Immersion freezing of potatoes with the aid of power ultrasound was investigated. The effect of power ultrasound on freezing rate was influenced by ultrasound power, exposure time and the freezing phase to which ultrasound was applied. The higher the ultrasound power and the longer the exposure time, the stronger the sonication was. However, the ultrasound power and exposure time should be chosen with the consideration of thermal effect of ultrasound. In this study, the freezing rate was improved greatly when 15.85 W ultrasound power was applied for 2 min (P < 0.05). When ultrasound was applied to the phase change period during freezing process, the freezing rate was increased significantly (P < 0.05). (C) 2002 Elsevier Science Ltd. All rights reserved.