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Microwave and convective drying of potato slices Bouraoui, Moez Mohamed 1991

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MICROWAVE AND CONVECTIVE DRYING OF POTATO SLICES By Moez Mohamed Bouraoui B. A . Sc., School of Agricultural Engineering, ESIER, Tunisia, 1988 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF T H E REQUIREMENTS FOR T H E DEGREE OF M A S T E R OF APPLIED SCIENCE in T H E FACULTY OF GRADUATE STUDIES T H E DEPARTMENT OF BIO-RESOURCE ENGINEERING We accept this thesis as conforming to the required standard T H E UNIVERSITY OF BRITISH COLUMBIA July 1991 © Moez Mohamed Bouraoui, 1991 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Ql_0~ tE-MtlfCP £/V£ The University of British Columbia Vancouver, Canada DE-6 (2/88) Abstract Potato slices were dried using microwave drying, combined microwave and con-vective drying, and convective drying. Drying conditions included several slice thick-nesses, power levels and air temperatures. The profiles of temperature, moisture con-tent and relative humidity, as well as shrinkage data were generated. Dried products were rehydrated and rehydration kinetics were determined. In this study, drying characteristics of the different drying methods are dis-cussed and microwave drying is compared with convective drying. Microwave drying has a potential for producing better quality dried products while reducing considerably drying duration. In addition, moisture diffusivity profiles were calculated by solving Fick's dif-fusion model using the solution proposed by Crank (1975). Multiple regression analysis shows that calculated diffusivity correlates well with the internal temperature and mois-ture content of the product. u Table of Contents Abstract ii Table of Contents iii List of Tables v List of Figures vi Acknowledgement ix 1 Introduction 1 2 Literature Review 3 2.1 Drying using heated air 4 2.1.1 Constant-rate period 5 2.1.2 Falling-rate period 5 2.1.3 Disadvantages of hot-air drying 8 2.2 Microwave heating 8 2.2.1 Power dissipated 10 2.2.2 Penetration depth 10 2.2.3 . Rate of rise of temperature 11 2.2.4 Electric field strength 11 2.3 Microwave drying 14 3 Materials and Methods 17 iii 3.1 Experimental apparatus 17 3.2 Tests 19 3.3 Analysis 20 4 Results and Discussion 22 4.1 Typical results 22 4.1.1 Microwave drying 22 4.1.2 Combined microwave and cool air drying 25 4.1.3 Combined microwave and hot air drying 33 4.1.4 Hot air drying 33 4.2 Effects of variables 42 4.2.1 Effect of sample thickness 42 4.2.2 Effect of microwave power 42 4.2.3 Effect of air flow 49 4.2.4 Effect of probe location 49 4.3 Rehydration 49 4.4 Shrinkage data 50 4.5 Regression analysis 50 4.5.1 Microwave drying tests 60 4.5.2 Combined microwave and hot air tests 60 4.5.3 Combined microwave and cool air tests 60 4.5.4 Hot air tests 61 4.5.5 " Comparisons 61 5 Conclusions 62 Bibliography 64 i v List of Tables 3.1 Drying types and conditions 19 4.2 Shrinkage data of 1.5 cm thick potato slices dried by different techniques (Average of three replications) 50 4.3 Multiple regression analysis to correlate diffusivity to internal temperature and moisture 56 v List of Figures 3.1 Side View of the Drying Apparatus 18 4.2 Temperature profiles for drying of a 1.5 cm-thick potato slice at full mi-crowave power 23 4.3 Moisture profile for drying of a 1.5 cm-thick potato slice at full microwave power 24 4.4 Drying rate profile for drying of a 1.5 cm-thick potato slice at full mi-crowave power 26 4.5 Diffusivity profile for drying of a 1.5 cm-thick potato slice at full microwave power 27 4.6 Relative humidity profile for drying of a 1.5 cm-thick potato slice at full microwave power 28 4.7 Temperature profiles for combined microwave and cool air drying of a 1.5 cm-thick potato slice 29 4.8 Moisture profile for combined microwave and cool air drying of a 1.5 cm-thick potato slice 30 4.9 Drying rate profile for combined microwave and cool air drying of a 1.5 cm-thick potato slice 31 4.10 Relative humidity profile for combined microwave and cool air drying of a 1.5 cm-thick potato slice 32 4.11 Temperature profiles for combined microwave and hot air drying of a 1.5 cm-thick potato slice 34 vi 4.12 Moisture profile for combined microwave and hot air drying of a 1.5 cm-thick potato slice 35 4.13 Drying rate profile for combined microwave and hot air drying of a 1.5 cm-thick potato slice 36 4.14 Diffusivity profile for combined microwave and hot air drying of a 1.5 cm-thick potato slice 37 4.15 Temperature profiles for hot air drying of a 1.5 cm-thick potato slice. . . 38 4.16 Moisture profile for hot air drying of a 1.5 cm-thick potato slice 39 4.17 Drying rate profile for hot air drying of a 1.5 cm-thick potato slice. . . . 40 4.18 Diffusivity profile for hot air drying of a 1.5 cm-thick potato slice 41 4.19 Temperature profiles for drying a 2cm-thick potato slice at full microwave power 43 4.20 Moisture profile for drying a 2cm-thick potato slice at full microwave power. 44 4.21 Drying rate profile for drying a 2cm-thick potato slice at full microwave power 45 4.22 Temperature profiles for microwave drying (power 5) of a 1.5 cm-thick potato slice 46 4.23 Moisture profile for microwave drying (power 5) of a 1.5 cm-thick potato slice 47 4.24 Drying rate profile for microwave drying (power 5) of a 1.5 cm-thick potato slice 48 4.25 Moisture profile during the rehydration of a microwave dried (full power) 1.5 cm-thick potato slice 51 4.26 Diffusivity profile during the rehydration of a microwave dried (full power) 1.5 cm-thick potato slice 52 vii 4.27 Moisture profile during the rehydration of a hot air dried 1.5 cm-thick " potato slice 53 4.28 Diffusivity profile during the rehydration of a hot air dried 1.5 cm-thick potato slice 54 vm Acknowledgement I would like to express my sincere gratitude to Dr. P.F. Richard (my supervisor) for his valuable guidance, help and understanding. I wish to thank Dr. K.V. Lo and Dr. T. Durance for sitting on my committee, providing good advice and reviewing this thesis. I thank S. Willetts (a summer student), J. Pehike and N. Jackson (Technicians) for their valuable help in setting up the experimental apparatus. I appreciate the financial support of the Tunisian government and of the Nat-ural Sciences and Engineering Research Council of Canada. I thank my parents and all the members of my family for their unconditional help and great love. I extend my thanks to all my friends, especially those in the Department of Bio-Resource Engineering, in Vancouver and in Tunisia. ix Chapter 1 INTRODUCTION Drying of solids has been conducted since ancient times. Drying of foods, for example, was achieved by natural energies such as the sun and wind. New drying methods have developed quickly in the second half of this century. The main objective of food drying is to preserve it by reducing water activity. In addition, reducing product weight and volume results in reduced transport and storage costs. Drying can, however, cause deterioration of the quality of the dried product. Despite this, dried foods are gaining popularity especially with the growing resistance to the chemical preservation of foods. There is also a growing demand for a wider variety of dried foods offering considerable convenience to the consumers. Therefore extensive research on drying techniques has been conducted to