Denaturation
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Denaturation is a process in which proteins lose their structure (terciary and secondary) by contact with and external force, stress, or compound such as High Heat, a strong base or acid, concentrated salts, or an organic solvent like alcohol. If proteins in a living cell are denatured, the cellular activity is disrupted and perhaps halted altogether. Non-living denatured proteins can show a range of characteristics, from loss of solubility to withering.

When food is cooked (subjected to heat), some of its proteins become denatured. This is why boiled eggs become hard and cooked meat becomes firm. A familiar example of denaturing proteins comes from egg whites, which are largely water. Fresh from the egg, egg whites are transparent and liquidy. Cooking the fluid whites turns them opaque and forms an interconnected solid mass.

The Latin seafood dish, ceviche, is prepared by chemically "cooking" or denaturing the raw fish in an acid but with no heat. In some cases denaturing is reversible.

The same sort of transformation can also be effected with a denaturing chemical. Pouring egg whites into a beaker of acetone will also turn egg whites translucent and solid. Often times the concept of denaturation is used to mean a food item (often sub-standard meats or alcohols) that has been heated or mixed with additives (like acetone) to make it unsuitable for human consumption.

Some condemned food is deliberately "denatured" when it is rendered inedible via the use of chemical and/or physical adulterants, which are intended to affect the food's palatability or appearance to a degree whereupon it becomes unappealing or even toxic to consume. An example of a chemical adulterant is Denatonium  (a commonly used bittering agent in poison), which can be applied to food used for laboratory purposes where human consumption is not intended.