Characterization of key chromophores formed by nonenzymatic browning of hexoses and L-alanine by using the color activity concept

Thermal treatment of an aqueous solution of D-glucose and L-alanine in the presence of the carbohydrate degradation product furan-2-aldehyde resulted in the formation of a variety of colored compounds, among which (Z)-2-[(2-f uryl)methylidene]-5,6-di(2-furyl)-6H-pyran-3-one (I), [E]- and [Z]-1,2-bis( 2-furyl)-1-pentene-3,4-dione (IIa/IIb), 4,5-bis(2-furyl)-2-methyl-3H-furan- 2-one (III), and (S,S)- and (SR)-2-[4,5-bis(2-furyl)-2-hydroxy-2-methyl-3(2 H)-pyrrol-1-yl]propionic acid (IVa/IVb) as well as 2-[(2-furyl)methylidene] -4-hydroxy-5-[(E)-(2-furyl)methylidene]methyl-2H-furan-3-one (V) were succe ssfully identified as the most intense by application of the color dilution analysis. To measure the contribution of these colorants to the overall co lor of the browned Maillard mixture, color activity values were calculated as the ratio of the concentration to the visual detection threshold of each colorant. By application of this color activity concept, 16.0% of the over all color of the Maillard mixture accounted for these five types of coloran ts, thus confirming them as key chromophores. On the basis of synthetic mod el experiments, the formation pathways leading to the chromophores IIa/IIb, III, and IVa/IVb were proposed.