Microwave-assisted air dehydration of apple and mushroom

Journal ofFood Engineering 38 (1998) 353-361 0 1999 Elsevier Science Limited. All rights reserved Printed in Great Britain OX%-877419816 - see front matter PII:

ELSEVIER

SO260-8774(98)00131-9

Microwave-assisted

Air Dehydration of Apple and Mushroom

Tomas Funebo* & Thomas Ohlsson SIK, The Swedish Institute for Food and Biotechnology, P.O. Box 5401, S-402 29 Goteborg, Sweden (Received 18 May 1998; accepted 24 September 1998)

ABSTRACT Microwave-assistedhot-air dehydration of apple and mushroom was performed with low-power microwave energy. The purpose of the investigation was to compare hot-air drying and microwave-assistedhot-air drying. The air velocity the microwave output power and the air temperature were the variables in the experiments. The microwave energy was suppled by either microwave applicators with transverse magnetic (TM) modes as dominant modes, or by a multimode cavity microwave oven. The quality parameters were rehydration capacity bulk density, and colour Low air velocity caused a browning of the products and a minimum air velocity of 1 m/s was identified. It was possible to reduce the drying time by a factor of two for apple and a factor of four for mushroom by using microwave-assisted hot-air drying. Rehydration capacity was 20-2570 better for TM applicator-dried apples and mushrooms than for multimode cavity dried ones. 0 1999 Elsevier Science Limited. All rights reserved.

INTRODUCTION

Background The market for dehydrated be worth US$ 260 million over US$ 125 million in potential, drying is still to

vegetables is large. In Europe the market is estimated to (Tuley, 1996). The world raisin production was valued at 1990 (Tulasidas et al., 1993). Despite the commercial a large extent unexplored when it comes to electromag-

*To whom correspondence should be addressed. E-mail: [email protected] 353

354

I: Funebo, T Ohkrson

netic methods in combination with hot-air drying. The electromagnetic methods are, for example, infrared (IR) radiation, high frequency (HF) and microwave (MW) heating and solar energy. Recently, modelling results from IR-assisted hot-air drying (Parrouffe et al., 1997) and HF heating in combination with hot air (Poulin et al., 1997) were published. Many results from modelling of microwave heating alone are available (e.g. Turner & Jolly, 1990; Jansen & van der Wekken, 1991; Barringer et al., 1995). In microwave-assisted hot-air drying there is much more to be done. The difficulty with the microwave process is the large number of factors which affect the microwave heat transfer behaviour; for example, the thickness of the food, the geometry of the food, the microwave heating (dielectric) properties of the food, the polarisation of the microwave energy. The heat capacity (C,) and the dielectric properties (E’ and E”)change with the moisture content and temperature, which also complicates the control of the process. Literature survey

During the first stage of drying of a hygroscopic material, the drying rate is constant because the surface of the material contains free moisture. This is called the COIZstunt rate period. Towards the end of the constant rate period, moisture has to be transported from the inside of the material to the surface, and the critical moisture content has been reached. After this time dry spots appear on the surface, and the drying rate decreases. This is called the falling rate period. When the surface is completely dry, the moisture is transported from the inner parts of the material as a result of concentration gradients between the interior of the material and the surface. This third drying period is called the second fulling rate period, and the drying rate is lower than previously (Mujumdar, 1995). It has earlier been proposed that microwave energy should be applied in the falling rate period or at a low moisture content for finish-drying (e.g. Huxsoll & Morgan, 1968; Smith, 1979). The reason for this is essentially economic. In the constant-rate period, the cost of removing water is regarded as too high when using electrical energy. With a total electrical efficiency of 50% for microwave energy, the cost per MWh is US$ 75 in Sweden today (IEA, 1996). The cost per MWh for heating up air with natural gas and a total heat efficiency of 90% is US$ 20.5 (IEA, 1996). Thus, the natural-gas heated hot-air process is only about 30% of the microwave energy cost. In the USA in 1979 the cost of hot air was 20% of the microwave process cost (Smith, 1979). Microwave energy contributes to the heat transfer by electromagnetic radiation and subsequent volumetric heating. Hot air transfers heat by convection. A combination of hot air and microwave energy improves the heat transfer compared to hot air alone. Several experimenters have reported microwave-assisted hot-air drying experiments with foodstuffs, where considerable improvements in the drying process have been evident (e.g. Huxsoll & Morgan, 1968 (apple and potato); Garcia et al., 1988 (banana); Torringa et al., 1993 (carrot)). The improvements are described as better aroma, faster and better rehydration than hot-air drying, and much shorter drying times. Reduction of the drying time for potato up to a factor of 60 has been reported (Bouraoui et al., 1994). Other results in the field of food drying indicate a drying time which is half or a third of the hot-air drying time (e.g. Riva et al., 1991 (mushroom); Tulasidas et al., 1993 (grapes); Prabhanjan et al., 1995 (carrot)). A modified domestic microwave oven or a small laboratory-scale system is often used for drying experiments (e.g. Yongsawatdigul & Gunasekaran, 1996; Prabhanjan et

