Michigan
State University Extension
Preserving
Food Safely - 01600494
10/13/97
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.