Abstract: In order to explore the enhancing effect of ultrasound on hot air drying process, the drying characteristics and quality changes of purple-fleshed sweet potato in an ultrasound-assisted hot air dryer were investigated at different drying temperatures (40, 50, 60 and 70 ℃) and ultrasound powers (0, 30 and 60 W). The Weibull distribution model was applied in the dynamic simulation of the drying process. The results showed that the increase in drying temperature and ultrasound power resulted in a significant reduction in drying time. The Weibull distribution model could fit the drying curves accurately under different drying conditions. Scale parameter (α) decreased in the range of 92.317 to 345.764 min with the increase in drying temperature and ultrasound power. Shape parameter (β) ranged from 0.817 to 1.032, indicating that the hot air drying of purple-fleshed sweet potato was controlled by internal diffusion resistance. The effective moisture diffusivity coefficient (Dcal) values increased in the range of 1.205 × 10-10 to 4.513 × 10-10 m2/s with the increase in drying temperature and ultrasound power. The activation energy decreased as ultrasound power increased. At the same ultrasound power, the contents of total phenolics and total flavonoids increased at first and then decreased with the increase in drying temperature. Both classes of compounds increased with the increase in ultrasound power at lower drying temperature, whereas the reverse trend was observed at higher drying temperature. Therefore, ultrasound-assisted hot air drying is an effective method to accelerate drying rate and simultaneously achieve better quality.

Key words: purple-fleshed sweet potato, ultrasound, hot air drying, drying characteristics, Weibull distribution model