Michigan State University Extension
Preserving Food Safely - 01600938
Whether food should be processed in a pressure canner or boiling-water canner to control botulinum bacteria depends on the acidity in the food. Acidity may be natural, as in most fruits, or added, as in pickled food. Low-acid canned foods contain too little acidity to prevent the growth of these bacteria. Acid foods contain enough acid to block their growth, or destroy them more rapidly when heated. 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 and figs. 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 or citric acid. Properly acidified tomatoes and figs are acid foods and can be safely processed in a boiling water canner.
Botulinum spores are very hard to destroy at boiling- water temperatures; the higher the canner temperature, the more easily they are destroyed. Therefore, all low-acid foods should be sterilized at temperatures of 240 to 250 degrees Fahrenheit, attainable with pressure canners operated at l0 to l5 PSIG. PSIG means pounds per square inch of pressure as measured by gauge. At these temperatures, the time needed to destroy bacteria in low- acid canned food ranges from 20 to l00 minutes. The exact time depends on the kind of food being canned, the way it is packed into jars, and the size of jars. The time needed to safely process low-acid foods in a boiling water canner ranges from 7 to 11 hours; the time needed to process acid foods in boiling water varies from 5 to 85 minutes. The practice of processing low-acid foods in a boiling water canner is NOT RECOMMENDED.