**3. Crop drying**

Many studies have investigated the application of microwave energy to speed up crop and wood drying [25, 26]. Higgins and Spooner [27] investigated alfalfa, which was microwavedried for 7, 8, 9 or 10 min in a microwave oven, compared with field and convective ovendried alfalfa. They found no differences in crude protein, *in vitro* dry matter digestibility or acid detergent lignin between the various drying methods. Microwave-dried alfalfa generally retained a higher proportion of the cell-wall constituents (neutral detergent fibre) than did field-dried alfalfa. Microwave dried Alfalfa that was treated for 7 minutes had significantly lower acid detergent fibre values than all other drying treatments.

Adu and Otten [28] studied the kinetics of microwave drying of white beans. They found that microwave drying was a falling rate process. When constant power was absorbed, seed temperature increased rapidly to a maximum value during the initial stages of drying and began to decrease gradually during the latter stages of drying. To maintain a constant drying temperature, the microwave power had to be increased progressively as the moisture content of the beans decreased due to drying.

This is linked to reductions in the dielectric properties of the beans as moisture is removed [29], which reduces the interactions between the microwave fields and the beans. The gradual decrease in seed temperature, when the drying rate decreases, is opposite to what is observed during conventional hot air drying. This may be caused by a progressively increasing heat of desorption during the drying process [30], which is a common phenomenon in hygroscopic solids. Thus, the microwave heating characteristics observed for white beans may apply to other hygroscopic solids, such as soils, wood, and fodder chaff.

Microwave drying is fast, as may be expected from the coupling of heat and moisture transport described earlier. The drying curve (Figures 1 and 3) exhibits a short relatively slow drying period, followed by a much faster almost linear relationship between applied microwave energy and moisture loss. This is followed by a more conventional falling rate drying period (Figure 1); however prolonged microwave treatment at high power leads to a phenomenon known as "thermal runaway", which causes charring (Figure 2). Microwave treatment profoundly affects the germination performance of grains, with any reasonable application of microwave power totally inhibiting grain germination (Table 2). The microwave drying curve can be described by:

48 The Development and Application of Microwave Heating

surface [23].

**3. Crop drying** 

chaff.

The temperature/moisture profiles in small-diameter cylinders, such as a plant stem, usually exhibit pronounced core heating [23, 24]. On the other hand, temperature profiles in large cylinders exhibit subsurface heating, with the peak temperature occurring slightly below the

Microwave heating in spheres is similar to that in cylinders. The microwave's electric field

*E E*

*n Ee t h i r r r <sup>t</sup> <sup>e</sup> ir ir e ki r k j ri r*

 *o o o o j fr*

<sup>2</sup> <sup>2</sup> 22 4 <sup>2</sup>

2 1

2 44 *<sup>o</sup> <sup>t</sup> <sup>r</sup> r r o o t t <sup>o</sup> <sup>o</sup>*

This analysis can be used, in conjunction with experimental data, to better understand how

Many studies have investigated the application of microwave energy to speed up crop and wood drying [25, 26]. Higgins and Spooner [27] investigated alfalfa, which was microwavedried for 7, 8, 9 or 10 min in a microwave oven, compared with field and convective ovendried alfalfa. They found no differences in crude protein, *in vitro* dry matter digestibility or acid detergent lignin between the various drying methods. Microwave-dried alfalfa generally retained a higher proportion of the cell-wall constituents (neutral detergent fibre) than did field-dried alfalfa. Microwave dried Alfalfa that was treated for 7 minutes had

Adu and Otten [28] studied the kinetics of microwave drying of white beans. They found that microwave drying was a falling rate process. When constant power was absorbed, seed temperature increased rapidly to a maximum value during the initial stages of drying and began to decrease gradually during the latter stages of drying. To maintain a constant drying temperature, the microwave power had to be increased progressively as the moisture

This is linked to reductions in the dielectric properties of the beans as moisture is removed [29], which reduces the interactions between the microwave fields and the beans. The gradual decrease in seed temperature, when the drying rate decreases, is opposite to what is observed during conventional hot air drying. This may be caused by a progressively increasing heat of desorption during the drying process [30], which is a common phenomenon in hygroscopic solids. Thus, the microwave heating characteristics observed for white beans may apply to other hygroscopic solids, such as soils, wood, and fodder

significantly lower acid detergent fibre values than all other drying treatments.

