Pressure canners for use in the home have been
extensively redesigned in recent years. Models made
before the 1970'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 high
temperatures applied for an adequate 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 10.5 lbs. provides an internal
temperature of 240 degrees F.
Two serious errors in temperatures obtained in
pressure canners occur because:
* 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.
* 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. 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, PSI.
Check dial gauges for accuracy before use each year
and replace if they read high by more than 1 pound at
5, 10, or 15 pounds of pressure. Low readings cause
over-processing 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. Gaskets on older model canners may require a
light coat of vegetable oil once per year. Gaskets on
newer model canners are pre-lubricated and do not
benefit from oiling. Check your canner's instructions
if there is doubt that the particular gasket 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 canning equipment
or from canner manufacturers. When ordering parts,
give your canner model number and describe the parts
needed.
* USDA Agriculture Information Bulletin No. 539 (rev. 1994)
* Meal-Master format courtesy of Karen Mintzias
