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. Studies have shown that the spores of C.botulinum will not germinate and grow at pH 4.8 and below. Therefore, pH 4.6 has been selected by the Food and Drug Administration as the dividing line between high acid and low acid foods. The application of mild heat will destroy all non-sporing bacteria and all sporing bacteria in the vegetative state in either low acid or high acid foods. However, in the low acid category high heat must be applied to kill the spores of C.botulinum and the spores of the other food spoilage organisms, hence the need for heat processing under pressure, as in the time-honored pressure cooker.