A freshly picked tea leaf is still active plant material. Its cells contain enzymes, polyphenols, amino acids, pigments, and many aromatic precursors.

As long as nothing interrupts this activity, the leaf continues to change.

However, when making green tea, the producer specifically aims to prevent significant enzymatic oxidation from transforming the leaf as it would in black tea.

Therefore, they must act quickly.

This intervention is based on a simple idea: use heat to significantly reduce the activity of the enzymes responsible for oxidation.

In China, this process is often referred to as Sha Qing, which is sometimes translated as “killing the green.” In Japan, fixation is very often done with steam.

Behind this technical step lies an essential part of green tea’s identity.

1. A fresh leaf continues to react after harvest

Picking a leaf does not instantly stop all biological activity.

After harvest, various processes continue within the plant tissues.

These include reactions involving enzymes such as polyphenol oxidase and peroxidase.

When cells are damaged and their components come into contact with oxygen, these enzymes help oxidize the phenolic compounds present in the leaf.

This is a fundamental process in making certain families of tea.

In black tea, these reactions are deliberately allowed to develop after withering and rolling.

In green tea, the opposite is true: the producer wants to limit them very early on.

2. Heat is primarily used to inactivate enzymes

Enzymes are proteins capable of accelerating certain chemical reactions.

However, their activity depends on specific conditions.

When exposed to a sufficiently high temperature, their structure can be altered to the point that they no longer function normally.

This is the phenomenon the producer exploits during fixation.

By quickly heating the leaves, they significantly reduce the activity of polyphenol oxidase and other enzymes involved in enzymatic browning.

This step largely explains why green tea retains more unoxidized catechins than black tea.

Scientific literature also describes fixation as one of the most characteristic steps in making green tea.

3. “Stopping oxidation” is a practical, but imperfect, phrase

It is often said that fixation “stops oxidation.”

To understand the principle, this phrase is useful.

But scientifically, it requires some nuance.

Heating is primarily intended to significantly inactivate the enzymes responsible for enzymatic oxidation.

This does not mean that no further oxygen-related chemical reactions can ever occur in the tea.

During processing, drying, and storage, other reactions can still occur slowly.

The final product can also change due to moisture, oxygen, temperature, or light.

4. Why is this step crucial for green tea?

Green tea is prized for characteristics very different from those of black tea.

Depending on its origin and processing, it can feature:

• fresh vegetal notes;
• grassy sensations;
• marine or saline notes;
• umami;
• hints of nuts;
• roasted notes;
• floral or slightly sweet characteristics.

To preserve a profile close to that of a fresh leaf, the producer must prevent enzymatic transformation from taking the same path as that of black tea.

Fixation therefore acts as a kind of fork in the road.

Before it, the raw material remains open to several possibilities.

After it, the tea resolutely enters the realm of green tea.

5. Steam or dry heat: two major traditions

Not all fixations are the same.

Two major methods are particularly well known: steam and heating in a pan or pan-firing.

Steam

This method is emblematic of many Japanese green teas.

The leaves are briefly exposed to hot steam.

The heat transfer is rapid and quickly inactivates the enzymes.

This method generally helps preserve highly vegetal, fresh, marine, or umami profiles, depending on the cultivar and tea style.

Pan-firing

This is particularly associated with the Chinese tradition.

The leaves are heated by coming into contact with a hot surface, either by hand or in specialized machines.

This heating is not only used to inactivate enzymes: it also influences the aromas.

The resulting teas can develop notes of chestnut, grain, nuts, or cooked vegetables.

Studies comparing different processes do indeed show that the fixation method influences the non-volatile composition and sensory profile of green teas.

6. Sha Qing: “killing the green” does not mean burning the leaf

The Chinese term Sha Qing can sound intimidating.

It is often translated literally as “killing the green.”

But it obviously does not mean that the producer aims to carbonize the leaf.

The goal is to stop its enzymatic activity quickly enough while preserving the components that interest the producer.

It is therefore a thermal compromise.

Not hot enough or not for long enough: inactivation may be insufficient.

Too hot or for too long: some heat-sensitive compounds may be degraded, and the aromas may become overcooked.

Studies on green tea production emphasize precisely this control of the time-temperature combination.

7. What would happen without fixation?

Consider a leaf intended to become a green tea.

If left too long in conditions that promote enzymatic activity, its profile will gradually begin to diverge from that desired for this family.

The catechins may be oxidized.

The color of the leaf may change.

The aromatic profile will change.

If the leaf is also rolled or heavily bruised, the rupture of the cells makes it easier for enzymes and substrates to meet.

This is precisely the principle exploited during the production of black tea.

Fixation therefore prevents the future green tea from continuing down this path.

8. Why does rolling often come next?

After fixation, the leaves can be rolled or shaped.

The producer can then manipulate the tissues more vigorously without causing the same level of enzymatic oxidation as if the enzymes were still fully active.

Rolling notably gives the tea its shape and alters the physical structure of the leaves.

