24 - High-Pressure Freezing

Abstract

The advantages of high-pressure freezing processes for product quality as compared to conventional methods stem from two main parameters: reduced duration of phase transition and less mechanical stress during formation of ice crystals. When freezing at constant pressure (atmospheric conditions and high-pressure assisted freezing), nucleation occurs only near the sample surface, in direct contact with the cooling medium. When the latent heat of crystallization is removed, ice crystals grow from the surface to the center of the product. The final ice crystals are needle-shaped and radially oriented. High-pressure-assisted freezing to ice I offers some advantages because the latent heat of the water decreases with pressure. However, the freezing point also decreases and so the temperature of the cooling medium must be lowered to keep the T between it and the sample constant. High-pressure-assisted freezing to dense forms of ice produces a quality improvement; however, to preserve that improvement thawing must take place under pressure to avoid solid–solid phase transition to ice I during expansion. Transferring this technology to an industrial scale is therefore a challenge because continuous storage under high pressure at low temperature seems to be a cost-intensive technology.