improve product quality and energy utilization. In convective drying, moisture is removed initially from the surface of the prod-uct thus creating a moisture gradient. This moisture gradient is the main mechanism responsible for outward moisture flow. As the surface dries out, it poses an increasing resistance to heat and moisture transfer. As such, drying duration is generally quite long and surface overheating could occur. All this could bring about case hardening, solute migration, as well as other damages to the sensory and nutritional characteristics of the product. Microwave drying involves the conversion of electromagnetic energy into heat. Microwaves heat the product volumetrically with selective heating of the liquid compo-nents. This results in the rapid internal vaporization of moisture. The resulting gas 1 Introduction 2 pressure gradient removes moisture from the product without overheating the atmosphere or the surface (Schiffman, 1987). Microwave drying produces a more even moisture profile and thus a better quality dried product. Furthermore, it reduces energy and maintenance costs and can also be combined with convective drying. This study investigates the performance of drying potato slices by microwave, convective, and combined microwave and convective processes. The main objectives of the work are to: 1. Generate such data as the profiles of temperature, moisture, relative humidity, and shrinkage for different drying modes and conditions. 2. Determine the rehydration properties and kinetics of the dried products. 3. Explore the heat and mass transfer mechanisms that govern microwave drying. 4. Compare microwave drying with convective drying. 5. Investigate the characteristics of moisture diffusivity. C h a p t e r 2 LITERATURE REVIEW Thermal drying is the application of heat in order to remove moisture from a prod-uct. It involves simultaneous heat and mass transfer in a multiphase system (Chen and Pei, 1989). By reducing water activity, drying allows a longer preservation of foods due to the limitation of microbial growth and enzyme activity. Another objective of drying is reduction of weight and volume which cuts down transport and storage costs. Drying is also frequently used to create the proper texture and flavour of such foods as raisins, prunes, etc. Drying could also offer more variety and convenience to the consumer. Hayashi (1989) indicated that humankind has been drying food since the old stone age. Natural energies such as sunlight and wind were used. These methods are slow and unreliable especially as they depend upon the weather. Since the end of World War II, mechanical drying has developed quickly. Early methods included trucked-tray drying, drum drying, and transfer ventilation drying. Nevertheless, such problems as protein denaturation, fat oxidation, destruction of vitamins, browning reac-tion by aminocarbonyls, and "off' taste can occur (Hayashi, 1989) resulting in deteriora-tion of eating quality and nutritive value of the food. In order to improve the quality of dried foods and the energy utilization, new drying methods were developed; spray dry-ing, flash drying, fluidized drying, vacuum drying, vacuum freeze drying, and microwave drying (Hayashi, 1989). Because foods constituents have different sensitivities to heat, the appropriate drier has to be carefully selected if good quality dried product is to be 3 Literature Review 4 obtained. Porter et al. (1973) cited nineteen types of driers that may be used to handle eight types of materials. A more recent classification of drying processes was done by Hayashi (1989). Rockland (1969) indicated that three types of bound water may exist in food products, namely: 1. Water molecules which are bound to ionic groups such as carboxyl and amino groups (the most difficult to remove); 2. Water molecules which are hydrogen-bonded to hydroxyl and amide groups, and 3. Unbound free water found in interstitial pores (the easiest to remove). Hot air drying and microwave drying will be described in this chapter. 2.1 Drying using heated air When hot air is blown over a product, the outer layer is heated by convection while conduction heats the remainder of the product (Buono and Erickson, 1985). Knowledge of drying mechanisms is indispensable when predicting methods for increasing the drying rate or for improving product quality (Chirife, 1983). Van Arsdel and Copley (1963) indicated some mechanisms that govern air drying. However, little work has been done to clarify which of the mechanisms prevail under different circumstances (King, 1977). Fellows (1988)^  suggested that water moves to the surface by the following mechanisms: 1. Liquid movement by capillary forces; 2. Diffusion of liquids, caused by differences in the concentration of solutes in different regions of the food; Literature Review 5 3. Diffusion of liquids which are absorbed in layers at the surfaces of solid components of the food, and 4. Water vapor diffusion in air spaces within the food caused by vapor pressure gra-dients. The convective drying process may be classified into two main categories: the constant-rate period and the falling-rate period. 2.1.1 Constant-rate period In this period, water moves from the interior of the food at the same rate as it evaporates from the surface and the surface remains wet. Moisture content at which the constant-rate period ends is termed critical moisture content. Internal moisture transfer is mainly attributable to capillary flow of free water, which is caused by the moisture gradient. The constant drying rate depends on such external conditions as temperature, humidity and flow rate of the convective medium (Chen and Pei, 1989). Labuza and Simon (1970), Chirife and Cachero (1970), Vaccarezza et al. (1974) and Alzamora et al. (1979) did not find a constant-rate period during air drying of apples, tapioca, sugar beet root, and avocado slabs, respectively. It can be deduced that the constant-rate period may be only important in cases where, for instance, the drying potential of air is very low or the moisture content of the food is very high (Chirife, 1983). 2.1.2 Falling-rate period When the moisture content of a product falls below the critical moisture content, the falling-rate period starts. The rate of moisture movement to the surface falls below the rate of water evaporation to the surrounding air. Hence, the surface dries out. Literature Review 6 Fellows (1988) indicated that non-hygroscopic materials (materials having a constant water vapor pressure at different moisture contents) have a single falling-rate period. Conversely, hygroscopic materials (materials in which the partial pressure of water vapor varies with the moisture content) have two falling-rate periods. The suggested transport mechanisms in the first falling-rate period were prin-cipally capillary flow, liquid diffusion and vapor-phase diffusion (Chirife, 1983). The plane of evaporation moves inside the food, and water diffuses through the dry solids to the drying air (Fellow, 1988). Chirife (1983) stated that Fick's law in terms of moisture gradient constitutes a good model for describing the drying characteristics of most foods in the initial phase of the falling-rate period. The Fick's diffusion model for unidimensional flow is (Yusheng, 1988): 8W 82W St = (2'5) where: W = Average moisture content (kg. water/kg. dry