Seven types of this bacterium are recognized and classified by the letters A through G. Types A and B are very heat resistant and are able to survive 5 or more hours in boiling water. It is a measure of the degree of acidity or alkalinity of the food. The scale of pH ranges from zero to 14. The mid-point or neutral point, neither acid nor alkaline, is at ph 7.0 . Distilled water is at pH 7.0 .
Most foods are acid but some are much more so than others.
A table can be constructed to show a number of foods ranging in pH from 2.2 to 8.0. Foods with a pH of 4.5 or higher are low acid foods. Those with a pH lower than 4.5 are high acid foods. All fruits are high acid. The pH of a food determines the type of bacteria that will grow in it and whether or not C.botulinum will grow and produce its toxin.