Abstract:

In order to improve the quality of dried products and reduce the drying time, red jujubes were subjected
to ultrasonic treatment before medium- and short-wavelength infrared drying. The optimal conditions for ultrasonic
pretreatment determined by response surface methodology were 40 kHz, 40 min and 350 W. The drying time to reach
a moisture content of 40% by medium- and short-wavelength infrared radiation was 9.55 and 13.33 h with and without
ultrasonic pretreatment, respectively, while the drying time for untreated samples by traditional hot air drying was 17.13 h.
After ultrasonic treatment for 40 min, red jujube peel produced a large number of cracks and became thin, only 38.8 mm in
thickness, and were easily separated into cuticular and epidermal layers, which was advantageous for moisture diffusion in
the drying process and reducing the drying time of red jujube dried by medium- and short-wavelength infrared radiation.
The quality of jujube products dried by medium- and short-wavelength infrared radiation after ultrasonic treatment was
significantly better than that of those without ultrasonic treatment and that obtained by traditional hot air drying. Jujubes
dried by infrared radiation after ultrasonic treatment contained the highest contents of total vitamin C, total phenols and total
flavonoids and the highest sugar/acid ratio, had the best color, and required the lowest energy consumption. Thus, infrared
radiation after ultrasonic treatment is an appropriate method for jujube drying.

Key words: ultrasonic treatment, medium- and short-wavelength infrared drying, jujube, drying time, quality