Water activity is the measurement of the equilibrium relative humidity of a material, that is the humidity that a hygroscopic material generates when it comes into balance with the air surrounding it in a sealed headspace. This “Available Water” is the unbound water able to come and go from a material by adsorption / desorption.
The water activity of the sample is equal to the relative humidity of air surrounding the sample in a sealed measurement chamber.
Water Activity can be expressed as either : –
Equilibrium relative humidity (erh) scaled 0- 100% erh units
Water activity ( aW) scaled 0-1 aW units. Most microbiologists tend to use aW units.
Air relative humidity is influenced by temperature so it follows that equilibrium relative humidity (water activity ) will be too. The higher the aW value, the greater the influence of temperature on the stability of water activity. Typically, a sudden change in ambient temperature of 2 deg C can result in several erh % shift in the reading of a previously stabilised sample. Whilst this can re- equilibrate over time, it may take several hours to do so and an erroneous reading may have been taken in the meantime.
The aw of a product may be critical to ensure microbial stability and safety, it may even be a legal parameter, often measured as part of Critical Control Point (CCP) validation, monitoring or verification. Water activity can be used for microbiological growth control, shelf-life, the stability of product composition (moisture migration), general product quality (texture, taste, potency & colour) . Some laboratories offer accredited testing for aw.