Use of Power Ultrasound to Improve Extraction and Modify Phase Transitions in Food Processing

In recent years, the physical and chemical effects of ultrasound in liquid and solid media have been extensively used in food processing applications. Ultrasound in liquids generates a number of physical forces. Vibration, pressure, and physical agitation are forces that can be generated in the absence of acoustic cavitation. In addition to these physical forces, acoustic cavitation generates microjets, shear forces, shockwaves, radical formation, and acoustic streaming. At lower frequencies (20-100 kHz), the physical effects dominate. At intermediate frequencies (200-500 kHz), chemical effects (formation of highly reactive radicals within the cavitation bubbles) are more dominant, as the number of active bubbles generated is higher. At higher frequencies (> 1 MHz), cavitation and the associated chemical effects are less likely and acoustic streaming effects are dominant. There are a number of food processing applications where these physical and chemical forces of ultrasound have been found to be effective. The present review summarizes selected areas of food applications such as extraction, crystallization, thawing, drying, and freezing where ultrasound is found to be beneficial in terms of increasing efficiency, reducing time, and increasing the yields. The reason for choosing these applications is that such areas are not critically reviewed in the existing literature.