Microwave Heating and Mass Transfer Characteristics of White Beans

J. agric. Engng Res. (1996) 64, 71 – 78

Microwave Heating and Mass Transfer Characteristics of White Beans Benjamin Adu; Lambert Otten School of Engineering, University of Guelph, Guelph, Ontario N1G 2W1, Canada (Receiy ed 23 March 1995; accepted in rey ised form 11 January 1996)

The drying characteristics of white bean seeds exposed to constant microwave (2450 MHz) absorbed powers ranging from 0?28 to 0?44 W / g (dry basis) was investigated with a single-mode (TE10) microwave drying apparatus. The effect of absorbed power level on moisture and temperature history during drying are presented. Microwave drying of white beans was found to be a falling rate process. At constant absorbed power, seed temperature increased rapidly during the initial stages of drying to a maximum value and began to decrease gradually during the latter stages of drying. To maintain a constant drying temperature the absorbed power had to be increased progressively with further reduction of moisture content. The gradual decrease in seed temperature when drying rate was decreasing is opposite what is observed during hot air drying. It was considered as resulting from a progressively increasing heat of desorption with drying, a phenomenon common to hygroscopic solids. Thus, the microwave heating characteristics observed for white beans is likely to apply to similar hygroscopic solids.

microwave process technology in the food and fibre industry. However, biological materials respond to microwave heating in a manner that is not well understood (Decareau1). Consequently, the design of microwave processes has been largely empirical, often with unpredicted and disappointing results due to lack of basic models for the heating characteristics of biological materials (Mudgett2). Adu and Otten3 investigated the microwave drying of soybeans and found that changes in hygroscopicity or moisture bond strength during drying significantly affected the heating and drying characteristics. The drying rate of granular materials in a deep bed depends on the drying rate of each single kernel within the bed. Thus deep-bed drying can be modelled from a model that describes the drying behaviour of a thin layer of seeds (Brooker et al.4). The basic definition of a thin layer, as one in which all seeds have the same moisture content and drying rate at any time in the drying process, applies equally to microwave drying. The drying rate of any seed kernel in the packed bed under microwave heating depends mainly on the power available to that particular seed kernel. The percentage of power available to each kernel in a packed bed depends on the penetration depth of the wave and the location of the kernel relative to the first kernel the wave encountered. Thus, the maximum thickness at which the power available to a kernel is not appreciably different from the power available to the first kernel encounted by the wave may be considered as the maximum thickness of the thin layer. The object of this study was to determine the microwave heating and drying behaviour (characteristics) of thin layers of seeds. White beans were chosen as the experimental material.

÷1996 Silsoe Research Institute

1. Introduction The application of heat in processing agricultural or biological materials remains a major processing method in the food and fibre industry. A biologically based solid food material may be classified as a moist, porous, hygroscopic solid with low thermal conductivity. Such materials offer high resistance to internal heat transfer. Consequently, conventional heating techniques that rely on the thermal conductivity of food material to transfer heat to inner locations where it is most needed are less effective than microwave heating as it generates heat within the material itself. Volumetric heating by microwaves is fast, effective, and eliminates conduction as the main mode of transferring heat to places where it is needed. This and other comparative advantages offered by microwave heating have resulted in the increasing use of 0021-8634 / 96 / 050071 1 08 $18.00 / 0

2. Materials and methods In an earlier thin-layer microwave drying study (Adu and Otten3), an experimental microwave drying 71

÷ 1996 Silsoe Research Institute

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Fig. 1. Schematic diagram of microway e apparatus. 1. Iron core dummy-load; 2. Bi-directional coupler; 3. Test cay ity; 4. Three-point tuner; 5. Bi-directional coupler . 6. Circulator; 7. Microway e generator; 8. Teflon pins; 9. Mettler balance.

apparatus (Fig. 1 ) was designed, constructed, and calibrated to monitor and record microwave absorbed power, seed temperature and mass loss with time. The experimental microwave apparatus has the following features. 1. It operates in the TE10 mode to eliminate spacial variability in electromagnetic field strength associated with multi-mode cavities; this was done to facilitate spatially uniform power distribution and absorption. 2. The sample container has a small width relative to the penetration depth of the wave. Thus the absorbed power, and hence drying characteristics, of seeds in the container are similar during a drying process. 3. There is immediate removal of lost moisture from the seed surface. 4. There is continuous monitoring and recoding of all relevant time-dependent data.

