Hydration Dynamics of Hydrocolloid Particles - Part 2: Molecular Dynamics Simulation and Correlation of Self-Diffusion Coefficients with Water Vapor Sorption Kinetics

In Part 1, experimental data showed that the vapor sorption rates of water for corn and wheat starches were reduced at high water activities (>0.4). Since water mobility in starch is not expected to decrease with increasing water activities, in the current study, we investigate the causes of this phenomenon through molecular dynamics (MD) simulations to predict the water sorption isotherms and self-diffusivity. It was shown that the self-diffusivity of water and the characteristic sorption parameter monotonically increase with increasing water activity (A_w). Three distinctive stages of water sorption were observed. At low values of A_w (0.6), the water self-diffusivity, and hence mobility, approached that of free water, and the characteristic sorption parameter continued to increase due to swelling. An increase of bifurcated (double) water-polymer hydrogen bonds was observed with increasing A_w that stabilized the starch structure. A more rigorous Darcy theory of unsaturated moisture transfer in porous media was used to obtain water permeability values from the experimental data that were reported in Part 1. The calculated water permeabilities increased monotonically with increasing A_w and correlated closely with the self-diffusivity of water following a simple exponential relationship. The predictions of the MD simulations agreed closely with the data reported in Part 1.