The crucial step in making black tea is to allow the juices in the rolled fresh leaves to darken from contact with the air. A similar process occurs when the flesh of a cut apple turns brown. The dark substances that form while the tea leaves are exposed to the air are produced by the chemical reactions of the tannins in the tea. The oxidizing stage of tea processing does not take long, no more than four hours. Green Tea Green teas, on the other hand, are very popular in Asia and are only just beginning to catch on here.
Green tea is made by preventing the tea leaves from ever oxidizing at all.
Instead, the leaves are steamed right after the withering stage, which destroys the enzymes that would otherwise cause the darkening. The steamed leaves are rolled and immediately fired. Thus, the dried tea leaves remain green, and the brewed tea, a pale green liquid, has a subtle, slightly bitter flavor, with grassy hints of the flavor of the fresh plant. Because the tannins do not go through the oxidizing process, which has a mellowing effect, green tea can be bitterer, more astringent than black, especially if it is steeped for a long time. That is, they are processed the same way that black teas are, but they aren't allowed to oxidize fully.