Comparison of critical storage parameters of the powders containing soy protein isolate and inulin, based on the concepts: Water activity and Temperature of glass transition

In this study water activity and glass transition concepts were used to evaluate stability of the two component powder systems containing inulin and soy protein isolate. Moisture adsorption isotherms of soy protein powder with 25, 50 and 75 g of inulin on 100 g powders additions at 25 °C were determined by static gravimetric method. The experimental data obtained were fitted to three mathematical models. All curves exhibit sigmoidal shape corresponding to type II isotherms. The monolayer moisture content for GAB (Guggenheim–Anderson–Boer) model obtained for powder systems varied between 0.0647 and 0.0739 g water/g dry solids. Thermal transitions of equilibrated powders were analyzed using differential scanning calorimetry and glass transition curves were plotted by Gordon–Taylor equation. The highest value of glass transition temperature of anhydrous solid was calculated for soy protein without inulin addition. In order to investigate optimum storage conditions diagrams including water activity, equilibrium moisture content and glass transition relationship were constructed by using Khalloufi–El-Maslouhi–Ratti equation. Critical moisture content values of the powders estimated based on both, water activity and glass transition concepts, were significant higher than corresponding them monolayer capacity values.