Fat, which does not contribute to the free water, is replaced by water or a gel to provide lubricity. These gels do not reduce a w and additional control methods will be necessary to prevent spoilage. In the former, it measures the thermodynamic free energy of water and in the latter it is used to define the lower limits of growth of food spoiling microorganisms. Microbiologists turned to water activity measurements upon discovering that microbial spoilage of food occurs at widely varying levels of water content. Scott, in the 1950's, applied the water activity concept to describe the water availability for microbial growth.
Water activity is a better index for microbial growth than water content.
Water activity better predicts the growth of microorganisms because microorganisms can only use 'available' water, which differs considerably depending on the solute. At the same molecular concentration, salt lowers the water activity more than sugar. When a binder substance is added to lower water activity, the result can be complicated. Ideally, an inert material could be added which would decrease water activity without any other effects such as increased ionic strength and decreased surface tension. In reality, the choice of binder substance can have a profound effect.