Low molecular weight colored compounds formed in xylose-lysine model systems

Aqueous solutions of xylose (1 m) and lysine monohydrochloride (1 m), initial pH 5.27, were refluxed for 1 h, either by control of the pH at 5 (by the addition of sodium hydroxide solution during heating) or without pH control (final pH 2.83). The ethyl acetate-extractable components were separated from each model system and represented 0.41 and 0.36 ± 0.05% (m/m), respectively, of the initial reactants for the systems heated with and without pH control. Analysis of the ethyl acetate-extractable components by TLC and HPLC (with diode array detection) showed some similarities, but also many differences between the two systems. Two and seven coloured peaks were analysed, respectively, in the systems heated with and without pH control, but only one was detected in both systems. A novel coloured Maillard reaction product (detected only in the model system heated without pH control) was isolated and purified by repeated semi-preparative TLC and HPLC, prior to analysis by electronic absorption and NMR spectroscopy and by low and high resolution FAB MS.