 B Convection 17.768 B Freeze-dried 10.231 C . .- Means with the same letter are not significantly different at the 0.05 level.
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)
Freeze 3: 1 . Dried Freeze 4:l - 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 of judges had difficulty in distinguishing the drying conditions used, since there were no significant dserences in rating over the various treatment combinations. Interestingly, the convectively dried product scored highest. The fact that some of the rnicrowave-dried bemes 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 tu be necessary to obtaining a microwave- dried strawberry of quality close to that of the 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 terms of time and power levels (including O W g*'), fit the data reasonably well, particularly for low moisture levels near the end of the drying period. This model could be venfied for intermediate power levels. OSMOTIC AND MICROWAVE DRYING OF BLUEBERZUES
7.1 RVZROD UCTION Since the experiments on microwave-drying of osmotically dehydrated strawberries were encouraging, it was decided to focus attention on blueberries, a berry fruit whose production and popularity are increasing. North America is the world's major supplier of blueberries, accounting for about 90% of production. The objective of this study was to fmd the osmotic dehydration rate of bluebemes pretreated with ethyl oleate and sodium hydroxide. The osmotically dehydrated bluebemes were dried and studied under convective, convective-microwave and freeze drying. Their drymg rates, rehydration ratios, colour, texture and sensory evaluation were compared.
7.2 MATERIALS AND METHODS The materials and methods used in the blueberry experiments were the same as those used in the experiments on osmotic dehydration and finish drying of strawbemes with microwaves. The levels of pretreatment with ethyl oleate and sodium hydroxide, fruit to sugar ratio in osmotic dehydration and air temperatures and the power levels used in the microwave drying were same as that of osmotic dehydration of strawberries (chapter 6.).
7.3 RESULTS AND DISCUSSION 7.3.1 Osmotic dehydration The details of osmotic dehydration at different periods of time are illustrated in Fig 7.1. It is obvious that the pretreatment is beneficial in osmotic dehydration of bluebemes, 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 bluebemes in osmotic dehydration under different F:S, pretreatments, and time.
. TRT Mean Time Mean
2% EO 15.11 A
No, EO 3.36 B
Means with the same letter are not significantly different at 0.05 level
The osmotically dehydrated in a 3:l ratio bluebemes treated with 2% ethyl oleate (EO)and 0.5% sodium hydroxide (NaOH) solution lost 20 % moisture (WB)in 24 h compared to untreated bemes which lose about 4 %. In 48 hours, treated bemes give up 24.62 % moisture compared to untreated 7.36 %. These figures are substantially lower than the moisture losses over time of the strawbemes. Table 7.1 indicates that the moisture loss in bluebemes was significantly higher when more sugar was used (3:l fruit to sugar ratio); however, this signifiant difference is very small(0.68 rn.c.1 and does not justify the additional quantity of sugar needed. 7.3.2 Drying kinetics The drying kinetics of blueberries at microwave powers of 0.1 W/g, 0.2 W/g and convective drying are shown 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 higher temperature and at the higher power level. As in the case of strawbemes, the bluebemes that had been osmotically dehydrated with the greater amount of sugar dried more slowly.
Table 7.2. Duncan's groupings for mean drying time, at difEerent temperatures, F:Syand microwave power lovels. Air Mean F:S Mean MW Mean Temp°C Wk
Means with the same letter are not significantly different at 0.05 level
Microwaves reduced the drying time by 50% (4.5 h) and 678 (6 h) over the convection case which took 9 h to dry, at power levels 0.1 and 0.2 W g-', respectively and inlet air temperature of 45°C. Yang et al. (1987) also found that the drying time increases as the fruit to sugar ratio goes up, in studies of freeze-drying. Fruit to sugar ratios of 4:1,3:1 and 2:l resulted in 15 h, 18 h and 24 h drying times to reach a moisture content of 16 %. Relative drying rates for different treatments shown in Table 7.3 varied from 1 to 3.83, these being in a lower and much smaller range than the relative drymg rates of strawbemes under the same conditions. Table 7.3 Drying tirne and relative drying rate of bluebemes osmotically dehydrated and microwave dried at different microwave power levels,
Treatment Drying Time Relative to MC 1 Drying Rate
Air temp F:S Power level, OC Wk
Control Figure 7.1. Osmotic dehydration of untreated and treated bluebemes at different durations of the.
6 '- Pt=O + PLzO.1 Wfg * PL10.2 WIg
Tîme min Figure 7.2. Micmwave drying of osmotically dehydrated bluebemes at different power levels. Tlme min Figure 7.3. Microwave drymg of blueberries osmotically dehydrated with F:S,3:l and 411 and dried at power levels 0.1 and 0.2 W/g.
- PL=O Wlg + PLrO.1 Wlg * PLZO.2 Wlg
O 1 2 3 4 Molrture Content kg/kg (DB)
Figure 7.4. Microwave drying rate of osmoticdy dehydrated blueberries at different power levels. Table 7.4. Rehydration and texture measurements of blueberries osmotically dehydrated followed by rnicrowave or freeze dried, under difEerent treatments. Treatment Rehydration Texture Ratio (Toughness) MPa Air temp F:S Power level Wk 35OC 3: 1 O
Freeze 3:l - dried Freeze 4:l - dried 7.3.3 Empirical mode1 of f!inish dryhg with microwaves
The same procedure as used for modelling the finish drying of strawbemes was used here. The resulting equation was:
The fits to the experimental data are shown in Figures 7.5 to 7.7 for convective drying and drying at the two power levels. Again, the fît was very good for the convection regime, and reasonably accurate for 0.1 W g-' and 0.2 W g-'. In general, the fit for bluebemes is better than that for strawbemes. This may be due to the longer duration of drying for the bluebemes and perhaps to a greater homogeneity in the case of blueberries, which are more spherical and have no central air gap.
7.3.4 Quality The rehydration ratios of bluebemes (Table 7.4) were significantly influenced by the air inlet temperature used in rnicrowave and convective drying, and by the fruit to 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 of rehydration ratios for bluebemes was lower (1.26to 1.43) and quite nmow compared to that of strawberries (1.62 to 2.12). 1 -Mode1 + Expt
O 120 24 O 36 0 483 600 Time. min
Figure 7.5 Predicted moisture content of bluebemes by the exponential model compared with the experimental values (PL= O W/g).
