Abstract: In order to investigate the enhancing effect of ultrasonic treatment on cold air drying of potato slices, an ultrasonic-assisted heat pump cold air dryer was applied to explore the influence of ultrasonic power and drying temperature on the drying process and the microstructure and several major nutritional components of dried potato slices. The results showed that the drying time was reduced significantly by increasing either ultrasonic power or drying temperature and that drying enhancement was more remarkable at a lower drying temperature than at a higher temperature. The drying rate initially remained constant and then declined during the ultrasonic-assisted drying process, indicating transformation from surface diffusion control to internal diffusion control. The application of ultrasonic treatment could enlarge the size and number of micro-channels on the material surface, being beneficial for moisture transfer. The Weibull distribution function could fit the drying process with high precision. The estimated value of moisture diffusion coefficient increased with increasing drying temperature and ultrasonic power, whereas the value of scale parameter α, characterizing the drying time, decreased with the increase in ultrasonic power. Both ultrasonic power and cold air temperature had significant effects on the contents of total phenols, total flavonoids and vitamin C in dried potato products (P < 0.05). It is clear that the application of ultrasonic treatment to cold air drying of potato slices can increase the contents of major nutritional components. The analytic hierarchy process was applied to obtain the optimal parameters as follows: drying temperature 10 ℃ and ultrasonic power 48 W, and the contents of total phenols, total flavonoids and vitamin C in dried potato slices prepared under these conditions were 296, 52 and 96 mg/100 g, respectively. Therefore, the application of ultrasonic treatment to the cold air drying process of potato slices can remarkably shorten the drying time and improve the product quality.

Key words: potato slices, ultrasonic treatment, cold air drying, drying characteristics, product quality