Boiling Water Canning or Pressure Canning?
We are definitely in canning season, and more and more people are interested in preserving local food because of the food quality and the “home economics.” Home canners ask, “Is it okay to can all food in a boiling water canner since I don’t have a pressure canner?” The answer is, “No!” According to the USDA Complete Guide to Home Canning, “whether food should be processed in a pressure canner or boiling-water canner to control botulinum bacteria depends on the acidity of the food.” The term “pH” is a measure of acidity and the lower the “pH”, the more acidic the food. The acidity of a food may be natural, as in most fruits, or added to the food to increase acidity as in pickled food. The acidity level in foods can be increased by adding lemon juice, citric acid, or vinegar. Acid foods, with a pH value 4.6 or lower, contain enough acid to block the growth of the botulinum bacteria or to destroy the bacteria more rapidly when heated. They include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters.
Low-acid foods have pH values higher than 4.6. Low-acid canned foods are not acidic enough to prevent the growth of the botulinum bacteria. 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.
Tomatoes usually are considered an acid food, but we know that some tomatoes have pH of above 4.6. Figs also have pH values slightly above 4.6. Therefore, if tomatoes or figs are properly acidified with lemon juice or citric acid, they may be canned as acid foods in a boiling-water canner.
However, botulinum spores are very hard to destroy at boiling-water temperatures; therefore, according to the USDA all low-acid foods should be sterilized at temperatures of 240° to 250°F, attainable with pressure canners. At temperatures of 240° to 250°F, the time needed to destroy bacteria in low-acid canned food is dependent of the food to be canned, the way it is packed into jars, and the jar size. The USDA Complete Guide to Home Canning, available online, or from your N.C. Cooperative Extension Center gives complete directions for safely processing food. The last canning workshops of the season will be held in the next two weeks on canning jams and jellies and using a pressure canner. Call your Extension Center at 688-4811 for more information or to register.