Canning

Canning puts summer in a jar: tomatoes, peaches, green beans and sauces kept on the shelf for the months when they are out of season. It is rewarding, and it is safe when it is done right. Done wrong, it can make people seriously ill, so the method matters as much as the recipe.

What canning is

Food is packed into jars, closed with lids and heated long enough to destroy the microorganisms that spoil food or make it unsafe, and to stop the enzymes that break it down. Heating also drives air out of the jar. As the jar cools, a vacuum forms and pulls the lid down into a seal that keeps new microorganisms out.

The seal keeps food safe only if the heating was right. A jar can seal perfectly and still be dangerous if it was not processed at the right temperature for the right time.

Why the method matters: botulism

The main concern is Clostridium botulinum, the bacterium that causes botulism. Its spores survive boiling, and inside a sealed jar of moist, low-acid food they can grow and produce a deadly toxin. The toxin cannot be reliably seen, smelled or tasted.

High-acid and low-acid foods

Acidity decides how a food has to be canned. The dividing line is a pH of 4.6.

Acid foods (pH 4.6 or lower) include most fruits, jams and jellies, fruit butters, pickles and sauerkraut.

Low-acid foods (pH above 4.6) include vegetables, meat, poultry, seafood, milk, and soups and sauces made with them.

Tomatoes and figs sit close to the line, so tested recipes add lemon juice or citric acid before canning them as acid foods.

The two canning methods

Boiling water canning

Jars sit on a rack in a large, deep pot and are covered with boiling water, which reaches 212°F at sea level. That is enough for acid foods, where the acidity keeps botulism spores from growing: fruits, jams, jellies, pickles and acidified tomatoes.

Pressure canning

Low-acid foods need about 240 to 250°F, which only a pressure canner can reach. A pressure canner is a heavy pot made for canning, with a locking lid, a vent and a dial or weighted gauge. It is not the same as a pressure cooker, and small pressure saucepans are not recommended for canning. Pressure canning is a must for vegetables, meat, poultry, seafood, and soups and sauces that contain them.

What you need

A boiling water canner, a pressure canner, or both, depending on what you plan to can. Canning jars (often called Mason jars) with two-piece lids: a flat lid, which must be new each time because its sealing compound only works once, and a screw band, which can be reused if it is not bent or rusty. A jar lifter, a wide-mouth funnel, a timer and a clean, dry place to store the finished jars.

Canning is not the same as vacuum sealing. A vacuum sealer removes air from a bag, but it does not heat the food, so vacuum-sealed food still has to be refrigerated or frozen.

The rules that keep jars safe

• Use a research-tested recipe, and don't change the ingredients or their proportions. Adding more onions, peppers or meat, or thickening a sauce, changes how heat reaches the center of the jar.
• Follow the recipe's jar size, headspace, processing time and pressure exactly.
• Adjust for altitude. Water boils at a lower temperature higher up, so times or pressures go up.
• Use canning jars and new lids. Screw bands can be reused if they are not bent or rusty.
• After processing, let jars cool undisturbed, check every seal, and store sealed jars in a cool, dark place.
• Never taste food from a jar that is leaking, bulging, spurting liquid, moldy or smells off.

Last reviewed November 2026.