Moisture diffusivity in food materials

► Self diffusivity of water in biopolymer matrices can be predicted using free volume theory. ► Self diffusivity of biopolymers in solution is predicted using generalised Stokes–Einstein relation. ► The Fickian mutual diffusivity can be linked to the self diffusivities via the Darken relation. ► Moisture diffusion in dense biopolymer and sugar matrices is showing universal behaviour. This paper investigates whether moisture diffusion can be predicted for food materials. We focus especially on mixtures of glucose homopolymers and water. The predictions are based on three theories: (1) the Darken relation, linking the mutual diffusivity to the self diffusivities, (2) the generalised Stokes–Einstein relation for the solute self diffusivity, and (3) the free volume theory for water self diffusivity. Using literature data obtained for the whole class of glucose homopolymer, we show that these theories predict the moisture diffusivity for the whole range of volume fractions, from zero to one, and a broad range of temperatures. Furthermore, we show that the theories equally holds for other hydrophilic biopolymers one finds in food. In the concentrated regime, all experimental data collapse to a single curve. This universal behaviour arises because these biopolymers form a hydrogen bonded network, where water molecules move via rearrangement of the free volume.