Quantitative studies on the role of browning precursors in the Maillard reaction of pentoses and hexoses with L-alanine

The time course of the formation of 1- and 3-deoxyosones, 2-osuloses, furan-2-aldehydes, as well as C₂, C₃ and C₄ α-dicarbonyl compounds, and hydroxycarbonyl compounds, upon the thermal treatment of aqueous solutions of glucose or xylose in the presence of L-alanine was investigated. 2-Osuloses and glyoxal were formed independently of the carbohydrate moiety, predominantly at the very beginning of the heating process, whereas the formation of 1- and 3-deoxyosones, 2-oxopropanal and hydroxy-2-propanone was favoured with increasing reaction times. In order to evaluate the role of these carbohydrate degradation products in the development of browning their activities as browning precursors were determined on the basis of a dosage/activity relationship by combining chemical/instrumental techniques and visual/sensory analysis. The browning precursors 3-deoxyosone, glyoxal and glycolaldehyde showed highest activities at the very beginning of heating carbohydrate/L-alanine solutions. In contrast to glucose, 2-oxopropanal was found to be an effective colour precursor even at the beginning of heating xylose in the presence of L-alanine. A prolongation of the reaction time led to a drastic decrease in the precursor activity of glyoxal, independently of that of the carbohydrate moiety, and, in parallel, to an increase in the participation of 2-oxopropanal and also furan-2-carboxaldehyde in browning reactions with L-alanine. In the present study, certain carbohydrate degradation products could be identified as browning precursors, and it could be demonstrated for the first time that the activity of these reaction intermediates in producing browning substances changes during the reaction time.