Buckwheat Husk as a Source of Melanin: A Multilevel Analysis of Its Ultrastructural Localization and Chemical Heterogeneity

Buckwheat husks are an abundant but underutilized agro-industrial residue containing melanin, a structurally complex biopolymer. This study aimed to link the subcellular localization of melanin in buckwheat husk with the chemical heterogeneity of sequentially extracted fractions, to support rational extraction and valorization strategies.

Melanin localization in buckwheat pericarp was examined by light and transmission electron microscopy with OsO₄–KI staining across fruit maturation stages. Ground husk was subjected to sequential alkaline extraction, and the resulting fractions were characterized by elemental analysis, Van Krevelen diagrams, ATR‑FTIR spectroscopy and size‑exclusion chromatography.

Melanin was confined to vacuolar lumina, with maximal accumulation in cells of the central pericarp layer and no deposition in cell walls or intercellular spaces. Sequential extraction revealed a pronounced chemical gradient: early fractions were more oxygen-rich (O/C = 0.46, H/C = 1.21) and contained trace sulfur (0.4%), consistent with sulfur‑containing components potentially related to pheomelanin‑like structures, whereas later fractions became progressively nitrogen‑rich and more aliphatic (O/C = 0.26, H/C = 1.79). Across fractions, melanin showed unusually high aliphatic carbon content, comparatively low nitrogen relative to reference melanins, and a recurrent oligomeric unit of approximately 3.5–5 kDa.

Buckwheat husk melanin is a hierarchically organized, chemically heterogeneous pigment whose vacuolar localization behind a lignocellulosic barrier governs its extractability. The gradient from more oxidized, sulfur‑containing fractions to more aliphatic, nitrogen‑rich domains explains selective recovery under alkaline conditions and provides a framework for designing integrated pretreatment and extraction schemes for high‑value valorization of buckwheat husk.