Differential Scanning Calorimetry of Nonfreezable Water in Solute—macromolecule—water Systems

Differential scanning calorimetry was employed to determine the amount of bound (nonfreezable) water in several model systems as a function of water activity (a w ). Water activity was controlled by varying total moisture content or by adding a solute, urea, to the aqueous phase of the model system. Since the amount of bound water is dependent on the nature of the components, correlations between bound water and a, are meaningful only for specific systems. In every case studied, bound water, as g H 2 O/g solids, decreased with decreasing a w , along what might be called a bound water isotherm. The results indicate that measurements of bound water should refer to a specified value of a w . In addition, a w of the solution phase appears to be a major contributor to the driving force for water binding by macromolecules.