Ultrasound can be defined as inaudible sound waves at a frequency
above 20 kHz. For food preservation, ultrasound waves of low frequency
(18–100 kHz; λ = 145 mm) and high intensity (10–1000 W/cm2) are
the most effective (Dujmic et al., 2013). The preservative effect of ultrasound
is associatedwith complex phenomena of gas cavitation, which explains
the generation and evolution of microbubbles in a liquid medium
(Galanakis, 2012). Cavitation takes place in the vicinity regions of a liquid
that are subjected to rapidly alternating high-amplitude pressures. During
the negative half of the pressure cycle the liquid is subjected to a tensile
force, and during the positive half it undergoes compression. The result
is the uninterrupted formation of microbubbles, the size of which increases
thousands of times (they expand) during the alternation of
the pressure cycles.