'-O h I Expt
0.0 ! I O 60 120 180 240 300 360 420 Time, min
Figure 7.6 Cornparison of the moisture content of bluebemes predicted by the exponential model and the experimental values (PL=0.1 W/g). 115. I i 1 , 1 r r O 60 120 180 240 300 360 420 Time, min Figure 7.7 Predicted moisture content of bluebemes by the exponential mode1 compared with the experimental values (PL= 0.2 W/g). Table 7.5. Duncan's groupings for mean Rehydration, at different temperatures, F:S, and microwave power levels. MW Mean Power Wk
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 0.2 W/g 1.3500 0.1 W/g 1.3425 O (Convective) 1.3192
Means with the same letter are not significantly different at 0.05 level
The toughness of dried blueberries (Table 7.4) was signincantly 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 bluebemes and those dried in other regimes (Table 7.7). The bluebemes were, in general, quite a bit softer than the strawbemes, toughness being in the range 0.19 to 0.37 MPa, compared to 0.26 to 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 0.2 W/g MW 0.2875 Freeze dried 0.2767 0.1 W/g MW 0.2642
Means with the same letter are not significantly different at 0.05 level
interesting to note that the freeze-dried bluebemes were intermediate with respect to toughness, whereas in the case of strawbemes, the freeze-dried sarnples were substantially more resistant to penetration than the others. Prolamate analysis and possibly structural studies could lead to explanations for these differences. The clifference in colour (Table 7.8) between the fresh blueberries and those dried convectively or in microwave regimes were influenced only by the Mt to 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 Table 7.8 Colour measurements of bluebemes osmotically dehydrated then microwave or fieeze dried under different treatments Treatment Air temp F:S Power level Wk 35OC 3:l O
0.1 0.2 Freeze 3: 1 - dried 4: 1 - Fresh fruit (Target colour) statisticdly significant, even though the human observer would not be able to make a distinction. The freeze-dried bemes gave a significantly higher colour difference (44.61, but again, this was only 1.8 units greater than the lowest (42.8), associated with the bluebemies dried at 0.1 W g1microwave power (Table 7.9). Table 7.9. Duncan's groupings for mean colour difference, at different temperatures, F:S, and drying regimes. Air Mean Regime Mean Temp°C
Means with the same 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 strawbemes. This might explain why the colour dflerences associated with the bluebemes are substantially greater than those associated with the strawbemes. The dried bluebemes resembled dark raisins. The assessments of the panel of 10 judges were analyzed, and it was found that there was a significant dflerence 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-dned bluebemes were quite similar to the freeze-dried bluebemes; however, they did dry much more rapidly. Table 7.10 Duncan's groupings for mean score by different judges. Judge Mean score Grouping
Means with the same letter are not significantly dserent at 0.05 leve1
Table 7.11. Duncan's groupings for mean score at dflerent regimes Regime Mean score Grouping
Means with the same letter are not significantly different at 0.05 level
121 7.4 CONCLUSIONS The blueberries osmotically dehydrated and microwave dried with convective air at a power level of 0.1 W g-' were of equal quality to the fieeze-dried product. Since they dried far more quickly, it remains to determine the energy requirements and economics for the process and compare them ta those for freeze-drying. The only drawback expected is the capital investment for appropriate microwave equipment. CONCLUSIONS, CONTRIBUTIONS TO KNOWIXDGE ANI) RECOMMENDATIONS FOR FURTHER WORX
8.1. Summary and Conclusions The principal objective of this study was to evaluate the feasibility of developing a microwave-based process for drying strawberries and bluebemes. The drying experiments and quality evaluations led to the following conclusions.
Whole strawberry and blueberry miit cannot be dried in the convective-microwave regimes used in this study without pretreatment. High skin resistance to moisture diffusion combined with rapid interna1 heat generation result in bursting of the fruits. Burning also occurs at higher microwave power levels. Punctunng the surface of strawbemes at several locations is not an appropriate way of improving diffusion of moisture, as the bemes bleed and sometimes burst at higher incident microwave energy levels.
2. Although sliced strawberries can be dried without bursting, there are substantial losses in quality attributes, probably due to volatilization of aromas.
3. The pureed product can be dried into a fhit leather; however, flavour and aroma are lost and the product is difficult to separate from the screen support. 4. The problem of skin resistance to diffusion can be overcome, as shown by other researchers, by dipping in &dine solutions of ethyl oleate for one minute at room temperature. A concentration of 1%ethyl oleate is sunicient to dissolve the wax, since no further improvement in the dryng rate is obtained at higher concentrations.
5. Dipping significantly improves both the convection and microwave drying rates of strawbemes and bluebemes.
6. Although dipping in the EONaOH 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 strawbemies prior to drying reduces loss of aroma, flavour and colour, leading to a dried product that is equivalent in sensory quality to freeze-dried strawbemes.
8. Osmotic dehydration removed significantly larger quantities of moisture from strawbemes than from bluebemes; however, if the osmotic step was carried out for longer than 24 hours, off- odours developed in both types of hit.
9. Strawbemes pretreated and osmotically dehydrated for 24 hours, dned in a 0.2 W g-l regime in approximately 1/10 of the time taken by convection drymg (45°C / 2 m s-'1. Under the same conditions, bluebemes dry in 1/3 of the time needed for convection. Bemes microwave dried at power level0.1 or 0.2 W go' were equd in quality to dipped, osmotically dehydrated and then freeze-dried bemes.
10. Bluebemes pretreated with 2% ethyl oleate and 0.5% sodium hydroxide, lost significantly more moisture (WB) during osmotic dehydration than untreated bluebemes.
11. Bluebemes and strawbemes that have been osmotically dehydrated with greater amount of sugar (3:l hit-to-sugar) lose more moisture ir, the same dehydration time, yet dry to the final state less quickly thereafter.
12. Power levels greater than 0.2 W g*' lead to burnt spots or general buming of the bemes, whether or not they have been pretreated.
13. The shrinkage ratio of strawberries has a straight line relation to the moisture ratio. The reduction in equivalent diameter is well-described by a reciprocal logarithmic fùnction.
Finally, one can conclude that it is possible to obtain high quality blueberries and strawbemes in a microwave drying regime if skin resistance to diffusion is reduced by an appropriate dipping treatment and if pigments and aromas are bound by infusion in sucrose. Since the freeze- drying industry uses infusion prior to freeze-drying, there is a potential time savings due to replacement of fieeze-drying with rnicrowave-drying for the finish drying. Detailed economic and energy analyses could be performed to evaluate the overall cost-benefit ratio associated with this technological alternative.
8.2. Contributions to knowledge This thesis has made original contribution to knowledge by providing basic and applied information on microwave-drymg of strawbemes and blueberries. The main contributions are as follows:
1. This study demonstrates that it is possible to obtain good quality dried strawbemes by using microwaves to assist convection, only if they have been previously pretreated to reduce skin resistance to moisture difision,
2. Strawbemes and bluebemes that were dipped in ethyl oleate and sodium hydroxîde and osmoticdy dehydrated and then microwave-dried were considered to be 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 to moisture diffusion in bluebemes and strawbemes. 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 èl, continuously applied mimwave power levels should not exceed 0.2 W g -'for a frequency of 2450 MHz.
5. It has been established that microwave drying at power level
126 the time for strawberries and Y3 of the time for bluebemes of the time required for convective drying at the same inlet air temperature.
6. The relationship between shrinkage ratio and moisture ratio of strawbemes is a straight line, whereas that between moisture ratio and equivalent diameter is a reciprocal logarithmic curve.
8.3. Recomrnendations for further studies The main points that are yet to be addressed are whether there are economic and/or energy benefits to substituting freeze-drying by microwave drying fier initial treatment of dipping in ethyl oleate/sodium hydroxide, followed by osmotic dehydration. Work by Shivhare (1991)also indicated that energy consumption in microwave-drying might be 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 timdenergy optimization combining different air inlet temperatures, inlet air velocities and pulsing modes of application of microwave energy. The fmit to sugar ratio should also be included in such a study since it influences the rate of finish drying. Since there is some absorption of sugars by the hits during osmotic dehydration, studies could be conducted to determine how these influence the drying rate to reduce it, as obsewed. The slower drying when more sugars are absorbed rnay be due to higher osmotic tension or to a change in dielectric properties associated with a different solids to liquids ratio, or to 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 possibility that lower concentrations yield the same results as 1%EO could be investigated. AIso, the concentration of NaOH may be adjusted. One aspect that was not covered in the work presented here was an investigation into the storability of the final product. Since this study showed that the internal temperature of the microwave-dried product exceeded 100°C during drying, there is reason to suspect that storage He could be afTected. On the one hand, this effect might be positive since the product appears to have been pasteurized; on the other, there may be a negative effective due to breakdown of ce11 structure due to the high temperatures. REFERENCES
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Table A4.1 Analysis of MW drging of slices
Table A49 Analysis of MW drying of Puree
Table A43 Analysis rehydration ratio of slices
Appendix B
Table k5.1 Analysis of MW drymg of whole strawbemes with pretreatment (Time)
Table A59 Andysis of MW drying of whole strawberries with pretreatment (Fkhydration)
Table A53 Andysis of MW drymg of whole strawbemes with pretreatment (Toughness)
Appendix C
Table A 6.1 Analysis of osmotic and MW drying of whole strawbemes (Tirne) 151
Table A 6.2 Analysis of osmotic and MW drying of whole strawbemes (Rehydration) 151
Table A 6.3 Analysis of osmotic and MW drying of whole strawbemes (Not including freeze àrying) for toughness 152
Table A 6.4 Aaalysis of osmotic and MW drying of whole strawbemes (this analysis includes the freeze drying numbers) toughness 152
Table A 6.5 Analysis of osmotic and MW drymg of whole strawbemes (this analysis does not include the freeze drymg nurnbem). 153
Table A 6.6 Analysis of osmotic and MW drymg of whole strawbemes (this analysis indudes the freeze drying numbers) for color difference. 153 APPENDIX D Table A 7.1 Analysis of osmotic and MW drying of whole Blueberries Dependent Variable : MCREM (Moisture content rernoved).
