High Pressure — Low Temperature Processing of Foods: A Review

Abstract

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 nucleation (ice I), of type III, V and VI ice crystal formation, or of pressure-assisted thawing are being thoroughly investigated. Fast data acquisition systems and specific microscopy techniques have revealed that pressure-shift freezing (PSF) induces significant undercooling, and enhances uniform ice nucleation throughout sample depth. Both physical and chemical parameters affect the ice crystal size distribution. These phase transition phenomena, as well as the pressure level, influence chemical constituents (proteins, enzymes) and structural elements (gels, emulsions, cellular tissues, microorganisms). Recent studies indicate that: 1) in PSF of muscle foods (meat, seafoods), the aggregation of myofibrillar proteins and the resulting toughness due to high pressure exposition appear to offset the benefit of small ice crystals; 2) the cell and tissue structure of some fruits or vegetables is less disrupted by PSF than by air-blast freezing, but it is doubtful whether this brings improved appearance, texture or water retention, especially for large samples; 3) high pressure-low temperature inactivation of food enzymes is not sufficient to replace thermal blanching; 4) microbial inactivation under pressure is somewhat enhanced at subzero temperatures in buffer solutions but perhaps less so in food products; 5) the benefits of pressure-assisted thawing in terms of enhanced rate and hygiene (subzero temperature, microbial inactivation) may compensate for increased equipment and packaging costs, especially for seafoods; 6) the formation of type III or V ice in biological samples cooled under pressure requires extensive undercooling; 7) progress is made in modelling pressure-assisted freezing and thawing, and in assessing the extent of pressure-shift nucleation.