Effect of Ultrasonically Induced Nucleation on the Drying Kinetics and Physical Properties of Freeze-Dried Strawberry

Power ultrasound has proved to be very useful in controlling crystallization processes since sonication can enhance both the nucleation rate and crystal growth rate by producing fresh and/or more nucleation sites. Therefore, in this study, power ultrasound was applied to control the freezing step of freeze-dried strawberry. The results showed that when power ultrasound was applied at different temperatures, it increased the nucleation temperature and shortened the characteristic freezing time. The application of power ultrasound in the freezing step increased the drying time in subsequently freeze-dried strawberry samples. This longer drying time was found to be due to increased resistance to moisture diffusion due to the formation of a network of small pores caused by sublimation of small ice crystals induced by the power ultrasound. Scanning electron microscopic images revealed that the freeze-dried sample frozen by ultrasound-assisted freezing (UAFD) had finer cellular structure compared to those frozen in other freezing conditions. UAFD samples had better textural hardness, while the rehydration capacity was lower compared to those of NRFD and NIFD samples.