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

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MICROWAVE AND C O N V E C T I V E 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 D E G R E E OF  M A S T E R OF A P P L I E D SCIENCE  in T H E FACULTY OF G R A D U A T E 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  degree  at the  thesis  in  University of  partial  fulfilment  of  of  department  this thesis for or  by  his  or  requirements  British Columbia, I agree that the  freely available for reference and study. I further copying  the  representatives.  an advanced  Library shall make  it  agree that permission for extensive  scholarly purposes may be her  for  It  is  granted  by the  understood  that  head of copying  my or  publication of this thesis for financial gain shall not be allowed without my written permission.  Department of  Ql_0~ tE-MtlfCP  The University of British Columbia Vancouver, Canada  DE-6 (2/88)  £/V£  Abstract  Potato slices were dried using microwave drying, combined microwave and convective 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 discussed 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 diffusion model using the solution proposed by Crank (1975).  Multiple regression analysis  shows that calculated diffusivity correlates well with the internal temperature and moisture 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  2.3 3  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  11  Electric field strength  Microwave drying  14  Materials and Methods  17 iii  4  3.1  Experimental apparatus  17  3.2  Tests  19  3.3  Analysis  20  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  4.2  33  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  4.2.4  Effect of probe location  flow  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 5  49  61  Conclusions  62  Bibliography  64 iv  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)  4.3  50  Multiple regression analysis to correlate diffusivity to internal temperature and moisture  56  v  List of Figures  3.1  Side View of the Drying Apparatus  4.2  Temperature profiles for drying of a 1.5 cm-thick potato slice at full microwave power  4.3  23  Moisture profile for drying of a 1.5 cm-thick potato slice at full microwave power  4.4  24  Drying rate profile for drying of a 1.5 cm-thick potato slice at full microwave power  4.5  26  Diffusivity profile for drying of a 1.5 cm-thick potato slice at full microwave power  4.6  27  Relative humidity profile for drying of a 1.5 cm-thick potato slice at full microwave power  4.7  28  Temperature profiles for combined microwave and cool air drying of a 1.5 cm-thick potato slice  4.8  29  Moisture profile for combined microwave and cool air drying of a 1.5 cmthick potato slice  4.9  18  30  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 cmthick 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.  . . .  4.18 Diffusivity profile for hot air drying of a 1.5 cm-thick potato slice  40 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 Natural 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. methods have developed quickly in the second half of this century. of food drying is to preserve it by reducing water activity.  The main objective  In addition, reducing product  weight and volume results in reduced transport and storage costs. cause deterioration of the quality of the dried product.  New drying  Drying can, however,  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 product 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 components.  This results in the rapid internal vaporization of moisture.  1  The resulting gas  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.  Chapter 2  LITERATURE REVIEW  Thermal drying is the application of heat in order to remove moisture from a product.  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. of World War II, mechanical drying has developed quickly.  Since the end  Early methods included  trucked-tray drying, drum drying, and transfer ventilation drying.  Nevertheless, such  problems as protein denaturation, fat oxidation, destruction of vitamins, browning reaction by aminocarbonyls, and "off' taste can occur (Hayashi, 1989) resulting in deterioration 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 drying, 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  obtained.  4  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). indicated some mechanisms that govern air drying.  Van Arsdel and Copley (1963)  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 gradients. 