Pressure-assisted freezing and thawing of foods: A review of recent studies

Phase transition phenomena take place in aqueous solutions, model foods and cellular tissues subjected to combined high pressure and low (subzero) temperature. The kinetics and mechanisms of pressure-shift ice nucleation, of type III, V or VI ice crystal formation, or of pressure-thawing are still poorly documented. Physical and chemical factors affect the ice crystal size distribution throughout sample depth. These phase transition phenomena, as well as the pressure level, influence both chemical constituents (proteins, enzymes) and structural elements (gels, emulsions, cellular tissues, micro-organisms). Recent studies indicate that: (1) in pressure-shift freezing of muscle foods, the aggregation of myofibrillar proteins and the resulting toughness due to high pressure exposition appear to offset the benefit of small ice crystal formation; (2) the cell and tissue structure of some fruits or vegetables is less disrupted by pressure-shift than air-blast freezing, but it is doubtful whether this brings improved appearance, texture or water retention; (3) high pressure-low temperature inactivation of enzymes is not sufficient to replace thermal blanching; (4) high pressure microbial inactivation is enhanced below 0degreesC, but probably not enough for practical applications; (5) the benefits of pressure-thawing in terms of enhanced rate and hygiene may not compensate for increased equipment and packaging costs; (6) progress is made in modelling pressure-freezing and thawing, and in assessing the extent of pressure-shift nucleation.