Relationships between hydration number, water activity and density of aqueous sugar solutions

摘要

Hydration numbers (n_h) of simple sugars have been investigated for decades using thermodynamic, spectroscopic as well as molecular modelling techniques. Results were shown to depend on the technique employed. The most reliable values only concern the first hydration shell assuming a maximum oxygen-oxygen distance below 2.8 A. As concentration increases, sugar-sugar interactions become preponderant and n_h decreases. Assuming that no long range structuring effect is exerted by the solute on water, it is possible to estimate the volume occupied by each of hydration water (with nearly 9% volume contraction) and bulk water from density measurements. Likewise, the volume occupied by non-hydrated sugar molecules in the aqueous medium allows finding for sugar density in the aqueous medium a value comparable to that of solid crystalline form. On the other hand, using the literature values of aqueous sugar solution densities, it was possible to calculate the hydration numbers at different temperatures and concentrations. These values of n_h show a noticeable decrease as temperature is raised and concentration increased. Decrease in n_h can be explained assuming a partial occupation of potential hydration sites (OHs) because of differences in H-bonds lifetimes on the one hand and molecular folding around glycosidic bond on the other. Calculation of water activity coefficients (f_w) based on n_h values was made for sucrose solutions. Results show the same trend as found previously [Starzak, M., & Mathlouthi, M. (2006). Temperature dependence of water activity in aqueous solutions of sucrose. Food Chemistry, 96, 346-370] for sucrose, i.e. a decrease of f_w with increasing molar concentration. Temperature effect on water activity coefficients and hydration numbers is also determined. It shows a decrease in hydration number as temperature is increased. In this paper, empirical relations are proposed to calculate water activity coefficients and hydration numbers at different concentrations and temperatures by use of accurate density values. These models were first applied to sucrose, the most documented sugar and applied to disaccharides (maltose, trehalose) and monosaccharides (glucose, fructose).