Mixed-strain fermentation enhances protein content, phenolic composition, antioxidant capacity, and free amino acid content in brewer's spent grain

Abstract

This study evaluated the potential of simultaneous solid-state fermentation (SSF) with Rhizopus oryzae and Trichoderma virens in enhancing the nutritional quality of brewer's spent grain (BSG). The effects of fungal ratio (1 × 10⁸ spores of R. oryzae to 1 × 10⁸ spores of T. virens), moisture content (27.0 – 73.0%), temperature (25 – 40 °C), and fermentation time (0 – 96 h) were investigated. Mixed-strain fermentation (1:1 fungal ratio) of BSG at 73.0% moisture and 28.7 °C resulted in the higher protein content (23.60 g 100 g⁻¹), than that present in unfermented BSG (20.82 g 100 g⁻¹). Under these conditions, antioxidant activity (AA) increased by 376.0% to 625.0%, as assessed by free radical scavenging methods and the ferric reducing antioxidant power assay. Enzyme activities increased during SSF (0 to 96 h), thereby modulating the phenolic profile of BSG. The contents of ferulic and vanillic acids were associated with total cellulase activity, whereas isovanillin and syringaldehyde contents were correlated with amylase, endoglucanase, and protease activities. The release of these phenolic compounds explained the increase in AA, with the highest values being observed after 72 h of mixed-strain fermentation. The free amino acid levels of BSG increased by 6.1 to 16.4 times after SSF. This study showed that mixed-strain fermentation using R. oryzae and T. virens is a viable alternative to enhance the nutritional value of BSG, yielding a value-added ingredient suitable for food formulations.