Modeling Desorption Isotherms and Thermodynamic Properties of Fresh Tea Leaves (Camellia sinensis var. assamica)

Abstract

Abstract Desorption isotherm of fresh tea leaves was studied using static gravimetric method of saturated salt solutions at temperatures of 15, 20, 25 and 30ºC and water activities of 0.07 to 0.97. Data were fitted to five mathematical equations. Using the best-fit mathematical model, thermodynamic properties of fresh tea leaves including net isosteric heat of sorption, differential entropy were estimated. The applicability of enthalpy – entropy compensation theory for fresh tea leaves was also evaluated. Results showed that the modified Oswin model was best fitted with the experimental data. Thermodynamics properties like net isosteric heat of sorption and differential entropy were found to decrease exponentially with the increase in moisture content. Net isosteric heat of sorption and differential entropy increased exponentially with decreasing moisture content of fresh tea leaves. Isosteric heat of sorption of fresh tea leaves was very close to heat of vaporization of pure water when the moisture content of tea leaves exceeded 40% (d.b.). The enthalpy-entropy compensation theory could be successfully applied for the moisture desorption behavior of fresh tea leaf samples and it was enthalpy driven spontaneous process.