Reviewed June 2003
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Canning preserves food by using heat to destroy the microorganisms that cause
spoilage. Heat forces air out of the jar. As the jar cools, a seal (vacuum)
forms. The processing times and temperatures in MU Extension publications have
been set through scientific research. For safe, high-quality home-canned food,
it's importantm that you follow these directions carefully.
Fresh foods spoil for a variety of reasons. Bacteria, molds and yeasts cause damage and so do food enzymes and contact with the air. Microorganisms live and multiply quickly on the surfaces of fresh food and inside bruised, insect-damaged and diseased food. Proper canning techniques will stop the growth and activity of microorganisms and can prevent spoilage and quality loss. Use these techniques to get safe food and high-quality results:
Don't even taste food not canned according to recommendations from MU Extension, USDA or Ball Blue Book (1989 or later editions only), even if you think it looks safe.
Pressure canning is the only canning method recommended for low-acid foods like meat, poultry, seafood and vegetables. Clostridium botulinum, the bacterium that causes botulism food poisoning, is destroyed in low-acid foods when they are processed at the correct time and temperature in pressure canners. Canning low acid foods in boiling-water canners is absolutely unsafe because the botulinum bacteria can survive this process. If Clostridium botulinum bacteria survive and grow inside a sealed jar of food, they can produce a deadly toxin. Even a taste of food containing this toxin can be fatal. Before eating canned foods, be sure of the following:
Whether you should process food in a pressure canner or boiling-water canner to control botulinum bacteria depends on the amount of acid in the food. The term "pH" is a measure of acidity. The lower the pH, the more acid the food.
Acid foods include pickles, most fruits and jams and jellies made from fruit. (In pickling, the acid level is increased by adding lemon juice, citric acid or vinegar.) Acid foods contain enough acidity either to stop the growth of botulinum bacteria or destroy the bacteria more rapidly when heated.
Low-acid foods don't contain enough acid to prevent the growth of botulinum bacteria. Process these foods at temperatures of 240 degrees Fahrenheit to 250 degrees Fahrenheit. To reach these high temperatures, you must use a pressure canner operated at 10 to 15 pounds per square inch of pressure (PSI). The exact time depends on the kind of food being canned, the way it is packed into jars and the size of jars.
Low-acid foods include red meats, seafood, poultry, milk, all fresh vegetables and some tomatoes. When you mix low-acid and acid foods, assume that the mixture remains low-acid.
Although tomatoes used to be considered an acid food, some are now known to have pH values slightly above 4.6, which means they are low-acid. To safely can them as acid foods in a boiling-water canner, you must add lemon juice or citric acid.
It's important that you know your altitude -- even in Missouri. Don't use process times recommended for canning food at sea level if you live at altitudes above 1,000 feet. Water boils at lower temperatures as altitude increases. Lower boiling temperatures are less effective for killing bacteria. You must increase either the process time or canner pressure to make up for lower boiling temperatures.
Don't taste foods that show any signs of spoilage, and never taste food from a jar with an unsealed lid. Some types of spoilage are easier to detect in jars stored without screw bands. When bacteria and yeast grow, they produce a gas that swells lids and breaks jar seals. Examine lids for tightness and vacuum. Lids with concave (curved inward) centers have good seals.
Next, hold the jar at eye level. While rotating the jar, look for streaks of dried food that have dripped down the exterior. Also, check for rising air bubbles and unnatural color in the food.
While opening the jar, try to smell unnatural odors, but do not actually sniff the jar contents. Look for spurting liquid and cotton-like mold growth (white, blue, black or green) on the food surface and underside of lid.
Treat all jars and cans of spoiled low-acid foods, including tomatoes, as if they contained botulinum toxin. Be especially careful if jars have come unsealed. Use tongs to lift jars off shelf, then place them in a newspaper-lined, heavy garbage bag. Try not to spill jar contents. If any spills occur, put on rubber gloves and thoroughly scrub area. Discard sponge or wash cloth in the garbage bag. Seal garbage bag tightly and place in the trash or bury in a nearby landfill.
Never open-kettle can or process jars of food in conventional ovens, microwave ovens or dishwashers. These practices do not prevent spoilage.
Steam canners are not recommend because safe processing times have not been adequately researched. Using boiling-water canner processing times with steam canners may result in spoilage. So-called "canning powders" are useless as preservatives and do not replace the need for proper heat processing.
Jars with wire bails and glass caps make attractive storage containers for dry foods, but don't use them for canning. One-piece zinc, porcelain-lined caps are also no longer recommended.
GH1451, reviewed June 2003