2.1. Sample preparation In porous hygroscopic solids, moisture may be free or bound to the solid matter depending on the moisture content. A high moisture contents, moisture exists in the capillaries in a relatively free state but at low moisture content, moisture is bound to the surface of the pores. There is a transition point, generally referred to as the critical moisture content, which indicates the two states of water in the solid. Under microwave conditions, the critical moisture content of seed lies between 30 and 40% d.b. depending on hygroscopicity (Nelson,6 Metaxas and Meredith7). In earlier studies (Adu and Otten,3 Adu and Otten8), it was observed that the initial moisture content of the sample affected the temperature and moisture content history of Natto beans during microwave drying when it was below the critical moisture content. Thus, to eliminate the effect of initial moisture content in this study, the samples were rewetted to a moisture content around 60% d.b. where moisture in food materials is known to be relatively free. Secondly, most hygroscopic food materials maintain a high

moisture content before processing. Thus, to increase the range of applicability of the result of this study, it was necessary to approach the moisture content of high moisture foods as much as possible. A sample of the Stinger variety of navy beans (Phaseolus Vulgaris L.), popularly known as white beans, was wetted to obtain seeds with moisture content between 60 and 70% d.b. and stored in sealed glass containers at 48C for about 14 d before being used in the experiments. The chemical composition of white beans is given in Table 1. 2.2 . Drying procedure Four drying conditions, each corresponding to a particular constant absorbed power level were investigated. The constant absorbed powers investigated were 0?28, 0?33, 0?39 and 0?44 W / g d.b. The incident, reflected, transmitted, and residual microwave powers were measured with microwave power meters and sensors (Marconi 6950 power meter and 6910 power sensors, Marconi Instruments, U.K.). The difference between the incident powers and the sum of the reflected, transmitted, and residual power is equal to the absorbed power. The mass of the sample was monitored with an electronic balance (Mettler 2400, Mettler Instrument Corp., NJ, USA) that provided both visual and digital output. The digital output was recorded at regular time intervals during the drying process. A fibre optic thermometer (Luxtron 950) was used for temperature measurement. Table 1 Chemical composition of white beans (Health Welfare Canada5) Carbohydrates Protein Oil Ash Seed shape 100 Seed mass Equilibrium m.c.

62% 21?1% 1?2% 3?8% ovate or ellipsoid 17 – 20 g 11?7% d.b.

MICROWAVE HEATING AND MASS TRANSFER CHARACTERICS OF WHITE BEANS

A typical test run involved filling the Teflon basket with about 100 g (330 seeds) of white bean seeds. The basket was tapped gently to settle the beans and placed on the Teflon pins in the test cavity. The microwave cavity cover was then secured in place. The fibre optic temperature sensing probe was then inserted into the centre of the seed bed to touch the surface of a seed located at the centre of the layer. The fan was turned on for 3 min to remove any surface water. Microwave heating then commenced with seed temperature, mass, and the incident, reflected, transmitted, and residual microwave power levels recorded at regular interval. Each test was run until the moisture loss rate was about 0?01 g / min / kg sample (d.b.) Preliminary tests showed that the fibre optic probe located in the thin-layer of seed introduces an additional but constant mass to the mass of the sample mass. To account for this additional mass, the mass of the sample was noted before and after the insertion of the probe. The difference in sample mass, with and without the probe was considered as the probe error. The probe error was accounted for by subtracting the additional mass it introduced from the recorded mass loss data. The ability to account for the ‘‘probe error’’ allowed mass loss and temperature change to be measured simultaneously for each test. There were three replicate runs for each experimental condition.

