Canning preserves food by heating sealed jars long enough to destroy the organisms that cause spoilage, then letting a vacuum seal form as the jars cool. Done right, home-canned food is safe on the shelf for a year or more.

The danger: botulism

The bacterium Clostridium botulinum forms spores that survive ordinary boiling. Inside a sealed jar — no air, moist, room temperature — those spores can wake up and make a toxin that is deadly in tiny amounts. You cannot see, smell or taste it.

The spores cannot grow in acid. That single fact divides all canning into two methods:

• High-acid foods (pH 4.6 or lower) — most fruits, jams and jellies, pickles, sauerkraut, and tomatoes with added acid — can be processed in a boiling-water bath at 212°F.
• Low-acid foods — all plain vegetables, beans, meat, poultry, fish, soups and stews — must be processed in a pressure canner, which reaches 240°F, the temperature needed to destroy botulism spores.

Tomatoes need help

Many tomatoes are close to the line, so tested recipes add acid to every jar: 1 tablespoon bottled lemon juice or ¼ teaspoon citric acid per pint, or 2 tablespoons bottled lemon juice or ½ teaspoon citric acid per quart. Use bottled juice — its acidity is consistent.

Use tested recipes

Processing times are worked out in a lab for a specific food, jar size and packing method. Changing a recipe — adding onions or peppers to salsa, thickening with flour, using bigger jars — can leave the center of the jar under-processed. Use recipes from the USDA Complete Guide to Home Canning, the National Center for Home Food Preservation, your extension office, or the current Ball guide.

Try it: open the free canning calculator on PGAMO Homestead Living and look up the method and time for something you want to put up this year.