Botulism!
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°F and 120°F; and less than 2
percent oxygen.
Botulism spores are on most fresh food
surfaces. Because they grow only in the
absence of air, they are harmless on fresh
foods.
Most 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.
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You must ensure the
recommended research-based process times, found
in these guides, are used.
The processing times in these guides 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°F.
Storing jars at 50°F to 70°F enhances retention of
quality.
Food Acidity
and Processing Methods
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 acidity 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 acidic 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. 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.
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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°F 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 "PSIG" designation is used hereafter in
this publication. At temperatures of 240°F to
25O°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 the 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.
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