Table A 7.2 Anaiysis of osmotic and MW drying of whole Blueberries Dependent Variable : DRYING TlME (Freeze dried not included).
Table A 7.3 Analysis of osmotic and MW drying of whole Blueberries Dependent Variable : REHY (Freeze dried not included).
Table A 7.4 Analysis of osmoüc and MW drying of whole Blueberries Dependent Variable : TOUGHNESS (Freeze dried not included).
Table A 7.5 Analysis of osmotic and MW drying of whole Blueberries Dependent VariaMe : DlFF (Freeze dried not included).
Table A 7.6 Analysis of osmotic and MW drying of whole Bluebenies Dependent Variable : SENSORY analyssis including freeze-dried and to see influence of Judges.
APPENDIX E Computer programme in HP-QBASIC to monitor and record the data used in the Data Acquisition System.
APPENDIX F Evaluation foms used in the sensory evaluation of strawbemes and bluebenies. APPENDIX A Analysis of MW 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 11 326 16.6666 2965.1515 388.17 0.O00 1 24 183.3333 7.63888 35 32800 R-Square C.V. hotMSE Time Mean 0.99441 4.252083 2.763853 65.0000 Source DF Type ISS Mean Square F Value AIR 2 ? 18.16666 359.08333 8.53 POWER 3 255447.222 85 149.0740 2023.34 AIR*POWE3R 6 2115.61111 352.601851- 8.38
Analysis of time to dry RYee
Table A49 Analysis of MW drymg of Puree Dependent Variable : Time DF Sum of Squares Mean of Squares F Value 11 25828 1.00000 23480.09090 557.94 24 1010.00000 42.08333330 35 259291.000 25929 1.000 R-Square C.V. Root MSE The Mean 0.996 105 5.682 190 6.48716682 114.166666 Source DF Type ISS Mean Square F Value Pr > F AIR 2 7 18.16666 359.08333 8.53 0.0016 POWR 3 255447.222 85149.0740 2023.34 0.0001 AIRSPOWER 6 2115.6111 1 352.601851 8.38 0.000 1 Analysis of rehydretion ratios of strawberry slices for convection, micmwave and fieeze dryhg methods.
Table A43 Analysis rehydration ratio of slices. Dependent Variable : Rehydration ratio DF Sum of Squares Mean of Squares F Value Pr > F 125.84 0.0001 10 0.05086667 0.00508667 14 2.61133333 R-Square C.V. Root MSE Time Mean 2.9933333 Source DF Type ISS Mean Square F Value Pr > F TRT 4 2.56046667 0.64011667 125.54 0.0016 Source DE' Type IIISS Mean Square FValue Pr> F TRT 4 2.56046667 0.64011667 125.84 0.0001 APPENDIX B
MW drying of whole strawbemes with pretreatment
Table A5.1 AnaIysis of MW drying of whole strawberries with pretreatment (Tirne) 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 Emr 24 1083.3333 45.138888 Corr.Total 35 33938.8888 R-Square C.V. Root MSE The Mean 0.968080 6.681429 6.71854812 100.5555555 Source DF Type ISS Mean Square F Value Pr > F TRT 3 3594.4444 1198,148148 26.54 0.000 1 POWER 2 26709.7222 13354.861111 295.86 0.0001 425.23 148 14 9.42 0.0001
Analysis of Rehydration
Table A.5.2 Analysis of MW drying of whole strawbemes with pretreatment (Rehydration) Dependent varialble: Rehydration
Source D Sum of squares Mean of squares F Value Pr > F F Model 14 6.31440 30 0.76640 Corrected total 44 7.08080 C.V. Root MSE Rehy Mean 6.869624 O. 159833% 2.32666 Source DF Type ISS Mean Square FVaiue Pr > F OLEATE 2 0.1812133 0.09060667 3.55 0.0414 TYPE 4 5.9102444 l.47756Ill 57.84 OtOOO1 0LEArn"TYP 8 0.2229422 0.2786778 1.09 0.3963 E Analysia of Toughness
Table A53 Analysis of MW drying of whole strawbemes with pretreatment Dependent varialble: Toughness
Source D Sum of squares Mean of squares F Value Pr > F F
30 0.01800 0.00060 Corrected total 44 0.987977 R-Square C.V. Root MSE Tough Mean 0.981784 8.998 126 0.02449490 0.27222 Source DF Type 1 SS Mean Square FValue Pr>F
TYPE APPENDIX C MW drying of whole drawberries osmotically dehydrated Table A 6.1 Analysis of osmotic and MW drying of whole strawbemes Dependent Variable : Time Source DF Sum of Squares Mean of Squares F Value Pr > F Mode1 9 3025006.8333 Error 26 2898.72222 Com.Tota1 35 3027905.5555 R-Square C.V. hotMSE Time Mean 0.999043 3.783028 10.655885 279.11111 Source DF Type 1 SS Mean Square F Value TEMP 1 65195.1111 65195.11111 584.77 SUGAR 1,777777 0.02 POWER 1406937.1944 12619.48 TEMP*SUGAR 7.3773 TEMP*POWER 633.73 SUGAR*POWER 17.07
Table A 6.2 Analysis of osmotic and MW drying of whole strawberries Dependent variahle : Rehydration Source DF Sum of Squares Mean of Squares F Value Mode1 9 o. 10090432 7.77 Error 26 0.33768332 0.01298782 Corre.Totd 35 1.245822222 R-Square C.V. Root MSE Rehy Mean 0.728947 6.235 119 O.ll3964lS 1.8277778 Source Type 1 SS Mean Square F Value Pr > F TEMP 0.0 l69OûûO 0.01690000 1.30 0.2644 SUGAR 0.00054444 0.0005444 0.04 0.8394 POWER O.'? 1637222 0.35818611 27.58 0.0001 TEMPSSUGAR 0,0016000 0.0016000 0.12 0.7284 TEMPfPOWER 0.08581667 0.42908333 3.30 0.0527 SUGARSPOWER 0.08690556 0.0345278 3.35 0.0509 Table A 6.3 Andysis of osmotic and MW drping of whole strawbemes (Not induding 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.Tota1 35 0.55240000 R-Square C.V. Root MSE Tough Mean 0.586079 20.686223 0.09377756 1.8277778 Source DF TypeISS Mean Square F Value Pr > F TEMP 1 0.0784000 0,0784000 8.91 0.0061 SUGAR 1 0.036 10000 0.036 1000 4.10 0.053 1 POWER 2 0.04605000 0.02302500 2.62 0.092 1 Tl3MP*SUGAR 1 0.0016900 0,0016900 1.92 O. 1774 TEMP*POWER 2 O. 14615000 0.07075000 8.31 0.0016 SUGAR*POWER 2 0.000 15000 0.0000750 0.0 1 0.9915
