Microwave-enzymatic cascade depolymerization of brewer's spent yeast β-glucan: From branch-on-branch structural...

Brewer's spent yeast, a substantial byproduct generated during beer manufacturing, represents an underutilized resource for sustainable valorization through β-glucan extraction. In this study, we developed an integrated approach combining citric acid-mediated microwave hydrolysis and enzymatic processing to convert yeast β-glucan into bioactive oligosaccharides. Optimal microwave treatment conditions (150 °C, 2 % citric acid, 30 min) applied to 4 % β-glucan solution generated a series of β-1,3-linked oligosaccharides with degree of polymerization spanning 2-5, achieving yields of 62 mg/g laminaribiose, 52.25 mg/g laminaritriose, 36 mg/g laminaritetraose, 26.25 mg/g laminaripentaose, and 13 mg/g gentiobiose. Complementarily, we selected and functionally characterized Gly30z (GenBank: WP_136480739.1), a novel β-1,6-specific endo-glucanase exhibiting maximal activity at pH 6.0 and 40 °C. This enzyme demonstrated precise cleavage of β-1,6-glycosidic linkages, yielding 21.78 mg/g gentiobiose and characteristic β-1,3-1,6-trisaccharides, thereby providing enzymatic evidence for the branched architecture of yeast β-glucan. The synergistic application of these physicochemical and biocatalytic strategies enabled comprehensive recovery of structurally distinct oligosaccharides containing both β-1,3 and β-1,6 glycosidic bonds. These findings establish a technically feasible pathway for transforming brewery waste into high-value functional carbohydrates, offering new opportunities for circular economy implementation in the beverage industry.