Tea oxidation (氧化, yǎnghuà)
Oxidation (氧化) is the enzymatic reaction of tea-leaf polyphenols with oxygen, darkening the leaf and deepening the taste. Degree of oxidation is the main axis of tea classification: from green (minimal) to red (maximal).
When tea leaf is crushed, torn, or rolled, its cells rupture, and the polyphenols meet the enzyme polyphenol oxidase and the oxygen of the air. Oxidation begins: catechins turn into theaflavins and thearubigins — the substances that give the liquor its coppery color, density, and sweet-fruity depth. The same process browns a cut apple; the tea maker's craft lies in steering it under control.
Degree of oxidation is the main axis of tea classification. Green tea knows almost no oxidation: its enzymes are killed by an early sha qing. White tea, like Bai Mu Dan, oxidizes slightly and unhurriedly, through simple withering. Oolongs are the kingdom of the middle: from light greenness to deep swarthiness, and the famous Dong Fang Mei Ren (Oriental Beauty) gets its honeyed character from leaves nibbled by leafhoppers — the bites trigger oxidation while the leaf is still on the bush. Red teas like Dian Hong are fully oxidized — in the language of wuxing, this is fire passing into earth, into warm bready ripeness. Do not confuse oxidation with microbial fermentation: shu pu-erh is dark not from oxygen but from the microbial work of wo dui.
For the practitioner this axis gives a simple intuition: the lower the oxidation, the more delicate the tea and the cooler the water it needs; the higher, the more boldly you can pour boiling water. And it also explains why the West's "black" tea and China's red tea are the same thing, named respectively for the color of the dry leaf and of the liquor.
Teas where this appears
Chinese tea glossary