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 principally 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  St  8W 2  =  '  (2 5)  where: W  =  Average moisture content (kg. water/kg. dry solid)  D  =  Diffusivity (m /s)  t  =  time (s)  x  =  Linear coordinate (m)  2  Assuming uniform initial moisture distribution and negligible external resistance, the solution expressed in terms of the average moisture content of the slab is (Crank, 1975):  Literature Review  7  W-W  8 ~  e  1  . ,  n  2  Dt,  ,  x  where: Wo  =  Initial moisture content (kg. water/kg. dry solid)  W  =  Equihbrium moisture content (kg. water/kg. dry solid)  L  =  Solid thickness (m).  e  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 dt  =  bcPp pdx  2  where: b  =  vapor-space permeability (kg. / Pa. m. sec.)  p  =  dry solid density (kg/m )  p  =  vapor pressure of water in food (Pa.)  3  {  '  ]  Literature Review  2.1.3  8  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 gradient 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). magnetron.  The microwave energy can be generated from a  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 electrons into a vacuum space between the cathode and the anode. created by a magnet surrounding the magnetron.  The magnetic field is  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. resultant friction creates heat which gets transferred to neighboring molecules.  The On the  other hand, charged ions such as chloride(-) and sodium(-f) flow toward the alternating electric field (Best, 1987). and Peterson, 1986).  Ions collision converts kinetic energy into heat (Decareau  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). gett (1985) cited the literature relevant to food dielectric properties.  Mud-  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 applications 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 properties 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- fE e" 14  2  (2.5)  where: P  =  power absorbed by unit volume, W / m  f  =  frequency, Hz  E  =  electric field strength, V / m  e"  =  dielectric loss factor  2.2.2  3  Penetration depth The depth of penetration of microwaves into a product is given by (Fellows, 1988):  Literature Review  11  x=  (2.6)  A  27TV e' tan £ where: x  =  depth of penetration, m  A  =  wavelength (in vacuum), m  e'  =  dielectric constant  tan 5  =  tan  2.2.3  = loss tangent.  Rate of rise of temperature The power required to raise the temperature of a material, subject to microwave  energy, from T to T (deg.C) in t seconds is given by (Metaxas and Meredith, 1983): 0  P = ^ ' where: P = C  =  p  p  power absorbed, W / m  r  - ° »  (2.7)  r  i  3  Specific heat of the material, J/kg deg.C  = density of the material, kg/m . Hence, 3  (1^1 = 4t  2.2A  pC  v  P  (2.8) '  Electric field strength Metaxas and Meredith (1983) mentioned that the electric field strength can be  determined through calorimetry:  P = 55.61 * lO- /^" = 14 J  p C p ( T  t  ~  T o )  (2.9)  Literature Review  12  Therefore, /  pC (T-T ) p  Q  ~  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  (1987).  13  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 microwave 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. a non-linear system by finite difference method.  They solved numerically  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 expelled 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 determination 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. and limitations of microwave drying.  Perkin (1979) discussed the advantages  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 material (wet cotton) heated by microwaves. gradient.  They found a very small mass concentration  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 different 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). drying rates and internal temperature histories.  The model effectively estimated  This model was also used by Chen  16  Literature Review  and al. (1990) to simulate microwave drying of hardwood veneer. the model for predicting drying times was satisfactory.  The accuracy of  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 exhibits negligible capillary moisture transfer (water-saturated packed bed of glass beads) have been conducted by Grolmes and Bergman (1990). the drying of bananas with microwave and air ovens. variable diffusion model.  Garcia et al. (1988) studied The drying data was fitted to a  Product temperature was not monitored, however.  So far there is little research reported on moisture diffusion determinations during microwave drying. crowave drying processes.  Such research is necessary for modeling development of mi-  Chapter 3  MATERIALS AND METHODS  3.1  Experimental apparatus Figure 3.1 depicts the experimental apparatus of this work.  Watt household microwave oven with a mode stirrer was used.  A modified 700  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 acquisition 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.  