Let’s Preserve 
Food Canning Basics
Adapted from Complete Guide to Home Canning. USDA bulletin No. 539.

University of Maine Cooperative Extension
Bulletin #4078

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 noted in Cooperative Extension publications have been set using scientific research. For safe, high-quality home-canned food, it’s important that you follow these directions carefully. 

 How Canning Preserves Food  

Fresh foods spoil for a variety of reasons. Bacteria, molds and yeasts cause damage, and so do food enzymes and contact with 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:

• Carefully select and wash fresh food.

• Prepare foods according to Cooperative Extension recommendations. You may need to peel some fresh foods, add acids (lemon juice, citric acid or vinegar) or use hot packs. See other bulletins in our Let’s Preserve series for more information.

• Use recommended jars and self-sealing lids.

• Process jars in a boiling-water bath or pressure canner according to instructions for the correct period of time.

 For Safety’s Sake  

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 botulism bacteria spores 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. Boiling food for 10 to 15 minutes destroys this toxin.

Boiling food before eating it is a way to inactivate the toxin if present. But using acceptable canning recommendations is the only way to be sure home-canned food does not have botulism toxin. Use the following precautions:

• Process low-acid foods in a pressure canner with an accurate gauge.
• Make sure process times and pressures match the size of the jar, style of pack and kind of food being canned.
• Be sure the jar lid is firmly sealed and concave (curved inward).
• Check that nothing has leaked from the jar.
• Check that no liquid spurts out when jar is opened.
• Check for unnatural or “off” odors.

Food Acidity Affects Processing Methods

Don't Use Unsafe Equipment and Canning Methods 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 recommended 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. They do not form a proper seal.

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 acidic 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 to either 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 to 250⁰F. These high temperatures are attainable only with pressure canners operated at 10 to 15 PSI. PSI means pounds per square inch of pressure. 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 the mixture is 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. For specific instructions on canning tomatoes, see the Let’s Preserve fact sheet on tomatoes.

Spot Spoilage by Careful Examination

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 screwbands. 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.

If You Suspect Spoilage, Handle with Care

Treat all jars of spoiled low-acid foods, including tomatoes, as if they contained botulinum toxin, and handle them in one of two ways:

1. If suspect glass jars are still sealed, place them in a heavy garbage bag. Close the bag, and place it in a regular trash container or bury it in a nearby landfill.
2. If suspect jars are unsealed, open or leaking, detoxify (destroy bacteria and toxin) as follows before disposal.

Detoxification process: Carefully place the containers and lids on their sides in an eight-quart or larger pan. Wash your hands thoroughly. Carefully add water to the pan until it is at least one inch above the containers. Avoid splashing the water. Place a lid on the pan, and heat the water to boiling. Boil 30 minutes to ensure that you have destroyed all bacteria and toxin. Cool and discard the containers, lids and food in the trash, or bury in soil.

Thoroughly scrub all counters, containers, and equipment that may have touched the food or containers; don’t forget the can opener, your clothing and hands. Place any sponges or wash cloths used in the cleanup in a plastic bag, and discard in the trash.

Published and distributed in furtherance of Acts of Congress of May 8 and June 30, 1914, by the University of Maine Cooperative Extension, the Land Grant University of the state of Maine and the U.S. Department of Agriculture cooperating. Cooperative Extension and other agencies of the U.S.D.A. provide equal opportunities in programs and employment.

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