3. Results and discussion 3.1. Mass transfer behay iour Drying data recorded with the apparatus showed very little scatter and replicate results did not vary significantly from each other. Table 2 shows an analysis of variance of typical test data. Since test replicates were not significantly different from each other, one set was randomly chosen to represent the behaviour of interest for the corresponding drying condition. The drying data were normalized by converting the Table 2 Analysis of variance for test replicate data at absorbed power of 0?33 W / g Source

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moisture contents to moisture ratios. The moisture ratio (MR ) is MR 5

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where M is the moisture content of seed, M0 is the initial moisture content, and Me is the equilibrium moisture content. Fig. 2 shows typical drying characteristic curves for the constant absorbed powers investigated. Each curve exhibits a decaying exponential trend with time. Drying curves that exhibit exponential decay behaviour are characteristic of drying processes with diffusion as the dominant mechanism of moisture transfer. Thus, diffusion appears to be the dominant mechanism of moisture loss in white bean seeds during microwave heating. Fig. 3 shows plots of drying rate versus time for the different absorbed power levels investigated. No constant drying rate period was observed for the drying conditions used in this study. In general, the drying rate increased with increasing constant absorbed power in the early stages of drying but appear to be independent of the absorbed power level at the latter stages. Secondly, each curve in Fig. 3 indicates a higher moisture loss rate at higher seed moisture contents with the moisture loss rate decreasing continuously with drying. This indicates that microwave drying of white beans is a moisture-limiting fallingrate process for moisture contents below the initial moisture content (about 60% d.b.) of this study. Shivare et al.9 observed a similar drying characteristic for corn thus the underlying cause for this microwave drying behaviour may be common to both corn and white bean seeds. Fig. 4 shows a typical plot of the seed temperature and the corresponding moisture loss history for white beans during microwave drying. The drying process exhibits a continuously decreasing temperature with decreasing moisture loss rate in the latter stages of drying. This is opposite to the trend observed during conventional heating. From the temperature curve, four seed temperature (heating) trends may be discerned. The first one is observed during the initial 2 to 3 min of power absorption, and is characterized by a rapidly increasing seed temperature and a constant heating rate. This is followed by a second stage which is characterized by a reduction in the rate of temperature rise. At third stage, the temperature attains a maximum. Following the maximum temperature stage is the fourth stage during which seed temperature decreases gradually with drying. The factors that dictate the sensible heating trends observed may be deduced from Fig. 5.

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Fig. 2. Drying cury es for y arious y alues of constant absorbed microway e power in W / g d .b. s , 0?44 W / g; h , 0?39 W / g , n , 0?33 W / g; d , 0?28 W / g

Fig. 5 shows a typical plot of the rate of temperature change versus seed moisture content, (power 5 0?28 W / g). The plot indicates that the initial rapid rise in temperature observed in stage one is associated with a warming up period when the relatively low seed temperatures result in very low moisture loss rate. Low moisture loss rate will result in less energy being used for evaporation hence the high rate of sensible heating. The second stage, characterized by a reduction in the rate of temperature rise marks the period when a higher seed temperature results in higher rates of moisture loss, and consequently higher evaporative cooling effect. At the third stage, the seed temperature reaches a maximum value. From energy conservation a steady temperature at this stage indicates that the heat generated must be equal to the sum

of the evaporative and combined heat losses. The duration of the steady maximum temperature period will probably depend on the heat, mass and power transfer dynamics. The fourth stage, which is characterized by the gradual decrease in temperature, especially when the drying rate is decreasing, indicates that there is a limiting factor to heat generation at the latter stages of drying. A white bean seed is a moist porous hygroscopic material and is therefore physico-chemically sensitive to moisture content. In hygroscopic materials moisture may be free or bound depending on the moisture content. At high moisture contents significant amounts of moisture exist as free water but below the critical moisture content, moisture exists mainly as bound water. The average bond strength of water molecules in hygroscopic

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Fig. 3. Typical moisture loss rates at y ersus time absorbed power ley els. n , 0?44 W / g; s , 0?39 W / g; h , 0?33 W / g; p , 0?28 W / g

MICROWAVE HEATING AND MASS TRANSFER CHARACTERICS OF WHITE BEANS

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Fig. 4 . Typical plot of simultaneous microway e heating and drying data. m , moisture content; h , temperature

solids generally increases with decreasing moisture content. Drying involves the removal of moisture existing in a material. Thus when moisture in a material is bound to the solid phase, the energy needed to remove it, i.e. heat of desorption increases with drying. At constant absorbed microwave power input, increased heat of desorption reduces the amount of thermal energy available for sensible heating. This will result in a temperature decrease at the latter stages of microwave drying. In essence, increasing moisture bond strength

due to drying reduces the energy available for sensible heating during microwave heating. A short abrupt decrease in heating rate is observed between the first and the second stages of microwave heating. This change in heating rate is most probably the result of a sudden increase in the rate of moisture loss, and hence latent energy demand which occurs when the seed coat cracks. Bean seeds and their coats are viscoelastic materials (Liu et al. ,10 Mensah, et al.11). As viscoelastic materials, stress levels in beans depends on the strain rate. High strain results in high

