Kinetic model of the glucose-glycine reaction

The kinetics of browning (measured in terms of the absorbance at 450 nm, A450) in the glucose-glycine and 3-deoxyhexosulose (DH)-glycine reactions, and the kinetics of loss of sulphite species (S(IV)) in glucose-glycine-S(IV) reactions, all at pH 5.5 (acetate buffer) and 70°C, were used to derive, from first principles, a kinetic model for the browning of glucose-glycine mixtures. A consecutive 3-step reaction mechanism, in which glucose is converted to DH, which in turn is converted into an unspecified intermediate (I), the precursor of melanoidins, is described by the following three rate equations, d [ D H ] d t = k 1 [ glucose ] [ glycine ] d [ I ] d t = k 2 [ DH ] ( [ glycine ] + 5.1 [ glycine ] 2 ) d A 450 d t = k 3 [ I ] where k1, k2 and k3 are rate constants. The validation of this model is discussed critically. Experiments show that absorbance, in this system, varies with (time)3 consistent with the integrated rate equations, but in contrast with the previously reported dependence on (time)2. © 1997 Elsevier Science Ltd