ture and the velocity of the air were controlled.  The tempera-  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  Front of microwave oven  Fluoroptic probes  Electric heating element  Weighing basket Computer for temperature and  Fan  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  3.2  Heating  4  Thicknesses  3  Hot air, Microwave, Microwave and hot air, and Microwave and cool air 1 cm, 1.5 cm, and 2 cm  Power settings Air temperatures Air flow rate  2 2 1  10 and 5 18°C and 65°C 0.032m /sec. 3  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. full power.  Power level 10 was  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.  For every drying test, three  Initial and final slice dimensions were measured.  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 diffusion model for unidimensial flow (neglecting the radial diffusion) was used:  where: W  =  Average moisture content (kg. water/kg. dry solid)  D  =  Diffusivity (m /sec.)  t  =  time (sec.)  x  =  linear coordinate.  2  The solution proposed by Crank (1975) was: W-W  Where:  e  8 ~  1  Dt  Wq  =  Initial moisture content (kg.water/kg. dry solid)  W  =  Equilibrium moisture content (kg. water/kg. dry solid)  =  solid thickness (m)  L  e  (3.12)  Simplifying this solution by taking only the first term of the series and by assuming that We = 0, gives:  Materials and Methods  21  0  =  ± [_ *Ri} exp  (  w  3-13  )  h  7T  Where 8 = %  Deriving the last equation with respect to time gives 88 dt  xD 8 L 2  2  (3.14)  Hence at given intervals of time, and knowing ^ and the average 8, the diffusivity values can be determined using the following equation: 88 D  T  2  = (-s»<^?)  <- > 3 15  Multiple regression analysis was also conducted in order to correlate the calculated diffusivity to such properties as the internal sample temperature, T and the sample moisture content, W (dry basis).  The relationship used was:  D = TW a  b  (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  RESULTS 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.  of moisture removal occurred while the temperature remained close to 100°C.  Most 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  Time (s.)  Surface temperature  — Internal temp.  400  450  24  Results and Discussion  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.  25  Results and Discussion  mechanism of moisture removal.  Moreover, surface temperatures were generally lower  than internal temperatures except at low moisture levels. effect of the surrounding cooler air.  This is due to the cooling  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. time.  Diffusivity values increased with  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 microwave  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 0.032m /sec.) 3  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 respectively.  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, Systat) showed that drying times did not differ significantly (0.05 was the chosen significance level) from those of microwave drying alone (P=0.624). did not differ significantly either (P=0.823).  The maximum drying rates  Results and Discussion  26  Drying rate profile Microwave drying  "sec.)  0.0120.01-  o 0.008water  OT  0.006-  Drying rate  d> 0.0040.002100  200  300 Time (s.)  400  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  Diffusivity profile Microwave drying 8000^ 7000o i—1  *  «3  600050004000-  >-> "> "tn 3  Q  3000200010000C  600  Figure 4.5: Diffusivity profile for drying of a 1.5 cm-thick potato slice at full microwave power.  Results and Discussion  28  Relative humidity profile Microwave drying 8070-  midi  >.  6050-  nCD 40> ca CD  OC  3020ii  10+ 0  300 Time (s.)  400  500  600  Figure 4.6: Relative humidity profile for drying of a 1.5 cm-thick potato slice at full microwave power.  Temperature profiles  o  o  o  Microwave and coo air drying 200  1-1  CD p>  <Tt-  o  O  cb  Q) 13 Q) ZJ  O o O  1» CD  a E  o % a P>  P-.  n o o  150  200  250 300 Time (s.)  350  -  era o P  Surface temperature  Internal temp.  400  450  500  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  0.2o-  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  Drying rate profile  Microwave and cool air drying 0.014  o cu 0.012 CO  «  •o5 o =  0.01  CO  cb 0.008 S CD  ch 0.006 g  0.004  03  c  0.002  Q 200  300 Time (s.)  400  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  Relative humidity profile Microwave and cool air drying  45-1  40-  humi  3530-  (D > CO CU  rr  2520 15  100  200  300 Time (s.)  400  500  600  Figure 4.10: Relative humidity profile for combined microwave and cool air drying of a 1.5 cm-thick potato slice.  