Water Activity (Aw)

Water activity (a_w) measures the amount of water in a food that is available to support microbial growth and chemical reactions. It is not the same as moisture content: two foods can contain similar amounts of water but have different water activity because water is bound or dissolved differently within their structure.

Water activity scale

Water activity is expressed on a scale from 0 to 1.0. Pure water has an a_w of 1.00. Lower values indicate less available water. Many bacteria require relatively high water activity to grow, while some yeasts and moulds can tolerate lower values.

Examples in food

• Fresh meat and fish: generally have high water activity and require strict time-temperature control.
• Honey and many jams: have lower water activity because dissolved sugars bind water, although their safety also depends on formulation, acidity and processing.
• Dried foods: drying reduces available water and can limit microbial growth.
• Salt-cured products: salt lowers water activity, but safe curing still depends on validated formulation and process controls.

Why water activity matters

Food manufacturers and safety professionals use water-activity measurements to help determine product stability, shelf life and whether additional controls are required. It is one part of a broader food-safety assessment that may also include pH, preservatives, packaging and storage temperature.

Chef’s safety note: Low water activity does not automatically mean a product is safe or shelf-stable. Use validated recipes, suitable measurement equipment and the applicable food-safety plan.

Key distinction: Moisture content measures total water; water activity measures the availability of water for microbial growth and other reactions.