A comprehensive perspective on melanoidins: definitions, polymerization, chromogenic mechanism, cross-linking, and bioactivity

• α-dicarbonyl compounds are crucial substrates for the formation of colored components. • Aldol, Michael and carbonyl-amino addition reactions are core polymerization pathways. • Continuous conjugated systems are the chromophores in colored components. • Residual carbonyl groups are a key site for crosslinking other components. • The diversified function is based on metal lion complexation and scavenging O - . Melanoidins, a class of Maillard reaction products (MRPs), are ubiquitous in thermally processed foods and exhibit a diverse spectrum of biological activities. These compounds are characterized by their heterogeneous, polymeric, and nitrogen-containing macromolecular structures, as well as their coloration. A critical challenge in the field lies in elucidating the intricate relationships between their structural attributes and functional properties, which is essential for establishing clear structure–function correlations. Carbonyl compounds, derived from food constituents, polyphenol oxidation, and endogenous metabolic processes, participate in the Maillard reaction (MR) with amino-group-bearing compounds to form colored components (CCs). Among these, α-dicarbonyl compounds act as key intermediates in the formation of CCs. Through chemical mechanisms including aldol condensation, Michael addition, and carbonyl-amine reactions, these precursors undergo polymerization, generating extended conjugated systems that function as intrinsic chromophores. Although melanoidins demonstrate multiple biological functionalities, their principal mechanisms of action are attributed to their capacity to scavenge negatively charged oxygen species and to chelate metal ions.