Aw

Water activity, written as a_w, measures how much water in a food is available to support chemical reactions and microbial growth. It is defined as the ratio of the vapour pressure of water above the food to the vapour pressure above pure water at the same temperature. The scale runs from 0 to 1.0; pure water has a water activity of 1.0.

Water activity versus moisture content

Moisture content measures the total amount of water in a food. Water activity measures how available that water is. Two foods can contain similar amounts of moisture but have different water activities because salt, sugar, proteins and other ingredients bind or interact with water differently.

Why water activity matters in food safety

Microorganisms need available water to grow. Lowering water activity through drying, adding salt or sugar, or other validated methods can limit the growth of certain bacteria, yeasts and moulds. Different organisms have different minimum water-activity requirements, so there is no single value that guarantees every food is safe.

Examples of water activity in foods

• Fresh meat and fish: generally have high water activity and require strict temperature control.
• Jams and preserves: high sugar concentration can reduce water activity, but safety depends on the recipe, acidity, processing and packaging.
• Jerky and dried foods: drying lowers water activity and can help prevent microbial growth when the process is properly validated.
• Crackers and dry pasta: contain relatively little available water, which limits microbial growth during appropriate storage.

How water activity is measured

A calibrated water-activity meter measures the equilibrium relative humidity above a food sample and converts it to an a_w value. Testing should follow the instrument’s instructions and a suitable sampling method.

Water activity as a preservation hurdle

Water activity is one factor in food preservation, often used alongside pH, temperature, salt, preservatives, packaging and heat treatment. A low a_w does not necessarily kill microorganisms or destroy toxins that may already be present. Use validated formulations and processes for shelf-stable foods.

Chef’s tip: do not confuse “dry” with “safe.” When developing a dried or preserved product, measure water activity with suitable equipment and follow a validated food-safety process.