Brewers' spent grain reborn via cellulase–xylanase hydrolysis and Lactobacillus acidophilus fermentation and their application in bread

A two-step strategy integrating cellulase/xylanase hydrolysis (pH 4.90, 50.5 °C, 5 h) and Lactobacillus acidophilus 336,636 fermentation (48 h) is developed to enhance the valorization of lignocellulosic brewer's spent grain (BSG). This treatment yielded significant improvements: total phenolic content increased by 2.5-fold ( p  < 0.05), flavonoids exhibited a 2.8-fold elevation (p < 0.05), and Ferric-Reducing Antioxidant Power (FRAP) demonstrated a twofold increase compared to untreated BSG. Hydration properties were also enhanced, with water-holding capacity improving by 61 % and soluble protein content rising by 78 %. Metabolomics analysis revealed upregulated pathways for isoquinoline alkaloid biosynthesis and phenylalanine metabolism, alongside the emergence of novel bioactives such as fraxin. In bread-making trials, substituting ≤15 % BSG for wheat flour maintained equivalent texture and volume ( p  < 0.05) while elevating dietary fiber content by 2.94-fold ( p  < 0.05), protein content by 1.15-fold (p < 0.05), and antioxidant activity by 2.11-fold (p < 0.05). However, exceeding 20 % substitution disrupted gluten networks, leading to reduced volume and sensory acceptability ( p  < 0.05). Mechanistically, synergistic release of phenolic acids and microbial β-glucan secretion during fermentation enhanced water-binding capacity and flavor development, enabling this strategy to convert beer waste into nutrient-dense, antioxidant-enriched functional ingredients for staple foods.