Microwave-assisted air dehydration

355

Bhartia et al., 1973). A system with small loads can create problems with repeatability of experiments, which we hoped to overcome by building a pilot-scale system with maximum loads in the region of kilograms.

al., 1995;

The present work

This work was a part of a project financed by the European Commission (EC). The aim of the EC project was to evaluate the feasibility of electrical methods in the drying of fruits and vegetables with the focus on quality parameters. Five European research partners from five countries participated and studied drying of the same fruits and vegetables with different drying methods. The drying methods were microwave drying, high-frequency (HF) drying, freeze-drying, and hot-air drying. Our role was to study microwave-assisted hot-air drying. The hypothesis of our research was that a fairly low microwave power could increase the drying rate many times by creating a ‘pumping’ effect of the microwaves. This way the largest amount of heat would come from inexpensive hot air, and the water would be transported out of the material by the pressure gradient created by the microwave energy. In our experiments we have seen that the major part of the energy/electricity consumption comes from the hot air while the microwaves appear to provide substantial benefits. A cost/benefit analysis is, however, outside the scope of this paper. The products SK dried with microwave-energy-assisted air-drying were apple and mushroom. We were also interested in a new kind of microwave applicator and how it could improve the microwave process by reducing edge overheating effects. Microwave heating still suffers from some unwanted overheating phenomena, which we also hoped to solve by improving the design of the microwave applicators. For the project we built a pilot-scale semicontinuous hot air and microwave oven as mentioned above. Objective The objective of this work was to improve the knowledge about important param-

eters in microwave-assisted air drying of foods, and to determine whether transverse magnetic (TM) applicators could improve the microwave drying process in terms of drying time and product quality compared to multimode-cavity drying.

MICROWAVE HEATING Heating mechanism

To heat food with microwaves is simply to use the fact that the dipole moment of polar molecules and/or electrical conductivity of ions makes these molecules move in the rapidly alternating electric field in a microwave oven. When the molecules rotate or change direction, the kinetic energy is dissipated as heat. The main polar molecule in foods is water. At the most common microwave heating frequency for foods, 2450 MHz, the heat comes mainly from dipolar rotation. A high concentration of ions in foods will, however, increase the absorption of energy at 2450 MHz.

