Food Chemistry 46 (1993) 121-127 Low molecular weight coloured compounds formed in xylose--lysine model systems# Jennifer M. Ames, Anton Apriyantono Department of Food Science and Technology, University of Reading, Whiteknights, Reading RG6 2AP, UK & Anna Arnoldi Dipartimento di Scienze Molecolari AgroalimentarL Universitd degli Studi di Milano, Via Celoria 2, 1-2013.? Milano, Italy (Received 28 January 1992; accepted 1 April 1992) 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 differencesbetween 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. INTRODUCTION The development of colour is the most obvious effect of the Maillard reaction, but very little is known about the chemical nature of the coloured compounds formed (Nursten, 1986). Coloured Maillard reaction products may be divided into two classes: the melanoidins, which are brown and possess molecular weights of several thousand daltons, and the low molecular weight structures which typically comprise 2-4 linked rings (Ames & Nursten, 1989). Much of the information available on the low molecular weight coloured Maillard reaction products is the result of studies carried out by Ledl and his co-workers in Munich and Stuttgart (Ledl, 1990; Ledl & Schleicher, 1990). However, several of Ledl's studies have been carried out in methanol or ethanol, rather than in aqueous solution, or using amines rather than amino acids. In fact, very few studies have reported structural data for non-volatile coloured compounds t Paper presented at the Royal Society of Chemistry sym- posium on 'Non-enzymic browning', December 1991. Food Chemistry 0308-8146/92/$05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain 121 formed from aqueous sugar-amino acid mixtures and the structures of only five such compounds have been elucidated worldwide (see Table 1) (Severin & Kronig, 1972; Ledl & Severin, 1978; Nursten & O'Reilly, 1983, 1986; Banks et al., 1988). An additional 29 compounds were isolated from an aqueous xylose-glycine model system and analysed by mass spectrometry only (O'Reilly, 1982). They possessed relative molecular masses in the region 149-448 D, and a furfuryl group was evident in several of them. No doubt, the lack of structural information available for coloured com- pounds formed in sugar-amino acid model systems is due to the highly complex mixtures of reaction products obtained, and the consequent problems encountered in isolating individual components in adequate amounts for analysis by IR, NMR and mass spectrometry. The Maillard reaction is known to be greatly influ- enced by the pH of the medium but, again, very little information is available regarding the nature of the coloured compounds formed in model systems heated at different pH values. A comparison of the volatile re- action products formed in a xylose-lysine model system heated either with the pH controlled at 5 or without pH control (final pH 2.83) revealed the formation of