Moisture diffusivity in concentrated and dry protein-carbohydrate films

Understanding moisture diffusivity behaviour over a wide range of moisture contents is pivotal for optimising drying operations. Generally, data on moisture diffusivity are scarce and the effect of matrix composition on moisture diffusivity at relevant temperature for drying processes is not yet well described. In this paper moisture diffusivity in protein-carbohydrate films is systematically investigated for a wide range of moisture contents at 80 °C. Diffusion data are obtained from controlled thin film drying experiments following the regular regime method and compared to theoretical models. Moisture diffusivity for binary maltodextrin-water and whey protein-water systems appeared similar and were reasonably well described with the Darken relation. Diffusivity was lower for casein-water systems at moisture contents above 0.15 kg water/kg, which may be explained by compartmentalization of water in the casein micelles. At low moisture contents all binary systems showed universal behaviour, which may be explained by random coil behaviour leading to similar water-molecule interactions. This behaviour could be well described by free-volume theory. In mixed systems of proteins and carbohydrates moisture diffusivity appeared strongly influenced by the presence of casein, probably due to their high voluminosity. Finally, it was surprisingly observed that diffusivity in multicomponent systems decreased sharply at lower water contents when compared to binary systems. This might be explained by a denser molecular packed system in the dry regime for multicomponent systems or water trapping by protein-carbohydrate complexes.