Mickey Parish, chair of the Nutrition and Food Science Department at the University of Maryland, explains.
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. (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. For instance, brine is the term for salt solutions used in curing or pickling preservation processes. 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.
There are numerous descriptions and permutations of curing which may include additional preservation techniques such as smoking or ingredients such as spices. However, all curing processes fundamentally depend on the use of salt and/or sugar as the primary preservation agent(s). 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.
There are several ways in which salt and sugar inhibit microbial growth. The most notable is simple osmosis, or dehydration. 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. 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. The result is a reduction of the so-called product water activity (aw), a measure of unbound, free water molecules in the food that is necessary for microbial survival and growth. The aw of most fresh foods is 0.99 whereas the aw necessary to inhibit growth of most bacteria is roughly 0.91. Yeasts and molds, on the other hand, usually require even lower aw to prevent growth.
Salt and sugar's other antimicrobial mechanisms include interference with a microbe's enzyme activity and weakening the molecular structure of its DNA. Sugar may also provide an indirect form of preservation by serving to accelerate accumulation of antimicrobial compounds from the growth of certain other organisms. Examples include the conversion of sugar to ethanol in wine by fermentative yeasts or the conversion of sugar to organic acids in sauerkraut by lactic acid bacteria.
Microorganisms differ widely in their ability to resist salt- or sugar-induced reductions of aw. Most disease-causing bacteria do not grow below 0.94 aw (roughly 10 percent sodium chloride concentration), whereas most molds that spoil foods grow at an aw as low as 0.80, corresponding to highly concentrated salt or sugar solutions. Yet other microorganisms grow quite well under even more highly osmotic, low aw conditions. For example, halophiles are an entire class of "salt-loving" bacteria that actually require a significant level of salt to grow and are capable of spoiling salt-cured foods. These include members of the genera Halobacillus and Halococcus. Food products that are concentrated sugar solutions, such as concentrated fruit juices, can be spoiled by sugar-loving yeasts such as species of Zygosaccharomyces. Nevertheless, use of salt and sugar curing to prevent microbial growth is an ancient technique that remains important today for the preservation of foods.