Using dynamic dewpoint isotherms to determine the optimal storage conditions of inert dust-treated hard red winter wheat

Water-solid interactions play a key role in determining the efficacy of inert dusts. The critical water activity (A_wc) for phase transition in amorphous materials is an important characteristic of amorphous inert dusts used as grain protectants. As water activity (A_w) rises above A_wc, amorphous dusts undergo a transition from glassy or vitreous state to rubbery state. Such a transition induces dramatic changes in material properties, texture and structure, and hence impact their performance as grain protectants. Full Dynamic Dewpoint Isotherms (DDI) of a synthetic amorphous zeolite intended for grain protection were generated using the Vapor Sorption Analyzer (VSA) to determine A_wc by investigating the relationship between moisture content and A_w at constant temperatures. Sorption experimental data was fitted using three sorption isotherm models: Guggenheim-Anderson-de Boer (GAB), Double Log Polynomial (DLP), and Brunauer-Emmet-Teller (BET). DLP model was the best model to estimate zeolite and wheat sorption isotherms. Full sorption isotherms of zeolite and wheat obtained at 25, 35, and 45 °C clearly showed the hysteresis phenomenon. The hysteresis loops were of type H3 for wheat, and of type H4 for zeolite powder. The intensity of hysteresis remained unchanged for wheat. However, the intensity of hysteresis decreased with increasing temperatures during water adsorption by porous zeolite powder. Monolayer moisture content values for each sorption direction were provided only by GAB and BET models and indicated a decrease in monolayer moisture content with an increase in temperature. The net isosteric heats of sorption and the differential enthalpy of zeolite estimated by the Clausius–Clapeyron equation and determined graphically decreased with increasing moisture content. Conversely, differential entropy of zeolite decreased with increasing zeolite moisture content. The optimal moisture content of inert dust for grain treatment was dependent on wheat moisture content and wheat storage temperature. This is the first time that a synthetic amorphous dust is being investigated for grain protection. Our results recommend the application of inert dusts at the optimal moisture content to mitigate moisture migration within the system “wheat-dust”, thus ensuring dust maximal efficacy.