Microwave, vacuum, and air drying characteristics of collard leaves

Abstract

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, 25g 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 1000W for microwave drying; 50, 75, 100, 125, 150, and 175 degrees C for air drying; and 0.4, 50, and 100mmHg at 50 and 75 degrees C for vacuum drying, respectively. Drying times ranged between 2.5 to 7.5min, 8 to 210min, and 35 to 195min for microwave, air, and vacuum drying, respectively. The data obtained compared well with a thin-layer drying model. Microwave drying at 750W provided optimal results with respect to drying time, color, and ascorbic acid content (vitamin C).