High pressure ice for food processing
During the freezing process, to avoid damage to cellular tissue and consequent poor quality frozen food, it is essential that the formation of large irregular ice crystals does not occur. The location of the ice crystals is also important as damage to cell components and membranes can ensue if ice crystals pierce membranes. Despite the importance of the freezing process to the food processing industry as a whole, scant information was available on the mechanisms during phase transitions under pressure and at sub-zero temperatures. Project partners of SAFE ICE devised a testing scheme to evaluate if increasing pressure at a certain temperature prior to expansion produced larger amounts of instantaneous frozen water. Furthermore, the importance of the supercooling phenomenon was studied and its effects on the quantity of ice formed after fast expansions. As a means of comparison, three methods of freezing in all were tested. The formation of ice during pressure shift freezing (PSF), conventional air freezing (CAF) and liquid immersion freezing (LIF) was evaluated. Three different pressures and temperatures were examined during PSF. Foods under test included gelatine gel, a common thickener and emulsifier in the food processing industry, and salmon. The results of the study showed that the PSF process significantly improved the formation of ice crystals in terms of size, regularity of formation and location. Further research into high pressure low temperature processing could help to optimise processing conditions with the overall aim of offering a superior food product to the consumer.