Microstructural change of potato tissues frozen by ultrasound-assisted immersion freezing

Power ultrasound has proved to be very useful in controlling crystallisation 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 assist the freezing process. The results showed that the freezing rate of potato sample was improved with the application of ultrasound, compared to that without ultrasound. Higher output power and longer exposure time to ultrasound would help to enhance the freezing rate. However, the heat produced when ultrasound passes through the medium limited the power applied and the exposure time. In this work, the freezing rate was fastest with an output power of 15.85 W and a treatment time of 2 min. The analyses conducted on the microstructure of potato tissue using cryo-scanning electron microscopy technique showed that the plant tissue exhibited a better cellular structure under ultrasonic power of 15.85 W. Much less intercellular void and cell disruption was observed. This was attributed to high freezing rate obtained under high ultrasonic level and thus the domination of intracellular small ice crystals.