DETERMINATION OF DRYING PARAMETERS, ASCORBIC ACID CONTENTS AND COLOR CHARACTERISTICS OF NETTLE LEAVES DURING MICROWAVE-, AIR- AND COMBINED MICROWAVE-AIR-DRYING

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-, air- and 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 APPLICATIONS Microwave-, 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 bothmethods. 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.