Moisture adsorption isotherms and thermodynamic properties of green and roasted Yerba mate (Ilex paraguariensis)

Moisture adsorption isotherms of green and roasted Yerba mate (Ilex paraguariensis) at 5, 25, and 40 °C were determinated by the static gravimetric method over the wide range of water activity from 0.05 to 0.9. GAB model fitted well the experimental adsorption data presenting sigmoid shape curves for both samples. Whereas the decrease of isotherms with increasing of the temperature was observed for green mate sample over the whole range of water activities, a crossing-over of isotherm at 40 °C was observed for roasted mate sample at 0.66 aw. Nevertheless, the moisture adsorption was significantly higher for green mate than that for roasted mate. The effect of temperature on the properties of sorbed water calculating from Curie's equation was only apparent for roasted Yerba mate. Claussius–Clapeyron equation showed that the adsorbed moisture is strongly bound to the surface of green Yerba mate in low moisture content in comparison with roasted Yerba mate. According to compensation theory, the sorption process for both green and roasted Yerba mate samples was enthalpy driven, and non-spontaneous. When stored at higher relative humidity (>66%), green and roasted Yerba mate should be kept at different temperature, i.e. higher temperature (40 °C) is favourable for green Yerba mate, whereas lower temperature (5 °C) is preferred for roasted Yerba mate.