A theoretical framework for β-glucan degradation during barley malting

Abstract

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.

Changes in saccharide, amino acid and S-methylmethionine content during malting of barley grown with different nitrogen and sulfur status

“…3(C)). These results indicate that the longer chains of β‐glucan are cleaved mainly by endo ‐1,3/1,4‐β‐ D ‐glucanase, which is in agreement with the work of Gianinetti 25. The degradation of β‐glucan in the later stages of germination and kilning was much slower than that in the earlier stages.…”

“…The degradation of β‐glucan in the later stages of germination and kilning was much slower than that in the earlier stages. Even well‐modified malt still contains a small amount of β‐glucan, and further depolymerisation seems to proceed more slowly 25. A recent report suggested significant involvement of exo ‐1,3‐β‐ D ‐glucanase in the degradation of β‐glucan, which developed during late germination 26.…”

Constitutive differences between steely and mealy barley samples associated with endosperm modification

“…Hence, even though β‐glucan breakdown is an absolute requirement for broad‐sense modification, it is only indirectly related to the proportion of starch that is subsequently made available for solubilisation. In fact, if modification is evaluated in terms of endosperm liquefaction,10 rather than β‐glucan breakdown, its extension appears to be more restricted, and skewed towards the area underneath the growing coleoptile,10, 11 a pattern that is quite different from that normally observed for cell wall modification 2, 3…”

“…Degradation of cell walls (mainly constituted by β‐glucans) is a two‐compartment process that represents the essential step of endosperm modification sensu stricto 2. Some predictions on the β‐glucan profile in the modifying grain can be derived from theoretical considerations,2 but actual analysis of β‐glucan levels in each of the two compartments (modified and unmodified endosperm) would be of great interest to check details of β‐glucan breakdown. Although precise separation of the two compartments is difficult, an easy way to partition the grain into two fractions that are closely associated with modification compartments is the friability test.…”

Impact of Low Hydration of Barley Grain on β-Glucan Degradation and Lipid Transfer Protein (LTP1) Modifications During the Malting Process

“…In addition, an efficient degradation of β-glucans is determinant for the subsequent action of other enzymes such as R and β-amylases that need to diffuse and access to their substrate. 11,12 Altogether, this emphasizes the importance of β-glucan degradation during malting.…”