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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°F.
Two serious errors in temperatures obtained in
pressure canners occur because:
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- 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
under-processing. 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.
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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.
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