356

T. Funebo, I: Ohlsson

Applicators and multimode cavity

In the experiments we used microwave applicators. The reason for using applicators is that an electric field parallel to an edge causes overheating of the edge. This is explained by refraction along the edge, which makes the microwave heating much stronger along edges than in other parts of the food. Applicators constructed to give a horizontal magnetic field (transverse magnetic, TM), with a large incidence angle of the microwaves, deal with the overheating problem. The solution is twofold. First, the TM polarisation of an electromagnetic wave with a large incidence angle gives an electric field component which is almost vertical. Most of the long edges in foods intended for microwave heating are horizontal (flat foods), which makes the electric field from TM applicators perpendicular to the horizontal edges, making it possible to avoid overheating of these edges. Second, a large incidence angle of TM waves reduces the amount of reflected energy from the surface of the food. A reduced amount of reflected energy increases the electrical efficiency of the process. For a more detailed discussion about microwave TM applicators, refer to Sundberg et al. (1996). A multimode oven is a closed volume, totally surrounded by conducting walls and large enough to permit more than one mode (pattern) of the electric field (Risman, 1991). In a multimode cavity, the microwaves are introduced through a slot in the wall or the ceiling, and the polarisation of the microwaves and dominant modes in the oven are more difficult to control. A domestic microwave oven is usually a multimode cavity. MATERIALS AND METHODS The foodstuffs were purchased at the local supermarket and stored at 4°C. The apples were always French (Golden delicious) and stored for a maximum of 1 week. Mushrooms (Agaricus bisponts) were used and stored for not more than 3 days. The apples and the mushrooms originated from several batches. Prior to drying, the apples and mushrooms were taken out of the storage and sliced in 5-mm slices with a cutting machine. The apples were not peeled and the core was not removed. Within lo-15 min of removing the foodstuffs from cold storage conditions, drying was started. The drying experiments were performed in a specially designed, semicontinuous, hot air and microwave oven (Fig. 1). Microwave applicators with TM modes as dominant modes were used in the major part of the drying experiments. The process judged to be the best was then repeated with a multimode oven set-up. Each time the oven temperature was pre-conditioned for the drying experiments. The maximum centre temperature in the food slices was monitored with optic fibres connected to a fibre-optic temperature measurement instrument (Luxtron 790). The control of the process was accomplished with the aid of a PID regulator, which used the mean maximum centre temperature of four food slices as input data. A maximum allowed food temperature was set in each experiment. Apples and mushrooms were dried in factorial experiments with three variables at two levels, and with three centre points (Table l), which resulted in 11 experiments for both mushroom and apple. The nature of factorial experiments is that replicates are excluded and a standard deviation can be calculated from the centre points of the design. The three variables were air temperature, maximum food-centre temperature, and air velocity.

351

Microwave-assistedair dehydration

The centre temperature of the food slices was used as an indicator of the absorbed microwave power. The microwave power level was changed continuously by the PID regulator, to keep the temperature at about the maximum allowed level. The foods were always placed in a single layer on the conveyor. Low-output power microwave energy, typically 0.5 W/g material or less, was used in combination with hot air. The kinetics of the drying were studied by taking out samples or continuous weighing of the total load in the oven. The drying process was regarded as being completed when 0.1 kg/kg was reached. The moisture content in the drying experiments was calculated on a dry basis (equation 1). x=

kg moisture (1)

kg dry matter

Process control

The difficulty in controlling the process has been a major problem in our experiments. Therefore, we feel that it is relevant to discuss why the centre temperature of the foods was used as a variable in the experiments. It has been suggested that

Controlunit Power-supplies Fig.

1.Schematic illustration

of the semicontinuous

hot-air/microwave

drying equipment.

358

2: Funebo, i7 Oh&son TABLE 1

The Design; a Randomised 23 Experimental Plan was Used for Both Mushrooms and Apples. The Food Temperature was Used as an Indicator of the Absorbed Microwave Power Abbreviation

mulap 1 mulap mulap mulap mulap mulap mulap mulap mulap mu/aplO mu/ap 11 apair muair mu MM ap MM