Table A 6.4 Analysis of osmotic and MW drying of whole strawbemes (this andysis includes the freeze drying numbers). Dependent Variable : Toughness Source DF Sum of Squares Mean of Squares F Value Pr > F Model 10 0.95976429 0.09597643 11.67 0.000 1 Error 31 0.25500000 0.0082258 1 Corre.Tota1 4 1 1.2 1476429 R-Square C.V. Root MSE TOUGH Mean 0.790083 18.01060 0.09069623 1.50357143 Source Type 1 SS Mean Square F Value Pr > F TEMP 0.7 1441429 0.35720714 43.43 0.0001 SUGAFt 0.03610000 0.0361000 4.39 0.0444 POWER 0.04605000 0.02302500 2.80 0.0763 TEMPSSUGAR 0.0016900 0.0016900 2.05 0.1618 TEMP'POWER O. 146l5ûûO 0.07307500 8.88 0.0005 SUGAFt*POWER 0.000 15000 0.0000750 0.0 1 0.9909 Table A 6.5 Analysis of osmotic and MW drying of whole strawbemes (this analysis does not include the freeze drymg numbers). Dependent Variable : DIFF Source DF Sum of Squares Mean of Squares F Value Pr s F Mode1 9 868.646425 96.5162694 4.197 0.0001 Error 98 2256.186459 23.02231081 Corre.Tota1 107 3 124.8328849 R-Square C.V. Root MSE DIFF Mean 0.277982 24.92874 4.789 15702 19.24749125 Source DF TypeISS 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 TEMPSSUGAR 1 1.1268481 1.12684810 20.05 0,8254 TEMP*POmR 2 206.407378 103.2036890 4.48 0.0 137 SUGAR*POWER 2 320.631606 160.3158032 6.96 0.0015
Table A 6.6 Analysis of osmotic and MW drying of whole strawbemes (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 Mode1 11 2239.81861 203.619874 9.867 0.000 1 Error 114 2253.790739 20.64728719 Corre,TotA 125 4593.609359 R-Square C.V. Root MSE DIFF Mean 0.487594 2530112 4.54392861 17.95939670 Source Type 1 SS Mean Square F Value Pr > F TEMP 1254.79% 1 627.397475 30.39 0.000 1 SUGAR 188.1 17315 188.117315 9.11 0.003 1 POwEFt 231.831437 115.91571876 5.61 0.0047 'iXMPSSUGAR 38.035930 19.01796549 0.92 0.4010 TEMP*POWER 206.407378 103.2036890 5-00 0.0083 SUGAR*POWER 320.63 1606 160.3 158032 7.76 0.0007 APPENDnL D MW drying of whole Bluebemes osmotically dehydrated Table A 7.1 Analysis of osmotic and MW drying of whole Bluebemes Dependent Variable : MCREM (Moisture content removed). Source DF Sum of Squares Mean of Squares F Value Pr > F Mode1 15 4441,58236833 Error 44 29.84535667 Corre.Total 59 4471.42772500 &Square C.V. Root MSE 0.993325 8.9 15744 0.82359 187 Source DF TypeISS Mean Square F Value SUGAR 1 5.41801500 5.418015000 7.99 TRT 1 2067.53 140 2067.53140167 3048.09 TIME 4 1738.602750 434.65068750 640.7948 SUGAR*TIME 4 1.95657667 0.489 14417 0.7273 SUGAR*TRT I 2.208001670 2.20800167 3.2673 TRT*TME 4 625.8656233 156.46640585 230.67
Table A 7.2 Analysis of osmotic and MW drying of whole Bluebemes Dependent Variable : DRYING TIME (Freeze dried not included). Source DF Sum of Squares Mean of Squares F Value Mode1 9 1044074.33333 116008.25925926 932.59 Error 26 3234.222222 124.393 16239 Corre-Total 35 1047308.55555 R-Square C.V. bot MSE 0.996912 2.961455 11.15316827 Source Type 1 SS Mean Square F Value TEMP 167826.777 167826.7777 1349.16 SUGAR 1573.44444 1573.4444444 12.6509 POWER 857486.8888 428743.44444 3446.688 TEMPSSUGAR 106.777777 106.777777 0.86 TEMP*POWER 16133.55555 8066.77778 64.85 SUGAR*POWR 946.8888889 473.- 3.81
154 Table A 7.3 Analpis of osmotic and MW drging of whole Blueberries Dependent Variable : REHY (Freeze dried not included). Source DF Sum of Squares Mean of Squares F Value Pr > F Model 9 O. 129050000 0.01433889 5.17 0.0005 Error 26 0.072072222 0.00277201 Corre.Tota1 35 0.201122225 R-Square C.V. Root MSE REHY Mean 0.641650 3.937256 0.05264987 1.3372222 Source DF Type 1 SS Mean Square F Vdue TEMP 1 0.01521111 0.015211111 5.49 SUGAR 1 0.07290000 0.072900000 26.30 POWER 2 0.00620536 0.003 10278 1.12 TEMP*SUGAR 1 0.00934444 0.00934444 3.37 TEMP*POWER 2 0.00493889 0.00246944 0.89 SUGAR*POWER 2 0.02045000 0.01022500 3.69
Table A 7.4 Analysis of osmotic and MW drying of whole Blueberries Dependent Variable : TOUGHNESS (Freeze dried not uicluded). Source DF Sum of Squares Mean of Squares F Value Mode1 9 0.11016389 0.0 1224043 3.85 Error 26 0.08260000 0.003 17692 Corre.Tota1 35 0.19276389 R-Square C.V. hot 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 0.000 1 SUGAR 1 0.00122500 0.00122500 0.39 0.5400 POWER 2 0.0 12 15556 0.00607778 1.9 1 O. 1678 TEMP*SUGAR 1 0.00202500 0.00202500 0.64 0.4319 TEMP*POmR 2 0.00575556 0.00287778 0.91 0.4 166 SUGAR*POWR 2 0.01520000 0.00760000 2.39 0.1113 Table A 7.5 Analysis of osmotic and MW drying of whole Bluebemes Dependent Variable : DIFF (Freeze dried not included). Source DF Sum of Squares Mean of Squares F Value Pr > F Mode1 9 68.27610705 7.58623412 5.99 0.000 1 Error 62 78.51864520 1.26642976 Corre.Totai 71 146.7947522 R-Square C.V. Root MSE DIFF Mean 0.465 113 2.602339 1.12535761 43.24407434 Source DF Type 1 SS Mean Square FVaiue Pr >F TEMP 1 1.03347122 1.03347122 0.82 0.3698 SUGAR 1 22.03100705 22.03 100705 17.40 0.0001 POWER 2 7.88783111 3.94391556 3.11 0.05 14 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 Blueberries Dependent Variable : SENSORY analyssis including freeze-dried and to see influence of Judges. Source DF Sum of Squares Mean of Squares F Value Pr > F Mode1 20 97.49047619 4.8745238 1 2.35 0.0024 Error 119 247.1595238 2.07697079 Corre.Total 139 344.6500000 R-Square C.V. Root MSE SENS Mean 0.2828680 24.22 134 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.O44 0.1348 TEMP*SUGAR 2 12.15238095 6.07619048 2.93 0.0575 'i'EMPSPOmR 2 1.86666666 0.93333333 0.45 0.639 1 SUGARfPOWER 2 37.0666667 18.5333333 8.92 0.0002 Computer programme in HP-QBASIC ta monitor and record the data in the data acquisition system.
