Analysis of simultaneous water absorption and water-starch reaction during soaking of amaranth grain

The soaking process of a grain amaranth variety (Amaranthus cruentus) was investigated in water between 30 and 90 °C. A mathematical model describing simultaneous unsteady water diffusion and first-order irreversible water-starch reaction is proposed to simulate soaking process of grains. The observed water absorption curves are used to estimate the effective diffusivities (Def) and reaction rate (k). The values of Def and k were between 10−12 and 10−11 m2 s−1 and 10−5 and 10−3 s−1, respectively. The dependence on temperature of Def and k was evaluated through an Arrhenius type equation with two distinct activation energies for diffusion (ED) and reaction (Ek) below and above 64 °C. The calculated values were ED = 31.0 kJ mol−1 and Ek = 24.5 kJ mol−1 below 64 °C and ED = 53.6 kJ mol−1 and Ek = 136.5 kJ mol−1 above 64 °C. These results suggest that at least, for the soaking temperatures above 64 °C, the absorption process is controlled by water-starch reactivity.