Effects of physical and chemical conditions on the in vitro oxidation of tea leaf catechins

The use of a model fermentation system containing purified green tea shoot catechins and partially purified polyphenol oxidase (EC 1.14.18.1) has provided important data on theaflavin and thearubigin formation. Low oxygen concentration during fermentation, as a result of inadequate aeration or high enzyme concentration, and enhanced by high temperatures, inhibits theaflavin and promotes thearubigin production. Analysis of catechin oxidation under low oxygen tension suggests that the quinones of epicatechin and epicatechin gallate, by virtue of their low redox potentials, are acting as electron carriers in the preferential oxidation of epigallocatechin, epigallocatechin gallate and theaflavin intermediates. Temperature changes between 20° and 30° had little effect on theaflavin values, whereas thearubigin production increased dramatically. Thearubigin-theaflavin ratios over this temperature range increased in oxygen from 1.5:1 to 2.5:1. The pH optima for theaflavin and thearubigin formation were 5 and 6, respectively. The optimization of this factor, together with oxygen concentration, temperature and enzyme concentration, for theaflavin formation, resulted in theaflavin levels 115% higher than those obtained from similar model fermentations occurring under the conditions associated with black tea manufacture in Malawi (pH 5.6; 30 × 10−8 kat of polyphenol oxidase; 27°; 20% oxygen).