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.
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