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stress, but the stress reduces to lower levels if sufficient time is allowed for stress relaxation. High rates of temperatures rise may result in high rates of seed expansion and / or moisture build-up between the cotyledons and the seed coat in moist seeds with intact seed coats. In essence, high rates of temperature rise result in high strain rates. In viscoelastic materials such as bean seeds, high strain rates results in high stresses that may exceed the ultimate strength of the seed coat, and thus causing a crack (Pfost12). Cracking provides an avenue for built-up moisture to escape under pressure resulting in a relatively higher evaporative cooling effect. Perkins13 modelled the microwave heating characteristics of high moisture materials and concluded that, during microwave drying, product temperature increased rapidly in the initial stages, assumed a constant value during the constant drying rate period and then increased rapidly again during the falling rate period. In fact, Perkin’s microwave heating model, except for the elimination of the part where material temperature approximately equals that of the drying air, is not different from a typical conventional heating model for a moist solid. The microwave heating trends observed for white beans (Fig. 3 ), and similar trends observed for Natto soybeans by Adu and Otten3 contradict Perkin’s model. Furthermore, Shivare et al. ,9 in a study on the microwave drying of corn, presented plots of outlet air temperature similar to the heating trends observed in this study. In conventional drying of foods, the temperature of the drying air becomes the ultimate temperature that the material can attain. In the falling rate period, the combined heat losses decrease with decreasing moisture loss rate, and the product temperature increases asymptotically towards that of the drying air. In contrast, microwave heating does not have a pre-set ultimate temperature. The observed heating trends for white beans, at constant absorbed power levels, are opposite to those of conventional heating during the latter stages of drying. With no pre-imposed final temperature for microwave heating, the implication of the continuous temperature rise model (Perkins13) is that a hygroscopic material will continue to increase in temperature so long as it absorbs microwave power. Thus, microwave heated products, if left for a sufficient time will ultimately burn-out. The parameter that controls the conversion of microwave energy to heat is the effective loss factor. The effective loss factor for foods and other hygroscopic materials at 2450 MHz is practically independent of moisture content below the critical moisture content (Metaxas and Meredith,7 Nelson6). Consequently, below the critical moisture content, drying

does not affect the amount of power absorbed by a hygroscopic materials to any appreciable degree, and hygroscopic materials may absorb significant amounts of microwave power even when completely dry. The ability of dry hygroscopic materials to absorb significant amounts of microwave energy will favour Perkins’ continuous temperature rise model if increases in latent heat consumption with decreasing rate of moisture loss did not occur during microwave drying. The observed temperature trend has important implications for both microwave heating and / or drying processes. One of the immediate implications is that low moisture hygroscopic materials are more likely to burn or char at the initial stages of microwave heating when moisture is relatively more abundant, less tightly bound, and the activation energy for heat generation is lower. In the foods industry, the implication is that different heating guidelines need to be formulated for high and low moisture foods because of their different microwave heating characteristics. For the same power absorption, higher temperatures will be achieved in high moisture foods than in low moisture foods. Fig. 6 shows a plot of seed temperature versus moisture content during drying for the constant absorbed powers investigated. The curves indicate that the maximum temperature attained by the seed at constant absorbed power depends on the absorbed power level. For each power level, the seed temperature attained a maximum value and then proceeded to decrease with drying. The implication of this microwave heating behaviour on public health cannot be over-stated judging by the fact that foods designed for consumption after microwave reheating is increasing. The maximum temperature that may be reached at constant or decreasing microwave power absorption depends on the changing dielectric and hygroscopic properties of each food during processing, and consequently, microwave food temperature histories are product-specific. Thus food mixtures intended to be processed by microwaves must be designed to have similar temperature histories and F values. The F value is the heating time required to achieve a desirable sterilization at a particular temperature. Microwave drying involves four simultaneously occurring processes; heat generation, moisture loss, sensible heating, and convective and radiative heat loss. From the microwave heating and drying curves, it was observed that sensible heating behaviour of white beans is affected by the moisture loss rate and moisture content at various stages of drying (Fig. 5 ). This is the case in all simultaneous heat and mass transfer processes involving hygroscopic materials.

