From the sigmoidal to the downward concave shape behavior during the hydration of grains: Effect of the initial moisture content on Adzuki beans (Vigna angularis)

• The initial moisture content of grains changed its hydration behavior.
• The seed coat permeability was affected by the moisture content.
• The water flow route during the grain hydration was described.
• Microstructural and X-ray results were correlated with the hydration kinetics.
• A model of hydration kinetics as function of initial moisture content was obtained.

This work studied the change of the hydration behavior of Adzuki beans (Vigna angularis) as a function of their initial moisture content. By studying the hydration kinetics at different initial moisture contents, it was demonstrated that the hydration behavior of this grain changes from the sigmoidal to the downward concave shape (DCS) when the initial moisture content is above ∼20% d.b. This change happens when the moisture content passes from zone II to zone III of the grain's adsorption isotherm. This was attributed to the transition from the glassy state to the rubbery state, especially the in seed coat components. The seed coat is impermeable when the moisture content of the grain is low, so the water ingress is only by way of the hilum. However, when the seed coat is above ∼20% d.b. the water enters not only via the hilum, but also by way of the seed coat. The hydration behavior of the grain was modeled using a sigmoidal mathematical model, whose parameters were evaluated as a function of the initial moisture content; the water absorption rate parameter demonstrated a sigmoidal increasing behavior. The time to reach the inflexion point of the curve, related to the lag phase, showed an exponential decay. The equilibrium moisture content was not affected. Finally, a general model, which was able to predict the moisture content as a function of the initial moisture content of the grain and the process time, was obtained.