solid) D = Diffusivity (m2/s) t = time (s) x = Linear coordinate (m) Assuming uniform initial moisture distribution and negligible external resis-tance, the solution expressed in terms of the average moisture content of the slab is (Crank, 1975): Literature Review 7 W-We 8 ~ 1 . ,n 2Dt, , x where: Wo = Initial moisture content (kg. water/kg. dry solid) We = Equihbrium moisture content (kg. water/kg. dry solid) L = Solid thickness (m). The second falling-rate period occurs when the partial pressure of water is below the saturated water vapor pressure (Fellows, 1988). The surface moisture con-tent reaches its maximum sorptive value, no free water exists and a receding evaporation front appears dividing the system into a wet region and a sorption region (Chen and Pei, 1989). Inside the evaporation front, the material is wet, i.e. the voids contain free water and the main mechanism of moisture transfer is capillary flow. Outside the front, no free water exists. All water is in the sorptive or bound water state and the main mechanisms of moisture transfer are the movement of bound water and vapor transfer (Chen and Pei, 1989). King (1968) suggested that water vapor pressure is the driving force for moisture transport. Chirife (1983) mentioned that if vapor pressure is the driving force, Fick's law should be written as follows: 3W = bcPp dt pdx2 { ' ] where: b = vapor-space permeability (kg. / Pa. m. sec.) p = dry solid density (kg/m3) p = vapor pressure of water in food (Pa.) Literature Review 8 2.1.3 Disadvantages of hot-air drying Fellows (1988) stated that the main disadvantages of hot air drying are: 1. Low rates of heat transfer due to the low thermal conductivity of dry foods, which results in long drying duration; 2. Damage to sensory characterises and nutritional properties caused by long drying times and overheating at the surface; 3. Oxidation of pigments and vitamins by hot air, and 4. Case hardening (formation of a hard impermeable skin) due to high moisture gra-dient between the interior and the surface of the food and to the solutes migration from the interior of the food to the surface. 2.2 Microwave heating Microwave energy is an electromagnetic radiation, the frequency of which is de-fined as being between 500 and 5000 MHz (Jones, 1986.). Microwave food heating usually employs specific frequency bands (2450 MHz, sometimes 896 MHz in Europe and 915 MHz in the USA) (Fellows, 1988). The microwave energy can be generated from a magnetron. Knutson et al. (1987) denned a magnetron as a cylindrical diode with the cathode located in the center and the anode around the circumference. When power is supplied, an electron emitting material at the cathode becomes excited and emits elec-trons into a vacuum space between the cathode and the anode. The magnetic field is created by a magnet surrounding the magnetron. The energy of the electrons becomes entrapped in the field and travels as waves through the magnetron to the antenna. The antenna transmits the oscillating waves to the wave guide (a hollow tube) in which they Literature Review 9 travel to the oven cavity (Knutson et al., 1987). A wave stirrer can disperse the waves and improve wave distribution uniformity (Ringle and David, 1975). Dielectric materials such as foods can react to an electric field because they contain charge carriers which can be displaced (Von Hippel, 1966). The two main mechanisms that govern microwave heating of dielectric materials are dipole rotation and ionic polarization. As an alter-nating field is applied, molecules carrying dipolar electrical charges such as water rotate as they attempt to align their dipoles with the rapidly changing electric field. The resultant friction creates heat which gets transferred to neighboring molecules. On the other hand, charged ions such as chloride(-) and sodium(-f) flow toward the alternating electric field (Best, 1987). Ions collision converts kinetic energy into heat (Decareau and Peterson, 1986). Microwave heating depends on the physical state of the material. In ice, for example, the movement of water molecules in a microwave field is restricted and, therefore, ice is a poor microwave absorber (Decareau and Peterson, 1986). Mud-gett (1985) cited the literature relevant to food dielectric properties. He indicated that the basic dielectric properties of foods are related to their chemical composition, modified by physical structure, and are highly frequency and temperature dependent. A better understanding of the relationship of food dielectric properties to food microwave heating characteristics is needed for the design, analysis and the development of new applica-tions in microwave food processing (Mudgett, 1985). The dielectric permittivity e* is a macroscopic parameter characterising the behavior of a material in a microwave field (Goyette et al. 1990). e = e - je where: (2.4) Literature Review 10 e = dielectric loss factor (which determines the absorbed power) e = dielectric constant (which is related to the amount of energy that can be stored in a material in the form of electric fields (Komolprasert and OfoH, 1988). The dielectric constant and the dielectric loss factor of foods are primarily determined by free water and salt contents and are also related to other electrical prop-erties that affect the coupHng of microwave energy and its distribution within the product (Mudgett, 1985). 2.2.1 Power dissipated Microwave heating involves the conversion of electromagnetic energy into heat. Metaxas and Meredith (1983) indicated that the heat transferred to each unit volume of material placed in the path of microwaves is given by (Decareau and Peterson, 1986): P = 55.61 * 10-14fE2e" (2.5) where: P = power absorbed by unit volume, W / m 3 f = frequency, Hz E = electric field strength, V/m e" = dielectric loss factor 2.2.2 Penetration depth The depth of penetration of microwaves into a product is given by (Fellows, 1988): Literature Review 11 x = A (2.6) 27TV e' tan £ where: x = depth of penetration, m A = wavelength (in vacuum), m e' = dielectric constant tan 5 = tan = loss tangent. 2.2.3 Rate of rise of temperature The power required to raise the temperature of a material, subject to microwave energy, from T 0 to T (deg.C) in t seconds is given by (Metaxas and Meredith, 1983): P = ^ ' r i - r ° » (2.7) where: P = power absorbed, W/m 3 Cp = Specific heat of the material, J/kg deg.C p = density of the material, kg/m 3. Hence, (1^1 = 4- (2.8) t pCP v ' 2.2A Electric field strength Metaxas and Meredith (1983) mentioned that the electric field strength can be determined through calorimetry: P = 55.61 * lO- 1 4/^" = p C p ( T ~ T o )  J t (2.9) Literature Review 12 Therefore, / pCp(T-TQ) ~ V 55.61 * l Q - " / « V ( 2 - 1 0 ) Jones (1986) reported that microwave heating is widely found in textile and paper-based industries where it is used for drying, and in the polymer industry where it is used for preheating granules or blocks of polymer prior to moulding as well as for welding PVC and similar materials. The lack of understanding of how microwaves interact with materials during heating, and the cost present major barriers to wider use of microwaves (Jolly and Turner, 1990). Still, the high rates of heating and the reduced damage to product quality increased the applicability of microwave heating in the food industry. Mudgett (1985) classified food