will keep. Generally speaking, a large number of bacteria are more difficult to kill than a
small number. Food products containing a large number of bacteria require higher
processing temperatures and longer process times. The types are more important than
the total number of bacteria.
b. Requirements for Canning Acid and Nonacid Foods. Canned foods are
classified as acid and nonacid foods. Acid foods are readily sterilized at 212F (100C).
Nonacid foods must be processed above 212F (100C). There are a number of
nonacid foods such as artichokes, broccoli, brussel sprouts, and mushrooms that are
ruined by processing above 212F (100C). These products are acidified to a point
below pH 4.5 by the use of citric acid and the processing temperatures are then
lowered. A pH of 4.5 will retard the development of Clostridium botulinum organisms.
All of the Clostridium botulinum organisms will not be destroyed in acidified nonacid
food, but they are inhibited by a pH below 4.5. The processing of all canned foods,
then, is based on the destruction of Clostridium botulinum or the inhibiting of these
organisms. As long as Clostridium botulinum organisms are held in a spore state, they
c. Three Sources of Contamination. Spoilage bacteria and pathogenic
bacteria in canned fruits and vegetables are derived from three different sources.
These sources are raw fruits and vegetables, canning plant equipment, and ingredients
added to canned foods. Fresh fruits and vegetables may contain a heavy bacterial
count, and washing and peeling will never remove all of the bacteria. Canning plant
equipment will become contaminated by normal processing procedures and must be
properly cleaned to prevent undue contamination of the canned product. Ingredients
such as sugar added to canned foods may often contribute to high bacterial counts.
The canner should assure himself that the bacterial content of these ingredients is
within acceptable limits.
d. Two Kinds of Swells in Cans. Biological swells and chemical swells are
commonly found in canned fruits and vegetables. One or both ends of a can will be
distended. This is caused by production of gases.
(1) Biological swells are caused by the production of gases by bacteria. A
high storage temperature will materially accelerate the rate of gas production. The
average analysis of gases found in biological swells is carbon dioxide, 75 to 97 percent;
hydrogen, 0.1 to 5.8 percent; and nitrogen, methane, or oxygen, 1.4 to 20 percent.
(2) Chemical swells are produced as the result of the reaction between acid
in the canned foods and the iron in the can. High storage temperature definitely
accelerates the rate of gas production in chemical swells. The average analysis of
gases found in chemical swells is carbon dioxide, 8.40 percent; hydrogen, 65.50