Melanoidins extinction coefficient in the glucose/glycine Maillard reaction

Abstract

Melanoidins (brown, nitrogenous polymers and co-polymers) are the final products of the Maillard reaction. The glucose/glycine melanoidins extinction coefficient was determined using 14C-labelled glucose at three different reaction conditions. The absorbance was measured at different wavelengths (420, 450, 470 and 490 nm) and the extinction coefficient determined for each. The value of the extinction coefficient can be used to recalculate browning, measured as absorbance units, into melanoidins concentration in terms of sugar molecules incorporated. The amount of 14C-labelled sugar molecules was estimated in melanoidins separated via dialysis with a cut-off value of 3500 Da. These melanoidins only represented ≈12% of the total colour formed. The extinction coefficient of the melanoidins remained constant during the observation period. At 470 nm, values of 0.65 (±0.02) l mmol −1 cm −1; 0.66 (±0.02) l mmol −1 cm −1 and 0.62 (±0.05) l mmol −1 cm −1, were obtained at 120 °C pH 6.8, 100 °C pH 6.8 and 100 °C pH 5.5, respectively. The difference is not significant. The extinction coefficient appeared to not to vary within the pH and temperature range studied. From the elemental analysis, the nondialysable melanoidins elementary composition seemed to be influenced by the reaction conditions, which was supposed to be related to the presence of side-chains on the melanoidin backbone. A trend was observed in the melanoidins C/N ratio: it decreased with increasing reaction pH as well as it changed to a lower level, of about 8, as the extent of browning increased.