Pressure canning is the only home method that makes vegetables, meat, poultry, fish, soups and beans safe at room temperature, because a sealed canner raises the steam temperature to about 240°F, which boiling water cannot reach. Vent the canner for 10 minutes of steady steam before sealing it, hold the pressure and time from a tested schedule for the exact food and jar size, and let the canner cool and depressurise on its own. A shortcut at any step can leave a jar that looks perfect and carries botulism.
Part of Canning & Pressure Preservation
Pressure canning is the heat treatment of sealed jars of low-acid food in a closed steam canner, at a temperature high enough to destroy botulinum spores.
Low-Acid Food Is Where Canning Kills
Steam in a sealed canner at the pressures in the parent article reaches about 240°F, against 212°F for boiling water at sea level.
Foodborne botulism is traced most often to home-canned low-acid foods such as green beans, corn, asparagus and meat that were water-bathed or under-processed, not to fruit jams (CDC surveillance; recalled). The reason is in Acid levels and safety thresholds: with no acid to hold the spores in check, the heat has to destroy them, and only steam above boiling does so. This article is the overview; the sub-articles deal with the foods, the schedule problem and altitude.
The foods in this class, from the parent article: all vegetables except pickled, meat, poultry, fish, soups, stews and broths, beans that are not pickled, and corn, peas, green beans and carrots.
What You Need
- A pressure canner that has passed inspection, with a verified gauge or weight (Salvaging pressure canners)
- Jars of the size named in the schedule and fresh lids
- A tested schedule for the exact food, jar size, pack and your altitude
- Heat you can hold steady for the full time, and someone to watch the pressure the whole time
Method: A Pressure Canning Run
These steps follow the parent article’s sequence.
Step 1: Put the rack and 2-3 inches of hot water in the canner.
Step 2: Pack hot food to the headspace in the schedule (the parent article gives 1 inch for all low-acid foods), release air bubbles, wipe rims, fit lids.
Step 3: Lock the lid and heat with the vent open. Let a steady stream of steam escape for 10 minutes. This drives air out; trapped air gives a falsely high pressure reading and a cooler canner.
Step 4: Close the vent or place the weight. When the gauge reaches the pressure in the schedule, start the clock.
Step 5: Hold the pressure steady by adjusting heat. If it drops below the schedule’s pressure, bring it back and restart the full time, as the parent article requires.
Step 6: When time is up, turn off the heat and let the pressure fall to zero by itself. Do not run cold water over the canner or open the vent to hurry it.
Step 7: Remove the weight, wait 10 minutes, and open the lid away from your face.
Step 8: Lift the jars out, leave them undisturbed for 12-24 hours, then check the seals.
Common Mistakes
- Skipping the venting. The gauge reads pressure that is partly air; the steam is cooler than the schedule assumed.
- Forcing the canner to cool. Sudden pressure change drives liquid out of the jars and can break them, and it also changes the heat the food receives.
- Using a schedule for a different jar size or food.
- Ignoring altitude. See Altitude adjustments.
Safety
Botulism and steam together
A jar that was under-processed gives no sign. Treat any low-acid jar with a bulging lid, spurting liquid, mould, cloudiness or an off smell as contaminated: discard it without tasting, and do not feed it to animals. The pressure canner itself is a pressure vessel; never open it while it holds pressure, and open the lid tilted away from you, because escaping steam burns instantly.
What’s Next
- Learn next: Meat, vegetables, soups, beans — what is different about each low-acid food
- Combine with: Pressure and time requirements — why the schedule is not something you can work out