Water activity in concentrated sucrose solutions and its consequences for the availability of water in the film of syrup surrounding the sugar crystal

A thermodynamically rigorous chemical model of water activity in the sucrose-water system is proposed. The model covers practically the entire composition range including very high sugar concentrations. It employs the concept of the semi-ideal solution according to which all the departures from ideal behavior are attributed to chemical reactions occurring in the system, while physical interactions are neglected. The model accounts for sucrose hydration and clustering as well as water association. All the reactions are assumed to attain chemical equilibrium. Two different mechanisms of sucrose hydration (linear and bridging) are included. The concept of independent binding sites, originally proposed by Schönert (1986), has been extended to describe both the probability of sucrose/water and sucrose/sucrose interactions. An equilibrium “mixture” model of liquid water, involving the formation of simple water clusters, has been incorporated in order to account for the water/water interactions. Experimental data (vapor pressure, boiling point elevation, equilibrium relative humidity, and heat of dilution) from selected literature sources have been used to validate the model and estimate its parameters.

The proposed model can be used as a mathematical tool in solving various practical problems encountered in the sugar and food industry, such as the behavior of crystalline sugar during conditioning and storage, stickiness, re-crystallization of amorphous sugar, its stability in foods and pharmaceuticals. It is postulated that the driving force for migration of water from sugar crystals should be defined in terms of the water activity rather than the nominal content of water in solution. Diffusion processes occurring in the film of syrup surrounding the sugar crystal are briefly outlined. The key role of sucrose hydration and clustering in establishing the driving force of monomeric water in the syrup film is discussed in detai