Characterisation of Polycondensation Products from Amino Acids and Lipid Oxidation Products

The Maillard reaction, or nonenzymatic browning, includes a complex network of reactions initiated by the condensation reaction of a carbohydrate and an amino compound, and is of utmost importance for the formation of flavour and colour in thermally treated foods. Besides volatile flavour compounds, also brown-coloured polycondensation products, named melanoidins, are produced. In latest years, the interaction of lipid oxidation products in the classical Maillard reaction pathway and vice versa is of particular interest, since it has been shown that the course of both reactions can be modified by the reactants, intermediates and products of the other (ZAMORA & HIDALGO 2005). Therefore, in parallel with the formation of model melanoidins, polycondensation products resulting from the interaction of amino acids with lipid oxidation products were studied. For this purpose, various coloured water-soluble high molecular weight and water-nonsoluble reaction products were isolated from the model reactions of glycine or lysine with a lipid oxidation product (hexanal, (2E)-hexenal, (2E,4E)-decadienal) in the presence or absence of glucose. They were characterised by UV-visible absorbance measurements, elementary analysis, and thermal degradation followed by SPME-GC-MS analysis (ADAMSet al. 2003). The UV-visible absorbance spectra before and after dialysis indicated that the most important contributors to the formation of water-soluble coloured material were constituents of the low molecular fraction. Elementary analysis data showed that a higher amount of nitrogen was incorporated in the high molecular weight fractions as compared to the water-nonsoluble fractions, except for the water-nonsoluble reaction products from amino acid/(2E, 4E)-decadienal interactions, which demonstrated the lowest C/N ratio found. Volatile carbonyl compounds, furans, a liphatic compounds, pyridines, pyrroles and benzene derivatives were the main groups of compounds identified in the thermal degradation profile of each fraction tested. Aldol condensation reactions of the carbonyl compounds were very important in the initial reaction steps. Especially pyridines seem typical indicators of amino acid-lipid oxidation product interactions.