Characterization of microwave vacuum drying and hot air drying of mint leaves (Mentha cordifolia Opiz ex Fresen)

Mint (Mentha cordifolia Opiz ex Fresen) was subjected to microwave vacuum drying and hot air drying, respectively. For microwave vacuum drying, three microwave intensities i.e. 8.0 W g−1, 9.6 W g−1 and 11.2 W g−1 were applied with pressure controlled at 13.33 kPa. For hot air drying, two drying temperatures of 60 °C and 70 °C were examined. Lewis’s, Page’s and Fick’s models were used to describe drying kinetics under various drying conditions. Effective moisture diffusivities were determined to be 4.6999 × 10−11, 7.2620 × 10−11, 9.7838 × 10−11, 0.9648 × 10−11 and 1.1900 × 10−11 m2 s−1 for microwave vacuum drying at 8.0 W g−1, 9.6 W g−1 and 11.2 W g−1, hot air drying at 60 °C and 70 °C, respectively. The microwave vacuum drying could reduce drying time of mint leaves by 85–90%, compared with the hot air drying. In addition, color change during drying was investigated. Lightness, greenness and yellowness of the microwave vacuum dried mint leaves were higher than those of the hot air dried mint leaves. From scanning electron micrographs, the microwave vacuum dried mint leaves had a more porous and uniform structure than the hot air dried ones. From rehydration test at 30 °C, rehydration rate constants of the dried mint leaves by the microwave vacuum drying at 9.6 W g−1 and 11.2 W g−1 microwave intensity were significantly higher than those by the hot air drying at 60 °C and 70 °C (p ⩽ 0.05).