Integrated pretreatment and fermentation strategies for L‐lactic acid production from brewers' spent grain using native Streptococcus infantarius and Bacillus coagulans

BACKGROUND Brewers' spent grain (BSG) is an abundant lignocellulosic byproduct of the brewing industry with strong potential as a renewable substrate for biotechnological production of lactic acid (LA). This study evaluated integrated pretreatment, enzymatic hydrolysis, and fermentation strategies to enable efficient production of optically pure L‐lactic acid (L‐LA) from BSG using native Streptococcus infantarius and thermophilic Bacillus coagulans strains. RESULTS Thermal, thermal/acid, and thermal/alkaline pretreatments were applied to BSG samples from two origins, followed by enzymatic saccharification using commercial cellulolytic and hemicellulolytic enzymes. Thermal/acid pretreatment was the most effective strategy, promoting extensive disruption of the lignocellulosic matrix and enhanced release of fermentable sugars while maintaining inhibitor concentrations compatible with microbial fermentation. S. infantarius showed limited sugar utilization and relatively low LA productivity under alkaline conditions. In contrast, B. coagulans strains efficiently co‐fermented hexose and pentose sugars from BSG hydrolysates, achieving L‐LA yields up to 0.82 g g −1 , volumetric productivities up to 0.74 g L −1 h −1 , and optical purities exceeding 99%, even in the presence of pretreatment‐derived inhibitors. CONCLUSION The combination of thermal/acid pretreatment and metabolically robust B. coagulans strains enables the sustainable conversion of BSG into high‐value optically pure L‐LA. These findings support the valorization of brewing byproducts within integrated biorefinery and circular bioeconomy frameworks.