Pressure canning
Pressure canning heats sealed jars to about 116°C (240°F) under steam pressure. It is the only safe home method for low-acid foods such as vegetables.
Also called Pressure canner.
Pressure canning is a way to preserve food in sealed jars by heating them in steam under pressure. At 10 pounds per square inch (psi) at sea level, the steam reaches about 116°C (240°F). That heat, held for a tested time, destroys the spores of the bacterium that causes botulism. Boiling water at 100°C (212°F) cannot do that, so pressure canning is the only safe home method for low-acid foods.
Low-acid foods include vegetables such as green beans, corn and carrots, dried beans, meat, poultry, fish, stock and soup.
How to do it
- Use a tested recipe from a reliable source, such as US government or university home-canning guidance. It gives the time and pressure for your jar size and altitude.
- Use a pressure canner, not a pressure cooker or electric multicooker. Check the gasket and vent, and have a dial gauge tested every year.
- Fill hot jars, leave the right headspace, remove air bubbles, wipe the rims and fit the lids.
- Put the jars on the rack with the amount of hot water the canner's instructions give, often 5 to 8 cm (2 to 3 inches).
- Lock the lid and let steam flow out of the open vent for 10 minutes. Then close the vent or add the weight.
- Start timing when the canner reaches the right pressure. If the pressure drops below it, bring it back up and start the timing again from zero.
- Turn off the heat and let the pressure fall to zero on its own. Wait 10 minutes, then open the lid away from your face.
- Leave the jars to cool for 12 to 24 hours. Check that every lid has sealed, then label and store them.
Why it works
Pressure raises the boiling point of water. At sea level, 10 psi on a weighted gauge gives about 116°C (240°F). At higher altitudes the air pressure is lower, so you need more gauge pressure to reach the same heat.
How to store it
Store sealed jars in a cool, dark, dry place and use them within a year for the best quality. Before you open one, check the seal and look for bulging lids, leaks or spoilage. Throw away any doubtful jar without tasting it.
Common mistakes
- Using an untested recipe, or changing one by adding flour, rice, pasta or extra fat.
- Canning mashed or pureed pumpkin or squash. US guidance has no safe tested time for it; can it in cubes instead.
- Cooling the canner with cold water or opening it before the pressure has dropped.
- Not adjusting for altitude.
- Thinking water bath canning for longer can replace pressure canning. It cannot.
Related terms
- Water bath canningWater bath canning preserves high-acid foods like jam, pickles and acidified tomatoes by boiling sealed jars. It is not safe for low-acid foods.
- BotulismBotulism is a rare but serious illness caused by a toxin that grows in low-acid, airless food, such as badly home-canned vegetables or garlic in oil.
- HeadspaceHeadspace is the empty gap between the food and the lid of a jar or container. The right amount helps canning jars seal and stops frozen jars cracking.
- Sterilize jarsSterilizing jars means heating clean glass jars to kill microbes before you fill them with jam or pickles. Boiling them for 10 minutes is the tested way.
- Pressure cookerA pressure cooker is a sealed pot that traps steam and cooks above boiling point, so beans, stews and stock are done in a fraction of the time.
- Shelf-stableShelf-stable food can be stored unopened at room temperature, like canned goods, dried pasta and UHT milk. Many of them need the fridge once opened.