Air temperature ec,

Food temperature (“C)

ii

ii

80 40

80 40 80

:: 40 40 80 Lz! 60 80 80 60

:i

80 40 60 40 no microwaves no microwaves

Air velocity (ml4

Comment

E 1:5 1.5 0.5 ;*: 1:5 :*50 0:5 1.0 1.0 1.5 1.0

hot air process hot air process MM = multimode MM = multimode

microwave energy should be applied in the final stage of a drying process, as discussed in the introduction of this article. However, at low moisture levels, the permittivity and the loss factor have low values. Low values of the dielectric properties mean that a small part of the energy is absorbed as heat. At a low moisture content the heat capacity (C,) is also low, because water has such a large influence on the heat capacity. The heat capacity is important to consider in microwave heating. A complication is the fact that a system with a low moisture content and low heat capacity often has a low thermal conductivity (2 in W/(m.K)). Empirical equations for Cp and J give this information (Buffler, 1993). This means that at a low moisture content, the temperature can be difficult to control. In our drying experiments, we found that the control of the process was difficult in the later stage when the moisture content was low, A slight change in power level caused the temperature in the food to rise rapidly. Therefore, we decided to concentrate on temperature control, and to switch the microwave energy off when the process became difficult to control. By temperature control, we mean that the temperature inside the foods was used as an indicator of the absorbed power. The loss factor, which affects the microwave power absorption as heat, is a function of temperature, moisture content, and bulk density, which makes the absorption of energy difficult to predict in a drying process. The loss factor (a”) has, in fact, a peak value at an intermediate moisture content for many foods, e.g. apple and mushroom (Funebo & Ohlsson, 1998). The absorption of microwave energy as heat is proportional to the loss factor, which complicates the control of a microwave drying process. The centre temperature of the foods turned out to be a good measure of the absorbed power. The output power was approximately 0.6-l W/(g food) initially, to be reduced in stages down to 0.1-0.3 W/g until the microwaves were shut off.

Microwave-assistedair dehydration

359

Quality evaluation

All quality evaluation has been performed according to the standard procedure of EC project CT-94-2254 (Book of Methods, 1995). In brief, the quality was evaluated as follows: Rehydration experiments were performed by immersing a known amount of dried produce into hot water for a period of time, water temperature 50°C for apples and 95°C for mushrooms. The apples and mushroom pieces were weighed before and after water immersion, and the rehydration capacity calculated (kg water/kg dry matter). The rehydration times were 1, 2, 5, 10, 20, and 40 min. The bulk density was measured by weighing and underwater weighing. The bulk density was then calculated. The L*a*b* measurements were performed with the aid of a Minolta colour reader CR10 with a 10 mm window. Multivariate analysis was performed on the data obtained using a computer program (Unscrambler 6.1, Camo AS, Bergen, Norway). RESULTS AND DISCUSSION When deciding which process conditions produced the best quality microwave-dried products we had to compromise between drying time and quality parameters. Good quality was defined as fast rehydration, high rehydration capacity, low bulk density, little shrinkage, and attractive colour. We did not accomplish optimisation of the drying process in terms of searching for the absolute optimum process with additional drying experiments. From each set of factorial experiments, one sample was judged to be better than the others. Drying rate

In the drying of apples, there was a correlation between increased centre temperature of the foods and increased drying rate (see Figs 2-5). The centre temperature of the food slices was used as an indicator of absorbed microwave power. Thus, the microwave output power had a crucial effect on the drying rate. In fact, the drying time for apples could be reduced to 60% of the air-drying time with the microwave process (cf. ap6 and apair in Figs 2 and 3 respectively). The drying time decreased from nearly 7 h to 4 h. The drying was finished when a moisture content of 0.1 kg/kg was reached. The air velocity influenced the drying rate. Microwave drying with high (1.5 m/s) and low (0.5 m/s) air velocity for apple revealed a difference in drying time with a factor 1.5-3 in favour of the higher air velocity (ap3;ap5, ap2;ap8, ap9;apll in Figs 2 and 3). The influence of air velocity on mass- and heat-transfer coefficients has been reported of previously (Ni & Datta, 1997). For apples, the reduction of drying time accomplished by high air velocity was well above the relative standard deviation, which was about 10% between the drying times. The standard deviation was obtained from the three centre points of the factorial design. We performed principal component analysis (PCA) to evaluate the impact of the process variables on the drying time. Air velocity was negatively correlated with the drying time. This

T. Funebo, T. Ohlsson

360

I 150

200

250

300

350

400

Time/minutes Fig.

