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Collard leaves (Brassica oleracea L. var. acephala) with an initial moisture content of 6.65 on percentage dry basis (%db) were dried by three different drying methods: microwave, air, and vacuum. Samples of fresh leaves, 25 g each, were dried until their moisture was down to 0.1 on a dry basis. The following drying levels were used in each of the drying processes: 350, 500, 650, 750, 850, and 1000 W for microwave drying; 50, 75, 100, 125, 150, and 175 C for air drying; and 0.4, 50, and 100 mmHg at 50 and 75 C for vacuum drying, respectively. Drying times ranged between 2.5 to 7.5 min, 8 to 210 min, and 35 to 195 min for microwave, air, and vacuum drying, respectively. The data obtained compared well with a thin-layer drying model. Microwave drying at 750 W provided optimal results with respect to drying time, color, and ascorbic acid content (vitamin C).

Grape leaves (Vitis vinifera L.) weighing 50 g (±0.08) with a moisture of 4.08 kg (moisture) kg −1 (dry matter) (75.35% wb) were dried using three different drying methods: microwave, air and combined microwave-air. Drying continued until the leaf moisture decreased to 0.14 (±0.01) kg (moisture) kg −1 (dry matter) (9.07% (±0.20) wb). Drying periods lasted 5-11, 30-70 and 1.5-6 min for microwave, air and combined microwave-air drying, respectively, depending on the drying level. In this study, measured values of moisture were compared with values of predicted obtained from several thin-layer equations; the Alibas Model was found to have the best fit. The optimum drying period, color and ascorbic acid content were obtained by using the combined microwave-air drying method with the optimum combination of 500 W of microwave power at a temperature of 75°C.

Nettle leaves (Urtica dioica L.), with a moisture of 4.41 (dry basis [d.b.]), were dried using three drying methods: microwave, air and combined microwave-air. Drying continued until leaf moisture decreased to 0.1 (d.b.). Three different microwave output powers, 160, 350 and 500 W, were used in the microwave-drying. Drying treatments in air-drying were 50, 75 and 100C, and 1 m/s fan speed. The combination drying, in which microwave-and air-drying were applied together, was nine different combination levels. Drying periods lasted for 6-14, 40-120 and 2-12 min for microwave-, airand combined microwave-air-drying, respectively, depending on the drying level. Energy consumption was 0.07-0.10, 0.25-0.34 and 0.08-0.16 kWh, respectively. In this study, measured values were compared with the predicted values obtained from Page's semiempirical equation. Optimum drying period, color and energy consumption were obtained when microwave-and air-drying were applied simultaneously and the optimum combination level was 500-W microwave applications at 50C. PRACTICAL APPLICATIONSMicrowave-, air-and combined microwave-air-drying have been industrially applied in the drying of foodstuffs, and pharmaceutical and biologic products. The use of a combined microwave-air dryer is common in the food industry. Applied examples include the serial or a parallel combination of both 1 Corresponding 213 methods. The application of the microwave as a heat source produces a fast drying rate and higher drying quality.Tunnel-type microwave-air dryers have been recently used in the industry, and they can be used for the drying of foods, wood and paper products, traditional medicine solid preparations, crude drugs and sliced herbal medicines. They have the virtues of rapid drying, well distributed, energy saving, high efficiency, easy to operate and control, safety and nonpollution, etc. 214I. ALIBAS

Green apple peels, (25.06 ± 0.06 g) with the moisture content of 3.05 ± 0.02 [kg (moisture) kg–1 (dry matter)], were dried using two dehydration methods, microwave, and natural drying until moisture content of apple peels was reduced to 0.092 ± 0.001 [kg (moisture) kg–1 (dry matter)]. Microwave drying periods at 1,000, 800, 600, and 400 W lasted for 17, 21, 27, and 33 min, respectively; whereas, natural drying lasted for 66 hr. The scope of this study, a comparison of measurements of moisture with that of the predicted results was made, obtained from 21 thin layer drying models. For both 600 and 800 W microwave drying, Modified Henderson and Pabis's model; whereas, at 1,000 W, Alibas's model, and at 400 W, Modified Jena and Das's model were considered to be the most appropriate models. The most suitable drying process according to color parameters, chlorophyll content, and nutrients was obtained at 400 W microwave drying. Practical applications The herbal tea industry is growing globally and it is constanrly looking for new flavors that can attact consumers' attention. Herbal teas, in addition to being delicious, are required to meet the daily nutritional needs of the consumers. One of the drawbacks of drying is the loss of aroma and nutrients, moreover, drying operations carried out in open fields, such as sun drying, leads to contamination and can cause more harm than benefit. Thus, for food safety, it is very essential for herbal tea production facilities to use appropiriate methods to produce high‐quality products. The present study on drying of apple peels for herbal tea contains important data on the selection of the most suitable drying method for the preservation of nutrients.

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