Study on the Antioxidant Activity of Fermented Broad Bean–Mulberry Composite Juice Based on In Vitro Digestion and Non-Targeted Metabolomic Analysis

Fermentation is a widely utilized technology that efficiently enriches bioactive compounds, thereby enhancing the bioactivity of food. This study aimed to investigate the release of the total polyphenol content (TPC) and flavonoid content (TFC), changes in antioxidant activity, and in vitro relative abundance trends of phenolic metabolites in the fermented composite juice of kidney beans and mulberries. An in vitro simulated gastric and intestinal digestion method was employed to examine the release patterns of polyphenols and flavonoids, along with alterations in antioxidant activity during the gastrointestinal digestion of the fermented composite juice. Non-targeted metabolomics LC-MS technology was employed to detect changes in the relative abundance and enrichment of phenolic metabolites during fermentation and digestion stages. The results indicated that after simulated digestion, the polyphenol content increased by 1.42-fold and the flavonoid content by 2.53-fold compared to pre-digestion. The scavenging rates for DPPH radicals, hydroxyl radicals, and ABTS radicals reached 85.44%, 94.77%, and 76.12%, respectively. Non-targeted metabolomic investigation through KEGG pathway enrichment showed associations between phenolic metabolites and antioxidant activity. During fermentation and digestion, daidzein, genistein, quercetin, and catechin may be the potential compounds contributing to the enhanced antioxidant activity of FBMJ. Among these, phenolic metabolites are significantly enriched in the biosynthetic pathways of flavonoids and flavanols. This study has elucidated the metabolic variations between the fermentation and digestion stages of fermented composite juice from a metabolomics perspective, providing preliminary in vitro research evidence and theoretical clues for developing it as a functional food with antioxidant potential.