Water activity is the vapor pressure of water in a substance as a fraction of vapor pressure of standard (Pure/Denoised) water at the same temperature. It can be measured using Resistive electrolytic hygrometers, Capacitance hygrometers or Dew point hygrometers, LuCan technologies uses dew point hygrometers.
Water activity is useful in product design and safety. Pure water has Aw 1,0 and dry and viscous products have low Aw (dried fruits Aw 0,60, honey Aw 0,5).
Products with high water activity attracts and support microbes, Aw 0,7 supports fungi while Aw 0,9 supports bacteria, low Aw does not kill microbes but prevents microbial growth which prolongs shelf life.
Water migrates from high to low Aw thus knowing your product Aw helps in product formulation especially if it is made of various products with different water activities.