Michigan State University Extension
Preserving Food Safely - 01600938
10/13/97
Whether food should be processed in a pressure canner
or boiling-water canner to control botulinum bacteria
depends on the acidity in the food. Acidity may be natural,
as in most fruits, or added, as in pickled food. Low-acid
canned foods contain too little acidity 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. They
include red meats, seafood, poultry, milk and all fresh
vegetables except for most tomatoes and figs. 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. 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 sterilized at temperatures of 240 to 250
degrees Fahrenheit, attainable with pressure canners
operated at l0 to l5 PSIG. PSIG means pounds per square
inch of pressure as measured by gauge. At these
temperatures, the time needed to destroy bacteria in low-
acid canned food ranges from 20 to l00 minutes. The exact
time depends on the kind of food being canned, the way it
is packed into jars, and the size of jars. 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. The
practice of processing low-acid foods in a boiling water
canner is NOT RECOMMENDED.