Tea Oxidation

The same plant, Camellia sinensis, is used to produce all types of tea, and the differences among the types come from the various processing steps that are applied. Tea is traditionally classified based on the degree or period of oxidation or fermentation the leaves have undergone.

Oxidation levels vary among teas. Based on the degree of oxidation, tea can be classified as:

Green tea 绿茶 (lǜ chá)

Yellow tea 黄茶 (huáng chá)

White tea 白茶 (bái chá)

Oolong tea 乌龙茶 (wū lóng chá) (semi-fermented tea)

Red tea 红茶 (hóng chá)(or Black tea in Western countries, fully fermented tea)

Dark tea 黑茶 (hēi chá) (or Aged tea, post-fermented tea).

Green tea has the lowest and Black tea (Red tea) has the highest levels of oxidation. Oolong lies between Green and Black tea, is partially oxidized.

Tea oxidation refers to a series of chemical reactions which turn the leaves brown. It is caused by air contact with enzymes in the tea leaves.  Much of the oxidation process revolves around polyphenols and the enzymes polyphenols - oxidase and peroxidase. A chemical reaction begins when the cells inside tea leaves are damaged and the components inside are exposed to oxygen and mix. This reaction converts the polyphenols known as catechins into natural flavonoids called theaflavins and thearubigins. Thearubigins and theaflavins influence the leaf and liquor color, taste, flavor, and aroma of tea. Theaflavins provide tea with its astringency, lively taste, and its yellow color. Thearubigins are red in color and are responsible for much of the staining effect of tea providing it with depth and body.

Depending on the type of tea being made, oxidation is prevented altogether (Green tea), or deliberately initiated, controlled and then stopped (Oolong tea). Tea producers use special methods and may choose when the oxidation should be stopped, which depends on the desired qualities in the final tea as well as the weather conditions (heat and humidity). For light Oolong teas, this may be anywhere from 5-40% oxidation, in darker Oolong teas - 60-70%, and Red tea (or Black tea in Western countries) is 100% oxidized.