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Acid foods are foods that contain enough
acid to have a pH of 4.6 or lower. Acidic foods can be
processed safely in a boiling water canner, usually without added
acid (lemon juice, vinegar or citric acid). This is necessary to
control botulinum bacteria. Acidity may be natural, as in most
fruits, or added, as in pickled food. Low-acid canned foods are not
acidic enough 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 up to 6.9. (non-acidic, or alkaline foods have pH values of 7.0
or greater) .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 or citric acid. To be safe, we simply recommend always adding 2 tablespoons of lemon juice to each quart of tomatoes or tomato products. 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 sanitized at temperatures of 240° to 250° F, attainable with pressure canners operated at 10 to 15 PSIG. PSIG means "pounds per square inch of pressure as measured by gauge". The more familiar "PSI" designation is used hereafter. At temperatures of 240° to 250° F, the time needed to destroy bacteria 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 jars, and the size of jars. When it is even possible*, 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. Note: * in many cases, no amount of water bath canning will kill the type of bacteria present, because the temperatures never rise high enough.
Summarizing, low acid or non-acidic foods must be:
Below is a summary of foods by pH. For a comprehensive list of the pH values of many fruits and vegetables, including some specific varieties, see this page.
Tomatoes are borderline - and must be considered a special case, with acid added!
These are considered to be LOW acid foods:
Almost ALL vegetables, such as: