Ensuring Safe Canned Foods (Part 1 of 2)

INGREDIENTS

INSTRUCTIONS

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 F and 120
degrees F * less than 2 percent oxygen
Botulinum 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 and making sure 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 degrees F. Storing jars at 50 degrees F to 70
degrees 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 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.
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
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 "PSIG" designation is used hereafter in this
publication. 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.
PROCESS ADJUSTMENTS 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 (Plate 2). 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 following canning directions in this series, 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.
======================================================= === * USDA
Agriculture Information Bulletin No. 539 (rev. 1994) * Meal-Master format
courtesy of Karen Mintzias
From Gemini's MASSIVE MealMaster collection at www.synapse.com/~gemini

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