1048 DfSP 1050 OfSP ' RICRQWtWE CONTROLLEa FOR ORYlMG EXP59fH~4Tw 1060 OISP ' JANUMY 1993' 1070 OlSP 1088 OISP . . .. - . . . 1@%l DIS? " 2. IIt?lpj RL~J~J<<':. 110a DISF l Il0 OfSP ltza GISE - > > FI = 3427~CLOCK" 3 130 OISP I GISF " > ; FZ = TIRE INTE~V~LS' l 159 O1 SP 11E3 OfSP ' > > F3 = SAEETY FMARETEES" ll7O OIS? 1 160 OIS? * Fa = STG?T DATA ACOUIjIÏiJM" 1 tg0 DISP 1x0ow > ) FS = SET AIR TEMPE=~T~EE- 1ZiQ OfSF 1228 DISP ' ) ' F6 = TARE THE SCALE' t220 DIS? 1220 DIS? i > F7 = PEINT COHWiTS" 125Q OLSF 12SO OXSP ' > > Fa = END EXPEPIMENT' 1270 r 1 Z8a ON KEY= I ,'CLOCE* GOSU0 2000 1290 ON KEY: 2 ,"TIHEr 60SU6 3a90 1396 Oh RE,$ 3,"SnFETY' GOSUE 4000 1310 ON KEY: 4,'START' 6OSUB 5300 1320 ON KEY: 5,'SET TERP' GOTO 14003 1330 ON KEYZ 6,'TARE' GOTO 15000 IjLQ ON KEY: 7,'CORMEMTS' GOTU 16000 1350 ON KEY3 8,'ENOœ GOTO 13000 IXQ ON KEY= 12,=nmifiEw- GNO toao 1370 KEY LABEL
i3Ê0 J t329 IF K=i TkEN 1280 ELSE 1000 1403 1 1500 OFFKEYS 1 510 CLEAR ista OFF KEY: t 1530 OF= KEY% 2 [ÇA@ off HE.{= 2 1550 OFF KEY: t 1569 OFF KEY% 5 1570 OFF KEY3 6 1SaQ OFF KE'lt 7 1550 OFF KEY% 8 1500 CRT 15 1 1610 PRINTER IS 2 1629 RETURN 1630 !SUBEN0 1640 i 2000 1, SET 3497A CLOCK :QI0 K-0 Z0Z9 GOSU0 1500 IOFFKEYS 2030 ON KEYS 1,'SET TIRE' GOSU8 2500 2040 ON KEYS A ,'HAINHENU' GOTO 2900 2050 CLEAR 2es0 KEY LABEL 2070 OUTPUT 709 r 'Tom 2080 ENTER 709 ; T0S 2090 DISP 2100 OfSP 'DATE: 1996/'rt@S11 ,23;'/';T0SC4,51 ti 1s DrsP *TIRE: ~TBSC~,I~J 2120 DISP 2130 WAIT SB00 2140 6ûTO ta70 2158 1 2580 ENTER NEW TIRE 6 OATE 251 û CLEAR 2528 ON ERROR SOTO 2618 IFORHhT ERROR
3750 OFF ERWE 3778 BEEP 37m OESF -IPIUALII TEME UR eko Fomw 3700 WAlT 2096 m0taTu 3569 3ElQ 1 3gm ! + mumi TO WN RENU 3910 GOSUE 1500 !OFFKEYS 392a K=8 3933 RETUFN 39LQ 1SUIEN13 59se ! 4000 t* SET SAFZTY PARAPIETERS 4010 K=Q 4OZQ Kl=l 4033 GOSUB 1530 .lOF=EEYS 4020 ON KEY: I ,"CH411GEm GOSU0 4500 4050 ON KEY2 J,'RAIMHENU' GOTO 4900 4om CLM 4373 OEfP 4080 UiSP "PiAX!f'!UR AIR TEI1PEP.flTUP.E IN CAVITY: ';TE3%;" 'C" 4002 0:SP sld0 0157 "3AXl?lM EEZLEITE3 POLU!: -;PMAY;' Watt" al 10 [?!Si 4128 OIS? "FINAL MhZS: "iPiASSOUT;' qn 301 4 1 JQ KEY LAEEL 4\50 1 4169 IF Flfl THEM 4152 4170 WTO sax 4180 1 4SQQ 1. CUANGE SAFETY PARAflETERS 4510 CL95 4529 gi=0 4538 ON E3ROR GOTO G50 tFORH6T ERROfl 4540 OISP 'ENTEE RAAIRUPi AIR TEMPE3ATURE' 4550 IWUT TPkX 4560 DISP 4579 OEP -E?~TE.F!i1AxInun RFLECTEO POWER IN ~dt:- 4580 INPUT PRAR 4590 019 'ENTE2 f IFl&t nASS Dr C" 4600 INPUT HASSCUT $6 19 OFF ERROR 4620 RETURN 4630 ! SUBEN0 464a ! 4650 IEEROR ON VALUE FORHAT maCLEM 4670 OFF E3ROR 4~x1EP 4600 OISP 'INVALID VALUE OR BA0 FORRAT' 4700 GOTO 4500 4710 1 4900 ,I RETURN TO RA f N REEIU 4910 60SU0 1500 IOFFKEYS 4920 K=d 4930 RETURU 4940 ISUEENO 4950 1 5000 1. START OATA 4COUISfTION 5010 CLEAA se20 sosue 1500 IOFFKEYS 5030 YIDI IOUHBY T0 CHECK IF PROGRAB WAS RUN 5040 1 5050 ON KEY8 8,'ENOm 60TO 13000 5060 KEY LMEL 5870 ! 5060 REC=f.iSCAPi+t !PIURCEP, CF RECORDS FOR OfiTFiF ILE 5090 ON ERROR 60TO 5130 51'26 CREATE FIS ,REC, 15a I CEEATE GATA FILE.EG SI i0 ASSIGN2 1 TC FIS Si20 60TO 5160 5 133 OFF ERROR -* 5140 PURGE FI3 SI 50 60TO 5090 516a OFF ERROR 5170 OUTPUT 709 ; 'T0' 5109 EbITER 709 i TS Si90 1 5100 IPRINT TO FIS INITIAL CONOIT?ONS 52 10 PRINT: 1 ,1 ; Tà ,TIHEONS ,Tf flEOFFS ,TIilESCANS ,NSCAi\I, tMkX ,Pnr;x 52ta ! 5238 !PR!MT INITIAL CONGITIONS TO PRINTER 5240 OUTcUT 700 T mTû- 5258 ENTEA 709 ; TS 5210 PRItlT "W78t.0S" !SET PRIMTER TO NORPiAL FONT 527a PRINT 'EXPE3IflENT STARTEO AT: ';TS 5280 PR IN7 'YISROW6VE 6ENERATOR TIRE OP4: ' ;TINEgNS 5250 PRINT 'MICROWAVE GENERATOR TIME OFF: ';TEMEOFFs ' 5300 PRIMT *fIHE INTE3VAL BETWEEN SCAN: " ;TIMESSANS 5310 PRINT 'TOTAL NUMES OF SCAN: 'iNSCAN 5320 PRïtJT 5338 PfiINT "flAX!RUM ;IR TEfiPE.?ATURE: ';TMAX;"Cœ 5240 PRINT 'flAXIRUM REFLECTEO POWER: 'iPMAX;' Watt' 535.2 PRlNT Sr'6a PRIPIT "0Z7SkZf' I SET PRINTER TO COMPRESSED FCrir 5365 PRINT 'TIME TEflP. AH8 TERP.IN TEMP.OUT RH.OUT RASS POWER,IN POWER.OU TM 5376 1 553a 1 8 SÏART SC9N 5510 T2~='00"&ïI~E~CA~~tl,213TIflESCAN~14,~1ISE7 FOonAÏ FOR TIRE INTERLJAL 5520 OUTPUT 709 1 'SI' 5530 ClEjR 703 5540 1 553 GOSU9 17000 IDfS?LAY OPERATING MENU SfS0 1 5570 FOR 2-1 TO NSCAN 5580 ALPhA 10.55 O DfSP USING '30 ,SA ,:O' ; L;' of ";NSCAN S~WGET nnE INTERRUPT FOR NEXT SCAN 5600 ON INTR 7 GOTO Si70 INEXT SCAN 5610 ENA8LE INfR 7 a 8 5620 OUTPUT 709 r 'SEaI6TIm&T2B 1 SET TIRE INTERVAL 0ETWÉEN SCAN 5630 1 5640 GOSUB 6000 iREAD SENSORS 5650 ! 