_by Nadine Fortna Tope_[6]
Use of a pressure canner for preserving low-acid foods is not new.
Pressure canners for home canning were first marketed in the early
1900's. In 1917, the U. S. Department of Agriculture announced that use
of a pressure canner at 10 pounds pressure (240° F) was the only safe
method for canning vegetables. Today's recommendations are essentially
the same.
A temperature of 240° F or 10 pounds pressure at sea level is needed to
kill spoilage organisms in a reasonable time, especially the spores of
_Clostridium botulinum_. These spores, if not killed, can produce the
most deadly toxin known to man.
The commercial canning industry stringently follows safe canning
practices. Their safety record is excellent. _A Complete Course in
Canning_ by Lopez (1975) says that since 1925 four deaths have been
reported from the consumption of more than 800 billion cans of
commercially processed foods. The record for home canning is much
worse--450 deaths in a fraction of the number of cans. Unsafe practices
were probably used in preserving the deadly home-canned food.
Methods like open kettle, oven, and boiling water bath canning for
low-acid foods are not sufficient to kill _Clostridium botulinum_
spores. Educators talk to many people even today who still use unsafe
practices because "that's the way my mother always did it", or because
they are unfamiliar with pressure canners.
In this chapter, the need for safe pressure canning procedures will be
discussed along with the rationale for using care in preparing low-acid
foods for canning.
Acidity of a particular food is the most important factor in determining
which canning method should be used--pressure or boiling water bath.
Acidity is measured and stated much the same way we express length or
weight. Acidity (pH) refers to acid strength, not the amount of acid
present.
For example, citric acid, an acid found in oranges, grapefruit, and
other citrus fruits, is a weak acid compared to hydrochloric acid, a
very strong acid. The measure used to express acid strength is pH. The
pH scale runs from 1 to 14 with 7 as the neutral point. Substances with
pH below 7 are called acidic, while those above 7 are called basic or
alkaline foods. The lower the pH, the more acid the food.
Acidity or pH of a food affects the length of time it must be processed
at a particular temperature to make it safe. The more acid the food (the
lower the pH), the shorter the time required for processing.
Almost all foods are acid in nature. Hominy is an example of a food that
is neutral or slightly alkaline.
Foods are further categorized as high acid or low acid because the _C.
botulinum_ spore will not grow at pH levels of 4.6 or below. High acid
foods (pH 4.6 or below) include tomatoes and all fruits except figs.
(See chart).
Those with a pH above 4.6 are the low-acid foods. All vegetables except
tomatoes and those that have been pickled or fermented are low-acid.
Safe processing times have been established at 240° F for low-acid home
canned products, since at this temperature the processing times are
reasonably short and texture of the resulting product remains good. The
heat-resistant _C. botulinum_ spore has been known to survive many hours
of boiling at 212°. Once food reaches 240°, the spore is killed when
held for the recommended number of minutes.