Kinetics and equilibria of tea infusion: Part 8-the effects of salts and of pH on the rate of extraction of theaflavins from black tea leaf
The rates of extraction of theaflavins from sieved Kapchorua Pekoe Fannings (600–710 μm) and from a Ceylon Broken Orange Pekoe Fine Leaf Blend have been measured at 80°C with a set of aqueous salt and buffer solutions spanning the pH range 3·1 to 8·1. There was little effect on adding 1:1 electrolytes but considerable decreases in rate occurred in the presence of calcium chloride. The rate constants in acid buffers were the same as in a normal unbuffered infusion but in alkaline buffers of pH 8 they were over 50% larger. This phenomenon was quantitatively accounted for by the partial dissociation of theaflavins at higher pH and the greater diffusion coefficient of the resulting ionic salt. The relevance of these findings to tea brewing and to industrial theaflavins extraction is discussed.