Mechanistic insights into polyphenol‑iron chelation-induced blue stain of lotus rhizome: Key compounds, structural characterization and distinction from enzymatic browning.

Postharvest lotus rhizomes develop either blue stain (BS) or enzymatic browning (EB), but their distinct mechanisms remain unclear. Using spectroscopy, enzyme assays, and widely targeted metabolomics, a comparative investigation of the polyphenolic compounds in the lotus rhizome peel subjected to BS and EB treatments was performed. Model system experiments demonstrated that ortho-dihydroxy polyphenols (L-dopa, catechins) chelate Fe2+/Fe3+ to form blue complexes via ligand-to-metal charge transfer, with 1:2 metal-to-ligand stoichiometry. In contrast, EB is driven by polyphenol oxidase. Metabolomics revealed that BS broadly activates the phenylpropanoid-flavonoid pathway (62 up- vs. 42 down-regulated metabolites) yet selectively depletes specific flavonoids (e.g., luteolin 7-O-glucuronide) due to chelation precipitation. Catechin monomers accumulate maximally in BS, while polymers decline. The essential distinction lies in the post-synthetic fate of ortho-dihydroxy polyphenols: chelation (blue) versus oxidative polymerization (brown). This work elucidates the molecular mechanism of lotus rhizome blue stain and informs targeted quality control.