Why does fixation require timely heating?

Start with the essentials

Three key points

1. Heat manages reaction progressTemperature and duration work with leaf size, moisture and loading.
2. Equipment and leaf temperatures differContact and transfer determine how a heat source affects material.
3. Later stages still matterShaping and drying follow oxidation control to complete the product.

Connect the process with the tea

The 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. When leaf temperature enters the high-heat zone above roughly 80 °C (176 °F) and exposure is sufficient, most PPO activity falls rapidly; the result still depends on heat transfer, leaf moisture and uniformity. In the study cited here, some PPO activity remained after steam fixation, and later drying reduced residual activity further[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]:

1. Substrates: fresh leaves are rich in catechins (EGCG, ECG, EGC) and simple phenols.
2. Catalysis: PPO uses molecular oxygen to oxidise catechins to o-quinones; POD can also participate when H₂O₂ is present.
3. Polymerisation: quinones are highly unstable and polymerise into theaflavins, thearubigins and even theabrownins — the pigments that darken black and oolong teas.
4. 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].

Enzyme characteristics and heat-inactivation kinetics

PPO and POD are metalloprotein oxidoreductases whose activity depends on temperature and pH[4]:

• Optimum temperature. Leaf PPO works best around 25–35 °C; activity drops quickly above 60 °C and most is lost above 80 °C.
• Optimum pH. PPO is most active around pH 5.0–7.0. Fresh leaf sap is naturally acidic, so once the cell is damaged the enzyme is already in a favourable range — which is why bruised leaf reddens so fast.
• Heat stability is condition-dependent. Leaf source, isoenzyme and assay conditions change the apparent heat stability of PPO and POD. In the Turkish green-tea study cited here, PPO was harder to eliminate completely; production guidance should not assume one universal order of inactivation.

Heat inactivation is not instantaneous — it follows an exponential decay as temperature and time increase. In pan-firing the pan may be 200–300 °C, but that is not the leaf temperature. Leaves touch the hot metal for only a few seconds before being tossed and cooled, so the actual leaf temperature rises quickly to roughly 80–100 °C. If the batch is too large or tossing is uneven, some leaves will be under-heated and “under-fixed”[2][5].

Heat-transfer comparison: pan-firing, steaming, drum and microwave

Different fixation methods differ in how heat reaches the leaf, and that directly affects enzyme inactivation, colour and aroma[1][2][5].

• Pan-firing. Direct contact with a hot metal surface drives off low-boiling grassy volatiles and triggers Maillard and sugar-amino-acid reactions, producing pyrazines and pyrroles — the source of Longjing’s chestnut/butter-bean note.
• Steaming. Steam transfers heat evenly and quickly, so chlorophyll is preserved better and the liquor stays greener. The moist heat also breaks down S-methylmethionine into dimethyl sulfide (DMS), giving steamed teas their characteristic seaweed-like note.
• Microwave. Polar molecules (mainly water) oscillate and generate heat throughout the leaf, giving fast, volumetric enzyme inactivation. Used alone it can under-develop aroma, so it is usually combined with hot air[2].
• Drum / continuous fixers. The main route for mass-market green tea: temperature, rotation speed and airflow are controllable, giving consistency but usually less aromatic finesse than hand pan-firing.

The colour science: why fixation “keeps green”

Green tea’s colour comes from chlorophyll. Fresh leaves contain chlorophyll a and chlorophyll b; their ratio sets the green tone. Under moist heat or acidic conditions the magnesium ion at the centre of the chlorophyll molecule is replaced by hydrogen, forming pheophytin, which shifts colour from fresh green to brown-olive. If the ester bond is also hydrolysed, pheophorbide forms, giving a more yellow-brown hue[4].

During fixation:

• Rapid heating inactivates the enzymes and reduces enzymatic browning.
• Prompt cooling (the tossing motion in pan-firing) prevents the leaves from stewing in their own heat, limiting pheophytin formation.
• Moisture control matters: after fixation the leaves are still hot, and if they are piled before cooling, residual heat drives further non-enzymatic reactions that yellow the leaf.

So “keeping green” is not just a colour issue — it is the result of temperature, time and cooling working together.

Fixation and drying as a relay

Fixation does not do the whole job. Three reasons why drying is still essential:

1. Residual activity can remain. Short fixation does not necessarily eliminate PPO and POD at the same rate or completely. Later drying reduces residual activity further, with the extent depending on the tea and drying conditions.
2. Moisture remains high. After fixation the leaves still contain about 55–60% water, which is enough medium for residual enzyme activity and microbial growth. Drying brings moisture down to 5–7%, truly “locking” the tea.
3. Aroma needs drying to develop. Fixation removes grassy notes and preserves the green base; the finer chestnut, floral and baked notes develop during drying through glycoside hydrolysis and Maillard reactions.

Think of fixation as pressing the shutter and drying as developing and fixing the photograph — both are needed for a lasting image.

Key compound dynamics

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].

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

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].

Fixation method, residual enzyme activity and aroma

This table reminds us that there is no single “correct” temperature — only a choice matched to the raw leaf and the later steps. Tender buds fear high heat; mature leaves need more heat to fix through.

Common misconceptions

1. “Fixation means drying the leaf.” Fixation only inactivates enzymes and partly removes water; rolling and drying still follow. Drying is what actually makes the leaf shelf-stable.
2. “The hotter the pan, the better.” A hot pan does not equal hot leaves; blind high heat scorches the surface while the inside remains under-fixed.
3. “Steaming is always better than pan-firing.” The two methods point in different aromatic directions: steaming gives green freshness, pan-firing gives chestnut depth. Neither is universally superior.
4. “After fixation the leaf cannot oxidise any more.” Under-fixation or later damp storage can leave enough active enzyme to cause colour and aroma deterioration.

See also

• Leaf chemistry — what is in the leaf before fixation
• Longjing green tea — the fixation science behind Longjing’s chestnut note
• Huangshan Maofeng — a baked green tea’s fixation and drying balance

Flavour sketch

Tea Science · core comparison

For the full six-tea-class comparison, see The Science of Tea: From Leaf to Six Tea Types.

References

1. R-060 — 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. https://www.sciencedirect.com/science/article/abs/pii/S0023643815300852
2. R-061 — 葛庆丰 等 (2011). 绿茶微波热风耦合杀青工艺研究. 食品科学, 32(12), 196–199. https://www.spkx.net.cn/EN/abstract/abstract23448.shtml
3. R-029 — 宛晓春 主编 (2003).《茶叶生物化学》(第3版). 中国农业出版社. ISBN: 978-7109066274.
4. R-031 — 施兆鹏, 夏涛 主编 (2016).《茶叶加工学》(第3版). 中国农业出版社. ISBN: 978-7109214347.
5. R-203 — Wang, Y., et al. (2021). Targeted metabolomics reveals dynamic changes during the manufacturing process of Yuhua tea, a stir-fried green tea. Beverage Plant Research, 1, 6. https://doi.org/10.48130/bpr-2021-0006