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.