microwave processes as follows, 1. Dehydration: to reduce moisture content 2. Blanching: to inactivate spoilage enzymes 3. Pasteurization: to inactivate vegetative microbes 4. Sterilization: to inactivate microbial spores 5. Cooking: to modify flavor and texture 6. Tempering: to raise temperature below freezing Products and packages suitable for use in domestic microwave ovens has been developed (Anon, 1987a,b). The most important industrial applications are thaw-ing, tempering, dehydration and baking. These were reviewed by Rosenberg and Bogl Literature Review 13 (1987). Fellows (1988) pointed out that applications involving foods with high moisture contents (for example blanching and pasteurization) are less successful. He attributed this to the low depth of penetration in large pieces of food and to evaporative cooling at the surface, which results in the survival of large numbers of micro-organisms. Lin et al. (1989) reported that food microwave heating may cause uneven cooking of large sized products, lack of browning, overheating, boil over, volcano effects due to steam build-up, among other problems. They attributed the occurrence of those problems to the lack of knowledge of the simultaneous heat transfer, moisture migration, chemical reactions and biological changes occurring during microwave heating. They used a finite element method to describe temperature distribution in slab and cylinder-shaped solid food products of high moisture contents during microwave heating. Datta (1990) stated that the three characteristic parameters determining the temperature profile are: the sample size in relation to microwave penetration depth, the boundary conditions (surface evaporation, convection, and radiation), and the sample shape. Ofoli and Komolprasert (1988) reviewed three modeling approaches used for the analysis of energy deposition in materials in a microwave environment: analysis of the heating potential, analysis of the power absorbed, and solution of the general energy equation. They discussed the importance of the electric field strength in the thermal modeling of mi-crowave systems, and outlined a procedure for its mapping. They also suggested the inclusion of moisture transfer terms in the analysis of food microwave heating. Dimen-sional analysis was employed to develop a predictive mathematical model for microwave heating analysis (Komolprasert and Ofoli, 1988). This model is limited, however, to water and other materials with similar physical, thermal and electromagnetic properties. Jolly and Turner (1990) developed a model for describing the power and temperature distributions in materials with varying dielectric properties. They solved numerically a non-linear system by finite difference method. The use of a dielectric layer, such as Literature Review 14 teflon, placed between the metal backing and material, allowed the control of the heat transfer and the associated temperature gradients (Jolly and Turner, 1990). 2.3 Microwave drying Microwave heating can heat even thick materials rapidly throughout their vol-ume. Besides, microwaves heat selectively the areas with high liquid content. Those properties, among others, brought about the commercial acceptance of the application of microwave heating in the drying of a number of products in the food, textile, wood and chemical industries (Chen and Schmidt, 1990). When a wet solid is exposed to microwave heating, its temperature may reach the boiling point of the liquid. The accompanying generation of vapor due to internal evaporation of moisture brings about a gas pressure gradient which can rapidly expel the moisture from the interior of the solid (Metaxas and Meredith, 1983). This process leads to very rapid drying without over-heating the atmosphere or the surface (Schiffman, 1987), and prevents case hardening since little solute migration in the liquid phase occurs (Knutson et al. 1987). Richard et al. (1990) found that in microwave drying most of the moisture could be rapidly ex-pelled while the product temperature remains close to the boiling point temperature of water. As far as food processing is concerned, microwave drying is mostly used for finishing operations of partly dried or low-moisture foods. Commercial appUcations include pasta-drying, finish drying of potato chips, and grain drying (Decareau, 1985). Mudgett (1989) reported the use of microwave heating for the drying of condiments, tomato paste, wild rice, snack foods, and bacon pieces. Bouraoui and Richard (1991) Literature Review 15 reviewed the utilization of microwave drying for the rapid moisture content determina-tion in foods. Despite the multitude of advantages that microwave drying has over the other drying methods, it is still not sufficiently applied in the food industry because of economic constraints and because of the lack of knowledge of the heat and mass transfer characteristics of microwave drying of foods. Perkin (1979) discussed the advantages and limitations of microwave drying. Several studies have been conducted in order to investigate the characteristics of microwave drying. Kudra et al. (1990) prepared a bibliography covering publications on dielectric drying. Lyons and Hatcher (1972) measured local temperatures, moisture content, and pressure within a wet porous mate-rial (wet cotton) heated by microwaves. They found a very small mass concentration gradient. Temperature gradients were also small. An analytical model of internal transport characteristics of a textile material during microwave drying was developed by Hatcher et al. (1975). Roussy et al. (1984) proposed a simple model for the mi-crowave drying of wet spherical particles of paper. The model was based on a first order kinetics law, the constant coefficient of which depended linearly on the square of the applied electric field. Bergman et al. (1987) studied the combined microwave and convective drying of a non-hygroscopic porous beds consisting of glass beads of differ-ent sizes. In beds of small bead sizes, drying rates were increased because of more uniform moisture distribution due to capillary effects. Dielectric drying of particulate materials in a fluidized bed improved the quality of the material being dried because of the uniform temperature maintained throughout the bed and because of the slow (as compared to dielectric drying alone) temperature increase in the course of drying (Kudra, 1989). Chen and Schmidt (1990) developed an integral model for simulation of dielectrically-enhanced convective drying behavior in hygroscopic (activated alumina spheres) and nonhygroscopic materials (glass beads). The model effectively estimated drying rates and internal temperature histories. This model was also used by Chen Literature Review 16 and al. (1990) to simulate microwave drying of hardwood veneer. The accuracy of the model for predicting drying times was satisfactory. Perkin (1990) used simplified modeling for the drying of non-hygroscopic capillary porous materials with dielectric and convective drying. This researcher also examined qualitatively the additional internal moisture flow mechanisms arising as a result of the volumetric heating. Experiments and simulations for dielectrically-assisted drying of a nonhygroscopic material that ex-hibits negligible capillary moisture transfer (water-saturated packed bed of glass beads) have been conducted by