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In physics and chemistry, freezing is the process of cooling a liquid to the temperature (called freezing point) where it turns solid. Melting, the process of turning a solid to a liquid, is the opposite of freezing. Consequently the freezing point is the same temperature as the melting point.
Freezing food for preservation
Freezing is a common method of food preservation which slows both food decay and the growth of microorganisms and, by turning water to ice, makes it unavailable for bacterial growth and chemical reactions.
However, freezing only slows the deterioration of food: it does not stop it, and while it may stop the growth of micro-organisms, it does not necessarily kill them. Many enzyme reactions are only slowed by freezing, so it is often important to stop enzyme activity before freezing, either by blanching or by adding chemicals.
Foods may be preserved for several months by freezing, but not indefinitely. Long-term freezing requires a constant temperature of -18 °C or less. Some freezers cannot achieve, or are not kept at, that temperature. If the temperature in a freezer fluctuates, the length of time foods can be kept is reduced considerably. Freezer doors should be kept closed as much as possible, and only a small amount of unfrozen food should be added at one time.
Freezing adversely affects the texture of many foods, and the texture of nearly all foods is damaged by thawing and re-freezing. Since water expands when it freezes, cell walls in food are often ruptured, resulting in food that is limp or pulpy when thawed. This is especially true of fruits and vegetables that have a high water content. Less damage is done to vegetables that are high in starch. Less damage is also done if the food is frozen quickly, so unfrozen food should be placed in the coldest areas, which are near the bottom of the freezer. Defects in the texture of thawed food can sometimes be obscured by cooking.
Ice cream is an example of a food which is intended for consumption while frozen.
See also: Food preservation
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