2. Microwave-assisted hot-air drying of apple. In this figure, the experiments with the high food temperature part of the experimental plan (microwave output power 0.1-l W/g) are shown. ap2 (40°C air, 80°C in food, 0.5 m/s air velocity), ap3 (80, 80, 1.5), ap5 (80, 80, 0.5), ap6 (60, 60, l.O), ap8 (40, 80, 1.5), and apMM (multimode cavity, 60, 60, 1.0).

0

100

200

300

400

500

600

Fig. 3. Microwave-assisted hot-air drying of apple. In this figure, the experiments with lower food temperature (microwave power, x0.1 W/g output power/g) are presented. ap4 (40°C air, 40°C in food, 1.5 m/s air velocity), ap7 (40, 40, OS), ap9 (80, 40, 1.5), apll (80, 40, 0.5), and apair (hot air 6O”C,1.0 m/s).

361

Microwave-assisted air dehydration

Drying limit

0

50

150

100

200

250

time

300

Time/minutes

fig.

4. Drying curves for mushroom. The curves in this figure are from the microwave

experiments with high food temperature (microwave output power 0.1-l W/g). mu3 (SOT air, 80°C in food, and 1.5 m/s air velocity), mu5 (80, 80, 0.5) mu6 (60, 60, 1.0) mu8 (40, 80, 1.5), muMM (multimode oven, 80,80, 1.5).

Drying limit

50

100

time

150

200

250

300

Time/minutes Fig. 5. Here the mushrooms dried with a lower food temperature are presented (microwave powerx0.1 W output power/g). mu4 (40°C air, 40°C in food, 1.5 m/s air velocity), mu7 (40, 40, 0.5) mu9 (80, 40, 1.5) mull (80,40,0.5), muair (hot air 80°C 1 m/s).

362

I: Funebo, ?: Oh&on

means that an increase in air velocity decreased the drying time (cf. ap3 and ap5 in Fig. 2). Microwave power absorption, observed as the centre temperature of foods, and air temperature were also inversely proportional to the drying time. The centre temperature of the food was more important than the air temperature. This is apparent in Figs 2 and 3. The drying curves in Fig. 2 have high food temperatures (high microwave output power) and dry more quickly. The drying curves in Fig. 3 have low food temperatures and dry more slowly, regardless of the air temperature. Mushroom was more easily dehydrated by microwave energy than apple. The resistance against diffusion was lower and mushrooms could be microwave and hotair dried down to 0.1 kg/kg in 40 min, avoiding browning of the dried product. The hot air process at 80°C took 170 min, and the drying time reduction was a factor of four better with microwaves and air compared to hot air alone (cf. mu3 and muair in Figs 4 and 5). The influence of air velocity was not as clear for mushrooms as for apples. The drying time to reach 0.1 kg/kg was reduced by one-third (mu3;mu5, mu4;mu7 in Figs 4 and 5) if the air velocity was increased from 0.5 to 1.5 m/s impinging air velocity cross current. In one case a low air velocity resulted in a shorter drying time than the high air velocity (mull;mu9 in Fig. 5). The relative standard deviation between the centre points of the factorial plan was approximately 15% for mushroom, which means that the influence of air velocity on drying time for mushroom is in the region of the experimental noise. PCA on mushroom showed that air temperature and air velocity were equally important for the drying time. Microwave power measured as food temperature was the important factor for the drying time of mushroom. Rehydration capacity The results from the rehydration experiments show a rehydration process for apple stabilising from 20 min and onwards at about 2.5 kg water/kg dry matter (see Fig. 6) which is about 40% of the initial moisture. Mushroom rehydration stabilised in about 5 min and the rehydration capacity was about 2 kg/kg, which is 20% of the initial moisture (see Fig. 7). Equilibrium was not reached even after 20-40 min of rehydration. The rehydration capacity was not better for microwave-dried mushrooms or apples compared to air-dried products. In the case of multimode cavity dehydration, the rehydration capacity for apples was 25% lower than for the TM applicator-dried and hot-air-dried samples. For mushroom, the difference was smaller; the rehydration capacity was less than 20% lower for multimode-dried samples compared to TM-applicator-dried samples. The processing conditions did not influence the rehydration capacity of mushroom. Both for apples and mushrooms the rehydration curves were cluster-like gatherings without a pattern (Figs 6 and 7). Bulk density

