General Canning Information
Ensuring Safe Canned Foods
Growth of the bacterium Clostridium botulinum in
canned food may cause botulism—a deadly form of food
poisoning. These bacteria exist either as spores or as
vegetative cells. The spores, which are comparable to
plant seeds, can survive harmlessly in soil and water
for many years. When ideal conditions exist for
growth, the spores produce vegetative cells which
multiply rapidly and may produce a deadly toxin within
3 to 4 days of growth in an environment consisting of:
- a moist, low-acid food
- a temperature between
40-degrees and 120-degrees F
- less than 2 percent
Botulinum spores are on most fresh food
surfaces. Because they grow only in the absence of
air, they are harmless on fresh foods.
bacteria, yeasts, and molds are difficult to remove
from food surfaces. Washing fresh food reduces their
numbers only slightly. Peeling root crops, underground
stem crops, and tomatoes reduces their numbers
greatly. Blanching also helps, but the vital controls
are the method of canning and making sure the
recommended research-based process times found in the
USDA's Complete Guide to Home Canning are used.
The processing times in this book ensure destruction
of the largest expected number of heat-resistant
microorganisms in home-canned foods. Properly
sterilized canned food will be free of spoilage if
lids seal and jars are stored below 95-degrees F.
Storing jars at 50-degrees to 70-degrees F enhances
retention of quality.
Food acidity and processing
Whether food should be processed in a
pressure canner or boiling-water canner to control
botulinum bacteria depends on the acidity of the food.
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
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 butters.
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
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-degrees to
250-degrees 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 in this
publication (the Complete Guide to Home Canning). At
temperatures of 240-degrees to 250-degrees 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. 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.
at high altitudes
Using the process time for
canning food at sea level may result in spoilage if
you live at altitudes of 1,000 feet or more. Water
boils at lower temperatures as altitude increases.
Lower boiling temperatures are less effective for
killing bacteria. Increasing the process time or
canner pressure compensates for lower boiling
temperatures. Therefore, when you use the Complete
Guide to Home Canning, select the proper processing
time or canner pressure for the altitude where you
live. If you do not know the altitude, contact your
local county Extension agent. An alternative source of
information would be the local district
conservationist with the Soil Conservation Service.