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Fresh tea leaves contain an enzyme called polyphenol oxidase (PPO) that oxidises tea polyphenols — the same reaction that reddens black tea. To keep green tea green, makers heat the freshly plucked leaf quickly to switch this enzyme off.
This step is called kill-green (fixation). Enzymes are proteins, and heat denatures them irreversibly — much like a cooked egg cannot return to liquid. Once deactivated, oxidation stops, chlorophyll and fresh-tasting compounds are preserved, and the liquor stays clear green.
How heat deactivates the enzyme
Drag the slider to raise leaf temperature; watch enzyme activity and leaf colour ↓
Schematic, not exact values. Real fixation also depends on moisture, time and leaf thickness.
Common methods are pan-firing and steaming (typical of Japanese green tea). Both aim for the same thing: heat fast and thoroughly to stop the enzyme before the leaf oxidises.
L2 · Deep DiveThe four steps of green tea
PPO and peroxidase (POD) catalyse the enzymatic oxidation of catechins. Rapid heating disrupts these enzymes’ protein structure and deactivates them, halting the oxidation cascade. Leaf temperatures above roughly 80 °C (176 °F), held long enough, are generally taken to deactivate most PPO. (Exact figures vary by study — needs verification with cited sources.)
“Green tea isn’t unchanged — it’s change with the pause button pressed.”
Typical flavour
Pan-fired green tea · flavour sketch
See also
- The Science of Black-Tea “Fermentation” — the same enzyme, the opposite treatment
The full process & parameters
| Step | Parameters | Role |
|---|---|---|
| Spreading | room temp, 4–8 h, ~10–15% water loss | sheds some water, softens the leaf; slight enzyme activity |
| Kill-green ⭐ | pan ~200–300 °C, 1–3 min | heat deactivates PPO/POD, halts oxidation, fixes the green; drives off grassy notes |
| Rolling | light→medium→heavy, 5–30 min | ruptures cells, shapes the leaf, aids extraction |
| Drying | bake / pan / sun, to 5–7% moisture | ends residual reactions, sets shape, develops aroma (Maillard) |
Representative ranges; they vary widely by cultivar, origin and process (synthesised from standard tea-processing texts).
References
- (Add an authoritative tea-biochemistry reference; register it in shared/references.md)