Concerning the bulk density, there was no obvious pattern within the sample series for either apples or mushrooms. The bulk density of dried apples was between 400 and 500 kg/m3 and that of dried mushrooms was in the region of 300-400 kg/m3. The bulk density results for the foods of this investigation were not affected by the process conditions. However, a slight trend can be derived from the apple bulk

Microwave-assistedair dehydration

363

2.5 is

-

E 02

ap2

-ap3 2

-

ap4

..r..aps --ap6 --tap7 ap9 --.-..apll -ap air A apMMM

01

a

5

10

15

20

25

30

35

40

Time/minutes Fig. 6. Rehydration capacity for apple at 50°C. ap2 (40°C air, 80°C in food, 0.5 m/s air velocity), ap3 (80, 80, 1.5), ap4 (40, 4O“C,1.5), ap5 (80, 80, 0.5, ap6 (60, 60, l.O), ap7 (40, 40, 0.5), ap9 (80, 40, 1.5), apll (80, 40, 0.5), apair (hot air 6O”C, LO), and apMM (multimode cavity, 60, 60, 1.0).

0

’ 0

I 5

10

15

20

25

30

35

40

Time/minutes

Fig. 7. Rehydration capacity for mushroom at 95°C. mu3 (SOT air, 80°C in food, and 1.5 m/s

air velocity), mu4 (40, 40, 1.5), mu5 (80, 80, 0.5), mu6 (60, 60, l.O), mu7 (40, 40, 0.5), mu8 (40, 80, 1.5), mu9 (80, 40, 1.5), mull (80, 40, 0.5), muair (hot air 8O“C, 1 m/s) muMM (multimode oven, 80, 80, 1.5).

364

T Funebo, I: Ohlsson

density. A long drying time promoted a higher bulk density, which is explained by the fact that a long drying time gives more time for the product to shrink. The shrinkage of the dried samples was calculated. The mean shrinkage for apple was 72% and for mushroom 87%. Colour of dried products

The L*a*b* values showed that the colour of hot-air dried apples was as good as that of microwave-dried apples (Fig. 8). The microwave-dried products were more influenced by browning reactions than the hot-air dried products. Uneven heating in microwave ovens is a well-known problem. Variations in the power density over the volume in a microwave oven makes heating uniformity difficult to achieve. The different moisture and temperature profiles in microwave-assisted drying compared to hot-air drying may be of importance for the browning reaction kinetics. The influence of low air velocity is seen in the a* value, which is an indicator of browning for apples. The PCA for apple shows that the a* value is correlated with the air temperature, and that the lightness (L*) is inversely correlated with the food temperature, that is, the higher the microwave power, the lower the lightness, L*. Colour measurements of mushrooms were not possible with the colour reader used. The lightest colour was found visually for mu3 and mu4 (see Table 1).

80

-

lP2

ZP3

aP7

apll

apai

Fig. 8. The L*a*b* mean values for apple from 5-10 measurements. ap2 (40°C air, 80°C in food, 0.5 m/s air velocity), ap3 (80, 80, 1.5), ap4 (40, WC, 1.5), ap5 (80, 80, 0.5, ap6 (60, 60, LO), ap7 (40, 40, 0.5), ap9 (80, 40, 1.5), apll (80, 40, 0.5), apair (hot air 6O”C, LO), and apMM (multimode cavity, 60, 60, 1.0).

