Food preservation

The basic idea behind all forms of food preservation is either:

• ·         To slow down the activity of disease-causing bacteria
• ·         To kill the bacteria altogether

I­n certain cases, a preservation technique may also destroy enzymes naturally found in a food that cause it to spoil or discolor quickly. An enzyme is a special protein that acts as a catalyst for a chemical reaction, and enzymes are fairly fragile. By increasing the temperature of food to about 150 degrees Fahrenheit (66 degrees Celsius), enzymes are destroyed. A food that is sterile contains no bacteria. Unless sterilized and sealed, all food contains bacteria. 

Different techniques used for preservation.

Refrigeration and Freezing

 In the case of refrigeration, the idea is to slow bacterial action to a crawl so that it takes food much longer  to spoil. In the case of freezing, the idea is to stop bacterial action altogether. Frozen bacteria are completely inactive.

Refrigeration and freezing are used on almost all foods: meats, fruits, vegetables, beverages, etc. In general, refrigeration has no effect on a food's taste or texture. Freezing has no effect on the taste or texture of most meats, has minimal effects on vegetables, but often completely changes fruits (which become mushy). Refrigeration's minimal effects account for its wide popularity.

Canning Food

 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. We generally think of "cans" as being metal, but any sealable container can serve as a can. 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).

Dehydrating Food 

Water content in the food is removed. Since most bacteria die or become completely inactive when dried, dried foods kept in air-tight containers can last quite a long time. e.g.

·         Powdered milk

·         Dried fruits and vegetables

·         Powdered soups and sauces

·         Pasta

·         Instant rice

Normally, drying completely alters the taste and texture of the food.

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. Freeze-drying is most commonly used to make instant coffee, but also works extremely well on fruits such as apples.

Salting and Pickling

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 was widely used to preserve meats, fruits and vegetables in the past, but today is used almost exclusively to produce "pickles,". 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.

Pasteurizing, Fermenting, Carbonating

Pasteurizing

It involves heating the food 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.

Fermenting

Fermentation uses bacteria like yeast to produce alcohol. Alcohol is a good preservative because it kills bacteria. When you ferment grape juice you create wine, which will last quite a long time (decades if necessary) without refrigeration.

Carbonating

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.

Chemical Food 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)

Another common preservative is sorbic acid. All of these chemicals either inhibit the activity of bacteria or kill the bacteria.

Food 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. Unlike canning, however, you do not significantly change the taste or texture of the food when you irradiate it.