Moisture sorption isotherms and heat of sorption of instant (soluble) green tea powder and green tea granules

Moisture adsorption isotherms of green instant tea powder and green tea granules were determined at 20, 30, 40 and 50 °C. A gravimetric static method was used under 0.11–0.90 water activity ranges for the determination of sorption isotherms that were found to be typical type II sigmoid. Experimental data were fitted to various mathematical models and found that Peleg model suits best in describing equilibrium moisture content–equilibrium relative humidity (EMC–ERH) relationships for instant green tea samples as well as green tea granules, over the entire range of temperatures. The net isosteric heat of sorption was determined from the equilibrium data at different temperatures. The isosteric heat of sorption varied between 48.54 and 44.71 kJ/mol at moisture levels varying between 1 and 9 g/g dry matter for instant green tea powder and 47.96 and 44.10 kJ/mol at moisture level varying between 0.2 and 1.4 g/g dry matter for green tea granules.