Moisture sorption isotherms, heat of sorption and vaporization of withered leaves, black and green tea

Moisture sorption isotherms (EMC/ERH) of withered leaves, black and green tea were determined at 20, 30, and 40 °C using the standard gravimetric static method over a range of relative humidity from 10% to 90%. The experimental sorption curves were fitted by six equations: modified Henderson, modified Chung–Pfost, modified Halsey, modified Oswin, modified Smith and GAB. The modified Halsey model was found to describe the sorption isotherms of withered leaves, black and green tea satisfactorily. The net isosteric heats of sorption and vaporization of water were determined from the equilibrium data at different temperatures. The net isosteric heat of sorption of withered leaves varied between 30.8 and 29.5 kJ/mol at moisture levels varying between 8% and 9% (d.b.). For black tea and green tea it varied between 25.5–16.8 kJ/mol and 34.8–20.7 kJ/mol, respectively, at moisture levels varying between 6% and 8% (d.b.).