Melanoidins are the final, brown, high molecular weight products of the Maillard reaction. The aim of the present study was to determine the average molar extinction coefficients of melanoidins formed in heated glucose-casein and fructose-casein systems. The value of the extinction coefficient can be used to translate spectrophotometrically measured browning (absorbance values) into melanoidin concentration. In the present study the melanoidins were quantified by measuring the concentration of sugar incorporated into the melanoidins, using (14)C-labeled sugar. The extinction coefficient of the melanoidins remained constant during the observation period as the absorbance at 420 nm increased to approximately 8 units, and it was calculated to be 477 (+/- 50) L mol(-1) cm(-1) in the glucose-casein reaction and 527 (+/- 35) L mol(-1) cm(-1) in the fructose-casein reaction. This difference is not significant. An increase of the number of sugar molecules per reactive amino group during the heating of glucose-casein and the fructose-casein mixtures was observed by the radiochemical method as well as by microanalysis of the high molecular weight fraction.