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, 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).

Keywords:

• Air drying
• Collard
• Microwave drying
• Vacuum drying

Notes

k, drying rate constant, min⁻¹; n, exponent; SEE, standard error of estimate; R ², coefficient of determination.

∗P < 0.01. ^NSNot significant.

Column mean values with different superscripts are significantly different.

∗P < 0.01.

L, brightness (+100)/ darkness (+0); a, redness (+50)/greenness (−50) coordinate; b, yellowness (+50)/blueness (−50) coordinate; C, chroma; α, hue angle.

Column mean values with different superscripts are significantly different.

∗P < 0.05. ∗∗P < 0.01.