Grolmes and Bergman (1990). Garcia et al. (1988) studied the drying of bananas with microwave and air ovens. The drying data was fitted to a variable diffusion model. Product temperature was not monitored, however. So far there is little research reported on moisture diffusion determinations during microwave drying. Such research is necessary for modeling development of mi-crowave drying processes. Chapter 3 M A T E R I A L S A N D M E T H O D S 3.1 Experimental apparatus Figure 3.1 depicts the experimental apparatus of this work. A modified 700 Watt household microwave oven with a mode stirrer was used. The oven cavity con-tained a nylon basket suspended from a top mounted balance (Electronic Analytical Sortorius Balance) for continuous monitoring of weights. During each drying test, the internal and surface temperatures of a potato slice were continuously measured by means of three Luxtron Fluoroptic temperature probes (Model 755). These are fibre-optic probes with a temperature sensitive phosphor mounted at the end of each probe. When excited with blue-violet fight, the phosphor responds with a deep red fluorescence, which varies with temperature (Anonymous, 1988). Temperature and weights were recorded in a personal computer (I.B.M.-compatible Campus 386) using a data acqui-sition system. Air flow could be supplied to the sample from a duct in the top of the microwave oven. Electrical resistances allowed the heating of the air. The tempera-ture and the velocity of the air were controlled. A Humeter was used to measure the relative humidity of the air leaving the microwave oven. Also, circulating water was introduced to protect the magnetron from the reflection of microwaves when the sample was dry. 17 Materials and Methods Side View of the Apparatus. Air out Balance Hot air in Fluoroptic probes Electric heating element Front of microwave oven Weighing basket Fan Computer for temperature and weight data acquisition Figure 3.1: Side View of the Drying Apparatus. Materials and Methods 19 Table 3.1: Drying types and conditions. Parameter Number Description Heating 4 Hot air, Microwave, Microwave and hot air, and Microwave and cool air Thicknesses 3 1 cm, 1.5 cm, and 2 cm Power settings 2 10 and 5 Air temperatures 2 18°C and 65°C Air flow rate 1 0.032m3/sec. 3.2 Tests Four types of drying were used: 1. Microwave drying 2. Combined microwave and hot air (65°C) drying 3. Combined microwave and cool air (18°C) drying 4. Hot air (65°C) drying Two microwave power levels were employed; 5 and 10. Power level 10 was full power. Microwave energy was emitted for only half the drying time when the microwave was set at power level 5. For each test, transverse potato slices (Russet potatoes, Solanum tuberosum) with thicknesses of 1 cm, 1.5 cm, and 2 cm were used. Potato was chosen because it is inexpensive, easily available, has a convenient geometry, and its drying characteristics have been well documented. Only one slice was dried at a time. Initial and final slice dimensions were measured. For every drying test, three replications were made. Table 3.2 summarizes the drying tests of this study. Materials and Methods 20 Once dried, sample rehydration in boiling water was undertaken. Sample weight was measured at selected time intervals (around two minutes for microwave and combined tests, and in the range of ten minutes for samples dried by convective drying). 3.3 Analysis Knowing the sample mass variation with time, the moisture content profiles and the drying rate profiles were calculated. In order to determine the diffusivity variation during drying tests, Fick's dif-fusion model for unidimensial flow (neglecting the radial diffusion) was used: where: W = Average moisture content (kg. water/kg. dry solid) D = Diffusivity (m2/sec.) t = time (sec.) x = linear coordinate. The solution proposed by Crank (1975) was: (3.12) W-We 8 ~ 1 Dt Wq = Initial moisture content (kg.water/kg. dry solid) Where: We = Equilibrium moisture content (kg. water/kg. dry solid) L = solid thickness (m) Simplifying this solution by taking only the first term of the series and by assuming that We = 0, gives: Materials and Methods 21 0 = ±exp[_w*Ri} (3-13) 7T h Where 8 = % Deriving the last equation with respect to time gives 88 x2D dt L2 8 (3.14) Hence at given intervals of time, and knowing ^ and the average 8, the diffu-sivity values can be determined using the following equation: 88 T2 D = ( -s»<^?) <3-15> Multiple regression analysis was also conducted in order to correlate the calcu-lated diffusivity to such properties as the internal sample temperature, T and the sample moisture content, W (dry basis). The relationship used was: D = TaWb (3.16) A statistical package (Systat) was used in order to determine the constants a and b of equation 3.16. Systat was also used to undergo an analysis of variance so as to test the variations of a and b for different drying conditions such as drying methods, power levels and slice thicknesses. A completely random design was employed. Chapter 4 R E S U L T S A N D DISCUSSION 4.1 Typical results Typical results of potato slice drying using different methods are presented and discussed in this section. 4.1.1 Microwave drying The results of microwave drying, at full power, of a 1.5 cm-thick potato slice are shown in Figure 4.2 and Figure 4.3 for temperature and moisture profiles respectively. Figure 4.2 shows the profiles of surface temperatures (from the probe placed just into the surface) and profiles of internal temperatures (from the two other probes located deeper into the slice). The figure shows that in the beginning, sample tempera-ture increased rapidly above 100°C and then stabilized at the boiling point temperature of water. When the boiling point of water was reached, moisture removal started. Most of moisture removal occurred while the temperature remained close to 100°C. This temperature stabilization can be attributed to evaporative cooling caused by internal evaporation. In the begining product temperature exceeded 100°C because internal evaporation was not enough to stabilize the temperature. At low moisture levels, product temperature rose again because the remaining water was too little to stabilize the temperature. It can be deduced from temperature profile that vapor flow is the major 22 Temperature profiles Microwave drying 18O1 0 50 100 150 200 250 300 350 400 450 Time (s.) Surface temperature — Internal temp. Results and Discussion 24 Moisture profile Microwave drying 1.2T Time (s.) Figure 4.3: Moisture profile for drying of a 1.5 cm-thick potato slice at full microwave power. Results and Discussion 25 mechanism of moisture removal. Moreover, surface temperatures were generally lower than internal temperatures except at low moisture levels. This is due to the cooling effect of the surrounding cooler air. Drying duration was about ten minutes. Drying rate profile is shown in Figure 4.4. High drying rate values were observed when the product temperature was near the water boiling point temperature. Then drying rates decreased with decreasing moisture contents. Diffusivity profile is shown in Figure 4.5. Diffusivity values increased with time. However, diffusivity values calculated at very low moisture contents should not be considered. This is because calculated diffusivity is inversely proportional to moisture content (see equation 3.15) and hence very small values of moisture content give values of diffusivity that are too high to be accepted. Figure 4.6 shows that relative humidity in the chamber increased up to 70 %. It decreased again when most of moisture was removed. 