Effect of Process Variables on the Sorption Characteristics of Nigerian Green Tea

Abstract

Green tea (Camellia sinensis) processing variables have been shown to influence variation in chemical constituents and acceptability of resultant tea products. This therefore suggests that basic green tea processing variables may influence quality of stored green tea. Thus, the present study investigated the effect of steaming time, drying temperature and time on the sorption characteristics of optimized Nigerian green tea. Green tea samples were conventionally produced and optimized using experimental runs. Static gravimetric method employed involve eight levels of water activities (0.1 - 0.8) using concentrated H₂SO₄ at temperatures of 27, 35 and 40 ^oC to simulate mini atmospheric condition. Plots of equilibrium moisture content (EMC) against corresponding water activity (a_w) were generated for isotherm curves. The adsorption data were fitted using common isotherm models (Caurie, Brunauer, Emmett and Teller (BET), Guggenheim Anderson and de Boer (GAB), Henderson, Oswin, Peleg and Smith). Adsorptive behaviour of optimized green tea samples showed equilibrium moisture content increases over elevated level of water activity at constant temperature. The sorption curved displayed sigmoid shapes characterized as Type II isotherm. Peleg model gave a better fit of the adsorption data with highest R² (0.9992) and least values of standard error of estimate (0.3529) and residual sum of square (0.4981). The GAB monolayer moisture content at 27, 35 and 40 ^oC ranged between 1.94 and 6.83, 2.94 and 6.47, and 3.31 and 6.65%, respectively. Thus, GAB equation suggest optimized green tea prepared by steaming for 120 s, dried at 70 ^oC for 150 min will retain safe storage moisture of 6.83% d.b under practical condition at 27 ^oC.