Partitioning of water in binary biopolymer mixtures at low water content

The dependence of the crystallinity of starch on water content has been used to monitor the amount of water associated with starch when it is mixed with egg white or xanthan gum. Measurements of the water sorption isotherms in the water activity range 0.2-0.9 predict that less water is associated with the starch in the mixture than would be obtained for starch alone at the same water activity. This is confirmed by the observed crystallinity. The effect can be quantified in terms of an interaction parameter xi which is the ratio of the water content of starch in the mixture to that of starch alone at the same water activity. For starch/egg white mixture xi = 0.79, whereas for starch/xanthan gum mixtures xi = 0.9.