Integrated non-targeted metabolomics, enzyme activities and morphological analysis to elucidate the mechanisms underlying the enhancement of activity and flavor of Monascus purpureus fermented Undaria pinnatifida.

Monascus purpureus-fermentation enhanced antioxidant activity and flavor of Undaria pinnatifida. This study employed an integrated approach combining non-targeted metabolomics, enzyme activity assays, and scanning electron microscopy (SEM) to elucidate the mechanisms. Results show that fermentation enhanced the activities of cellulase and pectinase, which effectively initiated the degradation of the cell wall. Morphological study by SEM confirmed this result. This upstream enzymatic change triggered the liberation of intracellular bioactive compounds. The increase in total phenolic and flavonoids led to a 1.2 to 2.2-fold improvement in antioxidant capacity (p < 0.05). Besides, non-targeted metabolomics further revealed the metabolic pathways of key phenolic compounds as phenylalanine and tyrosine metabolism. For the flavor property analysis, noticeable increases in alcohols, ketones, and acids, alongside the reduction in aldehyde corresponded to the reduction of the fishy odor. KEGG analysis indicated that the fermentation metabolism and transformation were primarily governed by the Embden-Meyerhof pathway (EMP), tricarboxylic acid cycle (TCA), and amino acid metabolism. The accumulation of bioactive non-volatiles such as BAIBA and GABA might be a new type of health-related component produced during fermentation. These findings establish the essential initiating role of microbially-driven cell wall degradation in triggering comprehensive metabolic remodeling, positioning fermented U. pinnatifida as a promising functional food.