Unless food is preserved, it begins to spoil soon after it is harvested. The main methods of food preservation are canning, freezing and drying.

Canning is the process in which foods are placed in jars and heated to a temperature that destroys microorganisms and inactivates enzymes. This heating, and later cooling, forms a vacuum seal. The seal prevents other microorganisms from recontaminating the food within the jar.

High-acid foods such as fruits and tomatoes can be processed or &8220;canned&8221; in boiling water, while low-acid ones such as vegetables and meats must be processed in a pressure canner at 240 &161;F (10 pounds pressure at sea level).

Pickling is another form of canning. Pickled products have an increased acidity that makes it difficult for most bacteria to grow. Pickled products are also heated in jars at boiling temperatures to destroy any other microorganisms present and form a vacuum in the jar.

Jams and jellies have a very high sugar content. The sugar binds with the liquid present making it difficult for microorganisms to grow. To prevent surface contamination after the product is made and thus possible yeast or mold growth, jams and jellies are either canned, frozen or refrigerated.

Freezing reduces the temperature of the food so that microorganisms cannot grow, yet some may still live. Enzyme activity is slowed down but not stopped during freezing.

Enzymes in vegetables must be inactivated by blanching, exposing to boiling water or steam for a specified time and then quickly cooled in ice water to prevent cooking.

Enzymes in fruits can cause browning and loss of vitamin C and are controlled by the addition of ascorbic acid. While peeling place the fruit in a solution of water and ascorbic acid.

Drying removes most of the moisture from foods. Thus microorganisms cannot grow and enzyme action is slowed. Dried foods should be stored in airtight containers to prevent moisture from rehydrating the products and allowing microbial growth.

For more information call Angela Treadaway at your Regional Extension Agent with the Alabama Cooperative Extension System.