Influence of ultrasound-assisted nucleation on freeze-drying of carrots

Ultrasound has been often used to assist nucleation process in liquid samples, while solid samples were barely taken into account. In this study, direct-contact ultrasound was used to realize the sonocrystallization for solid sample and the influence of ultrasound-assisted nucleation on freeze-drying process was investigated. First, a microscopic observation was performed to exam the effectiveness of direct-contact ultrasound on inducing nucleation for carrot slice at different supercooling degrees. Second, an ultrasonic vibrating plate was used to integrate the contact ultrasound into freeze-drying process and its influence on ice crystals size and sublimation rate was evaluated. The results showed that direct-contact ultrasound is applicable to stimulate nucleation in carrot slice at lower supercooling degree and obtain bigger ice crystals. Compared to the control sample, the sublimation time of the samples under sonocrystallization was significantly reduced, by 21.80%. Therefore, direct-contact ultrasound maybe a promising method to be applied in solid-sample freeze-drying process.