The energy of interaction between water and surfaces of biological reference materials

In the field of reference materials, long-term stability is of the highest importance. Particularly for biological matrices, any kind of deterioration must be avoided during shelf-life and several precautions are taken during production. It has been shown that several mechanisms of deterioration are associated with water activity and water content. Consideration of the availability of water for deteriorative processes will consequently lead to consideration of strong forces of interaction between water and the surface it is attached to. Data for the binding energy of water of four biological reference materials, cod fish muscle, whole grain wheat flour, whole milk powder and skim milk powder, will be presented as a function of the water content. Results obtained as isosteric heat of sorption, calculated from water sorption isotherms at several temperatures are compared with data resulting from a thermal analysis determination of the heat of evaporation of water from the surface. The latter approach combines the principles of differential scanning calorimetry and thermogravimetry. Findings are presented where the results for energy of interaction between water and surfaces, calculated as net isosteric heats of sorption, from water sorption isotherms, are confirmed by the heats of evaporation, determined from simultaneous thermal analysis. Furthermore, simultaneous thermal analysis has extended the range of validity towards dryness where data from water sorption isotherms become invalid.