Food Acidity

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The acidity level, or pH, of foods determines whether they should be processed in a boiling water canner or pressure canner. The lower the pH, the more acidic the food. Acidic foods have pH values below 4.6. These foods include pickles, most fruits, and jams and jellies made from fruit. (In pickling, the acid level is increased by adding lemon juice, citric acid, or vinegar.) Acidic foods contain enough acidity either to stop the growth of botulinum bacteria or destroy the bacteria more rapidly when heated. Acidic foods may be safely canned in a boiling water canner. Low-acid foods include red meats, seafood, poultry, milk, all fresh vegetables and some tomatoes*.  Low-acid foods have pH values higher than 4.6. They do not contain enough acid to prevent the growth of botulinum bacteria. These foods are processed at temperatures of 240 degrees F to 250 degrees F, attainable with pressure canners operated at 10 to 15 pounds per square inch (psi) of pressure. The exact time depends on the kind of food being canned, the way it is packed in jars, and the size of jars. Although tomatoes used to be considered an acidic food, some are now known to have pH values slightly above 4.6, which means they are low acid. To safely can them as acidic foods in a boiling water canner, you must add lemon juice or citric acid. When you mix low-acid and high-acid foods, assume the mixture remains low-acid and process in a pressure canner. Acidification To assure safe acidity in whole, crushed or juiced tomatoes, add two tablespoons of bottled lemon juice or 1/2 teaspoon of citric acid per quart of tomatoes. For pints, use 1 tablespoon of bottled lemon juice or 1/4 teaspoon citric acid. Acid can be added directly to the jars before filling. If the flavor is too acid to suit your taste, add a little sugar to offset the acid taste. Do not reduce the amount of acid.