Think of an apple when it is cut or bitten into and left out. What happens to the exposed surface? It turns brown eventually. This is oxidation. Tea, as with all leaves, will eventually turn brown if not exposed to heat and dried. So after tea leaves are harvested, they are exposed to heat as soon as possible to make green tea. Or they are left to wilt and oxidize to be made into either an Oolong or Black tea depending on the extent they are allowed to oxidize before they are dried. Exposing tea leaves to high temperature locks in the green freshness and prevents them from the first stage of oxidizing.Oxidation changes the molecular composition of the leaves which results in a broad spectrum of flavor and character.
Is processed with the minimum possible amount of oxidation - virtually none. The freshly picked, usually very young leaves are exposed to high temperatures as quickly as possible to lock in the fresh green qualities and prevent the naturally occurring wilting and coloration process of oxidation. The leaves are then fully dried according to the type of green tea and processing involved. Very few types of green tea are roasted after they are dried, most are unroasted.
Green teas contain the highest amounts of antioxidants and the lowest amounts of caffeine. However, those antioxidant compounds that caffeine coincides with are quite rejuvenating as well, making the effects of caffeine and the molecular compounds it occurs within an interesting enquiry.
Is the broad spectrum of teas between green and black teas. This spectrum is based on the degrees of oxidation and post-production roasting. So, oolongs are partially oxidized (10-75%) leaves that are either roasted after they are dried or not, depending on tea type. The degree of roasting also varies greatly among oolongs, adding a unique character and complexity to each type. Oolong leaves are often larger and more mature than green tea leaves providing substance and flexibility in the ways they are cured. Roasted oolongs are mellowed in their astringency and hence tend to be milder in their effects on the stomach.
Involves nearly full (90-99%) oxidation of the tea leaves, where the leaves undergo repeated rolling as they wilt to produce more even oxidation. After the leaves are properly oxidized they are dried without being exposed to high heat or any further roasting. Black tea leaves range from very young to mature depending on the type, and are mostly left in an open curled form, although tight rolling into a semi-spherical shape is becoming more popular in Taiwan for protection and preservation of the leaves.