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

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 T-g for approximately 2 h before measuring a(w) 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. (C) 2016 Elsevier Ltd. All rights reserved.