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Fig. 6. Plot of typical seed temperature y ersus moisture content for different absorbed power ley els. m , 0?44 W / g; n , 0?39 W / g; s , 0?33 W / g; h , 0?28 W / g

The proportion of microwave generated heat associated with each transport process in a moist hygroscopic solid will depend on the dynamics of the resistances associated with the transport processes. The resistances available during microwave heating of hygroscopic materials consist of the resistance to; heat generation, temperature change, and moisture loss. All these resistances are dependent on moisture content and temperature and may change with drying conditions. The processes that are dependent on moisture content, i.e. mass loss rate, heat generation, and rate of seed temperature change, influence each other as the material dries. Seed temperature in turn is dependent on the absorbed microwave power level (Fig. 6 ). This complex interdependence of simultaneously occurring phenomena, and the dynamics of each process resistance during the heating and drying process, is the controlling factor that shapes each heating curve. Thus any model capable of predicting simultaneous microwave heating and mass transfer characteristics needs to account for changes in these parameters, and changes in process resistances with changing process conditions.

the dominant mechanism of moisture transfer in white bean seeds during microwave heating. 2. For each power level there is a maximum temperature that was not exceeded during the microwave heating of moist white bean seeds. After the maximum temperature was attained, sensible heating of white beans decreased progressively with moisture content. The maximum temperature that could be reached depends on the absorbed microwave power level. 3. The gradually decreasing temperature for white bean seeds during the last stages of constant microwave power drying is opposite to that observed during conventional drying. References 1

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4. Conclusions 1. White bean drying rates are dependent on the absorbed microwave power level. At moisture contents below 60% d.b., higher seed moisture content results in higher drying rates. Drying rates progressively decrease with drying and appear to be independent of absorbed power during the latter stages of drying. Thus, microwave drying of white beans is mainly a falling rate process. Diffusion appears to be

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Decareau R V Microwaves in the Food Processing Industry. Orlando: Academic Press, 1985 Mudgett R Electromagnetic energy and food processing. Journal of Microwave Power and Electromagnetic Energy. 1988. 23(4): 225 – 230 Adu B; Otten L Simultaneous microwave heat and mass transfer characteristics of porous hygroscopic solids. Journal of Microwave Power and Electromagnetic Energy. 1993. 28(1): 41 – 46 Brooker D B; Bakker-Arkema F W; Hall C W Drying Cereal Grains. Wesport: The AVI Publishing Company, 1982 Canada Department of National Health and Welfare Nutrient value of some common foods. Bureau of Nutritional Science Report, 1988 Nelson S O Frequency and moisture dependence of dielectric properties of high-moisture corn. Journal of Microwave Power. 1978. 13(2): 213 – 218 Metaxas A C; Meredith R J Industrial Microwave Heating. London: Peter Peregrinus Ltd., 1983 Adu B; Otten L Modelling thin layer microwave drying

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of soybeans. Canadian Agricultural Engineering. 1994. 36(3): 135 – 141 Shivhare U S: Raghavan G S V; Bosisio R G Microwave Drying of Seed-Quality Corn in Packed Bed Using Surface Wave Applicator. St. Joseph: American Society of Agricultural Engineers. 1990. Paper No. 90-6057 Liu M; Haghigha K; Stroshine R L; Ting E C Mechanical properties of the soybean cotyledons and failure strength of soybean kernels. Transactions of the ASAE, 1990, 32(2): 946 – 952

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Mensah J K; Nelson G L; Hamdy M Y; Richard T G A Mathematical Model for Predicting Soybean Seed Coat Cracking During Drying. Transactions of the ASAE 1985, 28(2): 580 – 587 Pfost D L Environmental and variety factors affecting damage of seed soybeans during drying. Ph.D. Dissertation. The Ohio State University Library, Columbus, Ohio. 1975 Perkins R M Prospects of drying with radio frequency and microwave electromagnetic fields. Journal of Separation. Process Technology. 1979, 1(1): 14 – 23