Lactobacillus plantarum M3 Fermentation Enhances Mulberry Juice Antioxidant Capacity: Metabolomic Analysis

Mulberry, a plant highly valued for medicinal–edible features, was fermented with Lactobacillus plantarum M3 to enhance its bioactive profile. This study conducted a comprehensive evaluation of the antioxidant activity of fermented mulberry juice (FMJ) and identified key metabolites through an integrated approach involving non-targeted metabolomics, network pharmacology, RT-qPCR, and molecular docking. Under optimized conditions (28 °C, pH 5.5, 12°Bx initial sugar content, 48 h and 5% inoculum), fermentation significantly bolstered the antioxidant capacity of MJ. Specifically, superoxide dismutase (SOD) activity increased from 62.41 ± 0.11 to 84.99 ± 0.07 U/mL, while total phenolic content (TPC) surged from 1108.98 ± 2.90 to 2494.17 ± 7.05 mg GAE/L; DPPH radical scavenging activities were improved by 63.09%. Non-targeted metabolomics identified 195 secondary metabolites, primarily comprising alkaloids, flavonoids, and phenolic acids. Among these, protocatechuic acid, Albanin A, and apigenin exhibited significant dynamic shifts, indicating that they may play a pivotal role in regulating antioxidant capacity. Integrated network pharmacology, RT-qPCR validation, and molecular docking further elucidated that Albanin A and Moracin Q likely drive these enhanced antioxidant effects by activating the Nrf2 pathway, suppressing the NF-κB pathway, and upregulating SOD1 expression. These findings provide a theoretical basis for the development of high-potency functional mulberry products.