Mutarotation of d-fructose in aqueous-ethanolic solutions and its influence on crystallisation

The mutarotation of d-fructose in aqueous-ethanolic solutions was studied between 24 and 50 °C. The ethanol:water mass ratios of the solvent were 3:1, 6:1, and 9:1. The principal tautomers found in the solution were β-d-fructopyranose. β-d-fructofuranose, and α-d-fructofuranose, as is found with aqueous solutions of d-fructose. The furanose tautomers were more strongly favoured in the aqueous-ethanolic solutions, comprising approximately 60% of the fructose in solution for the range of solvent conditions studied, in comparison to 27% in aqueous solution at 24 °C. The rate of mutarotation from β-d-fructopyranose to the furanose tautomers was fivefold slower in solutions of ethanol:water ratio of 9:1 than in aqueous solutions. The tautomeric composition of d-fructose in aqueous-ethanolic solutions was shown to affect the crystallisation of the sugar, due to the slow rate of mutarotation of furanose forms to β-d-fructopyranose.