Convective, vacuum and microwave drying kinetics of mallow leaves and comparison of color and ascorbic acid values of three drying methods

Abstract

Mallow leaves (Malva sylvestris L.) with initial moisture of 5.02 +/- 0.003 on dry basis (82.5% on wet basis) were dried using three different drying methods, microwave, convective and vacuum. The leaves that weigh 75 g each were dried until their moisture fell down to 0.10 +/- 0.005 on dry basis (approximately 9% on wet basis). The following drying levels were used in each of the drying processes: 6.67, 8.67, 10, 11.33 W g(-1) microwave power density; 50, 75, 100 and 125 degrees C for convective drying; and 3, 7 kPa at 50 and 75 degrees C for vacuum drying. Drying periods ranged from 6-10, 26-150 and 38-130 min. for microwave, convective and vacuum drying, respectively. Effective moisture diffisuvities ranged from 2.04403 10(-10)-3.63996 10(-12) m(2) s(-1), 1.70182 10(-11)-1.10084 10(-10) m(2) s(-1) and 1.85599 10(-11)-5.94559 10(-10) m(2) s(-1) for microwave, convective and vacuum drying, respectively. According to ascorbic acid content and color parameters, the best microwave power density was found 10 W g(-1) with a drying period of 6.5 min.