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Home canning basics

How to can your own safe, high-quality food. Learn about processing time and methods, recommended canners, and cleaning and maintaining equipment.

Ensuring safe canned foods

How canning preserves foods

The high percentage of water in most fresh foods makes them very perishable. They spoil or lose their quality for several reasons:

  • Growth of undesirable microorganisms such as bacteria, molds and yeasts.
  • Activity of food enzymes.
  • Reactions with oxygen.
  • Moisture loss.
Fresh vegetables in red tray.

Microorganisms live and multiply quickly on the surfaces of fresh food and on the inside of bruised, insect-damaged, and diseased food. Oxygen and enzymes are present throughout fresh food tissues. These proper canning practices can minimize the effects of microorganisms:

  • Carefully selecting and washing fresh food.
  • Peeling some fresh foods.
  • Hot packing many foods.
  • Adding acids (lemon juice or vinegar) to some foods.
  • Using acceptable jars and self-sealing lids.
  • Processing jars in a boiling-water or pressure canner for the correct period of time.

When these practices are followed along with recommended methods, they control potential spoilage by:

  • Removing oxygen.
  • Destroying enzymes.
  • Destroying and/or preventing the growth of undesirable bacteria, yeasts, and molds.
  • Helping form a high vacuum in jars. Good vacuums form tight seals which keep the food in the jars and keep air and microorganisms from re-entering.

The most critical step in ensuring safety in canning is processing in a boiling-water bath or pressure canner. Processing destroys microorganisms and creates the desired vacuum for a good seal. Both a high temperature and sufficient time are required to be certain of adequate heat processing. This ensures that all parts of the food have received enough heat to reduce the number of microorganisms to an extremely small level. A safe food with a long storage life is then produced. The complete destruction of every microorganism would result in a product with unacceptable quality and little nutritional value. The best process is that which has a maximum effect on spoilage organisms and minimal effect on quality.

Reducing bacteria through canning

Growth of the bacterium Clostridium botulinum in canned food may cause botulism, a deadly form of food poisoning (Clostridium botulinum, University of Nebraska-Lincoln). These bacteria exist either as spores or as vegetative cells. The dormant spores can survive harmlessly in soil and water for many years. When ideal conditions exist, the spores produce vegetative cells which multiply rapidly. These cells may produce a deadly toxin within three to four days of growth in an environment consisting of:

  • Moist, low-acid food.
  • Temperature between 35 and 122 degrees F (depending on strain).
  • Less than 2 percent oxygen.

Botulism 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 canning method and using the recommended research-based process times.

The correct processing time destroys the largest number of heat-resistant microorganisms. Properly processed, canned food will be free of spoilage if lids seal and jars are stored below 95 F. To retain the best quality, store jars at 50 to 70 F.

To further reduce the risk of botulism, home canned low-acid and tomato foods should be boiled even if you detect no signs of spoilage. Boil foods for 10 minutes at altitudes below 1,000 feet. Add an additional minute of boiling time for each additional 1,000 feet elevation. In Minnesota, boil food for 11 minutes.

Food acidity and processing methods

Deciding whether to process food in a pressure canner or a boiling-water bath to control botulism bacteria, depends on the acidity in the food. Acidity may be natural, as in most fruits, or added, as in pickled food. Acid foods contain enough acidity to block bacteria growth, or destroy them more rapidly when heated. low-acid canned foods contain too little acidity to prevent the growth of these bacteria. To increase the acidity level of foods, add lemon juice, citric acid, or vinegar.

The term pH is an index of acidity. The lower its value, the more acid in the food.

  • 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 enough lemon juice, citric acid or vinegar is included 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.

Processing foods with an unknown pH

If you’re canning products with unknown pH as acid foods, they must be acidified to a pH of below 4.6 with lemon juice or citric acid. 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. When properly acidified, tomatoes and figs can be safely processed in a boiling-water bath.

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William Schafer, emeritus Extension specialist; Deb Botzek-Linn, former Extension educator;  Suzanne Driessen, Extension educator; and Kathy Brandt, Extension educator

Reviewed in 2018

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