Insights into melanoidin conversion into fluorescent nanoparticles in the Maillard reaction

The Maillard reaction is a well known chemical reaction in the food industry, in which melanoidins are generally considered as the final product. However, the exact final products of the Maillard reaction are far from being well understood. The conversion mechanism of melanoidins is of importance for explanation of the whole process of the Maillard reaction. In this paper, the conversion of melanoidins in the Maillard reaction was studied using glucose and lysine as raw materials. Our results showed that fluorescent nanoparticles (FNPs) can be formed after the hydrothermal reaction of melanoidins at 180 °C for 2, 4, 6 and 8 hours, respectively. Unlike melanoidins, the FNPs are highly water-soluble and strongly fluorescent and have a particle size of around 0.7-6.8 nm. X-ray photoelectron spectroscopy and 1H NMR spectroscopy analysis demonstrated that there are many functional structures like carboxyl, hydroxyl, aldehyde, amine and aromatic groups produced on the surface of the FNPs. Total elemental analysis indicated that the oxidization of the FNPs was intensified with the extension of reaction time. The thermogravimetric kinetics of the FNPs were significantly different from those of melanoidins. More heterocyclic and aromatic compounds were found in the pyrolysis products of the FNPs with the extension of reaction time. The results of this paper provided new insights into the conversion of melanoidins for further understanding of the Maillard reaction.