This will then influence how water penetrates the tissues when the tea is brewed.

You can explore this step further in: Tsuru No. 010 — Rolling Tea: Why Are the Leaves Rolled?

9. Fixation also affects the aromas

It would be a mistake to consider fixation as a simple enzymatic switch.

Heat also changes the material itself.

Some volatile compounds may disappear, while others appear or change in concentration.

The intensity and method of heating thus contribute to the final aromatic style.

Pan-firing, for example, can bring out warmer or roasted notes, while steam fixation may favor a more vegetal expression.

This distinction partly explains why two green teas made from similar cultivars can taste very different in the cup.

Processing matters as much as the raw material.

10. And what about the green color?

Color is obviously one of the most visible signs.

During tea processing, chlorophylls and other pigments can be altered by heat, acidity, and chemical reactions.

Properly controlled fixation helps preserve a greener character than that of a leaf left to oxidize heavily.

But once again, “greener” does not automatically mean “better.”

Certain techniques naturally produce yellower, more matte, or darker leaves.

Color must therefore always be interpreted in relation to the style of tea.

11. Are catechins preserved?

For the most part, yes.

One of the major consequences of enzymatic inactivation is that the catechins present in the leaves are far less transformed by oxidative enzymes than in black tea.

This explains why the polyphenolic profiles of green teas generally remain closer to those of fresh leaves.

But another simplification must be avoided: heat does not “preserve everything.”

High temperatures can themselves alter or degrade certain compounds.

Once again, expertise consists of finding a compromise.

12. Are all green teas fixed in the same way?

Absolutely not.

The duration, temperature, moisture of the leaf, type of machine, quantity of material, and heating method vary widely.

Some teas are fixed with steam.

Others are tossed in heated pans.

Some pass through drums.

The producer adapts the protocol to the raw material and the desired result.

Recent research even shows that different fixation techniques produce different profiles of catechins, amino acids, and other non-volatile compounds.

13. Japan and steam

Japan is particularly associated with steam fixation.

For many Senchas, freshly harvested leaves are quickly steamed to inactivate the enzymes.

They are then cooled, rolled, and dried through several successive operations.

Depending on the steaming time, the profile can change.

Some teas are very lightly steamed, while others undergo more intensive steaming.

This influences the structure of the leaf, the color of the infusion, and how the tea behaves in water.

14. China and dry heat

In many Chinese traditions, fixation is done by contact with a hot surface.

Historically, the work could be done by hand in large woks.

Today, machines also allow for control of the temperature and the movement of the leaves.

This method can give tea particularly distinctive aromatic characteristics: chestnut, seed, grain, hazelnut, or warm vegetal.

It is therefore not just about “blocking” something.

The producer is already beginning to build a profile.

15. And Oolongs?

Fixation also plays an important role in many Oolongs.

But the logic is different.

In this case, the producer first allows partial oxidation to occur, often through phases of withering and tossing the leaves.

When the desired level is reached, heat is then used to significantly slow enzymatic activity.

Fixation therefore occurs later than in green tea.

This helps explain why Oolong is often described as an extremely broad family: the producer precisely chooses how far to let the leaf develop before fixing its state.

16. And black tea?

In classic black tea production, the opposite is sought: to fully exploit enzymatic oxidation before final drying.

The leaves are withered, then rolled or bruised to bring enzymes and polyphenols into contact.

The catechins are then transformed into more complex compounds, including theaflavins and thearubigins.

These transformations contribute to the coppery color, body, and aromatic profile of black tea.

The final heating then serves more as a drying and stabilization step.

17. Fixation and drying: they are still not the same thing

As with withering, the technological objectives must be distinguished.

Fixation primarily aims to significantly reduce enzymatic activity.

Final drying primarily aims to reduce the moisture sufficiently to stabilize the tea and allow it to be stored.

Both steps use heat, which explains the confusion.

But they do not occur at the same time and do not serve exactly the same function.

To explore further: Tsuru No. 011 — Why Are the Leaves Dried and Roasted?

Key Takeaways

Heating the leaves immediately after harvest is one of the fundamental steps in making green tea.

This process is called fixing.

Its main purpose is to strongly deactivate enzymes, particularly polyphenol oxidase, which would otherwise contribute to the rapid transformation of polyphenols.

This allows green tea to retain more unoxidized catechins and a sensory profile different from that of black tea.

Fixing can be done in several ways, notably with steam or by heating in a vat.

These methods are not interchangeable: they profoundly influence the aromas, the structure of the leaf, and the final character of the tea.

And above all, fixing must not be confused with drying.

A few seconds of heat can determine
the entire direction a leaf will take.

Continue the Journey

Start with the step that often precedes the major transformations: Tsuru No. 037 — Withering Tea.

Then discover: Tsuru No. 010 — Rolling Tea.

Then: Tsuru No. 011 — Drying and Roasting.

And to understand what will ultimately end up in your cup: Tsuru No. 035 — What Really Extracts During Steeping.