(8)

*o oo*

(9)

4 4

2 2 2

*j fr*

distribution in the radial dimension of a cylinder can be described by [21]:

The resulting solution to equation (2) can ultimately be derived [21]:

*o o o oo o*

content of the beans decreased due to drying.

" 1 2

microwave heating affects agricultural and forestry products.

$$\text{MCC} = \left(\text{MC}\_i - \text{MC}\_f\right) \times e^{-\left(\frac{Em}{b}\right)^2} + \text{MC}\_f \dots \tag{10}$$

**Figure 1.** Wheat grain moisture content as a function of applied microwave energy

**Figure 2.** Charring of wheat during prolonged microwave treatment

Problems with thermal runaway during microwave drying can be overcome by using cyclic drying instead of continuous microwave heating [31]. In this technique, microwave energy is applied for a short time to induce rapid heating and moisture movement and then the product is allowed to equilibrate during a period with no microwave heating. This technique has been successfully applied to timber drying [31] (Figure 3). The resulting drying curve is still described by equation (10).


**Table 2.** Effect of microwave drying of 700 g samples of wheat in a 750 W, 2.45 GHz, domestic oven on germination percentages of grains

**Figure 2.** Charring of wheat during prolonged microwave treatment

drying curve is still described by equation (10).

Microwave Power (%)

germination percentages of grains

Problems with thermal runaway during microwave drying can be overcome by using cyclic drying instead of continuous microwave heating [31]. In this technique, microwave energy is applied for a short time to induce rapid heating and moisture movement and then the product is allowed to equilibrate during a period with no microwave heating. This technique has been successfully applied to timber drying [31] (Figure 3). The resulting

20 88% 42% 4%

50 46% 0% 0%

70 6% 0% 0%

100 0% 0% 0%

Control 90% 88% 94%

**Table 2.** Effect of microwave drying of 700 g samples of wheat in a 750 W, 2.45 GHz, domestic oven on

Microwave treatment time (minutes) 3.5 7.0 10.5

**Figure 3.** Wood moisture content as a function of the number of microwave energy cycles applied

Walde, et al. [32] studied the effect of microwave drying on the grinding properties of wheat. The microwave dried samples were crisp and consumed less energy for grinding compared to the control samples. The Bond's work index for the bulk sample was 2.26 kWh kg-1 compared to 2.41 kWh kg-1 for the control samples of equal moisture content. These studies indicated that microwave drying of wheat before grinding helps reduce power consumption in wheat milling. The microwave drying did not change the total protein content, but there were some functional changes in the protein, which was evident from gluten measurements.

Studies have also been carried out on the dry milling characteristics of maize grains, which were dried previously from different initial moisture contents (MCi) in a domestic microwave oven [33]. The MCi ranged from 9.6% to 32.5% on a dry sample basis. Drying was also carried out in a convective dryer at temperatures of 65 – 90 °C. The drying rate curve showed a typical case of moisture loss by diffusion from grains. The dried samples were ground in a hammer mill and the Bond's work index was found to decrease with increasing duration of microwave drying. There was no difference in protein and starch content between the different treatments. Viscosity measurements were made with 10% suspensions of the flour in water which were heated to 80 – 90 °C and allowed to cooled. Viscosity decreased with increasing microwave drying of the grains. The colour analysis showed that flour of the microwave-dried samples was brighter than the control and convective dried samples. Based on these and other studies, microwave drying of agricultural and forestry commodities appear to be a viable alternative to conventional methods, especially when rapid drying and high throughputs of moist material are desirable.
