Behaviour depending upon temperatur of D-fructose in aqueous solutions and melting process

Depending upon the temperature, the pyranose-furanose-equilibrium of D-fructose in aqueous solutions was determined quantitatively by gaschromatography. When the equilibrium of the tautomeric fructopyrandse and fructofuranose modifications was reached at temperatures of 0, 10, 20, 30, 40 and 50 °C, silylation was carried out and then gas-chromatographic separation followed. In the equilibrium of aqueous D-fructose solution, ß-D-fructopyranose and a- and ß-fructofuranose could be detected. However, the a-D-fructopyranose and the acylic form could not be observed. As the temperature rises, the proportions of a- and ß-fructofuranose experience a linear increase. They Show 4,1 and 11,1 % respectively at a temperature of 0 °C, and 12,3 as well as 31,9 % respectively at 50 °C. On the other hand, the portion of ß-fructopyranose decreases as the temperature rises i.e. from 84,8 % at 0 °C to 55,8 % at 50 °C. Therefore depending upon the temperature, a correlation between the pyranose-furanose-concentration, the specific rotation and the sweetness exists. Thus it was shown that the well-known temperature dependence of the sweetness for the D-fructose solution may be attributed to the conversion of pyranose into furanose and therefore the latter is less sweet. In addition, during the melting process the first change occurs in the conversion of the equilibrium of ß-D-fructopyranose into a- and (3-DIructofuranose. In the melted state, depending on storage conditions, a diversely rapid reconversion of D-fructofuranose into D-fructopyranose takes place. It may be concluded from these results that the first phase of the thermically caused fructose alteration or destruction appears to exist in the reversible conversion of fructopyranose into fructofuranose.