Biotransformation of phenolic compounds in Saccharina japonica by Monascus purpureus fermentation: Impact on composition, antioxidant profile, and in vitro bioaccessibility.

Phenolic compounds of Saccharina japonica (SJ) have low bioaccessibility due to macromolecule-bound forms. This study demonstrated that fermenting SJ with Monascus purpureus enhanced the phenolic bioaccessibility by converting bound macromolecular forms into free, low-molecular-weight forms. Fermentation increased both free and bound phenolics by 43% and 263%, respectively, with a corresponding increase in antioxidant activities. UHPLC-Q-Exactive Orbitrap-MS/MS detailed microbial biotransformation mechanisms, where complex phlorotannins and phenolic acids converted to salicylic acid, protocatechuic acid, caffeic acid, together with enrichment of luteolin, which was further transformed into low-molecular-weight phenolics during digestion. Fermentation improved digestive stability; intestinal-phase bioaccessibility relative to the corresponding undigested samples reached 134.5% (free) and 90.1% (bound), with absolute accessible phenolics approximately 1.6-fold those of digested unfermented controls. Thus, fermentation offers an effective strategy to convert inaccessible bound phenolics into bioactive, bioaccessible molecules, providing a new way to enhance the nutritional and functional value of SJ-based products.