ABSTRACT
Freezing is a process of bringing down the temperature of food below its freezing point; it is generally stored at temperatures below –5°C, more commonly –10°C or –18°C. Freezing is an ancient method of preservation, but today refrigerated and frozen foods occupy more than 50% of the oor space reserved for processed foods. Freezing results in the transformation of liquid water to solid ice. Liquid water is essential for the microbiological, enzymatic, and chemical activity in foods that reduces its storage life. The freezing of water to ice basically puts a stop to most of these activities. Freezing is an ice crystallization process. Since the bulk of the liquid phase in foods is water, this means that upon freezing, water transforms to ice crystals. Such transformations generally take place at freezing point, which is well dened for pure liquids (e.g., 0°C for water). In foods, in addition to water, there are numerous other constituents in varying amounts. For this reason, food never freezes at 0°C. Further, it can never have a dened single freezing point. Depending upon the nature of the
17.1 Introduction ......................................................................................................................... 323 17.2 Conventional Freezing ........................................................................................................ 324
17.2.1 Nucleation ............................................................................................................ 324 17.2.2 Crystal Growth ...................................................................................................... 324 17.2.3 Crystal size and Texture ....................................................................................... 324
17.3 High-Pressure (HP) Processing .......................................................................................... 325 17.4 Thermal Physics and Phase Transition under Pressure ....................................................... 326
17.4.1 Compression Heating ............................................................................................ 326 17.4.2 Phase Transition of Pure Water ............................................................................. 327 17.4.3 Phase Transition of Water in Foods ...................................................................... 327
17.5 Pressure-Shift Freezing (PSF) ............................................................................................ 328 17.5.1 Pressure-Shift Freezing (PSF) Kinetics and Microstructure of Ice Crystals ....... 329 17.5.2 Effects of Pressure-Shift Freezing (PSF) Process on Quality
Changes in Foods ................................................................................................. 330 17.6 Conclusions ......................................................................................................................... 334 References ..................................................................................................................................... 334
food and its composition, one can identify an initial freezing point. But as the food temperature is lowered to this point and as water starts to freeze, the solute concentration in residual unfrozen water begins to increase, which further depresses the freezing point of the residual water fraction. So the freezing point progressively decreases. Also, in theory, one cannot freeze all the water present in a food system, because there is always a certain fraction of it that is attached to the food constituents so tightly that it will not freeze. This constitutes “bound” water or unfreezable water. It is generally recognized that the bulk of ice crystallization takes place in the temperature range –1 to –5°C. This temperature zone is therefore termed the zone of maximum ice crystal formation.
