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
Botulinum spores are on most fresh food surfaces. Because they grow only in the absence of
air, they are harmless on fresh foods.
- a moist, low-acid food
- a temperature between 40° and 120°F
- less than 2 percent oxygen
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 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°F. Storing jars at 50° to 70°F
enhances retention of quality.
Temperatures for Food Preservation
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 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.
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°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° to 250°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.
Acidity of Foods
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. 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.
This document was extracted from the "Complete Guide to Home Canning," Agriculture Information Bulletin No. 539, USDA (Revised 2009).
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