Abstract: To investigate the effects of ultrasound-assisted sugar pretreatment on the quality and hygroscopicity of vacuum freeze-dried peach chips, we compared the moisture loss rate and solid yield of peach slices after ultrasound-assisted osmosis in four sugar solutions (sucrose, maltose, glucose and oligoisomaltose) as well as the microstructure, texture (hardness and brittleness), hygroscopicity and glass transition temperature of the resultant peach chips. The results showed that ultrasound-assisted osmosis increased the water loss rate and solid gain of peach slices, resulted in denser pores, cell rupture and structural collapse, and significantly increased the hardness of peach chips compared to the control group (P < 0.05). In addition, ultrasound-assisted osmosis reduced the hygroscopicity of peach chips, and the percentages of mass change after reaching the hygroscopic equilibrium with increasing relative humidity from 0 to 90% ranged from 55.16% to 59.63%, compared to 61.43% for the control group. Ultrasound-assisted osmosis increased the glass transition temperature to 64.27 ℃ compared to the control group (52.37 ℃). In conclusion, ultrasonic-assisted osmotic treatments with sugar solutions have significant influence on the product quality while reducing the hygroscopicity and changing the glass transition temperature of peach chips. The results of this study provide a technical reference for improving the quality of dried peach chips.

Key words: peach chips; vacuum freeze drying; osmotic dehydration; hygroscopicity; glass transition temperature