![]() Yeasts and molds, on the other hand, usually require even lower a w to prevent growth. The a w of most fresh foods is 0.99 whereas the a w necessary to inhibit growth of most bacteria is roughly 0.91. The result is a reduction of the so-called product water activity (a w), a measure of unbound, free water molecules in the food that is necessary for microbial survival and growth. This has the effect of drawing available water from within the food to the outside and inserting salt or sugar molecules into the food interior. Salt or sugar, whether in solid or aqueous form, attempts to reach equilibrium with the salt or sugar content of the food product with which it is in contact. The most notable is simple osmosis, or dehydration. There are several ways in which salt and sugar inhibit microbial growth. Incidentally, these processes not only prevent spoilage of foods, but more importantly serve to inhibit or prevent growth of food-borne pathogens such as Salmonella or Clostridium botulinum when properly applied. However, all curing processes fundamentally depend on the use of salt and/or sugar as the primary preservation agent(s). There are numerous descriptions and permutations of curing which may include additional preservation techniques such as smoking or ingredients such as spices. Examples of foods preserved with salt or sugar include the aforementioned corned beef as well as bacon, salt pork, sugar-cured ham, fruit preserves, jams and jellies, among others. For instance, brine is the term for salt solutions used in curing or pickling preservation processes. (Pieces of rock salt used for curing are sometimes called corns, hence the name "corned beef.") Curing may utilize solid forms of salt and sugar or solutions in which salt or sugar is mixed with water. Protection of foods from microbial spoilage using salt (usually sodium chloride) or sugar (usually sucrose) has ancient roots and is often referred to as salting, salt curing, corning or sugar curing. Mickey Parish, chair of the Nutrition and Food Science Department at the University of Maryland, explains.
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