Mechanistic insights into polyphenol‑iron chelation-induced blue stain of lotus rhizome: Key compounds, structural...

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.