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

### PRESSURE CANNERS

        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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