Hydration Dynamics of Hydrocolloid Particles. Part 1: Experimental Investigation of Water Vapor Sorption and Bulk Diffusion

Water vapor sorption data at a range of temperatures (25–70 °C) have been measured for starch (corn and wheat) and non-starch (carrageenan and xanthan gum) hydrocolloid particles in the form of a thin slab. The results revealed that the hydration kinetics may be more accurately described by a sigmoidal model rather than a Fickian model that has been employed in previous studies. The sigmoidal model takes into account the vapor sorption at the surface before the onset of bulk diffusion while the Fick's model does not. The bulk diffusion coefficient increased with water activities initially (≤0.3) but decreased at relatively higher activities (>0.3). Additionally, the data were characterized by a non-dimensional parameter, D^∗=Dτ/L² in the range of values of 0.04∼0.05, which was termed the non-Fickian diffusion parameter, where τ is the characteristic time of the surface sorption, D is the bulk diffusion coefficient and L is the thickness of the layer of particles. The thermal dependence of D could be described on the basis of an Arrhenius activation energy process. The molecular basis for the decrease of the bulk diffusion coefficient at higher water activities will be further discussed in Part II using molecular dynamics simulations.