Kinetics of tea infusion: the effect of the manufacturing process on the rate of extraction of caffeine

The temperature effect on the rate of infusion of caffeine from South African black tea has been determined over a temperature range with the help of high performance liquid chromatography (HPLC). The activation energy was found to be 40.3 kJ mol−1 which is similar to the reports for other teas. The leaf size used was 1.40–2.00 mm. The rate constant at 80°C was determined to be 0.99 min−1 using a steady state model (Spiro and Jago, 1982). This value is slightly larger than the expected value for tea with similar particle size. Using the rate constants, diffusion coefficients of caffeine over a temperature range have been calculated. These have been compared with the corresponding values of caffeine in water at similar temperatures giving hindrance factors of 86 to 54 over the temperature range. These results indicate that the diffusion of caffeine through the leaf matrix is a greatly hindered process. A detailed comparison of kinetic and equilibrium data reported in the literature has been carried out between different teas manufactured through different processes. It is clear that the manufacturing method has a direct influence on the infusion of caffeine from the tea leaf.