Ultrasound Pretreatment of Apple Slice Prior to Vacuum Freeze Drying

To improve the vacuum freeze drying rate and to reduce the energy consumption of apple slice, apple slice were treated with different ultrasonic frequency, time and temperature. The vacuum freeze drying rate of treated apple slice was determined and, meanwhile, the rehydration ratio of samples with pre-treatment were determined to compare with those of untreated samples. The results indicated that ultrasonic pre-treatment can significantly improve the drying rate of apple slice, and increase the rehydration rate of the dried apple slice. Under a 100 kHz for 5 min at 25C, the drying rate of pre-treatments is 1.25 times higher compared with that of the control group.