The contribution of coloured Maillard reaction products to the total colour of browned glucose L-alanine solutions and studies on their formation

Thermal treatment of aqueous solutions of glucose and L-alanine in the presence of furan-2-carbox-aldehyde (mixture I) resulted in the formation of a variety of coloured compounds, amongst which (1R,8aR)- and (1S,8aR)-4-(2-furyl)-7-[(2-furyl)methylidene]-2-hydroxy-2H,7H,8aH-pyrano[2,3-b]pyran-3-one (1a/1b) and 3, 5-dihydroxy-2-[(E)-(2-furyl)methylidene]methyl-5, 6-dihydropyran-4-one(2) were identified as the most intense by application of the colour dilution analysis (CDA). To study how the colorant formation is influenced by the solvent, the Maillard mixture was then heated in a water/methanol mixture (mixture II). Besides 1a/1b and 2, additional coloured compounds were detected in mixture II, amongst which (E)- and (Z)-6-hydroxymethyl-2-methoxy-4-[(2-furyl)methylene]-2H-pyran-3-one(3a/3b), (E)- and (Z)-2-methoxy-4-[(2-furyl)methylene]-2H-pyran-3-one (4a/4b) as well as (1R,8aR)- and (1S,8aR)-4-(2-furyl)-7-[(2-furyl) methylidene]-2-methoxy-2H,7H, 8aH-pyrano[2,3-b]pyran-3-one (5a/5b)could be distinguished from the less colour-active by application of CDA. To measure the contribution of these colorants to the overall colour of the browned Maillard mixtures I and II, colour activity values were calculated as the ratio of the concentration to the visual detection threshold of each colorant. By application of this colour activity concept, 3.3% of the total colour of the Maillard mixture II was shown to be caused by the 2H,7H,8aH-pyrano[2,3-b] pyran-3-one chromophore (1a/1b and 5a/5b). Based on a labelling experiment with glucose-6-[C-13(1)], the formation pathway leading to this key chromophore, involving a retro-aldol cleavage of the C(6) carbon from the hexose skeleton, was clarified.