4.1.2 C o m b i n e d m i c r o w a v e a n d cool air d r y i n g The results of combined microwave (at full power) and cool air (18°C at 0.032m3/sec.) drying of a 1.5 cm- thick potato slice are shown in Figures 4.7 to 4.10 for the profiles of temperatures, moisture content, drying rate, diffusivity, and relative humidity respec-tively. These results are very similar to those of microwave drying with the exception that relative humidity did not exceed 45%. For the same power level (full power) and slice thickness (thickness=1.5 cm), analysis of variance (using a statistical package, Sys-tat) showed that drying times did not differ significantly (0.05 was the chosen significance level) from those of microwave drying alone (P=0.624). The maximum drying rates did not differ significantly either (P=0.823). Results and Discussion 26 0.012-"sec.) 0.01-o OT 0.008-water 0.006-d> rate 0.004-Drying 0.002-Drying rate profile Microwave drying 100 200 300 400 Time (s.) 500 600 Figure 4.4: Drying rate profile for drying of a 1.5 cm-thick potato slice at full microwave power. Results and Discussion 27 8000^ 7000-o i—1 6000-* 5000-«3 4000->-> "> 3000-"tn 3 2000-Q 1000-0-C Diffusivity profile Microwave drying 600 Figure 4.5: Diffusivity profile for drying of a 1.5 cm-thick potato slice at full microwave power. Results and Discussion 28 80-70-60->. midi 50-n CD > 40-ca CD 30-OC 20-ii 10+ 0 Relative humidity profile Microwave drying 300 400 500 600 Time (s.) Figure 4.6: Relative humidity profile for drying of a 1.5 cm-thick potato slice at full microwave power. o o o 1-1 CD p> <Tt-O O o O o % a P> P-. n o o o cb Q) 13 Q) ZJ 1» CD a E 200 Temperature profiles Microwave and coo air drying 150 200 250 300 350 400 450 500 Time (s.) -era o P Surface temperature Internal temp. Results and Discussion 30 Moisture profile Microwave and cool air drying 1.2 1 CO "o 0.8-ra v- 0.6-"c " E 0.4-"o 0.2-o- 400 500 600 Time (s.) Figure 4.8: Moisture profile for combined microwave and cool air drying of a 1.5 cm-thick potato slice. Results and Discussion o cu CO « •o 03 c Q 0.014 0.012 =5 0.01 o CO cb S 0.008 CD ch 0.006 g 0.004 0.002 Drying rate profile Microwave and cool air drying 200 300 400 Time (s.) 500 600 Figure 4.9: Drying rate profile for combined microwave and cool air drying of a 1.5 cm-thick potato slice. Results and Discussion 32 45-1 40-35-humi 30-( D > C O 25-C U rr Relative humidity profile Microwave and cool air drying 20 15 100 200 300 400 500 600 Time (s.) Figure 4.10: Relative humidity profile for combined microwave and cool air drying of a 1.5 cm-thick potato slice. Results and Discussion 33 4.1.3 Combined microwave and hot air drying The results of combined microwave (at full power) and hot air (65°C at 0.032m3/sec.) drying of a 1.5 cm-thick potato slice are presented in Figures 4.11 to 4.14. Figure 4.11 shows that the duration of temperature stabilization was shorter than that in the case of microwave drying. For unknown reasons, irregularities were observed in the temper-ature profiles at the end of drying. Figure 4.13 and Figure 4.12 show that drying rates were higher and that drying duration was a little shorter when compared with microwave drying alone. From analysis of variance (0.05 was the chosen significance level), these differences were not significant (P=0.182 for drying rates and P=0.079 for drying times). Diffusivity profile is shown in Figure 4.14. Relative humidity values remained close to 14%. 4.1.4 Hot air drying The results of hot air (65°C at 0.032m3/sec.) drying of a 1.5 cm-thick potato slice are shown in Figures 4.17 to 4.18. Drying duration was longer than 900 minutes. A constant rate period was observed. Drying rates were generally more than sixty times lower than those of microwave drying (see Figure 4.17 and Figure 4.16). Figure 4.15 shows that surface temperatures were, most of the time, higher than internal temper-atures. Most of moisture is thought to be removed in liquid form because most of the drying occurred while sample temperatures were below 50°C (well below the boiling point of water). The calculated diffusivities are shown in Figure 4.18. a 2 ? cm C . a o o !T o <T> tn p 2 ft CD M P trl-n *d >-» o o o-5' o ex. o a-Temperature profiles Microwave and hot air drying 180-150 200 250 300 350 400 Time (s.) 450 Surface temperature Internal temp. Results and Discussion 35 1.2-1i CD Tn °o 0.8-0.6-IS" 0.4-'0 0.2-Moisture profile Microwave and hot air drying 600 Time (s.) Figure 4.12: Moisture profile for combined microwave and hot air drying of a 1.5 cm-thick potato slice. Results and Discussion 36 o cu CO * T 3 CO c Q 0.014 0.012A =3 0.01 o CO CD £ 0.008 £ ro C D 0.006 2 0.004 0.002 Drying rate profile Microwave and hot air drying 300 Time (s.) 600 Figure 4.13: Drying rate profile for combined microwave and hot air drying of a 1.5 cm-thick potato slice. Results and Discussion Diffusivity profile Microwave and hot air drying 8000n -7000-o o i-H 6000-* OS 5000-CS 4000- • >> "S 3000-• )iffusi 2000- • • • 1000- - - " " 0- * i * * — i 1 1 1 1 1 1 1 50 100 150 200 250 300 350 400 450 500 Time (s) Figure 4.14: Diffusivity profile for combined microwave and hot air drying of a 1.5 cm-thick potato slice. Temperature profiles Hot air drying 50 1 5 0 100 200 300 400 500 600 Time (min.) Surface temperature Internal temp. Results and Discussion 39 M o i s t u r e p r o f i l e Hot air drying 1.2T -Time (min.) Figure 4.16: Moisture profile for hot air drying of a 1.5 cm-thick potato slice. Results and Discussion 40 Drying rate profile Hot air drying 0.012 100 200 300 400 500 600 700 800 900 1000 Time (min.) Figure 4.17: Drying rate profile for hot air drying of a 1.5 cm-thick potato slice. Results and Discussion o o S Q Diffusivity profile Hot air drying 100 200 300 400 500 600 700 800 900 1000 Time (min.) Figure 4.18: Diffusivity profile for hot air drying of a 1.5 cm-thick potato slice. Results and Discussion 42 4.2 Effects of variables 4.2.1 Effect of sample thickness Results of microwave drying (at full power) of a 2 cm-thick potato slice are dis-cussed. The following figures show the profiles of moisture content, drying rate, and temperatures. When compared with microwave drying of thinner slices, the following conclusions were drawn: 1. The thicker the slice, the longer the drying duration. 