Pressure shift freezing

Kalichevsky-Dong, T., Ablett, S., Lillford, P.J., and Knorr, D. (2000). Effects of pressure-shift freezing and conventional freezing on model food gels. Int. J. Food Set. Tech., 35, 163-172. 

Chevalier et al. (1999) reported that high-pressure thawing of blue whiting was quicker and resulted in lower drip volume in comparison to conventional thawing. Chevalier et al. (2001), based on the analysis of physicochemical properties of fillets (such as color, lipid oxidation, and protein stability), later concluded that 140 MPa was the optimum pressure level for pressure-shift freezing of turbot fillets. 

Chevalier, D., Le Bail, A., and Ghoul, M. 2001. Evaluation of the ice ratio formed during quasi-adiabatic pressure shift freezing. High Pressure Research 21 227-235. 

Zhu, S., Le Bail, A., Ramaswamy, H.S., and Chapleau, N. 2004a. Characterization of ice crystals in pork muscle formed by pressure-shift freezing as compared with classical freezing methods. Journal of Food Science 69 E190-E197. 

Koch, H., Seyderhelm, I., WiUe, P, Kahchevsky, M.T., and Knorr, D. 1996. Pressure-shift freezing and its influence on texture, color, microstructure and rehydration behaviour of potato cubes. Nahrung-Food 40 3) 125-131. 

Otero, L., Martino, M., Zaritzky, N., Solas, M., and Sanz, P.D. 2000. Preservation of microstructure in peach and mango during high-pressure-shift freezing. Journal of Food Science 65(3) 466 70. 

Prestamo, G., Palomares, L., and Sanz, P. 2005. Frozen foods treated by pressure shift freezing Proteins and enzymes. Journal of Food Science 70(1) S22-S27. 

Another technique—pres sure-shift freezing—also shows promise. In this technique the material is subjected to high pressure (200 MPa) and cooled to -15 C. Under these conditions the water does not freeze. However, when the pressure is released suddenly, many small ice crystals form. This has two results the small ice crystals do not rupture any structures present, but by dehydrating the unfrozen material the remaining stmcture is aggregated and stiffened by the introduction of secondary cross-links. At low concentration of solids there are too few interconnecting chains for there to be a load-bearing continuum, and the material tends to flocculate and settle out. 

At normal pressures the freezing process to the glassy state shifts toward lower temperatures with decreasing cooling rate. Therefore, at infinitesimal small rates (curve c). 

We saw in Section 10.5 that the vapor pressure of a liquid rises with increasing temperature and that the liquid boils when its vapor pressure equals atmospheric pressure. Because a solution of a nonvolatile solute has a lower vapor pressure than a pure solvent has at a given temperature, the solution must be heated to a higher temperature to cause it to boil. Furthermore, the lower vapor pressure of the solution means that the liquid /vapor phase transition line on a phase diagram is always lower for the solution than for the pure solvent. As a result, the triplepoint temperature Tt is lower for the solution, the solid/liquid phase transition line is shifted to a lower temperature for the solution, and the solution must be cooled to a lower temperature to freeze. Figure 11.12 shows the situation. 

Increasing the pressure lowers the melting temperature for ices and raises the melting temperature of acetonitrile, and this causes a shift in the relative positions of the invariant points. In Figure 14.20b, the pressure is 100 MPa. The (liquid + liquid) region is still above the water freezing curve, but the freezing... 

When a solution freezes, the solid is usually pure solvent. Thus the solid-vapor equilibrium (sublimation) P-T curve is unaffected by the presence of solute. The intersection of this curve and the liquid-vapor curve is the triple point (nearly the same temperature as the freezing point, which is measured at atmospheric pressure). Since a solute lowers the solvent vapor pressure, the triple point is shifted to lower temperature, as shown in Figure 11-2. Detailed calculations show that the decrease in freezing point for a dilute solution is proportional to the total molal concentration of solutes... 

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