Definition:
Oxidation in tea is the enzymatic reaction in which polyphenol oxidase (PPO) enzymes in the tea leaf react with oxygen to convert catechins into yellow-orange theaflavins and red-brown thearubigins — progressively browning the leaf and fundamentally changing its chemical composition and flavour profile, and serving as the primary processing variable that distinguishes green teas (unoxidized), oolongs (partially oxidized), and black teas (fully oxidized) from one another. It is halted by heat (kill-green or steaming) in green and oolong teas, and allowed to proceed fully in black tea.
In-Depth Explanation
The biochemical mechanism: Tea leaf cells contain polyphenol oxidase stored separately from the catechins it acts upon. When the leaf is physically damaged — by bruising, cutting, crushing, or rolling — cell walls rupture and PPO contacts the catechin-rich cell contents. In the presence of ambient oxygen, PPO catalyzes the conversion of colorless, astringent catechins (particularly EGCG, EGC, ECG, EC) into the colored compounds that define black tea: theaflavins (bright yellow-orange, responsible for brightness and brisk astringency) and thearubigins (dark reddish-brown, responsible for deep colour and smoothness).
Natural vs. enzymatic oxidation: The tea term “oxidation” technically refers to enzymatic oxidation (PPO-driven), not the purely chemical oxidation by atmospheric oxygen. The two are distinct processes. When oxidation is stopped early (by kill-green), only minimal enzymatic activity occurs. When left to proceed, oxidation continues until PPO is deactivated by the high heat of the final drying step.
Oxidation level and tea type:
- Green tea: Oxidation halted at <5% by immediate kill-green
- Yellow tea: Oxidation halted early, then slow men huang transformation (not oxidation)
- White tea: Very light passive oxidation (5–15%) during withering; no intentional bruising
- Oolong: 15–85% oxidation — the widest spectrum of any category
- Black tea: 85–100% oxidation before drying
Temperature and humidity: Oxidation rate is accelerated by warmth and humidity. Withering rooms for black tea are typically kept at 20–25°C with moderate humidity to enable controlled, even oxidation. Lower temperatures slow the reaction; very high temperatures (above 55°C) begin to denature PPO and halt oxidation prematurely.
Post-fermentation vs. oxidation: True microbial fermentation (as in puerh) is distinct from enzymatic oxidation. The tea industry confusingly uses “fermentation” to describe oolong and black tea oxidation (a Chinese language carryover), and “post-fermentation” or “fermented tea” for puerh — which involves actual microbial action.
See Also
For Japanese tea processing context:
Related Terms
Research
- Owuor, P.O., et al. (2008). The impact of relative humidity and temperature on the enzymatic oxidation quality parameters of black tea. Food Chemistry, 107(1), 214–220.
[Documented that optimal oxidation conditions for black tea lie at 22–25°C and 70–80% relative humidity, with theaflavin content (quality marker) peaking at controlled oxidation endpoints.]
- Sharma, V., et al. (2018). Polyphenol oxidase activity and catechin transformation during green, oolong, and black tea production from the same Assam cultivar. Journal of Agricultural and Food Chemistry, 66(28), 7344–7352.
[Quantified the progressive conversion of EGCG through successive oxidation stages and showed theaflavin/thearubigin ratios as reliable markers of oxidation level.]