Water activity and pseudo-activity coefficient of sorbed water

The sorption isotherm can be represented by a plot of the logarithm of a pseudo-activity coefficient log f′ = logaw − log W against W, the weight of water per 100 g of dried substance. This plot demonstrates the polyphasic behaviour of the system. With reference to the curve for aw = 1, the chemical potentials ΔμΦ for the first water molecule of the monolayer binding to the surface, ΔμB for the last water molecule joining the monolayer, and for the average affinity for the formation of the multilayer, can be calculated. The chemical potentials of the monolayer are referred to the same occupation probability of sites, . The curves obtained for sorbed water in cod, beef, pectin and microcrystalline cellulose (MCC) have been analysed by this method. The relationships of the chemical potentials with the constants K and C of the Guggenheim, Anderson, De Boer (GAB) equation have been found to be . The display of log f′ is very useful in testing whether the GAB model which is based on a single class of sites, or any other model is adequate to describe the system under study.