A measure of acidity or alkalinity that, alongside water activity, determines whether a food supports the growth of pathogens.
pH measures how acidic or alkaline a food is on a scale from 0 to 14, where 7 is neutral, lower is more acidic and higher is more alkaline.
At or below pH 4.6 the growth of many dangerous bacteria, including the organism that causes botulism, is controlled, which is why acidified foods target this level.
Acidic conditions inhibit most foodborne pathogens, so pH is used with water activity to judge whether a food is potentially hazardous and to validate preserved products.
It is measured with a calibrated pH meter, and results are recorded as part of the food safety program to show each batch met its validated limit.
A sufficiently low pH, often combined with low water activity, can prevent pathogen growth and allow a food to be stored safely at ambient temperature.