Mechanistic Insights into Lotus Rhizome Bluing: In Situ Visualization and Molecular Identification of Polyphenol-Iron Chelates in Damaged Cells

The blue stain of lotus rhizome, a major postharvest issue, is hypothesized to result from polyphenol-iron complexation; however, direct in situ evidence and molecular-level identification of the pigments have been lacking. This study employed a multimodal analytical approach to unravel this process from the cellular to the molecular level. Transmission electron microscope and electrolyte leakage results demonstrated that cellular damage was a prerequisite, enabling the colocalization of polyphenols and Fe³⁺ at lesion sites, as visualized by scanning electron microscopy with energy-dispersive X-ray spectroscopy and confocal fluorescence microscopy. Raman spectroscopy and X-ray photoelectron spectroscopy verified complex formation, while mass spectrometry identified specific chelates. The mechanism involves polyphenol-mediated reduction of Fe³⁺ followed by chelation. Ascorbic acid effectively mitigated bluing by competitively reducing/chelating Fe³⁺, disrupting this redox cycle. This work definitively establishes the mechanism and proposes ascorbic acid as a viable antidiscoloration strategy.