Home Canning: Ensuring Safe Canned Foods
Growth of the bacterium Clostridium botulinum in canned food may cause botulism — a deadly form of foodborne illness. These bacteria exist either as spores or as vegetative cells. The spores, which are comparable to plant seeds, can survive harmlessly in soil and water for many years. When ideal conditions for growth exist, the spores produce vegetative cells that multiply rapidly and can produce a deadly toxin within 3 to 4 days of growth in an environment that consists of:
- a moist, low-acid food
- a temperature between 40°F and 120°F
- less than 2 percent oxygen
Botulinum spores are on most fresh food surfaces. Because they grow only in the absence of air, they are harmless on fresh foods.
Food Acidity and Processing Methods
Whether food should be processed in a pressure canner or boiling-water canner to control botulinum bacteria depends on the acidity of the food. Acid foods contain enough acidity to block their growth, or destroy them more rapidly when heated.
Acidity may be natural, as in most fruits, or added, as in pickled foods and tomatoes. Low-acid canned foods contain too little acidity to prevent the growth of these bacteria. The term "pH" is a measure of acidity; the lower its value, the more acid the food. The acidity level in foods can be increased by adding lemon juice, citric acid, or vinegar.
Low-acid foods have pH values higher than 4.6. They include red meats, seafood, poultry, milk, and all fresh vegetables except for most tomatoes. Most mixtures of low-acid and acid foods also have pH values above 4.6, unless their recipes include enough lemon juice, citric acid, or vinegar to make them acid foods. Acid foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters.
Although tomatoes usually are considered an acid food, some are now known to have pH values slightly above 4.6. Figs also have pH values slightly above 4.6. Therefore, if they are to be canned as acid foods, these products must be acidified to a pH of 4.6 or lower with lemon juice, vinegar, or citric acid. Properly acidified tomatoes and figs are acid foods and can be safely processed in a boiling-water canner.
Botulinum spores are difficult to destroy at boiling-water temperatures — the higher the canner temperature, the more easily they are destroyed. Therefore, all low-acid foods should be processed to temperatures of 240°F to 250°F, attainable with pressure canners operated at 10 to 15 PSIG (pounds per square inch pressure as measured by gauge).
At temperatures of 240°F to 250°F, the time needed to destroy bacterial spores in low-acid canned food ranges from 20 to 100 minutes. The exact time depends on the kind of food being canned, the way it is packed into the jars, and the size of jars. The time needed to safely process acid foods in boiling water (212°F) varies from 5 to 85 minutes.
*Provided by K-State Rapid Response Center. May 2012. Revised by Karen Blakeslee, M.S., Extension Associate, K-State Department of Animal Sciences and Industry.