Michigan State University Extension
Preserving Food Safely - 01600938


     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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