Modelling the hydration process of wheat grain with layer-dependent diffusion coefficients

This study develops and validates a multilayer diffusion model of wheat grain hydration that incorporates layer-dependent diffusion coefficients for bran, endosperm, and germ. The moisture transport is formulated using Fick's law with two interface formulations: (i) classical continuity of the concentration and flux and (ii) an interlayer resistance formulation that permits concentration discontinuities. Diffusion coefficients and geometric parameters were determined experimentally; A 3D grain model (structured-light scanning, COMSOL Multiphysics) informed the computational domain. Numerical solutions combined eigenfunction expansions with finite-difference discretisation near the interfaces. Across eight winter wheat varieties, the diffusion coefficients spanned 11.6 - 20.5 × 10-12m2.s-1 (mean 16.27 ± 3.08 × 10-12m2.s-1 ). Relative to the continuity model, the resistance model reduced the early-stage endosperm over-prediction by ~ 0.6-1.0 % (absolute) and lowered the whole-grain RMSE by ~ 20-30% over 0-240 min. These results support the role of thin moisture-retaining films as active barriers and yield smooth, real-time-ready outputs suitable for the automated control of pre-milling hydration; the framework is extensible to full 3D transient simulations.