Microwave-assistedair dehydration

365

Microwave applicators

It is obvious from the results of the drying time, rehydration, bulk density, and colour, that a microwave-assisted hot-air drying process for apple and mushroom does not benefit from microwave applicators with TM modes as dominant modes. Microwave drying with multimode cavity set-up produces dried products of equally good quality, which is an interesting result. The explanation is that edge overheating is not a problem in microwave drying. Foods are usually put close together on the conveyor in drying. Simulation of microwave heating has shown that objects not further away from each other than a few centimetres are not affected by edge overheating (Sundberg et al., 1998). The theoretical results and the experimental results presented in this article agree very well. Food pieces shrink and become more separated as the drying process proceeds. In the final stage of drying, the separation may be more than 2 cm. Edge diffraction is, however, not a problem when the permittivity is low, which also makes TM applicators redundant for quite dry foods (low permittivity). It should be pointed out that the experiments were designed with low-power microwave energy, typically less than 0.5 W output power per gram food, and a low load under each applicator (x 100 g or less). The higher efficiency of TM applicators compared to the multimode oven set-up can be observed in the apple drying results. The difference between TM-applicator drying and multimode drying is obvious in the early stage of apple drying (Fig. 2). Applicators resulted in faster drying initially, probably because the load of apples was relatively high (x200 g) under each applicator in the beginning of the drying process. Thus, a process with high-output power or/and a high load under each applicator may produce totally different results from ours. To increase the microwave output power above the levels in our experiments will certainly be difficult when a high-quality dried product is desired. CONCLUSIONS Apple dried at 60°C air temperature and 60°C in the centre of the apple pieces together with an air velocity of 1 m/s resulted in an L*a*b* colour which was close to that of fresh apples, and the drying time was decreased by a factor of two compared to the hot-air drying time. For these reasons, this process was judged to be the best compromise for microwave drying of apple. Other process conditions reduced the drying time more or preserved the colour better, but the products were then of inferior quality or the drying time was too long. The best drying process for mushrooms was judged to be 80°C air temperature, 80°C food temperature, and 1.5 m/s air velocity. The drying time for mushrooms dried under these conditions was one-quarter of the hot-air drying time. Food temperature was used as an indicator of the absorbed microwave power. The food temperature induced by microwave power was more important than the air temperature. The air velocity in the cross current should be more than 1 m/s. A low air velocity (0.5 m/s) caused browning in apples and mushrooms. The air velocity also affected the drying rate. Microwave drying with high (1.5 m/s) and low (0.5 m/s) air velocity for apple revealed a difference in drying time with a factor of 1.5-3 in favour of the higher air velocity. For mushroom the difference between different air velocities was

366

T Funebo, T. Ohlssan

less obvious. The drying time of mushroom was reduced by lo-20% when using a high air velocity. The rehydration capacity was about 25% higher for TM-applicator-dried apples and 20% better for mushrooms compared to multimode-cavity-dried apples and mushrooms. Drying with food pieces close together does not require TM applicators. The main reason is that edge overheating is not a problem when food pieces are separated not more than approximately 2 cm. ACKNOWLEDGEMENTS The European Commission is gratefully acknowledged for financial support within EC project CT-94 2254, in which the present work has been performed. REFERENCES Barringer, S. A., Davis, E. A., Gordon, J. & Ayappa, K. G. (1995). Microwave-heating temperature profiles for thin slabs compared to Maxwell and Lambert law predictions. J. of Food Science, 60(5), 1137-1142. Bhartia, P., Stuchly, S. S. & Hamid, M. A. K. (1973). Experimental results for microwave and hot air drying. J. of Microwave Power, S(3), 245-252. Book of Methods (1995). Available from SIK or Erik Torringa at ATO-DLO, PO Box 17, 6700 Wageningen, The Netherlands. Bouraoui, M., Richard, P. & Durance, T. (1994). Microwave and convective drying of potato slices. J. of Food Process Engineering, 17, 353-363. Buffler, C. R. (1993). Microwave Cooking and Processing, Engineering Fundamentals for the Food Scientist, Van Nostrand Reinhold, NY, USA. ISBN O-442-00867-8. Funebo, T. & Ohlsson, T. (1998). Dielectric properties of some fruits and vegetables. J. of Microwave Power and Electromagnetic Energy, submitted. Garcia, R., Leal, F. & Rolz, C. (1988). Drying of bananas using microwave and air ovens. International Journal of Food Science and Technology, 23,73-80.

