Characterization of the most intense coloured compounds from Maillard reactions of pentoses by application of colour dilution analysis

Thermal treatment of an aqueous solution of xylose and l-alanine in the presence of furan-2-carboxaldehyde, one of the major pentose dehydration products, led to a rapid colorization of the reaction mixture. To characterize the key chromophores formed, a screening procedure, which is based on the determination of the visual threshold of coloured high-performance liquid chromatography (HPLC) fractions, was developed to select the most intense coloured compounds in the complex mixture of Maillard reaction products. This so-called colour dilution analysis (CDA) revealed 20 coloured fractions, amongst which four fractions were evaluated with by far the highest colour impacts. The identification experiments were therefore focused on the compounds evoking the intense colour of these four fractions. Compound 12 was characterized as a mixture of the previously unknown orange-coloured (1R,8aR)- and (1S,8aR)-4-(2-furyl)-7-[(2-furyl)methylidene]-2-hydroxy-2H,7H,8aH-pyrano[2,3-b]pyran-3-one (12a/12b) by several 1D- and 2D-NMR techniques, liquid chromatography–mass spectrometry (LC–MS) as well as UV–Vis spectroscopies. The other three key chromophores 7, 17 and 9 were identified as the yellow coloured 2-[2-furyl)methylidene]-4-hydroxy-5-methyl-2H-furan-3-one (7), the red coloured 2-[(2-furyl)methylidene]-4-hydroxy-5-[(E)-(2-furyl)methylidene]-methyl-2H-furan-3-one (17) and the red coloured (S)-4-[(E)-1-formyl-2-(2-furyl)ethenyl]-5-(2-furyl)-2-[(E)-(2-furyl)methylidene]-2,3-dihydo-α-amino-3-oxo-1H-pyrrole-1-acetic acid (9).