2. The thicker the slice, the lower the drying rate. 3. At low moisture level, the thicker the slice the higher the temperatures reached. These conclusions are also valid for combined microwave and convective dry-ing. 4.2.2 Effect of microwave power The results of microwave drying (power setting 5) of a 1.5 cm-thick potato slice were as follows. Figure 4.22 shows jagged temperature profiles due to the fact that at power setting 5, the magnetron was on for only half the drying time. In comparison with microwave drying at full power, microwave drying at power setting 5 is characterized by (see Figure 4.23, Figure 4.24, and Figure4.22): 1. Longer drying duration 2. Lower drying rates 3. Lower product temperatures at low moisture levels These characteristics were also observed in the combined microwave and con-vective drying tests. Temperature profiles Microwave drying 180n 160-° 0 50 100 150 200 250 300 350 400 450 Time (sec.) Surface temperature — - Internal temp. Resulis and Discussion 44 1.2-1 CD "55 'o 0.8-ro j ~ 0.6-c (D 0.4-00 'o 0.2-0--0 Moisture profile Microwave drying - r 200 300 400 500 600 700 800 Time (s.) Figure 4.20: Moisture profile for drying a 2cm-thick potato slice at full microwave power. Results and Discussion 45 0.009 d 0.008 CD i | 0.007-1 ° 0.006 j 0.005-1 5 0.004H 3 2 0.003 co o) 0.002-1 0.001 Drying rate profile Microwave drying 100 200 300 400 500 600 700 800 Time (S.) Figure 4.21: Drying rate profile for drying a 2cm-thick potato slice at full microwave power. Temperature profiles Microwave drying 180 ° 0 100 200 300 400 500 600 700 800 900 Time (s.) Surface temperature Internal temp. Results and Discussion 47 Moisture profile Microwave drying 1.2i Time (s.) Figure 4.23: Moisture profile for microwave drying (power 5) of a 1.5 cm-thick potato slice. Results and Discussion 48 0.01 "Jj 0.009 *OT 0.008 •g 'o 0.007H CO -5? 0.006-« 0.005-3 0.004 IJ 0.003-I c 0.002 Q 0.001 Drying rate profile Microwave drying 200 400 600 800 Time (s.) 1000 1200 Figure 4.24: Drying rate profile for microwave drying (power 5) of a 1.5 cm-thick potato slice. Results and Discussion 49 4.2.3 Effect of air flow When cool air flow was combined with microwave heating, the results were very similar to those of microwave drying alone. The values of relative humidity, however, were more limited. On the other hand, the combination of hot air with microwave heating resulted in a reduction of temperature stabilization period and in the stabilization of relative humidity in the chamber. 4.2.4 Effect of probe location Except at low moisture levels, surface temperatures were lower than the internal ones during microwave and combined drying tests. At low moisture levels, even internal temperatures were quite different. This was thought to be due to the nonhomogeneity of the product and probably to the change in the positions of the temperature probes caused by the product shrinkage. 4.3 Rehydration Rehydration tests were undertaken as an indicator of dried product quality. Mi-crowave and combined microwave and convective dried potato slices were rapidly and almost completely rehydrated. This is thought to be due to the fact that the pressure build up during microwave drying caused opening of pores and allowed complete rehy-dration. Rehydration kinetics following microwave and combined drying tests were very similar. Figure 4.25 and Figure 4.26 show the profiles of moisture and diffusiv-ity, respectively, during the rehydration of a microwave dried, 1.5 cm thick potato slice. Diffusivity decreased with time as opposed to diffusivity profiles in microwave drying, which increased with time. This was due to differences in the mechanisms of moisture diffusion between drying and rehydration. In contrast, the rehydration of hot air dried Results and Discussion 50 Table 4.2: Shrinkage data of 1.5 cm thick potato slices dried by different techniques (Average of three replications). Drying tests Radial shrinkage (%) Thickness shrinkage (%) Microwave (Full power, 1.5 cm-thick slice) 16.2- (3.032") 22.2 (3.811) Microwave (Power 5, 1.5 cm-thick slice) 13.6 (0.48) 17.8 (3.851) Hot air ( 1.5 cm-thick slice) 14.6 (3.466) 44.45 (3.851) * Mean. ** Standard deviation. potato slices did not exceed 80% (see Figure 4.27 and Figure 4.28) and was much slower. This was thought to be due to case hardening and closing of surface pores brought about by hot air drying. 4.4 Shrinkage data Table 4.2 presents typical results of shrinkage (values are averages of three replica-tions). The degree of shrinkage that occurred during microwave drying was very close to that occurring during combined microwave and convective drying. Furthermore, slice shrinkage in microwave drying at power setting 5 was less than that at full power. More thickness shrinkage took place during hot air drying alone despite lower temperatures. Therefore, in comparison with convective drying, microwave drying allowed better rehy-dration while causing less product shrinkage. 4.5 Regression analysis A statistical package (Systat) was used in order to correlate the calculated dif-fusivities, D with the internal sample temperature, T (deg.C) and the sample moisture Results and Discussion 51 Moisture profile (rehydration) 6 8 10 12 Time (min.) Figure 4.25: Moisture profile during the rehydration of a microwave dried (full power) 1.5 cm-thick potato slice. Results and Discussion 52 Diffusivity profile (rehydration) 120y o 1 1 100-o s usands) 80-60-_>> o [> H ' co S ta 40-Q 20-T 1 1 1 1 " 1 1— 1 T 3 4 5 6 7 8 9 10 11 12 Time (s) Figure 4.26: Diffusivity profile during the rehydration of a microwave dried (full power) 1.5 cm-thick potato slice. Results and Discussion Moisture profile (rehydration) 0.9j 0.8-CD 0.7-to 0.6-'o CO 0.5-'c 0.4-3 00 0.3-"o 0.2-0.1-0* 0 30 Time (min.) Figure 4.27: Moisture profile during the rehydration of a hot air dried 1.5 cm-thick slice. Results and Discussion 54 Diffusivity profile (rehydration) Time (min.) Figure 4.28: Diffusivity profile during the rehydration of a hot air dried 1.5 cm-thick potato slice. Results and Discussion 55 content, W (dry basis). The model used is: D = TaWb (4.17) Where: a and b are constants . Diffusivity unit is (m2 x 10lo/sec.) for microwave and combined drying tests. It is (m2 x lO10/min.) for convective drying tests. Table 4.3 summarizes the multiple regression results of this model for different drying tests. R2 is the squared multiple regression coefficient. Pa indicates the sig-nificance of temperature (if Pa is very small, then temperature has a significant effect on the model). P 0 indicates the significance of moisture (if Pj, is very small, then moisture has a significant effect on the model). The values of R2 indicate that diffusivity correlates very well with the tem-perature and moisture. Moreover, the values of Pa and P(, were very small (generally less than 0.05, the chosen significance level), which means that both the temperature and moisture have significant effect on the model. Results and Discussion 56 Table 4.3: Multiple regression analysis to correlate diffusivity to internal temperature and moisture. Drying tests a b R2 Pa Pb n Microwave (full power, 1 cm-thick slice) 1 1.466 -0.293 0.998 0.000 0.000 19 2 1.347 -0.500 0.985 0.000 0.002 23 3 1.389 -0.404 0.988 0.000 0.002 22 Microwave (Power 5, 1 cm-thick) 1 1.300 -0.403 0.988 0.000 0.000 34 2 1.227 -0.531 0.989 0.000 0.000 40 3 1.264 -0.432 0.995 0.000 0.000 41 Microwave (full power, 1.5 cm-thick) 1 1.576 -0.400 0.997 0.000 0.001 19 2 1.545 -1.025 0.988 0.000 0.002 20 3 1.475 -0.205 0.995 0.000 0.159 21 Microwave (Power 5, 1.5 cm-thick) 1 1.388 -0.943 0.983 0.000 0.000 37 2 1.393 -1.039 0.990 0.000 0.000 40 3 1.422 -1.147 0.985 0.000 0.000 39 Microwave (full power, 2 cm-thick) 1 1.776 -1.427 0.991 0.000 0.015 18 2 1.857 -2.156 0.991 0.000 0.000 21 3 1.682 -0.994 0.996 0.000 0.000 21 Microwave (Power 5, 2 cm-thick) 1 1.450 -1.380 0.977 0.000 0.000 49 2 1.487 -0.864 0.987 0.000 0.000 43 3 1.503 -0.705 0.990 0.000 0.000 45 Results and Discussion 57 Table 4.3 (continued) Drying tests a b R2 Pc a Microwave (full power, 1 cm-thick) and hot air 1 1.410 0.179 0.996 0.000 0.237 15 2 1.434 -0.341 0.993 0.000 0.074 17 3 1.443 -0.794 0.969 0.000 0.003 16 Microwave (Power 5, 1 