Nutrition promotion of brewer's spent grain by symbiotic fermentation adding Bacillus velezensis and Levilactobacillus brevis

Brewer's spent grain (BSG) is the main byproduct of the beer industry and is commonly used as a low-value animal feed or food ingredient. This study investigated the role of symbiotic fermentation with Bacillus velezensis and Levilactobacillus brevis in improving the nutritional value of BSG. The addition of B. velezensis increased the cell growth of L. brevis by over thirty-fold higher than that in the single-strain system. Simultaneously, cellulase and protease activities were substantially increased, and more soluble sugars were produced. Total amino acid, glutamic acid, and γ-aminobutyric acid (GABA) levels increased by 52.2%, 155.1%, and 144.1%, respectively. Conversely, lactic acid production decreased. Genetic analysis showed that the glycolysis-related gene pykA, pentose phosphate pathway gene gnd, and GABA-related gene gad were upregulated, while the heterolactin-related gene xpkA was downregulated, explaining that B. velezensis might inhibit heterolactin fermentation while enhancing GABA biosynthesis. This study provides a potential strategy for the high-value utilization of BSG in the food or feed industry.