Exploring biotransformation of phenolic compounds during lactic acid fermentation of grape marc and their fate following simulated in vitro digestion

Grape marc (GM), a winemaking byproduct, has excellent antioxidant properties. Using GM as a lactic acid bacteria (LAB) fermentation medium could contribute to health benefits while managing waste volume. This study optimised a LAB fermentation system using GM and investigated different strains' impact on phenolics content and antioxidant capacity during fermentation and simulated gastrointestinal digestion. Our findings indicated that while raw GM could support growth, its intrinsic carbon limitation and high phenolic load necessitated yeast and glucose to 'prime' the metabolic pathways toward efficient homofermentative lactic acid production and biotransformation. During fermentation, flavonoids and tannins were significantly degraded, while the recovery and release of measurable anthocyanins was enhanced through enzymatic modification of the matrix. LC-ESI-QTOF-MS/MS analysis revealed changes in specific phenolics during degradation and transformation. In vitro digestion increased the total phenolic content and antioxidant activity of fermented GM compared to unfermented samples. These findings indicate that fermentation enhances the functionality of phenolics, making LAB-fermented GM a promising phenolic-rich functional ingredient for health-promoting applications in food, pharmaceuticals, and cosmetics, while also contributing to sustainable development. Furthermore, this biotransformation strategy demonstrates significant potential for extending to other phenolic-rich agro-industrial residues, such as olive mill wastewater, supporting broader circular economy goals.