5663 GOSN 7000 iC3NTROL HfCROWAVE ON/OFF 5678 1 %RANCHfNG ChUSEO 8Y INTR 7 5680 OFF TIVERS 1 5584 IF K2<>i THEN t(6 1-PISUH/NI !COHPUfE !€AN POE8 INC. 5687 IF K2ol THEN L(?bPRSUtt/NI !COWUTE HEAN POUE2 REF. s69a nr=i 5700 IF KZ-i THEN 5740 ICONTINUOUS OPERATION 5710 OUTPUT 709 i m~~o,r-~a~~~.~m! SUITCH OFF nrcRowfiuE 5728 WAIT (000 5730 OUTPUT 789 I '004.2' 5748 OFF TIPIERS t 5758 P-SPOLL(709) 5760 SThTUS 7.1 1 A 5778 NEXT 1 5780 l 5738 CfEfiR 709 5888 MORT10 7 . 5843 ! ma0 REXI SENSORS 601 0 OLfTFiJT 709 ; 'TO: 6020 ENTEZ 709 r TlS !TIRE OF REAOIPIG 6030 ALPHA IS,S5 @ OIS? USlNG '1AA" ; TIS 6040 ! FiEAO REFERENCE TEHPERATURE, CHSPINEL 19 6056 07=G ITHERNOCOUPLE TYPE-T 6060 02319 ! CHANNEL FOR TEHP. REF. 607g F l S=VAt%(OZ 1 6080 IF LEN(PlS)-i THEN PlJ=*@'&PlS 6030 OUTPQT 709 ; 'VRSVNlVAlVFI~JDÇVC0~IS0UW0"! 3697 SETUP 6 TRG 6lQQ OUTPUT 783 i 'AC"LPfJ&'VT3' 1 CLOSE CHANNEL 19 8 FE60 UOLTiXE 6 1 16 ENTER 703 i P 6120 OS-I0.P 6130 ! 61 00 ITC ARBIENT AIR 6150 Pl-1Z 616a GOSUE 8000 ICLOSE CHfiNNEL PI & READ VOLTASE 6 170 6021?E %8i3 1 TEPIPERATURE TRAEJSFORM 6190 L( 11-9 6199 ! 6200 FOR JI -2 TG 3 ! TC IN AND OVT 6210 TC-0 6Zt0 FOP J2=l TO 3 IiïEAU OUER 3 TC 6230 Pl=J1+8 6240 GOSU6 8000 tCLO5E CHANNEL Pl & REAO VOLTAGE 6250 GOSCB 9008 ITEMPERATURE T8ANSFgRM
6253 TC+/3+iC fl 6270 NEXT Jt 6259 L(J 1 1-P 6290 NEXT JI 650a 1 6210 FOR J-7 TO 9 1 1 83 -5 1 SCPLE 6120 PI=J ICHHNNEL FOR TC
6350 GOSUS 8008 f CLOSE CHANNEL Pl HND READ VOLTAGE 6360 IF f-7 THEN GOSU0 O200 1 RH TRANSFORI 639 IF J-8 THEN 60SU8 9300 1 MAS5 TRANSFORiï 6368 JC-J-3 6370 L( JC)-FOUT 1 STORE CALCULATEO VALUES IN ARRAY L( ) . 6380 NEXÏ J 6350 1 6400 IPRINT DATA tO .€O AND PRINTER 641 0 PRINT3 1 r T 1 S ,Li 1 IPRfNT COLLECTE0 DATA TO €0 6420 PRINf USING 'ldA,XX,ût40.4O,X)* ; TlS,t(l ),L(~),L(~),L(~),L(S),L(~~.L(~) 6430 ALPHA 12,55 8 OlSP USIN6 '40.0 ,X ,2A' ; HASSOUT;' g' 6440 ALPHA 1ï,S5 8 OISP USING '4O.O.X ,2A' ; L(S)i' q' 6450 RETL'RN 660 ISUBENO 6470 ! 7000 1. CONTROL flICROUtVJE SEfiERfiTOR 7005 NI4 !RESET WRfABLE TO 0 7006 PISUR-0 IRESET UhRIABLE TO 0 7007 PRSUR-e !RESET VARIABLE TO 0 7010 imEON 7015 MIWItl 7020 HUONW&( TIREONSt 1 ,2 1 1+68+URL(TtPtEONSC4, ) ! SECONOS ON 7030 OFF TIRERS i 7840 IF Kt-! THEN 60T0 7068 lCONt1NUOUS OPERMION 7050 ON TIRER# t.muoweee soro 7170 !TO trnE OFF 7060 OUTPUT 709 z 'OC4,I0 ICLOSE RELAY SI , HfCROWAUE ON 7070 ONFS-' ON' 7800 flLPHh 8.55 8 OfSP USIN6 '3Am i ONFS 7898 UAIT 1009 . .. * 162 7100 OUTPUT 709 i "004,l' !OPEN PELA7 $1 fil0 HI=I !SET COUNIE4 TO COMPUTE HERN POWE= 7120 ECISUE 7320 !CHECE IF TEWERATUtE ZFFE 7150 GOSUE 7430 1 UPOATE RkSS 7140 GOïUE 7558 !CHECK IF REFLECTEO POE? SeFE 7 1 aS IF ERERGENCY THEN 78QQ ! EMERGENCY CLCISE 7150 GOTO 7120 !WAIT FGR ON TIHE!?=l OR FOR NEXT lNTR 7 7160 ! 7170 ITIME OFF 71 60 OFF TIRE23 1 71 94 PISUfl=PISUM+L( 6 1' 7 157 PRSUR=PRSUiî+L( 7 7100 RWOFF-VAL(CI~~EOFF~CI,~~)*~~+VAL(TIHEOFFSI~~~I~!SECONOS OFF 7200 ON TIMER3 I,MWOFF*IBBO GOTO 7010 !TO TItîE ON 7210 OUTPUT 709 t 'OC4,Z" !CLOSE RELAY 82, MiCROWRVE GFc 7220 ONFS='OFF' 7230 htPHA 8,55 @ OISP USINC "419' i ONF9 7240 ALPliA 9,55 3 DISP USiNG '40.O,X,ZAU ; O;' W" 7250 fiLPHa 10.55 @ OISP VSfNG '4O.D,X,2A' ; 0;' W" 7250 UfilT Y000 7270 OUTPUT 709 ; "GW,?' !OPEN RELAY :2 7288 GOSUE 73Zd ICHECK IF TEflPEfifiTURE %FE .7290 GOSUE 7450 1 UFOATE RAS3 7300 GOTO 7270 IWAIT FOR ON TIMERS1 7310 ! 7320 !CHESE FOR 5AFE TEMPERfiTURE 7330 Pl-IO 7340 GOSUS 8000 ICLOSE CH $19 REFERENCE TEHPERATURE. REAO mV 7550 03=id*F 7360 Ft=5 JOUTLET hIR TEHPEWiTURE 7370 60Sü8 8000 ICLOSE CHtPi , RENI mV ,7300 GOSUS 9000 ICONVERT TEMPERATURE 7390 TAiR-P 7480 ALPHA lt ,55 B OISP USING '40.0,X ,ZA' ; TAIfi;"'C" 7310 IF PlTMAX THEPJ EtlERGENCYrt ISET FLAC ON THAX EXCEE3EO 7120 IF FITMAX THE.% GOTO 7500 ITO EMERGENCY CLOSE 7430 RETURN 7440 JSUSENO 7450 I .7460 1 UPOPTE RASS 7470 P 1-0 7480 GOSUE 80~01 CLOSE CHZB a RE~OvouaGE 7490 GOSU8 9300 ICONVERT MAS5 7500 fiASS-POUT 7510 ALPHA 13.55 @ OISP USIN6 '40.0,X,ZAm ; MASSI* g' 7520 RETURN 7sàa ! 7540 ! 