Pressure [[Canning]] is the only safe method for preserving low-acid foods including vegetables, [[Meat Curing]]s, poultry, seafood, and soups. The pressure canner heats the contents to 115 to 121 degrees Celsius (240 to 250 degrees Fahrenheit), which is above the boiling point of water. This temperature is necessary to destroy Clostridium botulinum spores, which produce a deadly toxin in low-acid, anaerobic environments.
A pressure canner has a heavy, locking lid, a pressure gauge, a vent port, and a safety relief valve. Dial gauge canners display the pressure on a dial. Weighted gauge canners maintain a preset pressure (5, 10, or 15 pounds) and release excess pressure through a rocker. Dial gauges are tested for accuracy before each canning season. The canner's rack prevents jars from touching the bottom.
The canner is filled with the recommended amount of water, typically 2 to 3 inches deep. The jars are placed on the rack, the lid is locked, and the canner is heated to vent steam for 10 minutes. The vent is then closed, and the canner is brought to the correct pressure. Processing times are based on the food type, jar size, and packing method (raw pack or hot pack). Altitude adjustments are required above 300 meters (1,000 feet): 1 extra pound of pressure for every 300 meters above 300 meters.
The pressure is stabilized at the target level for the entire processing time. Fluctuations in pressure during processing cause liquid loss from jars. The canner is not opened until the pressure returns to zero naturally. Quick cooling by running water over the canner or releasing the vent early can cause jar breakage or burns. The lid is removed by tilting it away from the face to avoid steam burns.