Applications of ultrasound

Major mechanical effects of ultrasound are provided when the power is sufficiently high to cause cavitation. Like any sound wave, ultrasound is propagated via a series of compression and rarefaction waves induced in the molecules of the medium through which it passes. At sufficiently high power, the rarefaction cycle may exceed the attractive forces of the molecules of the liquid and cavitation bubbles will form. Such bubbles grow by a process known as rectified diffusion, that is, small amounts of vapor (or gas) from the medium enters the bubble during its expansion phase and is not fully expelled during compression. The effectiveness of ultrasound as a food processing tool has been proven in the laboratory and there are a number of examples of scale-up. In most cases, commercially available frequency is used, that is 20 or 40 kHz, and this has proved quite satisfactory. In such cases, the variable parameters are temperature, treatment time, and acoustic power. Little attention has been paid to the use of different frequencies except in a few cases. One such is the use of ultrasound in food preservation using the bactericidal action of sonication combined with other techniques such as heat, ultraviolet light, and the use of a biocide.