A measure of the free water available in a food for microbial growth, used to judge whether a food needs temperature control.
A measure of the free water available in a food for microbial growth, used to judge whether a food needs temperature control. Water activity, written as aw, measures the amount of free water in a food that is available to support the growth of microorganisms and chemical reactions. It is expressed on a scale from 0 to 1.0, where pure water has an aw of 1.0. It is not the same as moisture content, because some water is bound to salt, sugar or other components and cannot be used by microbes. Water activity is a key factor in deciding whether a food is potentially hazardous and needs temperature control. Most bacterial pathogens cannot grow below an aw of about 0.85, and many stop well above that, which is why drying, salting and adding sugar make foods more stable. Combined with pH, water activity is used to classify foods and to justify storing certain products at ambient temperature. For a food manufacturer, controlling water activity is a way to make a product shelf stable or to keep it out of the temperature danger zone requirements. Cured meats, hard cheeses, dried fruit, jams and confectionery all rely on low water activity, sometimes together with low pH, to stay safe without refrigeration. Businesses that make such products typically measure water activity with a calibrated meter and record the results as part of their food safety program, using the values to validate that a product does not support pathogen growth. Under the Food Standards Code, a food that is shown by water activity and pH not to support the growth of pathogens or the production of toxins may fall outside the definition of potentially hazardous food. A smallgoods maker producing a dried salami measures the finished product at a water activity of 0.88 and a pH of 4.9, records both against the validated limits in its food safety program, and uses the combination to demonstrate the salami is shelf stable and not a potentially hazardous food.
Frequently asked questions
Is water activity the same as moisture content?
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.
What water activity stops bacterial pathogens growing?
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.
Why is water activity important for food safety?
It is one of the main factors that determines whether a food is potentially hazardous and needs temperature control, alongside pH.
How is water activity measured?
It is measured with a calibrated water activity meter, and results are recorded to validate that a product does not support pathogen growth.
Can low water activity make a food shelf stable?
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.
