Determination of Drying Parameters, Ascorbic Acid Contents and Color Characteristics of Nettle Leaves During Microwave‐, Air‐ and Combined Microwave–air‐drying

Abstract

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

Microwave, Air and Combined Microwave-Air Drying of Grape Leaves (Vitis vinifera L.) and the Determination of Some Quality Parameters

“…The results most similar to those of fresh grape leaves with respect to L, a, and b values were obtained at the 75°C, which confirmed the findings of several previous studies [10,39,47,48]. A significant loss occurred for the L, a, and b values of grape leaves dried under different combinations Several researchers found to similar findings [10,34,48]. Among all the drying trials, the color criteria most similar to the those of fresh leaves occurred at the 500 W-75°C, while the color criteria least similar to those of fresh leaves was noted at the 100°C.…”

Effect of drying methods on the chemical composition and colour of peppermint (Mentha × piperita L.) leaves

“…Chroma values of the microwave dried peppermint leaves did not decrease. In turn, good retention of colour also was observed in nettle (Alibas, 2010) and coriander (Sarimeseli, 2011). The colour saturation or chroma (C*) parameter permits the determination of the strength of response to the hue of a colour in a qualitative manner, through interpretation of its intensity and depth (Gozdecka, 2006).…”

The effect of high power airborne ultrasound and microwaves on convective drying effectiveness and quality of green pepper

“…Application of microwaves reduces also the degree of vitamin C degradation, i.e. by combined microwave-air-drying the ascorbic acid content was retained up to 98% [9].…”