A measure of the free water available in a food for microbial growth, used to judge whether a food needs temperature control.
No. Moisture content is the total water present, while water activity measures only the free water available to microorganisms, so a food can be moist yet have a low water activity.
Most bacterial pathogens cannot grow below a water activity of about 0.85, and many stop growing at higher values, which is why drying and salting preserve food.
It is one of the main factors that determines whether a food is potentially hazardous and needs temperature control, alongside pH.
It is measured with a calibrated water activity meter, and results are recorded to validate that a product does not support pathogen growth.
Yes. A sufficiently low water activity, often combined with low pH, can keep a food safe at ambient temperature and outside the potentially hazardous food definition.