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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 [1]. 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 [1][2].
L2 · Deep DiveThe green-tea process
Fixation is the master switch for green tea quality. 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; the actual degree of inactivation depends on the heating method. Studies show PPO is more heat-resistant than POD, and after steam fixation some activity may remain, with final drying (100–120 °C / 212–248 °F for 60–90 min) completing the inactivation[1][2].
“Green tea isn’t unchanged — it’s change with the pause button pressed.”
Biochemistry: how heat silences the enzymes
The enzymatic oxidation cascade runs as follows[4]:
- Substrates: fresh leaves are rich in catechins (EGCG, ECG, EGC) and simple phenols.
- Catalysis: PPO uses molecular oxygen to oxidise catechins to o-quinones; POD can also participate when H₂O₂ is present.
- Polymerisation: quinones are highly unstable and polymerise into theaflavins, thearubigins and even theabrownins — the pigments that darken black and oolong teas.
- Fixation intervention: high heat (pan-firing leaf temperatures 80–100 °C; steaming at 100 °C) rapidly disrupts the PPO/POD structure, inactivating the active site and truncating the oxidation chain within minutes[1].
Öztürk et al. (2016) showed that fixation reduces PPO activity by over 90% and POD activity even more; steam fixation leaves more residual PPO than pan-firing, indicating that dry-contact heat is more effective at complete deactivation[1].
Key compound dynamics
| Compound | Fresh leaf | Change after fixation | Quality significance |
|---|---|---|---|
| Catechins (EGCG, etc.) | High | Slight thermal degradation, but mostly preserved | Freshness, astringency, antioxidants |
| Chlorophyll | High | Partly converted to pheophytin | Determines leaf and liquor greenness |
| Amino acids | Medium | Slightly increased by mild protein hydrolysis | Freshness, sweetness |
| Soluble sugars | Medium | Stable or slightly increased | Sweetness, body |
| Grassy volatiles (hexanal, leaf alcohol) | High | Largely driven off | Removes green/grassy off-notes |
| PPO/POD activity | High | Sharply reduced | Halts enzymatic oxidation |
Reference: Qiu et al. (2022) found that after fixation chlorophyll, protein and lipid metabolites decrease, while amino acids, soluble sugars and glycosidic aroma precursors increase, laying the biochemical foundation for green tea’s freshness[3].
Parameters and quality defects
These are common ranges, adjusted in practice for leaf condition, equipment and style[5].
| Step | Parameters | Role |
|---|---|---|
| Spreading | room temp, 4–8 h, ~10–15% water loss | sheds some water, softens the leaf; slight enzyme activity |
| Fixation | pan ~200–300 °C, leaf temp 80–100 °C, 1–3 min [1][2] | 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 [1] | ends residual reactions, sets shape, develops aroma (Maillard) |
Under-fixation causes:
- residual enzyme activity → red edges, dull yellow liquor;
- grassy volatiles not removed → green/grassy aroma;
- too much moisture → storage-related staleness.
Over-fixation causes:
- excessive chlorophyll degradation → grey-yellow dry leaf, yellow-brown liquor;
- scorched edges → smoky/burnt off-notes;
- loss of amino acids and sugars → reduced freshness.
Representative styles
| Style | Fixation method | Key difference | Flavour direction |
|---|---|---|---|
| Longjing (pan-fired) | high-temperature pan-firing | tossing, pressing, covering motions carry leaf temperature quickly past the enzyme optimum | toasted chestnut, fresh and sweet |
| Enshi Gyokuro (steamed) | steam | even steam penetration, gentler temperature rise | seaweed-like, vivid green |
| Huangshan Maofeng (baked) | pan-fired then baked | drying dominated by baking with a gentle gradient | clear high aroma, fresh and mellow |
| Japanese sencha | steam + strong firing/rolling | deep steaming gives a deeper green leaf and liquor | marine, umami-forward |
The steamed-vs-pan-fired aroma divide has a molecular footnote: the “seaweed” note of steamed green tea comes largely from dimethyl sulfide (DMS), produced as S-methylmethionine breaks down in the moist heat of steaming; the strong heat of pan-firing and roasting drives most DMS off, which is why the seaweed note is faint in pan-fired green teas[4].
Flavour sketch
Green tea · flavour sketch
Tea Science · core comparison
| Dimension | Green tea (fixation) | Black tea (oxidation) | Oolong (shaking/withering) |
|---|---|---|---|
| Transformation pattern | Early heat suppresses enzymatic oxidation | Extensive enzymatic oxidation (not a universal measured percentage) | Controlled, local and partial enzymatic oxidation |
| Dominant force | heat deactivation of enzymes | PPO/POD enzymatic oxidation | mechanical damage + enzymatic oxidation |
| Core products | preserved catechins, chlorophyll | theaflavins, thearubigins | floral terpenes, some theaflavins |
| Liquor colour | clear green | bright red | golden to orange |
For the full six-tea-class comparison, see The Science of Tea: From Leaf to Six Tea Types.
References
- Öztürk, B., et al. (2016). Change of enzyme activity and quality during the processing of Turkish green tea. LWT-Food Science and Technology, 65, 318–324. (PPO is more heat-resistant than POD; steam fixation only partially inactivates PPO, with final drying completing the inactivation.)
- Ge, Q., et al. (2011). Green tea fixation by simultaneous microwave and hot air treatment. Food Science, 32(12), 196–199. (1200 W microwave + 50 °C hot air for 60 s almost completely eliminates PPO activity.)
- Qiu, Y., et al. (2022). Non-targeted metabolomics reveals dynamic changes in green tea during fixation. Food Chemistry, 385, 132648. (After fixation amino acids, soluble sugars and aroma precursors rise while chlorophyll and lipids fall.)
- Wan, X.-C. (2003). Tea Biochemistry (3rd ed.). China Agriculture Press. (Classic textbook account of green tea fixation enzymology and quality formation.)
- Shi, Z.-P. (Ed.). Tea Processing Science. China Agriculture Press. (Systematic treatise on green tea processing parameters and equipment operation.)