Huxsoll, C. C. & Morgan, A. I. (1968). Microwave dehydration of potatoes and apples. Food Technology, 22,47-51.

IEA (1996). Energy prices and taxes. The International Energy Agency. Jansen, W. & van der Wekken, B. (1991). Modelling of dielectrically assisted drying. Journal of Microwave Power and Electromagnetic Energy, 26(4), 227-236.

Mujumdar, A. S. (1995). In Handbook of Industrial Drying, A. S. Mujumdar (Ed.), 2nd edn, Chapter 1, p. 21. Ni, H. & Datta, A. (1997). Moisture distribution and loss in microwave heating of foods: effect of food structure, initial moisture level and surface conditions. In Proceedings of the 7th Engineering and Food Conference, ICEF, C37-C40, ISBN 1-85075-814-X. Parrouffe, J. M., Dostie, M., Navarri, P., Andreiu, J. & Mujumdar, A. S. (1997). Heat and mass transfer relationship in combined infrared and convective drying. Drying Technology, 15(2), 399-425. Poulin, A., Dostie, M., Proulx, P. & Kendall, J. (1997). Convective heat and mass transfer and evolution of the moisture distribution in combined convection and radio frequency drying. Drying Technology, 15(6-8), 1893-1907. Prabhanjan, D. G., Ramaswamy, H. S. & Raghavan, G. S. V. (1995). Microwave-assisted convective air drying of thin layer carrots. Journal of Food Engineering, 25,283-293. Risman, P. 0. (1991). Terminology and notation of microwave power and electromagnetic energy. Journal of Microwave Power and Electromagnetic Energy, 26(4), 243-250.

Microwave-assistedair dehydration

367

Riva, M., Schiraldi, A. & Di Cesare, L. (1991). Drying of Agaricus biospoms mushroom by microwave-hot air combination. Lebensmittel Wissenschafr und Technologie, 24,479-483. Smith, F.J. (1979). Microwave-hot air drying of pasta, onions, and bacon. Microwave Energy Applications Newsletter, 12(6), 6-12.

Sundberg, M., Risman, P. O., Kildal, P. S. & Ohlsson, T. (1996). Analysis and design of industrial microwave ovens using the finite difference time domain method. Journal of Microwave Power and Electromagnetic Energy, 31(3), 142-157.

Sundberg, M., Kildal, P. S. & Ohlsson, T. (1998). Moment method analysis of a microwave tunnel oven. Journal of Microwave Power and Electromagnetic Energy, 33(l), 36-48. Torringa, E. M., Nijhuis, H. H., van Remmen, H. H. J. & Bartels, P. V. (1993) Electromagnetic energy in the drying of vegetable produce, In Proceedings of the Fourth International Congress on Food Industry, Cesme, Turkey, pp. 331-344. Turner, I. & Jolly, P. (1990). The effect of dielectric properties on microwave drying kinetics. Journal of Microwave Power and Electromagnetic Energy., 25(4), 211-223. Tulasidas, T. N., Raghavan, G. S. V. & Norris, E. R. (1993). Microwave and convective drying of grapes. Transactions ofthe ASAE, 36(6), 1861-1865. Tuley, L. (1996). Swell time for dehydrated. International-Food-Zngredients,4, 23-27. Yongsawatdigul, J. & Gunasekaran, S. (1996). Microwave-vacuum drying of cranberries: T;t13Energy use and efficiency. Journal of Food Processing and Preservation, 20(2),