Background and objectives: In this study, 165 lactic acid bacteria (LAB) strains were isolated from three traditional Qu (LQ, XQ, and DQ) starters and screened for isoflavone glycoside biotransformation ability. The selected and identified strains were used as starter culture to enrich isoflavone aglycone in soybean sourdough bread (SSB). The in vitro gastrointestinal digestion, nutritional, and baking quality of soybean sourdough bread were determined. Findings: Based on yield, three strains identified as Pediococcus pentosaceus (D14, X11; 0.4940 and 0.5243 mg/g, respectively) and Lactobacillus plantarum group (L1, 0.6113 mg/g) showed the highest (p < 0.05) yield. The Lactobacillus plantarum group was confirmed to be L. plantarum using species-specific multiplex PCR based on recA gene. L. plantarum was used as starter culture in soybean sourdough fermentation and bread preparation. In sourdough fermentation, sucrose was the preferred carbon source, as it supported cell growth and exhibited the highest β-glucosidase enzyme activity (9.86 U/g). The aglycone content of SSB was increased under L. plantarum (5.93 mg/100 g). Similarly, the amino acid content was increased compared with soybean bread (SB) and wheat bread (WB). In vitro gastrointestinal digestion, SSB showed significant high release, absorption, and bioavailability of aglycone in bread. Baking quality (specific volume and texture) and overall acceptability of SSB were higher (p < 0.05) than SB and closer to WB. Conclusions: The use of β-glucosidase from LAB strains during sourdough fermentation enhanced the bioactivity and overall acceptability of sourdough bread. Significance and novelty: This added new knowledge and the application potential as commercial starter in sourdough fermentation applications in the food industry. K E Y W O R D Saglycone, bread, lactic acid bacteria, sourdough, soybean isoflavone

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