This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
0.1 Jars and lids that are not sterile. Food placed in unsterilized jars
or even air contact with non-sterile lids provide a bacteria source that
can ramp up in numbers VERY rapidly. Processing times are deemed for
uncontaminated food and vessels.
1. Fresh food was decayed, unwashed, unpeeled or untrimmed. This results
in a high microbial load. A larger than normal number of microorganisms can
take a longer processing time for complete sterilization than is usually
2. Food packed too tightly in jars. Temperature in the geometric center
of the jar was not high enough long enough to result in complete
sterilization of the food. Pack food loosely, prepare according to USDA
Guidelines (1/2 inch slices, halves, etc.) then use the recommended time,
3. Jars became un-sterile soon after being filled. If lids are not placed
on jars and processing is not started immediately after jars are filled,
microorganisms may start to grow and reach very high levels prior to
4. Inaccurate heat-processing time was used; this may occur if old
recommendations are used (food is underprocessed) or if the timing
was interrupted (power failure, pressure fluctuation, etc.)
5. Food was not processed at the correct temperature:
A. Pressure Canner (240F, 115C).
1. Failed to test dial gauge yearly.
2. Failed to exhaust canner 10 min with full steam flow.
3. Failed to make an adjustment for elevation (11 PSIG versus 10 PSIG
in Illinois due to average 1000 above sea level altitude)
4. Failed to keep pressure accurate (high enough).
B. Boiling Water Bath Canner
1. Water was not covering jar tops by 2" or more.
2. Water was not maintained at a rolling boil.
3. Processing time was too short.
4. Failed to make an adjustment for altitude (addition of 2 minutes
for every 1000 ft above sea level).
6. Use of Open Kettle Canning, Microwave Canning, or Oven Canning Methods.
These methods do not get the canned food hot enough long enough to destroy
microorganisms so the food may spoil, may contain dangerous microorganisms
and their toxins, or both.
7. Improper cooling of jars after processing:
A. Failure to remove jars from canner at the end of processing time or
when gauge reads "0". As jars cool, they may suck water (containing microbes
or spores) back into the food.
B. Failure to properly cool jars. Very slow or very rapid cooling may
interfere with formation of a seal.
8. Use of paraffin to seal jelly jars. Paraffin is no longer recommended
for sealing jams, jellies or preserves. Mold, which is the most common
spoiler of sweet spreads, can send "roots" down along the edge of the
paraffin and produce toxic substances into the spread.
9. Improper storage of home-canned foods:
A. Home canned foods which are exposed to temperatures in excess of 95F
may spoil. Sterilization recommendations used for home canning do not
necessarily kill some of the "thermophiles" or heat-loving microorganisms.
These organisms tolerate high temperatures and will grow at high temperatures.
If they are still present, they may grow and spoil the food, or alter the food
so that other microorganisms can grow.
B. Home canned foods which are stored in the sunlight may get very hot
inside--the light goes in, changes to heat as it is absorbed by the food,
allows the air in the headspace to expand breaking open the seal allowing
microorganisms to come in.
C. Keeping very acid foods (pickled or fermented products, some juices)
for a long period of time may give the food acid time to eat away at and
deteriorate the lid resulting in pinholes which allow microorganisms to get
into the jar. Discard any home canned food with damaged or flaking metal on
D. Lids on home canned foods stored in a damp place may rust through
allowing microbes to get into the food.
Prepared by Susan Brewer/Foods and Nutrition Specialist/Revised, 1992