Michigan State University Extension
Preserving Food Safely - 01600494
Pressure canners for use in the home have been extensively redesigned in recent years. Models made before the l970's were heavy-walled kettles with clamp-on or turn- on lids. They were fitted with a dial gauge, a vent port in the form of a petcock or counterweight and a safety fuse. Modern pressure canners are lightweight, thin-walled kettles; most have turn-on lids. They have a jar rack, gasket, dial or weighted gauge an automatic vent/cover lock, a vent port (steam vent) to be closed with a counterweight or weighted gauge and a safety fuse.
Pressure does not destroy microorganisms, but water under pressure causes the steam formed to get very hot. High temperatures applied for a specific period of time do kill microorganisms. The success of destroying all microorganisms capable of growing in canned food is based on the temperature obtained in pure steam, free of air, at sea level. At sea level, a canner operated at a gauge pressure of 10 lb. provides an internal temperature of 240 degrees Fahrenheit.
Two serious errors in temperatures obtained in pressure canners occur because:
1. Internal canner temperatures are lower at higher altitudes. To correct this error, canners must be operated at the increased pressures specified in this publication for appropriate altitude ranges. 2. Air trapped in a canner lowers the temperature obtained at 5, 10 or 15 pounds of pressure and results in underprocessing. The highest volume of air trapped in a canner occurs in processing raw-packed foods in dial-gauge canners. These canners do not vent air during processing. To be safe, all types of pressure canners MUST be vented 10 minutes before they are pressurized.
To vent a canner, leave the vent port uncovered on newer models or manually open petcocks on some older models. Heating the filled canner with its lid locked into place boils water and generates steam that escapes through the petcock or vent port. When steam first escapes, set a timer for 10 minutes. After venting 10 minutes, close the petcock or place the counterweight or weighted gauge over the vent port to pressurize the canner.
Weighted-gauge models exhaust tiny amounts of air and steam each time their gauge rocks or jiggles during processing. They control pressure precisely and need neither watching during processing nor checking for accuracy. The sound of the weight rocking or jiggling indicates that the canner is maintaining the recommended pressure and needs no further attention until the load has been processed for the set time. The single disadvantage of weighted-gauge canners is that they cannot correct precisely for higher altitudes. At altitudes above 1,000 feet, they must be operated at canner pressures of 10 instead of 5, or 15 instead of 10 pounds pressure.
Check dial gauges for accuracy before use each year and replace if they read high by more than 1 pound at 5, l0, or l5 pounds of pressure. Low readings cause overprocessing and may indicate that the accuracy of the gauge is unpredictable. Gauges may be checked at most county Cooperative Extension offices.
Handle canner lid gaskets carefully and clean them according to the manufacturer's directions. Nicked or dried gaskets will allow steam leaks during pressurization of canners. Keep gaskets clean between uses. Older canner models may require to be lightly coated with vegetable oil once per year. Newer models are pre-lubricated and do not benefit from oiling. Check your canner's instructions if there is doubt that the particular canner lid you use has been pre-lubricated.
Lid safety fuses are thin metal inserts or rubber plugs designed to relieve excessive pressure from the canner. Do not pick at or scratch fuses while cleaning lids. Use only canners that have the Underwriter's Laboratory (UL) approval to ensure their safety.
Replacement gauges and other parts for canners are often available at stores offering canner equipment or from canner manufacturers. When ordering parts, give your canner model number and describe the parts needed.