Abstract: In this work, hot air and microwave were combinedly used to dry yacon and its drying characteristics were explored under varying conditions of sample thickness, air temperature, and microwave power. Quadratic regression orthogonal rotary combination design was used to optimize total color change (ΔE) and drying time with respect to hot air temperature, water content at conversion point and microwave mass specific power. The results showed that the optimal process parameters of hot air drying were sample thickness of 2－4 mm, air temperature of 70 ℃ and those of microwave drying were sample thickness of 4 mm and microwave mass specific power of 2 W/g. The importance of factors affecting ΔE resulting from hot air combined with microwave treatment dropped in the following sequence: microwave mass specific power, hot air temperature and water content at conversion point, and water content at conversion point was the most important factor affecting drying time, followed by hot air temperature and microwave mass specific power. The optimal parameters for drying yacon by hot air combined with microwave treatment were sample thickness of 4 mm, hot air temperature of 68.1 ℃, water content at conversion point of 61.0% and microwave mass specific power of 2.6 W/g. Under the optimal process conditions, the color change ΔE  was 21.53, the drying time t 172 min, the rehydration rate RR 4.12, and the shrinkage rate SR 84.35%.

Key words: yacon, hot air, microwave, drying characteristics, combinatorial drying