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.                 

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