cm-thick) and hot air 1 1.346 -0.736 0.986 0.000 0.000 28 2 1.291 -0.746 0.980 0.000 0.003 26 3 1.344 -0.439 0.983 0.000 0.003 29 Microwave (full power, 1.5 cm-thick) and hot air 1 1.561 -0.363 0.988 0.000 0.076 19 2 1.532 -1.017 0.973 0.000 0.015 18 3 1.538 -0.488 0.979 0.000 0.035 18 Microwave (Power 5, 1.5 cm-thick) and hot air 1 1.477 -0.289 0.992 0.000 0.009 35 2 1.459 -0.182 0.994 0.000 0.046 36 3 1.474 -0.507 0.993 0.000 0.000 35 Microwave (full power, 2 cm-thick) and hot air 1 1.626 -0.648 0.977 0.000 0.007 23 2 1.623 -0.665 0.991 0.000 0.002 24 3 1.661 -0.660 0.988 0.000 0.034 19 Results and Discussion 58 Table 4.3 (continued) Drying tests a b R2 Pa Pb n Microwave (Power 5, 2 cm-thick) and hot air 1 1.508 -0.589 0.992 0.000 0.000 46 2 1.504 -0.609 0.993 0.000 0.000 47 3 1.468 -0.623 0.982 0.000 0.000 50 Microwave (full power, 1 cm-thick) and cool air 1 1.684 -0.322 0.996 0.000 0.001 20 2 1.392 -0.428 0.982 0.000 0.009 21 3 1.425 -0.249 0.991 0.000 0.007 22 Microwave (Power 5, 1 cm-thick) and cool air 1 1.311 -0.391 0.983 0.000 0.001 32 2 1.261 -0.375 0.985 0.000 0.000 38 3 1.241 -0.211 0.983 0.000 0.068 37 Microwave (full power, 1.5 cm-thick) and cool air 1 1.574 -1.408 0.984 0.000 0.004 19 2 1.524 -0.366 0.996 0.000 0.004 21 3 1.593 -0.385 0.998 0.000 0.000 21 Microwave (Power 5, 1.5 cm-thick) and cool air 1 1.426 -0.646 0.985 0.000 0.000 36 2 1.363 -0.788 0.976 0.000 0.000 42 3 . 1.360 -0.482 0.988 0.000 0.000 43 Results and Discussion 59 Table 4.3 (continued) Drying tests Microwave (full power, 2 cm-thick) and cool air 1 2 3 Microwave (Power 5, 2 cm-thick) and cool air 1 2 3 Hot air (1 cm-thick slice) 1 2 3 b R2 Pa Pb 1.607 -0.373 0.991 0.000 0.013 28 1.588 -0.649 0.994 0.000 0.002 23 1.702 -0.599 0.993 0.000 0.000 21 1.499 -1.287 0.987 0.000 0.000 47 1.412 -1.071 0.977 0.000 0.000 44 1.626 -2.204 0.976 0.000 0.000 44 1.300 -0.115 0.999 0.000 0.000 89 1.685 -0.109 0.999 0.000 0.000 108 1.208 -0.036 0.999 0.000 0.443 74 Hot air (1.5 cm-thick) 1 1.351 0.225 0.998 0.000 0.000 112 2 1.228 -0.194 0.998 0.000 0.000 109 3 1.522 -0.265 0.998 0.000 0.000 109 Hot air (2 cm-thick slice) 1 1.318 0.906 0.998 0.000 0.000 122 2 1.319 0.932 0.995 0.000 0.000 130 3 - _ 1.389 0.258 0.998 0.000 0.000 126 Systat was also used to undergo an analysis of variance for the determined values of the constants a and b. Results and Discussion 60 4.5.1 Microwave drying tests For the same thickness, a- values of full microwave power tests significantly differed from those of power 5 tests (p-values less than 0.05), contrarily to b-values (p-values greater than 0.05). For full microwave power tests, a-values of three thicknesses (1, 1.5 and 2cm) were significantly different (P=0.002). Significant difference (P=0.034) was also found between b-values of the three thicknesses. 4.5.2 Combined microwave and hot air tests For the same thickness, a-values of full microwave power tests significantly dif-fered from those of power 5 tests. Except for 2cm thick slices (p=0.01), b-values of full microwave power tests were not significantly different from those of power 5 tests. For the three thicknesses,and for fulT microwave power tests, a-values were significantly different (P=0.000) whereas b-values were not (P=0.464). 4.5.3 Combined microwave and cool air tests Except for a-values of 1.5 cm thick slices (P=0.004), a-values of full microwave power tests did not differ significantly from those of power 5 tests. Also, for the same thickness b-values of full microwave power tests were not significantly different from those of power 5 tests. For the three thicknesses, and for full power tests, a-values were not signifi-cantly different (P=0.344) and neither were b-values (P=0.464). Results and Discussion 61 4.5.4 Hot air tests For the three thicknesses, a-values were not significantly different (P=0.924) whereas b-values were significantly different (P=0.018). 4.5.5 Comparisons At full microwave power and for the same thicknesses, microwave tests results were compared to combined microwave and cool air tests results. a-values were not signifi-cantly different. Except for 2 cm thick slices (P=0.048), b-values were not significantly different. At full microwave power and for the same thicknesses, microwave tests results were compared to combined microwave and hot air results. No significant difference was found between the a-values. Except for 1 cm thick slices (P=0.000), b-values were not significantly different. Chapter 5 CONCLUSIONS Experiments on microwave drying, combined microwave and convective drying, and convective drying were undertaken. These experiments generated profiles of tem-perature, moisture content and relative humidity, as well as shrinkage data. Fick's diffusion model was solved in order to calculate moisture diffusivity profiles during drying tests and during rehydration tests. Multiple regression analysis was used to relate the calculated diffusivities to the product temperature and moisture content. From the results of this study, the following conclusions can be drawn: 1. Results of microwave drying were very similar to those of combined microwave and convective drying, in contrast to convective drying alone. 2. During microwave drying and combined drying most of the moisture was expelled while product temperatures remained close to the boiling point of water. This temperature stabilization was attributed to evaporative cooling. The stabilization period was shorter in the case of combined microwave and hot air drying. 3. In microwave drying and in combined drying, vapor flow is thought to be the principal mechanism of moisture removal, whereas liquid flow is thought to be the major mechanism of moisture transfer in convective drying. 4. In contrast to convective drying, in microwave drying and in combined drying internal temperatures were generally higher than surface temperatures. 62 Conclusions 63 5. Drying rates in microwave drying and in combined drying were more than sixty times higher than those in convective drying. This resulted in much shorter drying times. 6. The lower the microwave power setting, the longer the drying time and the lower the product temperatures reached at low moisture levels. 7. The thicker the product, the longer the drying duration and the higher the internal temperatures of the product. 8. In microwave drying and in combined drying, no case hardening was observed. 9. The shrinkages were less than that in convective drying. 10. Results suggest that products dried by microwave or combined drying can be com-pletely rehydrated. Generally, less than 80% rehydration occured following convec-tive drying. 11. Good correlation was found between calculated diffusivities and product tempera-ture and moisture content. For future studies, it is suggested that Fick's law be solved considering vapor pressure gradient as the driving force of moisture removal in microwave drying. More-over, product nonhomogeneity and shrinkage should be considered in microwave drying modeling. Microwave drying has potential to produce better quality products with re-duced drying time. Further work is required to explore taste and microstructure of dried foods. Bibliography [1] Alzamora, S.M., Chirife, J., Viollaz, P. and Vaccarezza, L.M. 1979. Developments in Drying, A.S. Mujumdar (Ed), Science Press, N.J., Quoted in Chirife, J. 1983. 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