Michigan State University Extension
Preserving Food Safely - 01600938
10/13/97

### FOOD ACIDITY AND PROCESSING METHODS

        Whether  food should be processed in a pressure  canner

    or
     boiling-water  canner  to  control  botulinum   bacteria

    depends
    on the acidity in the food.  Acidity may be natural,

    as
     in most fruits, or added, as in pickled  food.  Low-acid

    canned
     foods  contain  too little acidity  to  prevent  the

    growth
    of these bacteria.  Acid foods contain enough acid to

    block
     their  growth,  or  destroy  them more  rapidly  when

    heated.
      The term "pH" is a measure of acidity;  the  lower

    its
     value,  the more acid the food.  The acidity  level  in

    foods
     can be increased by adding lemon juice, citric  acid,

    or
    vinegar.

        Low-acid  foods have pH values higher than  4.6.   They

    include
     red  meats, seafood, poultry, milk  and  all  fresh

    vegetables
    except for most tomatoes and figs.  Most mixtures

    of
     low- acid and  acid foods also have pH values above  4.6

    unless
    their recipes include enough lemon juice, citric acid

    or
    vinegar to make them acid foods.  Acid foods have a pH of

    4.6
     or  lower.  They include fruits,  pickles,  sauerkraut,

    jams,
    jellies, marmalades and fruit butters.

        Although tomatoes usually are considered an acid  food,

    some
     are  now known to have pH values slightly  above  4.6.

    Figs
    also have pH values slightly above 4.6.  Therefore,  if

    they
    are to be canned as acid foods, these products must  be

    acidified
    to a pH of 4.6 or lower with lemon juice or citric

    acid.
      Properly acidified tomatoes and figs are acid  foods

    and
    can be safely processed in a boiling water canner.

        Botulinum  spores are very hard to destroy at  boiling-

    water
     temperatures; the higher the canner temperature,  the

    more
     easily  they are destroyed.  Therefore,  all  low-acid

    foods
    should  be  sterilized  at temperatures of 240  to 250

    degrees
     Fahrenheit,   attainable  with   pressure   canners

    operated
    at  l0  to  l5  PSIG.  PSIG means pounds per square

    inch
     of  pressure  as  measured   by   gauge.    At   these

    temperatures,
    the time needed  to destroy bacteria  in  low-

    acid
    canned food ranges from  20 to l00 minutes.   The exact

    time
    depends on the kind of food  being canned,  the  way it

    is
    packed into jars, and  the size of jars.  The time needed

    to
    safely process low-acid foods in a boiling  water  canner

    ranges
    from 7 to 11 hours;  the  time needed to process acid

    foods
    in boiling water varies from  5 to 85  minutes.    The

    practice
     of  processing  low-acid  foods in a boiling water

    canner
    is NOT RECOMMENDED.

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