Food preservation is the science of extending the shelf life of food while maintaining as much as possible of its nutritional quality and avoiding the growth of unwanted micro-organisms. There are a number of different ways of preserving foods. Some involve storage methods, some involve packaging, and some involve treating the food in particular ways prior to or during storage, either cooking it in whole or part or combining it with other foods or substances. This science has been practiced in a variety of ways for hundreds of years.
Chilling food is one way of maintaining it. Some foods do well with the temperature level that most people associate with refrigeration, while other food items are best kept frozen. Cold cellars or root cellars were historically used to keep produce, and they are still used today. In the United States, it is common to find a combination refrigerator and freezer as a standard appliance in most kitchens. Even more specialized is the hydrator tray offered in most refrigerators to control the moisture for storing fruits and vegetables, and the butter container that keeps butter and margarine slightly warmer than the rest of the refrigerator to improve its spreadability.
Storage methods that are used in food preservation include vacuum sealing, canning, and bottling. These techniques seal food in sterile bags, jars, or cans, and canning is often combined with other preservation methods, such as pickling or the making of preserves, jams, or jellies. Wrapping food carefully, for example in plastic wrap and then aluminum foil, is another deterrent to food spoilage.
Other food preparation methods besides pickling and preserving that contribute to preserving the life and quality of foods include the following:
- Drying, which can be done in the air, or using a specialized dehydration device;
- Salting or curing, which uses osmosis to draw the moisture out of meat, and is sometimes achieved with chemicals called nitrates and nitrites;
- Smoking, a drying process often used with meat and fish that uses heat to remove moisture from food but stops short of cooking it;
- Irradiation, subjecting food to ionizing radiation in, for example, the form of X-rays or gamma rays, which is a fairly new method of treating food to kill micro-organisms and insects and reduce spoilage.
Additionally, preservation can be accomplished through pasteurization, heating foodstuffs in order to effect certain micro-organisms; lactic fermentation, a preservation method somewhat like pickling that reduces the pH value; using lye as a preservative—but note that it increases alkalinity to stop micro-organisms from growing; and preservation with ethanol.
These methods can be individually applied or in some cases combined, and are chosen depending on the particular food to be preserved.