National University of Singapore
Food Science & Technology
Fundamentals of Ultrasound Processing
Sound waves can be classified as infrasound (< 20 Hz), audible sound (20 Hz – 20 kHz) and ultrasound (> 20 kHz). They are propagated through a medium in a series of compressions and decompressions (rarefactions). Sound travels through air at a speed of 330 ms^-1 with a wavelength of about < 0.015 m. The wavelength and the frequency of sound waves are indirectly related which means that at high frequencies, the sound waves have shorter wavelengths. The wavelength of any waves is the distance between 2 peaks or 2 troughs and the relationship between the wavelength (λ), velocity (c) and frequency is depicted as follow (Equation 1),
Ultrasonic waves are similar to sound waves. The only difference is that the former’s frequency is too high to be heard by the human ear. Ultrasound processing has recently received a greater attention in food application in areas such as food preservation and as a processing aid. Ultrasound processing can be categorised into low intensity ultrasound and high intensity ultrasound. Ultrasound processing at low powers (< 1 W·cm^-2) and high frequency (> 1 MHz) has been used for non-invasive and non-destructive analysis of food as the power intensity is not great enough to cause any physical or chemical alteration in the material the wave passes through. Whereas, high intensity ultrasound processing requires high power (> 4 W·cm^-2) and low frequency (≈ 40 kHz).
The energy imparted to a fluid by ultrasound depends on the intensity (I, W·m^-2) of the sound. The relationship between intensity, speed (c) and energy density (ω) of the wave is expressed in the following equation:
The energy density is expressed as follows
Here, ρL represents the density of the nutrient broth and P is the difference between the maximum and minimum pressure at the wave peak and trough (denoted as pressure amplitude of the sound wave).
