Modeling moisture desorption isotherms and thermodynamic properties of fermented tea dhool (Camellia sinensis var. assamica)

The desorption isotherms of fermented tea dhool were determined at 50, 55, 60, and 70 degrees C by using static gravimetric method of saturated salt solutions within the water activity range of 0.05-0.81. Results indicated that the equilibrium moisture contents were decreased with the increase in temperature. A modified Oswin model was well fitted with the experimental data. Desorption isotherm curves obtained were type II sygmoidal. Thermodynamic properties like net isosteric heat of sorption and differential entropy were found to decrease exponentially with the increase in moisture content. Monolayer moisture content was also decreased linearly with the increase in temperature. The enthalpy-entropy compensation theory could be successfully applied for the moisture desorption behavior of fermented tea dhool samples and it was an enthalpy-driven spontaneous process.