Salt plays a critical role in curing food. Food may be preserved in other ways, such as air-drying or smoking, but these methods are not as reliable or as tasty as curing with salt. Salt curing can be done by pickling in brine, or by spreading salt over a food and hanging in the air. It may also be combined with other methods, such as smoking.
Salt plays several different roles in curing. The first is that it creates changes in the osmotic pressure of food cells, which pulls water out of the food and kills most pathogens. All food cells are filled with water that contain minerals, mostly salts. When you immerse food into salt brine, the salt content of the fluid outside the cells becomes greater than what is inside the cell.
The cell's normal functions rely on maintaining a balance of osmotic pressure between the fluids inside and outside the cell . So as salt concentration outside a cell rises water rushes out of the cells to dilute the salt and bring the organism's osmotic pressure back into balance. This dries out the cells, and since microbes that cause spoilage need water to live, they are destroyed in the process. At the same time salt-cured food becomes a less hospitable environment for future growth of harmful (or toxic) bacteria.
Perhaps the most dangerous bacteria that grows in curing meat is Clostridium botulinum, which causes botulism, a state of partial or full paralysis that can lead to respiratory failure. Because C. botulinum only produces toxins in anaerobic environments, it is not a hazard in cured meats other than very large dense hams and dry-aged meats in casings such as some sausages. The only way to prevent its growth is to use special curing salts with nitrites and nitrates.
Most cured meats (bacon, hot dogs, bologna) can be made nitrate-free without sacrificing safety, though they will lack the bright color and tang of traditionally cured meat. When nitrites interact with the amino acids in meats in our stomachs or in high-heat cooking, potentially carcinogenic nitrosamines are formed. However, during curing most of the nitrite breaks down to nitric oxide, a harmless chemical found naturally in the body. Excessive nitrite salt in cured meat combines with lactic acid to oxidize the pigment myoglobin, producing the green oxymyoglobin. This is known as nitrite burn. "Greening" of curing meats can also results from the accumulation of hydrogen peroxide produced from the reaction of lactic acid with atmospheric oxygen. In either case, the meat is likely inedible.
The tangy flavor of cured meats is, in part, the taste of nitrite, but the powerful flavors associated with curing are also products of the natural aging processes that happen over time. As food sits in a cure, enzymes inside the cells of the food break down protein into savory amino acids (like meaty-tasting glutamic acid), and fats into flavorful compounds that range from floral and citrusy to grassy and buttery. Wet-cured products are not quite as flavorful as dry-cured food because their flavors are diluted with water. The flavor of cured meats is highly concentrated, since 18 to 25 percent of the meat's original moisture is lost during the curing process.