On the Influence of the Carbohydrate Moiety on Chromophore Formation during Food-RelatedMaillard Reactions of Pentoses, Hexoses, and Disaccharides

The influence of the carbohydrate moiety on the formation of 2-[4-oxo-3-(pyrrolidin-1-yl)cyclopent-2-en-1-ylidene]furan-3(2H)-one chromophores during food-related Maillard reactions from pentoses, hexoses, and disaccharides is reported. The orange compounds 1a,b and 2a,b, detected in a roasted xylose/ L-proline mixture, were identified as (2E)/(2Z)-4-hydroxy-5-methyl-2-[4-oxo-3-(pyrrolidin-1-yl)cyclopent-2-ene-1-ylidene]furan-3(2H)-one and (2E)/(2Z)-5-methyl-2-[4-oxo-3-(pyrrolidin-1-yl)cyclopent-2-en-1-ylidene]-4-(pyrrolidin-1-yl)furan-3(2H)-one, respectively, by 1D- and 2D-NMR, LC/mass, and UV/VIS spectroscopy, as well as by synthetic experiments. Studies on their formation revealed that 1a, b and 2a, b are formed upon condensation of pentose-derived 4-hydroxy-5-methyl- (3) and 5-methyl-4-(pyrrolidin-1-yl)furan-3(2H)-one (4), respectively, with 2-hydroxycyclopenta-2,4-dien-1-one (5) and L-proline (Scheme 1). Further condensation reaction of 1a, b with furan-2-carbaldehyde yielded the red (2Z)-2-{(5Z)-5-[(2-furyl)methylidene]-4-oxo-3-(pyrrolidin-1-yl)cyclopent-2-en-1-ylidene}-4-hydroxy-5-methylfuran-3(2H)-one (6) as an additional novel Maillard chromophore. Replacement of the pentose by glucose in the mixture with L-proline led, after dry-heating, to the identification of the structurally related colored (2Z)/(2E)-2-[5-hydroxy-5-methyl-4-oxo-3-(pyrrolidin-1-yl)cyclopent-2-en-1-ylidene]-4-hydroxy-5-methylfuran-3(2H)-one (7a/7b) and to the characterization of 2,4,5-trihydroxy-5-methylcyclopent-2-en-1-one (10) and 5-hydroxy-5-methylcyclopent-3-ene-1,2-dione (11) as key intermediates in chromophore formation from hexoses. Comparative studies on disaccharides revealed that not 7a/7b, but the colorless 4-(α-D-glucopyranosyloxy)-2-hydroxy-2-methyl-6H-pyran-3(2H)-one (8) and 2-(α-D-glucopyranosyloxy)-4,5-dihydroxy-5-methylcyclopent-2-en-1-one (9) were formed amongst the major degradation products of maltose (Scheme 4). The aglycons of 8 and 9 could not be liberated under food-related heating conditions, thus, inhibiting the formation of the color precursors 10 and 11 and, in consequence, of 7a/7b (Scheme 6). These data strongly suggest that the 1,4-glycosidic linkage of disaccharides is responsible for their lower efficiency in browning development compared to pentoses or hexoses.