Ultrasound technology in food processing: a review

Ultrasound is a non-thermal food processing technology. The sound wave in which the frequency is above the limit of human audibility i.e., greater than 20 KHz referred to as an ultrasonic wave. The energy generated by sound waves of 20000 or more vibrations per second. Ultrasonic vibrations can be produced in any sort of the material- gaseous, liquid and solid. Ultrasound is broadly classified into two group’s as Low Power Ultrasound and High Power Ultrasound. Low Power Ultrasound uses frequencies higher than 100 kHz and intensities below 1 W•cm-2. Low Power Ultrasound cause no physical or chemical alterations in the properties of the material through which the wave passes. High Power Ultrasound uses frequencies between 18 & 100 kHz and intensities higher than 1 W•cm-2 (typically in the range 10-1000 W•cm-2). High Power Ultrasound is capable of altering material properties (e.g. physical disruption, acceleration of certain chemical reactions).Classic ultrasound equipment consists of three components: Electrical power generator, transducer and emitter. The power generator takes the energy from the electrical source. The transducer converts electrical energy into mechanical energy. The emitter is incharge of delivering sound energy into the medium through radiation of the waves. When sound waves enter a medium, sound is transmitted as sinusoidal waves and energy is propagated throughout the system in the form of vibration. This vibration is composed of cycles of compression and expansion moving in the media particles. When the energy (i.e. vibration) reaches an optimum level (depending on the characteristics of the medium such as volume, temperature, composition) an important increase of pressure takes place in the medium. This increase generates thousands of bubbles (cavitation). Cavitation can be transient or stable, a difference that depends on the size of the bubbles produced during cavitation and the speed of bubble growth. Cavitation is responsible for cell disruption, breakdown of microstructures, and production of free radicals in the medium etc.Cavitations create regions of very high temperature (5500° C) and peaks of pressure (50000kPa). (Zoran et al., 2013)