33  Results and Discussion  4.1.3  Combined microwave and hot air drying The results of combined microwave (at full power) and hot air (65°C at 0.032m /sec.) 3  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 0.032m /sec.) drying of a 1.5 cm-thick potato slice 3  are shown in Figures 4.17 to 4.18. constant rate period was observed.  Drying duration was longer than 900 minutes.  A  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 temperatures.  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? cm2 C. a o  o o  <T>  tn  p  2CD  Temperature profiles  !T  ft M P trl-  Microwave and hot air drying  180-  n  *d >-» o  o o-  5'  o  ex.  o  a-  150  200 250 Time (s.)  Surface temperature  300  350  Internal temp.  400  450  Results and Discussion  35  Moisture profile  Microwave and hot air drying 1.2-  CD  Tn  °o  1i 0.80.6-  IS"  0.4-  '0 0.2600  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  Drying rate profile Microwave and hot air drying 0.014 o cu 0.012A CO  * T3 =o3 0.01 CO  CD  0.008 ££ ro CD  0.006  2 0.004 CO  c  0.002  Q  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 7000o  o  i-H  *  OS CS  >>  )iffusi  "S  600050004000-  • •  30002000-  •  --""  10000-  •  * i*  50  *—i  1  1  1  1  1  1  100 150 200 250 300 350 400  •  1  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 Time (min.)  Surface temperature  400  Internal temp.  500  600  Results and Discussion  39  M o i s t u r e profile 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  Diffusivity profile Hot air drying  o  o  S  Q 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  4.2  42  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 drying. 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 convective drying tests.  Temperature profiles Microwave drying  180n 160-  °0  50  100  150  200 250 Time (sec.)  300  350  Surface temperature — - Internal temp.  400  450  Resulis and Discussion  44  Moisture profile Microwave drying  1.2CD  1  "55  'o 0.8ro  j~  c  0.6-  (D  00  0.4-  'o  0.20-0  -r  200  300  400 500 Time (s.)  600  700  800  Figure 4.20: Moisture profile for drying a 2cm-thick potato slice at full microwave power.  Results and Discussion  45  Drying rate profile Microwave drying 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 0.001 100  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 Time (s.)  Surface temperature  600  700  Internal temp.  800  900  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.  48  Results and Discussion  Drying rate profile Microwave drying 0.01 "Jj 0.009 * 0.008 OT  •g  'o 0.007H CO  -5? 0.006« 0.0053 0.004 IJ 0.003-I c 0.002 Q 0.001 200  400  600 Time (s.)  800  1000  1200  Figure 4.24: Drying rate profile for microwave drying (power 5) of a 1.5 cm-thick potato slice.  Results and Discussion  4.2.3  49  Effect of air flow When cool air flow was combined with microwave heating, the results were very  similar to those of microwave drying alone. were more limited.  The values of relative humidity, however,  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. temperatures were quite different.  At low moisture levels, even internal  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 rehydration.  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 cmthick 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 rehydration 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 Time (min.)  8  10  12  Figure 4.25: Moisture profile during the rehydration of a microwave dried (full power) 1.5 cm-thick potato slice.  Results and Discussion  120y 1  52  Diffusivity profile (rehydration) 1  100-  o  s _>>  [> 'co  S  usands)  o 80-  60-  o H  40-  ta Q  20-  T 3  1  4  1  5  1  6  1  "  7  1  8  9  1—  1  10  11  T  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 0.8CD  0.7-  to  0.6-  'o CO  'c 3 00  "o  0.50.40.30.20.1-  0* 0  30  Time (min.)  Figure 4.27: Moisture profile during the rehydration of a hot air dried 1.5 cm-thick slice.  54  Results and Discussion  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 = TW a  (4.17)  b  Where: a and b are constants .  Diffusivity unit is (m x 10 /sec.) for microwave and combined drying tests. It is 2  lo  (m x lO /min.) for convective drying tests. 2  10  Table 4.3 summarizes the multiple regression results of this model for different drying tests.  R is the squared multiple regression coefficient. 2  P indicates the siga  nificance of temperature (if P is very small, then temperature has a significant effect on a  the model).  