The effects produced by high power ultrasound when travelling
across a medium are diverse and their relative importance depends
on the characteristics of the medium. In general, ultrasound produces
alternating compression and decompression of the media.
In liquids, when ultrasonic power attains a threshold, the rarefaction
cycle may exceed the attractive forces and, from existing gas
nuclei, cavitation bubbles could appear (Soria and Villamiel,
2010). These bubbles could maintain a stable increasing and
decreasing size giving rise to the so-called ‘‘stable cavitation’’ generating
a micro-agitation of the medium. However, the bubbles can
also grow and collapse generating very high local temperatures
(5000 K) and pressures (1000 atm), which produce, in turn, high energy
shear waves and turbulence in the cavitation zone. This last effect
is known as ‘‘transient cavitation’’ (Leighton, 1998). The
implosions are asymmetric if produced near a solid surface generating
a microjet that hits the solid (Mason, 1998). This is the main effect
observed in the use of high intensity ultrasound in cleaning
operations. Moreover, the microjets hitting the solid food surface
may produce an injection of fluid inside the solid (Mason and
Cordemans, 1996). The intensity of cavitation and its effects depend on the characteristics of the medium, such as viscosity, and/or process
variables, like ultrasonic intensity, ultrasonic frequency or
pressure.