Tea oxidation (氧化, yǎnghuà)
In tea production, oxidation usually refers to reactions involving leaf compounds, oxygen and the plant’s own enzymes. These change colour and flavour; processing controls how they develop.
Rolling or shaking damages leaf tissue, bringing substances that were separated within cells into contact. Catechins form other compounds, including theaflavins, which contribute to black tea’s colour and taste. This does not account for every aspect of aroma.
Green tea is heat-fixed early; oolong undergoes partial oxidation, and black tea more extensive oxidation. White tea also changes during withering. The six categories cannot be placed on a single precise percentage scale: yellow tea involves men huang, while dark teas involve particular microbial processes.
Tea terminology often calls oolong and black tea oxidation “fermentation.” This differs from microbial activity in wo dui, although the processes can interact. Calling black tea “fully fermented” does not mean every compound is literally 100% oxidised.
Oxidation level cannot tell you the caffeine content or one correct water temperature. Cultivar, leaf size, roast, dose and time also matter. A recipe for the particular tea, adjusted to taste, is more useful for brewing.
Related teas
Chinese tea glossary