High power ultrasonic thawing of frozen foods

Ultrasonic thawing tests were undertaken on frozen meat and fish samples to assess the effectiveness of high intensity ultrasound for thawing frozen foods, previous work having indicated that ultrasonic absorption is greatest near the initial freezing point where most of the freezing occurs. Ultrasonic frequencies from 0.22 to 3.3 MHz and intensities up to 3 W cm−2 were investigated. Overheating near the surface was found to be a problem at high intensities and at high and low frequencies, due to the increase in attenuation with frequency, and the onset of cavitation at low frequencies. Using frequencies and intensities around 500 kHz and 0.5 W cm−2, surface heating was minimized, and beef, pork and cod samples were thawed to a depth of 7.6 cm within about 2.5 h. Thawing times were consistent with those predicted by an ultrasonic thawing model. The effects of muscle fibre orientation and inclusion of a fat layer were found to agree with known attenuation coefficients and characteristic acoustic impedances.