Title: Ensuring Safe Canned Foods (part 1 of 2)
Categories: Canning, Information
Yield: 1 guide
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.
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* USDA Agriculture Information Bulletin No. 539 (rev. 1994)
* Meal-Master format courtesy of Karen Mintzias
