A theoretical framework for β-glucan degradation during barley malting

During malting, barley germinates and produces hydrolytic enzymes that de-structure the endosperm, making the grains soft and friable. This process starts close to the embryo and spreads throughout the whole grain. It is leaded by the degradation of cell walls, which are mainly constituted of beta-glucans. Fast and extended breakdown of beta-glucans occurs by means of an expanding reaction front driven by beta-glucanase, and appears to follow pseudo-first-order kinetics. Endosperm permeabilization to macromolecules is closely linked to the dismantling of cell walls, thus that access to beta-glucans by beta-glucanase itself is limited. It is shown that the kinetics of beta-glucan degradation during malting are consequent to this condition, and can be explained according to an anomalous evolution of the reverse quasi-steady-state approximation (rQSSA) for enzymatic reactions. In fact, kinetics based on the rQSSA include a transient phase wherein fast substrate depletion is indeed of pseudo-first-order. In the germinating barley, the conditions in which the physical modification of the endosperm occurs are shown to be suitable for the fast transient to persist in dynamic equilibrium while it progressively expands throughout the grain, depleting most beta-glucans and, then, establishing the overall kinetics of beta-glucan breakdown.