The role of peptides and proteins in melanoidin formation

High-molecular-weight (HMW) coloured compounds called melanoidins are widely distributed, particularly in foods. It has been proposed that they originate through the Maillard reaction, a non-enzymatic browning reaction, due to the interaction between protein or peptide amino groups and carbohydrates. The melanoidin structure is not definitively known, and they have been generally defined as HMW nitrogen-containing brown polymers. In order to gain information on the nature of melanoidins, a simple in vitro model was chosen to investigate the products of the reactions between sugars and peptide/proteins. This approach would elucidate whether melanoidin formation is due to the binding of different sugar units to a peptide/protein or vice versa. With this aim, the reactivity of two different peptides, EPK177 and physalaemin, and a low-molecular-weight (LMW) protein, lysozyme, was tested towards different saccharides (glucose, maltotriose (MT), maltopentaose and dextran 1000) in aqueous solutions at different temperatures. The incubation mixtures were analysed at different reaction times by MALDI/MS. Furthermore, in order to verify the possible role of sugar pyrolysis products in melanoidin formation, the products arising from the thermal treatment at 200 degrees C of MT were incubated with lysozyme, and the reaction products were analysed by the same MS approach. The obtained results allowed the establishment of some general views: melanoidins cannot simply originate by reactions of sugar moieties with proteins. In fact, the reaction easily occurs, but it does not lead to any coloured product, as melanoidins have been described to be; melanoidins cannot originate from the thermal degradation products of glycated proteins. In fact, the thermal treatment of glycated lysozyme leads to a severe degradation of the protein with the formation of LMW species, far from the view of melanoidins as HMW compounds; experimental evidence has been gained on the melanoidin formation through reaction of intact protein with the pyrolysis products of MT. This hypothesis has been supported either from MALDI measurements or from spectroscopic data that show an absorption band in the range 300-600 nm, typical of melanoidins.