Kinetics and equilibria of tea infusion. Part 3.—Rotating-disc experiments interpreted by a steady-state model
The kinetics of the extraction of a soluble constituent from tea leaf have been treated by a steady-state model. This leads to an overall rate constant made up of 3 main contributions which arise from the diffusion of the constituent through the leaf, its transfer across the leaf/water interface, and its diffusion away through the Nernst layer. That the last step was not the rate-determining one was shown by rotating-disc experiments. Koomsong tea dust was glued on to large discs and the rate of caffeine extraction at 80 °C measured at various rotation speeds. The rate was found to be independent of the speed. The viability of the experimental procedure was checked using discs coated with similarly-sized copper powder: here the rate of attack by dilute dichromate increased with increasing rotation speed in semi-quantitative agreement with the Levich equation.