Physical Properties of Foods and the Effect of Water on Them-Water Activity and Glass Transition

Water sorption isotherms give information on the interaction between water and solid components in foods. Using solution thermodynamics, the thermodynamic parameters of both water and solutes can be estimated. The integral Gibbs free energy, Δ G s , is considered to be a suitable parameter for evaluating the interaction between solid and water from the thermodynamic point of view. The thermodynamic parameters obtained from water sorption isotherms relate to the degree of reduction in the glass transition temperature ( T g ), accompanied by water sorption. The difference between the chemical potential of solid (adsorbent) between a solution and a pure solid, Δ G as , which is obtained from water sorption isotherms by solution thermodynamics, is correlated well with Δ T g ( ≡ T g T g0 ; where T g0 is T g of dry material). This indicates that plasticizing effect of water on foods can be evaluated through the parameter Δ G as .