Volatile reaction products from a heated xylose-lysine model system

Abstract

Aqueous solutions of xylose (1 molal) and lysine monohydrochloride (1 molal) were refluxed without pH control (initial pH 4.9) for 240 min by simultaneous steam distillation-solvent extraction using a modified Likens and Nickerson apparatus. The solvent extract was replaced by fresh solvent after 15, 30, 45, 60, 120 and 180 min refluxing and subsequently concentrated to give a series of volatile isolates. Isolates were analysed by gas chromatography (GC) and GC-mass spectrometry, and a total of 44 reaction products could be identified. 2-Furfural constituted over 99% (w/w) of the volatiles in each isolate. The most numerous reaction products were monocyclic furans (20 being identified), whereas only eight nitrogen-containing compounds were noted. Yields of total reaction products increased up to 120 min refluxing, and then decreased, but if 2-furfural is excluded from the calculations, yields of the remaining reaction products increased up to 180 min before decreasing. Individual classes of compounds also tended to follow this general rule, notable exceptions being the nitrogen-containing components. Monocyclic pyrroles, 2,3-dihydro-1 H-pyrrolizines and pyridines were identified only after 45, 45 and 60 min refluxing, respectively, whereas yields of 1-(2-furfuryl)pyrroles increased throughout the heating period.