Agriculture Reference
In-Depth Information
Current standards for home canning come from research by
the USDA that is continually updated. Most of the standards
haven't changed for decades, because the research methods
are quite thorough. The USDA researchers deliberately
introduce viable heat-resistant bacteria spores into foods in
home canning jars and then use temperature sensors inside the
jars as they are canned. After canning, the cans are kept at the
precise temperature necessary for best bacterial growth for
several months and then opened in a sterile environment and
tested for presence of the bacteria or any other spoilage.
The USDA standards published around World War I allowed
for up to 2% spoilage, but the standards published since that
time require 0% spoilage. This means that foods canned at
home using current USDA guidelines are completely safe.
Actually, the times and temperatures provided by the USDA
also contain a safety factor. This means that if experimenters
achieved 0% spoilage at 237 degrees for 11 minutes, the
standards specify 240 degrees for 15 minutes. Times and
temperatures are always rounded up, never down.
There are two methods of canning: boiling water bath and
steam pressure. The choice of method depends on the level of
acidity of the food being canned. This is because the length of
time that spoilage organisms will survive at a given
temperature is longer in foods that are less acidic. So less
acidic foods get canned using the steam pressure method that
produces a temperature of 240 degrees; more acidic foods get
canned in a boiling water bath that produces temperatures of
212 degrees. The length of time specified for canning is based
on how long it takes the heat to fully penetrate a particular
food in a particular-sized jar. The standards are written for
half-pint, pint, and quart jars. If a mixture (such as stew) is
