Teas are described as fermented, but this is dissimilar to alcoholic fermentation and is really oxidation. Oxidation is carried out by oxidase, an enzyme that reacts with oxygen in the air when the leaves have been broken and its cells exposed to the air. Oxidase enzymes change the chemicals in the freshly harvested tea leaves to make the different types of tea.
To determine what the tea taste will be, tea processors must master the process of oxidation/ fermentation. First, oxidation should be done as quickly after harvesting as possible, hence most tea factories are close to where the tea leaves are picked.
Second, the degree of oxidation determines the robustness of the tea and the coppery-colour of the infusion, with the lightest and least oxidised teas being green teas. In green teas, oxidation has been stopped by disabling the enzymes that cause oxidation, either by steaming the leaves or firing them. At the other end of the scale, black teas are fully encourage to oxidise - the tea leaves are exposed to humidity (80 - 90%) and a low ambient temperature of 22 - 23oC (71 - 74oF). A light and controlled oxidation gives oolong teas.
- Gascoyne, K. Marchand, F., Desharnais, J., Américi, H. (2014) Tea: History Terroirs Varieties, Ontario, Canada, Firefly Books. ISBN 9781770853195.