Kinetics of the Sulphite- Inhibited Maillard Reaction

The kinetics of the inhibition of Maillard browning of glucose and glycine by suIphur(IV) oxospecies, S(IV), are reported in detail in water, and solutions containing either 40% w/w ethanol, 40% w/w polyethylene glycol 400 or up to 81.5% w/w glycerol. The progress of the reaction was followed by measuring free and reversibly bound S(IV), production of H+ and formation of sulphate ion. Equilibria between S(IV)-species were investigated by Fourier Transform Infrared (FTIR) Spectroscopy.
Kinetic data show that despite earlier reports demonstrating a lack of an effect of pH on this reaction, maintaining of reaction mixtures at a constant pH causes considerable increases in rate.
Overall the kinetics support the following 3-step mechanism,

[diagram available in .pdf]
S(IV) glycine
hydroxysulphonate hydroxysulphonate hydroxysulphonate

where DH, DDH and DSH represent 3-deoxyhexosulose, 3,4-dideoxyhexosulos-3-ene and 3,4-dideoxy-4-sulphohexosulose. Some autoxidation of S(IV) also takes in practice. The kinetically measured dissociation constant of the hydroxysulphonate of DH is found to be 0.28 M and this is the first report of kinetic evidence for such an interaction in the S(IV)-inhibited Maillard reaction. Hydrogen ion production during the reaction is the result of a condensation reaction between glucose and glycine, and autoxidation of S(IV).
FTIR spectroscopy shows that addition of glycerol as a humectant does not cause the expected conversion of HSO3-t0 S2O52- in solution. Instead new species, e.g. ion pairs such as NaS205_, are formed and S2O52' is probably less important in concentrated foods, than once thought. The effect of humectant is to increase both k] and k2 (aw 1-0.43) and the effects are explained in terms of equilibria involving water as a reactant and interactions between transition states of the solvent.