Kinetic study of the mutarotation of D-glucose in concentrated aqueous solution by gas-liquid chromatography

Growth is the most important step of industrial seeded crystallisation. The overall laws that govern α-D-glucose monohydrate crystal growth should depend upon mutarotation. This parameter could become a limiting factor, as it is the case for diffusion or the integration of molecules at the surface of crystal. Consequently, it was considered essential to obtain additional information on mutarotation kinetics, under temperature and concentration conditions close to solution saturation. Composition of D-glucose concentrated solutions was studied, using GC procedure after trimethylsilylation in pyridine. This is commonly used to obtain directly the anomeric composition of α and β-D-glucopyranose in solution. Moreover, minor components, such as D-glucofuranose, can be evidenced. GC has one main and obvious advantage over polarimetric measurement as it can be applied to turbid solutions, which is the case for concentrated syrups. Admitting that mutarotation is a first order kinetics reaction, it was found that concentration has no significant effect on the rate constant for mutarotation, whereas increasing temperature enhanced the mutarotation rate of D-glucose solution. Results show that the proportion of the α-anomer at equilibrium increases slightly with both saturation and temperature, whatever the anomer (α or β) in the original solution. These results agree with previous studies by Kraus and Nyvlt, who have established kinetic parameters of mutarotation at higher temperatures.