Effect of ultrasonic pretreatment on the properties of freeze-dried carrot slices by traditional and infrared freeze-drying technologies

The effect of ultrasound pretreatments at different power levels on drying time and quality parameters of freeze-dried carrot slices was studied. The drying time was significantly reduced (p < 0.05) for both traditional freeze-drying (FD) and infrared freeze-drying (IRFD) with ultrasound pretreatment; the reduction was more evident for IRFD. The drying time was reduced from 698 min to 593 min for FD, whereas it was reduced from 577 min to 459 min for IRFD. The higher ultrasonic power seriously destroyed the cell wall, resulting into more void structure. At ultrasonic power of 150 W, 240 W, and 300 W, the drying time was reduced by 20.7%, 23.7%, and 22.6%, respectively, during IRFD, when compared with FD. The retention of β-carotene was significantly higher (p < 0.05) at 240 W and 300 W; the value was, respectively, 22.7% and 32.0% higher than the control value. The LF-NMR study showed the different states of water and the increase in ultrasound power caused the increase in the relaxation time for free water, indicating more easily drying. However, there was no significant difference (p > 0.05) in color, texture, and sensory properties between FD and IRFD samples.