Water activity (aw) refers to the free or available water within a food which is not bound to food molecules, i.e. water which is available in foods to support the growth of bacteria, mold and yeast. Water activity has a scale from zero, i.e. (0% water available) to one (100% water available).
Routine water activity measurement of food products is not as common place as pH measurement in the food industry. However, with advances in instrumentation and methodology, its use is becoming more widespread. Typically, water activity is measured on macerated food. It is recommended that food business operators measure water activity according to EN ISO 21807:199416 or other applicable methods.
Water activity is not the same as the moisture content of foods, a variety of foods may have exactly the same moisture content and yet have quite different water activities. While some high moisture foods may have high water activity this is not always the case, e.g. jams have high moisture contents but the moisture is bound to the sugar in the jam and unavailable for microbial growth, giving jams a low water activity.
Most foods have a water activity above 0.95 and that will provide sufficient moisture to support the growth of bacteria, yeasts, and mold. The amount of available moisture can be reduced to a point that will inhibit the growth of microorganisms.Sugar and particularly salt are used to preserve foods by decreasing the water activity by either binding free water and making it unavailable for microbial growth, e.g. sugar in jams, or by exerting osmotic pressure directly on microorganisms, e.g. salt used in brine for some cured meats. Food processing techniques such as drying, freezing, curing and cooking will also influence the water activity. This will impact on the ability of microorganisms to grow and subsequently, the safety and shelf-life of the food.
Water activity testing can now be performed quickly and reliably thanks to the Aqualab range of instruments available through Lennox Laboratory Supplies.