Food preservation is one of the oldest technologies used by human beings. The idea behind all forms of food preservation is to slow down the activity of disease-causing bacteria, and to kill the bacteria altogether.

Refrigeration and Freezing

Refrigeration and freezing are probably the most popular forms of food preservation in use today. In the case of refrigeration, the idea is to slow bacterial action to a crawl so that it takes food much longer (perhaps a week or two, rather than half a day) to spoil. In the case of freezing the idea is to stop bacterial action altogether. Frozen bacteria are completely inactive.

Canning

Since 1825 or so, canning has provided a way for people to store foods for extremely long periods of time. In canning you boil the food in the can to kill all the bacteria and seal the can (either before or while the food is boiling) to prevent any new bacteria from getting in. Since the food in the can is completely sterile, it does not spoil.

One problem with canning, and the reason why refrigeration or freezing is preferred to canning, is that the act of boiling food in the can generally changes its taste and texture (as well as its nutritional content).

Dehydration

Since most bacteria die or become completely inactive when dried, dried foods kept in air-tight containers can last quite a long time. Normally drying completely alters the taste and texture of the food, but in many cases (such as raisins) a completely new food is created that is just as good as the original. Soup and milk are easy to dry and last for years.

Freeze Drying

Freeze drying is a special form of drying that removes all moisture and tends to have less of an effect on a food's taste than normal dehydration does. In freeze drying food is frozen and placed in a strong vacuum. The water in the food then sublimates - that is, it turns straight from ice into vapor.

Salting

Salting, especially of meat, is an ancient preservation technique. The salt draws out moisture and creates an environment inhospitable to bacteria. If salted in cold weather (so that the meat does not spoil while the salt has time to take effect), salted meat can last for years.

Pickling

Pickling was widely used to preserve meats, fruits and vegetables in the past, but today is used almost exclusively to produce "pickles", or pickled cucumbers. Pickling uses the preservative qualities of salt (see above), combined with the preservative qualities of acid, such as acetic acid (vinegar). Acid environments inhibit bacteria. To make the pickles you find on your hamburger, cucumbers are soaked in a 10% salt water brine for several days, then rinsed and stored in vinegar to preserve them for years.

Pasteurizing

Pasteurization is a compromise. If you boil a food you can kill all bacteria and make the food sterile, but you often significantly affect the taste and nutritional value of the food. When you pasteurize a food (almost always a liquid) what you are doing is heating it to a high enough temperature to kill certain (but not all) bacteria and to disable certain enzymes, and in return you are minimizing the effects on taste as much as you can. Commonly pasteurized foods include milk, ice cream, fruit juices, beer and non-carbonated beverages.

Carbonation

Carbonated water is water in which carbon dioxide gas has been dissolved under pressure. By eliminating oxygen, carbonated water inhibits bacterial growth. Carbonated beverages (soft drinks) therefore contain a natural preservative.

Cheese Making

Cheese is way of preserving milk for long periods of time. In the process the milk in cheese becomes something completely unlike milk, but cheese has its own interesting and delicious properties. Cheese making is a long and involved process that makes use of bacteria, enzymes and naturally formed acids to solidify milk proteins and fat and preserve them. Once turned into cheese, milk can be stored for months or years. The main preservatives that give cheeseits longevity are salt and acids.

Chemical Preservation

There are three classes of chemical preservatives commonly used in foods: benzoates (such as sodium benzoate) nitrites (such as sodium nitrite) Sulphites (such as sulphur dioxide)

Irradiation

Nuclear radiation is able to kill bacteria without significantly changing the food containing the bacteria. So if you seal food in plastic and then radiate it, the food will become sterile and can be stored on a shelf without refrigeration.