Fructosylglycine assembles into melanoidin with more glycine than glucose while heating

In this study, melanoidins formed from fructosylglycine and heated mixtures of glycine and glucose were analyzed and compared using spectroscopic techniques including UV/Vis, FTIR, EPR, NMR, as well as elemental analysis (EA). EA revealed that melanoidin formed from fructosylglycine incorporates a higher proportion of glycine compared to melanoidin produced through the direct reaction of glycine and glucose upon heating. FTIR spectra identified carbonyl or carboxyl groups with distinct bands at ~ 1749–1759 cm⁻¹, contributing to the extended π-electron system observed at 170–200 ppm in NMR spectra. EPR measurements demonstrated a higher abundance of unpaired electrons in fructosylglycine-derived melanoidin. The UV/Vis, FTIR, and NMR data indicated that the backbones of fructosylglycine-derived melanoidins contain a greater number of conjugated π bonds. Therefore, we conclude that the melanoidin skeleton synthesized from fructosylglycine includes more amino acid residues, owing to enhanced activation of nitrogen in the secondary amine of fructosylglycine compared to the primary amine of glycine. EPR results further reveal a positive correlation between melanoidin absorbance spectra, the size of their π-electron system, and antioxidant activity. These findings suggest that the nucleophilic attack of glycine’s amino group on glucose’s carbonyl group is facilitated in secondary amines, indicating a potential pathway to enhance melanoidin formation by incorporating secondary amines during food processing.