Canning is a method of preserving food that uses heat to destroy harmful microorganisms, inactivate enzymes that cause spoilage, and create an airtight vacuum-sealed jar. Foods such as fruits, vegetables, stews, sauces and relishes are suitable for canning.

There are two methods of processing canned foods, depending on the pH (acidity level) of the food. Acidic foods, such as fruits, pickles and some tomatoes, are canned by the boiling water-bath method, which involves cooking the food in canning jars in a 212°F water bath for a period of time specific to the food. Low-acid foods, such as vegetables, meats, poultry and fish, must be processed in a steam-pressure canner, which produces a superheated temperature of 240°F. During the canning process air in the jar is forced out, which creates a vacuum that seals the lid to the jar. This seal is necessary to prevent contamination of the food by harmful organisms.

For additional information and detailed instructions about canning, contact the United States Department of Agriculture, a county agricultural extension agent or consult literature from one of the large canning jar manufacturers. Since canning information has been updated in recent years, make sure that any printed materials are current.

Not all varieties of tomatoes have enough acid to allow them to be safely processed by the boiling water-bath method. Often newer hybrids are lower in acid. Before canning tomatoes, always determine the tomato variety and ask the county agricultural extension agent if it is safe to process by this method. When in doubt, process tomatoes in a steam-pressure canner.