The desorption isotherms of fermented tea dhool were determined at 50, 55, 60, and 70 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.

Thin layer drying characteristics of fresh tea leaves were investigated to quantify the rate of moisture transfer of fresh tea leaves during the withering process within the temperature range of 20 -35 o C, and a relative humidity range of 40 -90 % at 1.2 ± 0.3 m/s airflow rate. Five different mathematical models available in the literature were used with the drying data for fresh tea leaves using a constant climatic chamber. Results indicated that the Two-term model gives better predictions for moisture transfer than others. The desorption process of tea leaves occurred in falling rate period. The effective diffusivity of water in tea leaves varied from 3.3409 -5.4669 × 10 -10 m 2 /s over the temperature range investigated with an activation energy of 1477.75 kJ/kg. The temperature dependence of diffusivity coefficient was described satisfactorily by a simple Arrhenius type relationship.

One-dimensional heat and mass transfer mathematical model was developed to simulate moisture content of tea leaves during trough withering. Model solutions were approached using finite difference method with appropriate boundary conditions. A computer program written in QBASIC was used to calculate the real time moisture content of tea leaves and other psychometric parameters of conditioned air during withering. Four experiments were performed using commercial type of withering trough to validate the developed model. Leaf samples were drawn from top, middle and bottom layers of the leaf bed in the trough for one-hour time interval for 12 h of the test period. Simulated moisture data calculated by the program was compared with the experimental data. Time and space increments of the model were chosen minimizing the estimation error of moisture content. Results showed that the experimental and simulated moisture data were in close agreement for top, middle and bottom layers of the withering trough with standard errors in the range