Effect of radiofrequency assisted freezing on meat microstructure and quality

Freezing can affect food quality inducing structural changes upon the formation of ice crystals at both intra and extracellular levels. Despite a number of different innovations in freezing technology, the issue still remains that of controlling crystallization processes and hence, ice crystals size. Beside traditional approaches to induce fast and ultra fast freezing, more recently different strategies based on pressure or ultrasound assisted freezing have been proposed. While the former is based on pressure/temperature drops to promote instant nucleation, the latter allows the size of ice crystals to be controlled by application of an acoustic stress. It is likely that similar effects can be expected as a consequence of the application of electromagnetic stresses, such as those deriving from radiofrequency (RF) treatments. The present research was addressed to exploit water dipole rotation induced by low power RF to control ice crystal size during freezing. To this aim, freezing of pork meat was performed in a RF pilot equipment properly modified to allow food immersion in a liquid nitrogen spray. During freezing, continuous or pulsed RF treatments with low potential (2kV) were applied. Freezing under nitrogen flow was also performed as control. In the presence of RF, freezing was achieved in times comparable to those associated to conventional nitrogen flow freezing. The thawing losses of meat decreased in the order: control freezing > freezing + continuous RF > freezing + pulsed RF. The analyses conducted on the microstructure of meat showed that the tissue exhibited a better cellular structure when RF was applied. Much less intercellular voids and cell disruption was observed. This was attributed to the ability of RF to depress the freezing point thus producing more nucleation sites. As compared to the control samples, RF frozen meat presented smaller ice crystals, which were mainly located at intracellular level.