Drying Characteristic of Blanched Purple-Fleshed Sweet Potato Under Microwave Vacuum Drying

Abstract

The commercial microwave oven was modified in order to make a lab scale microwave vacuum dryer. The dryer was used to investigate drying characteristic of purple-fleshed sweet potato slices. Two levels of power and two kinds of blanched pre-treatment was applied to the tuber slices. The experimental results showed that the lab scale microwave vacuum dryer was successfully made from the commercial microwave oven. The drying time under microwave vacuum drying was done in the very short amount of time, e.g 7 minutes to reach approximately 7.5% of moisture content in dry basis. Two drying phases were found in the drying curve which was the constant drying rate and falling drying rate. The best model for explaining the dying characteristic of the tuber slices was the Page model with R square (R2) value more than 0.995. Highest effective diffusivity (3.83E-07 m2/s) was obtained at 800 W with steam blanching pre-treatment. Steam blanching pre-treatment resulted in a more similar color of the dried slices compared to the fresh color of the tuber slices. Combination of the microwave vacuum drying and steam blanching pre-treatment resulted in the very fast drying process and better color retention compared to the combination of the microwave vacuum drying with hot water blanching.