The time required for heat to penetrate to the center of the food in a
container (the slowest heating point) is extremely important. Heat is
transferred through food in containers by two mechanisms: conduction and
convection. The mechanism involved depends on the consistency and amount
of liquid in the food. The heat penetration rate is also influenced by
size of the container, type of heating medium (wet steam vs. dry air),
ratio of solid to liquid, kind and size of solid material in container,
amount of fat, and amount of salt and sugar.
For example, pumpkin or squash can be home canned in two forms: strained
or cubed. University of Minnesota research has shown that the time
required for the center of a pint jar of strained squash (which heats by
conduction) to reach sterilization temperature is three to four times as
long as for a pint jar of cubed squash (which heats by convection). The
same is true of creamed corn (heats by conduction) and whole kernel corn
(heats by convection).
Methods and recipes recommended by Extension agencies take into account
all of these factors and must be followed precisely to assure a safe and
wholesome product.
Why is it necessary to heat-process pint jars of string beans at 240° F
for 20 minutes in a pressure canner when tomatoes can be successfully
heat-processed in a boiling water bath? This brings up the second method
of preserving food at home, controlling the food's acid content. This
method is most commonly used in combination with heat processing.
Most foods contain naturally occurring organic acids. Some foods contain
more of these acids and are called acid or high acid foods. These
organic acids have the ability to limit, inhibit, or prevent the growth
of many of the micro-organisms producing spoilage and disease. The
degree of inhibition is related to the amount of acid present.
A method used for measuring acid content is called pH. A measure of pH
is a determination of the hydrogen ion concentration which reflects the
amount of acid or alkali present in the system. A scale from 0 to 14 is
used. A pH of 7 is considered neutral, above 7 alkaline, below 7 acidic.
Very few foods have a pH above 7.
The classification of foods in the acid range below 7 is extremely
important. Above pH 4.6 most of the spoilage type micro-organisms can
grow, as well as the dreaded _Clostridium botulinum_ (see discussion
following on botulism). In foods with a pH greater than 4.6, it is
necessary to heat-process the food at temperatures above boiling to
obtain the desired level of sterility.
There are some types of bacteria that produce entities called endospores
or spores which are extremely resistant to environmental stresses. They
are a means of assuring survival in bacteria, although not themselves a
reproductive mechanism. One growing or vegetative cell will produce one
spore, which under proper growth conditions will germinate and produce
one cell. This one cell continues to grow and can produce millions of
bacterial cells. Destruction of the resistant spore necessitates the use
of temperatures above that of boiling water (212° F).
_BOTULISM._ One of the most notorious of the spore-forming bacteria is
_Clostridium botulinum_. When growing in food this bacteria can produce
a deadly poison which causes botulism, a deadly illness. The mortality
rate is 56 percent. This bacteria and its spore are present in soil
throughout the world and as a result contaminates most of the food we
eat. But the spore only germinates and grows where there is suitable
food, no air, and a pH above 4.6. These conditions exist in canned low
acid foods.
To assure botulism-free home-canned foods, it is absolutely essential
that low-acid foods be canned in a pressure canner at temperatures above
212° F. The poison produced by this bacteria is one of the most potent
poisons known to humans. It has been estimated that 1 cup (8 ounces) is
sufficient to kill all the humans on earth. It is not something to take
chances about. All home canning procedures recommended by Federal and
State Extension agencies are adequate to destroy this dreaded
spore-forming bacteria.
[Illustration: pH VALUE OF VARIOUS FOODS]