Effects of Sequential Fermentation with Saccharomyces bayanus and Lactobacillus brevis on the Metabolite Composition and Antioxidant Activity of Chinese Yam Juice

Chinese yam (<i>Dioscorea opposita</i> Thunb.) is a nutrient-rich tuber with recognized health benefits, yet its application in beverage products remains limited due to processing and formulation challenges. In this study, a sequential fermentation strategy was adopted, using <i>Saccharomyces bayanus</i> followed by <i>Lactobacillus brevis</i> to enhance microbial viability and metabolic activity in Chinese yam juice. Samples were collected as an unfermented control (CY), yeast-fermented juice (SP), and sequentially fermented juice (LB). Microbial analysis showed that sequential fermentation supported high LAB viability, reaching 8.92 log CFU/mL in LB, accompanied by a progressive decrease in pH from 5.67 (CY) to 4.27 (LB). Untargeted LC-MS/MS metabolomics identified 1442 metabolites and revealed distinct shifts in the metabolic composition of CY, SP, and LB, indicating stage-dependent modifications of metabolic pathways. Targeted analyses confirmed substantial depletion of sucrose and maltose during fermentation, while trehalose accumulated from undetectable levels in CY to 5.23 mg/g in SP and 7.49 mg/g in LB. Organic acid profiling demonstrated marked increases in lactic and succinic acids, consistent with microbial carbohydrate metabolism. Total phenolic and flavonoid contents increased by 58% and 30%, respectively, while antioxidant capacity (DPPH, ABTS, and FRAP) improved by up to 120% after sequential fermentation. The final fermented beverage (LB) contained a low ethanol concentration of 0.8% (<i>v</i>/<i>v</i>). Sensory evaluation indicated that sequential fermentation improved the overall flavor, aroma, and acceptability of the Chinese yam juice. These findings demonstrate that sequential fermentation with <i>S. bayanus</i> and <i>L. brevis</i> effectively enhances the bioactive composition and antioxidant potential of Chinese yam juice, supporting its development as a functional fermented beverage.