7550 i* CHECK FOR SAFE REFLECTEO POWER 7560 Pl-30 IREFLECtEO POUER 7570 GOSU0 8000 ICLOSE CH830 8 REAO U 7580 GOSUB 9600 iTRAHSFORR POWER REFLECTEO -7598 L(7 )=L( 7 )@( Ur-l )/llt+POUT /RI ! COl'tPUTE HEAN AND TRANSFER 7618 ALPHA 10.55 O OlSP USIN6 '40.O,X,2AW I POUT;' W' 7620 Pi-20 iINCIDENT POUER 7630 GOSU0 0000 1ClOSE CH820 6 REAO V .7640 60SU8 9500 !TRMlSFOAll POWER INCIOENT 7650 L(6 1-C( 6 >a(HI -1 i/HI+POüT/HI ! COHPUTE HEAN AN0 TRANSFER 7570 ALPH& 9.55 @ OISP USIN6 '40.0,X,2A0 I POUT;' id' 7688 IF L(7 )>PMAX THEN EilER6ENCY-2 !SET FLAG ON PnAX EXCEEDED 7700 HI=nI+i 7705 !PRINT NIiRIiPOUfiL(6)8PISUR 7710 RETURN 7720 ISUBEND 7730 î .7880 !a ERER6ENCY CLOSE 163 7818 OFF EFHUF 7329 OFF TIîlERit 1 7933 nI=i 7520 OUTPUT 709 i 'Si" IRESET ItJTEEFACE 8 3097A 7650 OUTPUT 709 ; 'OC4,3 ,dg t CUT OFF POWEFi TG PIiCiWWAE AIJD Hf3TEFiS .78E0 WSiT 10QQ 7970 OUTPUT 700 ; 'SI' 7830 RESET 7 7892 CLEAR 7895 IF YI01 THEN 12000 790a 60SU9 6000 1 READ SENSORS AN0 PRINT OATA -7910 IF EfiE2GENCY=0 THEN 12000 tTRANFER OATG Ti3 FLGPPY 7920 PRINT 'THE PROGRNI HAS INTERUPTEO THE FROGRESfION GF THE EXIERIflENTM 7930 PRINT 'SINCE ONE OF THE SAFETY musWAS EXCEEOE~ 79c0 PFiINT 7950 IF EMERGENCY=l THEN PRINT "AIR TEHPEFMTURE IN THE OUEN WAS ABOU€: ";THAXrm' c- 7950 IF EME??5EX'!-t THEN PEINT 'RERECTEO FOWEP EYCEEEO: 'tPVAX;'Wa!t' 796s IF EMERGENCY-3THEN PRINT -PROOUCT HAS REACHED rTs FINAL MASÇ OF -;MASSOUT; - a- 7970 GOTO 12000 ITRANSFER OATA T0 FLOPPY 798n 1 Eo00 1 CLOSE CHhPJNEL Pl 8010 PIS=VALJ(PI U020 IF LEN(P1S)-I THEN PIS-'69"SPl§ 8030 OUTPUT 709 ; 'VR5UN1UAlVFIUDSVC0US0VWO' 1 3497 SETUP 8040 OUTPUT 709 ; "AC'&Pl3&'VT3' ICLOSE CHANNEL Pl & RE30 VOLTAGE eos0 ENTER tas ; F 8960 RETURN 8070 ISUeENO eos0 1 9000 1. TRANSFORR VOLTAGE TO TEffPERATURE 981 0 09-P 9020 Pl =O8 @ PZ-33 9030 05-5 ITC TYPE-T 9040 Ol=PI+a(03,41 )&?2*(R(03,6t )+PZ*(R(SjI43)+PZ,R(03 ,da1)) 9050 02-7 5060 IF O I CR( 05,0*5+4 THEN 02-3 5070 IF O1 9580 !@ TRhNSFORfl VOLTAGE INTO POUER INCIDENT 9518 POUT~CNPI(P) 9515 IF POUT<@ THEN POUT-B 9528 RETURN * - -- - 164 9533 8 SIJ2EFIQ 9548 1 .960Q ! * TFXEtf)ZM VGLTiIC-E IMTO PWEE REILECTEC 95 10 POUT =FNPR P 1 9615 IF POiJf '0 THED FOUT4 9520 RETURti 9630 ! SUeEKO 9510 1 1000a 1- TC COEFFICIENTS 10010 Or=-Z7@ 1003 oz=4as ieaza w=o 10040 CS-70 10050 ! 10050 RESTO~Ela149 10070 03=E ITYPE-T 10080 FOR 014 T0 8 1 8 SESEENTS OF CQEFFSCIEFITS FOR C-L lOQ9O FOR OC=l TO 5 1 5 COEFFIC!EPJTS 10i0O RE30 R( O3.Ç*Qi +O2 ) 101 10 NEAT 02 101za NEXT 01 l013Y 1 YCEfFiCfENTS FOR TYPE-T THERMOCOUPLE 101 40 CSTA 950757 ,-S5Ua 1200000 ,Z116760QOQ00@000,-O.WS?5759 ,-?70 101 58 OATh 335!35 1 .-3aO6 1 W080.I 6270WQ090000,-0 -0QE2JEàZ ,-26d -4% 10163 OATA 157764 ,-32-%7S3000,SZ4OS8000e@0,-0.006I 8652 ,-:Sa. 946 101 70 OATA 8W!0 ,5 ,-400OZZB0,l8?ï7I68000 ,-6.88SOîl8S .-Z:S .959 101e0 OATd 55021.0,-8913320,i4t3130000,-0.00S3~?S5,-l8~.??6 iQIW OAT4 36537.7 ,-2ZddZW,171 456088 ,-0.00364106,-10!. 42& 10200 OAT4 2587 1 ,9 ,-E6J%7,2Z?9 1000 ,-S. 0666Se-08Ç ,- 1.29*22 la2la OATA [email protected] ,-l?ZZ??,ZSg99â0,0.00769tIZ,163.SI 10229 1 COEFFICIENTS C-L 10230 OhT3 1.70 i 8Ee-007,3.8ESSe-OOS ,Z39gS7?e-008,l 6-1 1 ,0 1 KA0 RETUE5 tg250 1 SUeENC 10250 1 1100a 1. 3697+ HA5 FnlLEO EELF TEST il010 CLEriC? 11320 0157 11830 O!SP 11040 OfSP '3497A HAS F7ILEO IT'S SELF EST* 1 10Sd &EE? 11060 WAlT 5000 11073 GOTO 11053 1 1080 EN0 .Il090 ! 12000 10 OATA TRANSE3 FROH .€O TO FLOPPY 12010 6ûTO 17500 ITERUINhTE OR RESURE 7 12020 OFF ERROR 12030 K-0 1;1040 CLEnR 12050 IF Yi10 THEN 12260 12060 DISP 12870 OISP 'INSERT OISK IN OlSK OAIVE' 12080 OISP 'ICdNTl' 12090 PAUSE i2100 FO-'O'&TSEI ,2l&TS~4,53LTSC7.8lLTSC10,11 I lZtt0 ON ERROR 60T0 12370 IFILE ALREAOY EXIST 12120 OISP 12150 OISP 'DATA TRANSFER IN PR06RESSm 12140 OfSP 12iS0 ! 12160 COPY FIS T0 FI 12178 ! 12 100 BEEP 12198 U4IT 1000 12208 BEEP 122 i 8 OIS? - I KAN~F~~cunrlt i t9- 12220 PRINT 'FILENAME: ";tS 12233 PF.'!klT I 2zaa PR INT lZZÇ8 FtINT "'012" 12250 OFF EEESR 12270 YI-1 IGATA HA5 EEEN SAVE0 i 22a0 DISP m~~~~~~~~r~~EE~OEO- 12290 DISP .i2380 OISP 'HIT C63EAEI' 12310 PITCH=Z000*RND+503 12328 LENGTH=I 0*P.FiCtZ 12330 ewP ITCH ,LENGTH rmo WTO mi0 12350 Er19 12360 ! 12370 OFF ERROF 12380 ~~~~~~8F3C2.11 12390 GOT0 lZllQ 12400 ! 13800 1- ENGEO B'! CSER 13016 OFC TIRER3 1 15at0 CLEM J3a3Q FFINT "PROCEAM TEtflINATED 8Y USE8" 13040 6010 7800 1 EnERGENCY CLOSE i GO TO SAVE OATS ON FILE 1 Z0SQ ! 