High Pressure Freezing And Thawing
High pressure freezing, and, in particular, the pressure shift freezing, increasing attention as a method of freezing for high quality or freezesensitive products. The pressure in the shift of freezing food is cooled high pressure, temperatures below zero. Since the freezing temperature decreases, the phase transition will not happen. When the product temperature has more or less in equilibrium pressure is released suddenly. Food now supercooled several degrees and nucleation occurs spontaneously during the supercooled product, causing instant increase of temperature. There may be a short period of equilibrium, in which a little water remains supercooled (Otero and Sanz, 2000). Single nucleation provides uniformly small crystal size and minimum texture damage. High pressure thawing were also investigated as a quick method of thawing. By lowering the freezing point, the difference between the product and the ambient temperature increases, consequently, a greater flow of heat.
High pressure thawing and freezing effect of pressure on the thermodynamic properties (latent heat, freezing temperature, thermal conductivity) should be taken into account.
Freezing temperature is decreased by the pressure, according to Clapeyron equation. Latent heat is also reduced. Thermal conductivity below zero will vary, as there is no ice.
Chourot et al. (1997) modeled high pressure thawing of the infinite cylinder with clean water, using FDM with the CrankNicolson quite apparent specific heat approach. Latent heat is assumed to contribute to a triangular peak spanning 1Kbased. Thermal conductivity is assumed to be constant above and below the range of change of phase, and vary linearly over this range. The total latent heat, and thus, phase transition, temperature given as a polynomial function of pressure. The whole process of thawing occurs under pressure.
Denis et al. (1997) modeled pressure shift freezing with the use of FDM for the explicit activation and obviously, the specific heat of the wording. At the time of discharge pressure, temperature increase from Ti to Tnew calculated the enthalpy of balance. Product is supposed to be in uniform temperature when the pressure is released; however, in a subsequent paper (Denis et al., 2000) this restriction is relaxed and energy balance is made from node to node...
