Whether food should be processed in a pressure or boiling-water canner to control Clostridium botulinum bacteria depends on the acidity of the food. Acid may be natural, as in most fruits, oar added, as in pickled food. Low-acid canned foods contain too little acid to prevent the growth of these bacteria.
Acid foods contain enough acid to block bacterial growth or to destroy them more rapidly when heated. The term pH is a measure of acidity; the lower the value, the more acid in the food. The acid 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 butter. Although tomatoes usually are considered an acid foods, some are now known to have pH values slightly above 4.6.
Therefore, if they are to be canned as acid foods, tomato products must be acidified to a pH of 4.6 or lower with lemon juice or citric acid. Properly acidified tomatoes are an acid food and can be safely processed in a boiling-water canner.
Botulism Poisoning Risk
The risk of botulism poisoning determines the choice of either boiling water or pressure canners for canning foods. In foods that are acid (pH 4.6 or lower) the microorganism that causes botulism cannot grow. Therefore it is safe to use a boiling water bath canner. All other foods must be canned using tested pressure canning processes.
Clostridium 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 processed at temperatures of 240ºF to 250ºF, attainable with pressure canners operated at 10 to 15 pounds of pressure.
At these temperatures, 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 the jars and the size of the jars. The time needed to safely process low-acid foods in a boiling-water canner ranches from 7 to 11 hours; the time needed to process acid foods in pressure canner varies fro 5 minutes to 85 minutes.
Is It Safe to Can "Low Acid" Tomatoes?
This question is frequently asked. It is safe to use "low acid" tomatoes in canning. Follow the same procedures as with regular tomatoes. Flavor in tomatoes is a complicated process. Flavor is regulated by the balance of acid, sweetness (or sugar), and some 37 other compounds found in tomatoes that are known to have a taste of flavor. Slight differences in any of the flavor compounds and differences in taste. As the tomato ripens, some of the acid-flavor is replaced so that is becomes milder in flavor.
It has been shown, in several USDA tests, however, that tomato acidity varies very little among tomato varieties. There is some decline in acidity when the tomatoes reach a very complete ripeness - approaching overripe. They have made it clear that we need to articulate the difference between acid-tasting tomatoes and the actual acid content (which doesn't vary much among any variety of tomato).
As K-State Research and Extension Specialists in Horticulture conducted flavor evaluations of tomatoes, another thing became quite clear. we asked 50 people to rate over 20 varieties of tomatoes for flavor (including sweetness and acidity), texture (firmness or softness), as well as visual appearance. We asked them to rate these factors on a 1 to 10 scale. It was interesting to note that there were ranges for all these factors from 2 to 10. What one person reacted to with a low rating, another reacted with a high rating. As we began to look at the results, there was much more variability among the raters than there was among the tomatoes.