14000 i* SET AIR TERPZRATURE 14010 K-0 1i02a GOSU0 1500 IOFFEEYS 14038 CLEM I4aad ON KEY= 4,'fYAINt'lENU" G3TO 14Sa0 14EQ CtE3R l4Q60 KEY LA0EL 14073 Pt-19 ITC RE33ENCE TEPiPE3ATURE tJ08a GOSUe EGO0 1 CLOSE CHNWEL $1 9 & READ VOLTAGE 14090 Of-10-P 1 TE!lPE?ATUOE COflPENSATlOM lAl00 Pt=? fTC IN flICROWAVE CAVfTY iAll0 6OÇUB 8000 1 CLOSE CHANNEL 01 B flEAD VOLTAGE 11120 GOSU8 9000 1 TRANSF3Riï VOLTAGE IN70 TEMPERATURE 14130 TAIR-f 14140 CLEeR 14150 OIS? USfNG 'IfA,AO.ZD,X,tA' I 'AIR TEMPERATURE: 'rP:"C" 14160 UAIT 5000 14 176 GOTO i 4078 1 REM TC AGA EN 14180 !SUBEN0 14120 1 14500 1. REÏURN TO MIN nEw 14510 60SU0 1500 IOFFKEYS 14520 CLEM 14525 Ki8 14530 60T0 1800 !T0 HAIN HENU 14540 lSU8EUO 14550 ! .15000 !O TARE THE SCALE ISOIQ n=0 tS0S0 60SU0 1588 IOFFKEYS 15830 ON KEY8 l,'TAREm 60T0 15508 15840 ON KEYS 4,'PIAfNHENUm 60TQ 15900 15058 KEY LABEL if068 Pl-0 !SCALE 15870 60SU0 8000 !CLOSE CH88 b REAO OVll 15080 GOSU0 9380 !TRANSFORU VOLT IN MASS 15890 CLEAR lSlû00ISPUSING'SA,2X,4O.2O.X,ZA' i 'HASS-'tPOUTt'um . 1511@ UAIT 2000 1 Si28 SOTO 1 S878 166 ' ! t* Tfi!?E TEE SCALE CLEAR OECP "ThRIN5 THE SCALE' OIS= "PLEhSE WAiT" DELa-O OE~rl=a FOo 1-1 TO 10 Pl4 !SCALE GEL!e 8900 !CLOSE Ch+9 & RE30 OYn 60SUB 9369 VRWSFOWI VOLT IN RA55 OEtP=OELP+POUT/i0 NEXT I OELH4ELP 60TO 15000 ITO TARE HENU 1 SL'BEND I !, RETURN TO M!NMENU GOSUE 1500 IOFFKEYS K 4 GOTO i a00 I TO 3hIN RENU 1 St'BEkLl I 1. PRINT COMRENTS 60SUE 1500 !OFFKEYS u. -0 ON KEY$ 4 ,*RAINRE?U* GOTO 16000 CLEAR KEY LABEL I 1 - ENTE CC)P?HEFiTS OISP 'ENTE3 COPiHENTS itNO HIT ERETiJf?Nl. (RAXlHUtl OF 7Zl CHRS)' OISP Of5P 'TIPE EN0 TO TERHIPlhÏE SESSION' OIS? IFiPUf C3MMENTs IF COMiïENTS-"EFIO' OR COt?flENTS=" end' THEN 16 900 PR I NT conmm GOTP 16553 I 1 EETURN TO RAINRENU GOSJB tsaa IOFFKEYS K -0 G3TO 1000 ITO RHIN MENU 1 SUBEN0 I ! GENERATENG OIf PLAY CLEM ALPKA 3.23 OISP 'PiICROUWE ORYIN6 SYSTEN" ALPH4 5.30 OlSP 'OPERATIN6 CON01f IONS' ALPHA 8 ,t8 OfSP USIN6 'Km I 'GENER4TOR STATUS: ' ALPHA 9.30 OfSP USING 'Km r 'INCXOENT POWER:' AtPHfi 10.29 DISP USING *Km 1 'REFLECTEO POWER:' ALPHfi 11 ,29 OISP USING *Km I 'hIR TEflPERATURE:' hCPHA 1 t -33 DISP USIN6 'Kg 1 'FINAL RASS:" &PHfi 1 3.33 DISP USfN6 'Kg I 'ACTUdL ilASS:* CILPHCI t 4 *4e DISP USIN6.- 'Kg i 'SCAM:' 172 13 ALF..i;1 1s Irftc? Usff'lrj 'E" ; 'f!nE OF LAST SC,iFi:* 17230 RLPHA 16,3i 1724E OlSP USIP!E "K" ; "TiE INTE!?LIAL:' 17250 AL1EA 14.3 1 1726C 0151 USIPJG 'K' I "SAFETY FAR5HETEES' 17270 ALPHA 21 ,2t r 720a orsp usm t -nk:;Inun AIR TEMPERATURE:- 1 72% ALPHA 22 ,Z I 17300 OISP USINY 'Km r 'RAXIRUI'l REFLECTED POWE4:' 17310 ALPHA 16.61 17320 CfSF UÇlNG 'ÇAb t TIHESCANS t 7330 ALPHrî 2 1 ,5E !7340 OISP UÇINC- '30.O,X,ZA' i TMAX;"Cn if350 ALPHA 22 ,ES l73G0 OISP USIEiG w30.0,X.ZA' 6 PnfiXr' Watt' 17370 1 17380 RESTORE If460 17330 FOE 1111 TO 6 1740Q REAO LINE 174:3 r3n Ji-10 TO 75 1752Q AtPHF3 LINE ,JJ 17430 OISF USINE 'u* r '-• 17440 NEXT Jf 17450 NEXT II IïG6 DATk 2,4.6 .l8,20,23 17470 RETURN ISTART SCAN 17430 ISUEENO 17430 ! 17500 iRE3UME OR TEEHINATE THE EXPERIMENT 17510 CLEAR 175x1mue ISYO ~OE=EEYS 1753 KI-l 17540 Clf? 17550 OISP 'FI = RESURE EYPE3fWENT' 17550 C15P 37578 OIS? 'FJ = EN0 EXPE3IENT' 17580 ! 17590 ON KEY: l,'REÇUt!E' GCTO 17800 17600 ON t:EYZ 8,'ENOm 6010 12020 'SAVE OAT4 AN0 EN0 TEE EXPERItlENT 17610 KEY LheEL 13620 ! 17630 BE? 17648 wArT Z~GM f765a GaTO 17630 17660 1 1 7888 i REjURE EXPEE lRE?lT 17818 60SUB 1500 IOFFKEYS 17020 CLEAR 17838 OISP 17848 OXSP 'RESET RICROUAVE AN0 HEATER'S BRAKERS' 17850 DISP 17860 OISP 'HhKE THE NECESSARY hOJUSTHENTS TO THE EXPERIPIENTm 17870 DISP 17880 OXSP 'THEN, HIT ICOUNTI' 17090 ! 17980 PAUSE 17910 6OSUO 17000 !OISPLAY RENU 17926 60T0 5750 IRESURE PROGRAH 17938 1 Name...... , ...... Date...... ,...... SENSORY EVALUATION OF STRAWBERRIES Taste the Suawbeny samples and didc how much you lilcc or dislilcc. Use the appropriate sale given to shoyour attitude by checlûng at the point (~fRemernber you are the only one who can tell us whac you lilcc. Lihc Exrrcmelv 1 i Lile Modcratelv 1 1 1 1 Lilte Slightlv 1 Neither Like Nor diriikc I 1. 1 11 Dislikce Moderarelv 1 1 1 1 SENSORY EVALUATlON OF BLUEBERRIES Taste the Blueberry sanoles and ciieck how much you liks or dislik . Use the appropriate scale given to show your âtiitude by checking Et the point Remember you tre the only ona who cmtell us what you like. r 88755 1 35601 1 26535 1 93044 ( 1 Lik- Extremely 1 1 1 Like Vev Much 1. 1 1 1 Like Moderztely 1 1 1 Like S lig ht ly 1 Neither Like Nor dislike 1 1 1 1 1 .O Dislike Slightly 1 1 1 1 Dislike Moderaely 1 1 1 1 Dislike Very Mucn 1 I l I 1 I 1 I I 1 Dislike Exrremely 1 1 1 1 1