P indicates the significance of moisture (if Pj, is very small, then moisture 0  has a significant effect on the model). The values of R indicate that diffusivity correlates very well with the tem2  perature and moisture. Moreover, the values of P and P(, were very small (generally less a  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 and moisture. a b Drying tests R Microwave (full power, 1 cm-thick slice) 1 1.466 -0.293 0.998 2 1.347 -0.500 0.985 1.389 -0.404 0.988 3 Microwave (Power 5, 1 cm-thick) 1 1.300 -0.403 0.988 1.227 -0.531 0.989 2 1.264 -0.432 0.995 3 Microwave (full power, 1.5 cm-thick) 1.576 -0.400 0.997 1 1.545 -1.025 0.988 2 1.475 -0.205 0.995 3 Microwave (Power 5, 1.5 cm-thick) 1.388 -0.943 0.983 1 1.393 -1.039 0.990 2 1.422 -1.147 0.985 3 Microwave (full power, 2 cm-thick) 1.776 -1.427 0.991 1 1.857 -2.156 0.991 2 1.682 -0.994 0.996 3 Microwave (Power 5, 2 cm-thick) 1.450 -1.380 0.977 1 1.487 -0.864 0.987 2 1.503 -0.705 0.990 3 2  internal temperature n  Pa  Pb  0.000 0.000 0.000  0.000 0.002 0.002  19 23 22  0.000 0.000 0.000  0.000 0.000 0.000  34 40 41  0.000 0.000 0.000  0.001 0.002 0.159  19 20 21  0.000 0.000 0.000  0.000 0.000 0.000  37 40 39  0.000 0.000 0.000  0.015 0.000 0.000  18 21 21  0.000 0.000 0.000  0.000 0.000 0.000  49 43 45  Results and Discussion  57  Table 4.3 (continued) Drying tests  a  b  R  Pa  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  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  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  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  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  2  c  Microwave (full power, 1 cm-thick) and hot air  Microwave (Power 5, 1 cm-thick) and hot air  Microwave (full power, 1.5 cm-thick) and hot air  Microwave (Power 5, 1.5 cm-thick) and hot air  Microwave (full power, 2 cm-thick) and hot air  Results and Discussion  58  Table 4.3 (continued) Drying tests  a  b  R  P  P  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  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  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  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  1  1.426  -0.646  0.985  0.000  0.000 36  2  1.363  -0.788  0.976  0.000  0.000 42  1.360  -0.482  0.988  0.000  0.000 43  2  a  b  n  Microwave (Power 5, 2 cm-thick) and hot air  Microwave (full power, 1 cm-thick) and cool air  Microwave (Power 5, 1 cm-thick) and cool air  Microwave (full power, 1.5 cm-thick) and cool air  Microwave (Power 5, 1.5 cm-thick) and cool air  3  .  Results and Discussion  59  Table 4.3 (continued) Drying tests  b  R  P  2  a  P  b  Microwave (full power, 2 cm-thick) and cool air 1  1.607  -0.373  0.991  0.000  0.013  28  2  1.588  -0.649  0.994  0.000  0.002  23  3  1.702  -0.599  0.993  0.000  0.000  21  1  1.499  -1.287  0.987  0.000  0.000  47  2  1.412  -1.071  0.977  0.000  0.000  44  3  1.626  -2.204  0.976  0.000  0.000  44  1  1.300  -0.115  0.999  0.000  0.000  89  2  1.685  -0.109  0.999  0.000  0.000 108  3  1.208  -0.036  0.999  0.000  0.443  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  1  1.318  0.906  0.998  0.000  0.000 122  2  1.319  0.932  0.995  0.000  0.000 130  1.389  0.258  0.998  0.000  0.000 126  Microwave (Power 5, 2 cm-thick) and cool air  Hot air (1 cm-thick slice)  74  Hot air (1.5 cm-thick)  Hot air (2 cm-thick slice)  3  -  _  Systat was also used to undergo an analysis of variance for the determined values of the constants a and b.  Results and Discussion  4.5.1  60  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 significantly different (P=0.344) and neither were b-values (P=0.464).  Results and Discussion  4.5.4  61  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. cantly different.  a-values were not signifi-  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. found between the a-values. significantly different.  No significant difference was  Except for 1 cm thick slices (P=0.000), b-values were not  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. temperature stabilization was attributed to evaporative cooling.  This  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 completely rehydrated. Generally, less than 80% rehydration occured following convective drying. 11. Good correlation was found between calculated diffusivities and product temperature 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 reduced drying time. foods.  Further work is required to explore taste and microstructure of dried  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. Fundamentals of the drying mechanisms during air dehydration of foods, In: Advances in Drying, vol. 2, A.S. Mujumdar (Ed), Hemisphere Publishing Co., Washington, 73-102.  Anon, 1987. Microwavable foods - industry's response to consumer demands for con venience, Food Technology, 41, 52-62. Anon, 1987. Ingredients and packages for microwavable foods, Food Technology, 41, 102-104.  Anonymous, 1988. Accurate temperatures in microwave ovens are difficult to achieve, Food Engineering, 60, 175-179. Bergman, T.L., Evans, T.A. and Schmidt, P.S. 1987. 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