Brewer's Spent Grain Flour: Chemical Composition, Functional Properties, and Influence on Gut Microbiota

Brewers' spent grain (BSG), which accounts for approximately 85% of the by-products generated during beer production, is a valuable source of dietary fibre, proteins and antioxidant compounds. This study aimed to characterise the chemical composition, techno-functional properties, antioxidant capacity and potential prebiotic effect of BSG flour as a sustainable functional ingredient. Dietary fibre composition, mineral content, and extractable and non-extractable (poly)phenol fractions were determined. The prebiotic potential of BSG flour was evaluated using an in vitro fermentation model with human faeces. Microbial metabolic activity was assessed through the production of short-chain fatty acids (SCFAs), lactate and ammonium, alongside changes in antioxidant capacity during fermentation, while microbiota composition was analysed by 16S rRNA amplicon sequencing. BSG flour showed high levels of insoluble fibre, mainly hemicellulose and arabinoxylans, as well as proteins and non-extractable (poly)phenols, particularly hydroxycinnamic acid derivatives. In vitro fermentation led to a significant increase in SCFA production, particularly acetate and propionate, indicating active degradation of fibre polysaccharides. These metabolic changes were accompanied by enhanced antioxidant capacity and shifts in microbiota composition, including an increased relative abundance of Bifidobacterium species. Overall, this study suggests that BSG flour could be used as a novel ingredient for the development of dietary-fibre-rich foods with potential gut health benefits.