A new method to determine “equilibrated” water activity and establish sorption isotherm by erasing surface history of the samples

• Measure of unreproducible “apparent aW” due to the handling surface history.
• Simulations on maltodextrin highlights water diffusion facilitated close to Tg.
• “Equilibrated aW” by thermal treatment at Tg erases moisture gradients in powders.
• Robust sorption isotherm built more rapidly based on thermal treatment.

Building sorption isotherms usually requires several weeks for the samples to equilibrate at different relative humidities. Such experimental durations are very long in view of present industrial timeframes. In this work, we propose a rapid and robust method using wise thermal treatment that significantly accelerates equilibration of samples without inducing chemical or physical degradation. To demonstrate the relevance of our approach, we used maltodextrin powders considered as model systems and coupled experimental work with theoretical prediction of water diffusion. By testing different temperatures, we prove that a thermal treatment at Tg for approximately 2 h before measuring aw allows in most cases getting rid of moisture gradients within the powders; hence, we propose to introduce the notion of apparent vs. equilibrated water activity. By using equilibrated water activity data we find sorption isotherms which are less curved in the low water activity range in comparison with classical methods and propose to discuss the differences in view of the long time it takes for powders to fully equilibrate without thermal treatment. These results impact not only the methods and the accuracy of the data but also their practical use for instance in predicting shelf-life stability or modelling drying processes.