Freezing curves
Freezing food begins when the food is placed in contact with a cold environment, which can be solid (for example, heat exchanger plates at temperatures from -30 to -40C, solid carbon dioxide (dry ice) when 78.5C), liquid (immersion in cooling mixture or cryogenic liquids such as liquid nitrogen in 196C) or gas (air flow, gaseous nitrogen or CO2). The surface of the food cools faster than the centre of the food, because the heat from the bowels of the food must reach the surface by conduction.
Hypothermia
Below the initial point of freezing, say, supercooled. This is the metastable liquid state, fluid may remain in this state for a very long time before the birth of the first of the crystal is. After that the crystals grow and spread throughout the volume quickly. Pure water (without additives, such as dust particles, which will act as nucleation centers) can be supercooled about 40C. At lower temperatures, the water freezes because of homogeneous ice nucleation and growth. In food products, the degree of supercooling is much less than in pure water because of heterogeneous nucleation of ice. Hypothermia is an important character, as it is one of the mechanisms by which living plants and animals cope with the freezing temperatures or minimize the damage from their tissue that ice formation can cause...
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Food refrigeration and freezing