Tsuru No. 041 — Why Cool Tea Leaves After Heating?
Tsuru 鶴 Collection
Tsuru No. 041 — Why Are Tea Leaves Cooled After Heating?
When Stopping the Heat Becomes Just as Important as Applying It
The heat has just done its work.
Moments earlier, the leaves were still subjected to a high temperature. Steam, a heated vat, a drum, or a wok: depending on the tea and tradition, the producer has just used heat to profoundly alter the activity of the leaf.
But when the leaves leave the machine or the hot surface, one thing is certain: they do not instantly become cold.
They still carry heat.
And this residual heat can continue to act.
In several green tea manufacturing processes, the leaves are therefore spread out, cooled, or left to rest before rolling. Recent experimental protocols explicitly describe cooling the fixed leaves before they pass through the roller. Other methods, on the contrary, may process the leaves while they are still hot. It is therefore not a universal rule, but a real manufacturing parameter.
Central question:
Why is the temperature of the leaves controlled after fixation, and what can this transition change in tea manufacturing?
Fixation Does Not Exactly Stop at the Machine Exit
Fixation is one of the major steps that define green tea. Its principle is to use enough heat to greatly reduce the activity of the enzymes in the leaf, especially those involved in enzymatic oxidation.
This heating can be achieved in different ways: steam, a heated drum, hot air, or a combination of several techniques. The fixation methods also influence the composition and sensory profile of the resulting tea.
But removing the leaf from the heat source does not mean that its temperature drops immediately.
Like any heated material, it continues for a while to release the accumulated energy.
It is precisely this transition period that the producer must master.
A Leaf That Is Still Hot Remains a Material in Transformation
After fixation, the leaf still contains water and retains a high temperature.
It is therefore not yet a stable product.
Its suppleness, moisture, temperature, and mechanical behavior will continue to evolve before rolling and drying.
Several scientific green tea manufacturing protocols thus use a cooling phase before rolling. In a study on different fixation methods, the fixed leaves were left to cool for one hour before passing through a rolling machine.
Another recent study also describes leaves spread out and cooled for one hour after fixation before rolling.
This shows that cooling can be an integral part of the processing protocol.
Why Spread the Leaves?
When very hot leaves remain clumped together in a mass, their heat dissipates more slowly.
The leaves located in the center can then retain a higher temperature than those located on the surface.
Spreading them out increases the contact surface with the air and thus promotes a more even drop in temperature.
Depending on the equipment, this step can take the form of simply resting at room temperature, spreading on a conveyor belt, or passing through a device that allows the leaf to cool and soften. A study on green teas from Hubei, for example, describes a specific cooling and softening phase immediately after fixation.
Cooling Does Not Mean Stopping All Transformation
This is an important nuance.
Fixation mainly acts on the enzymatic activity of the leaf, but tea manufacturing obviously does not stop there.
Rolling, shaping, and drying will still profoundly modify the plant material.
The thermal steps also have an effect on the aromatic and non-volatile compounds of the tea. Recent research shows that fixation itself constitutes a major transformation phase of metabolites and aromatic compounds.
Cooling is therefore not a chemical switch.
It rather allows controlling the transition between two operations.
Rolling Often Follows Immediately After
After fixation, many green teas are rolled or shaped.
This step changes the shape of the leaves and their physical structure. It also contributes to how they will behave during the following operations and, later, during infusion.
The temperature of the leaf at the time of rolling can therefore be part of the technical parameters of the process.
Some protocols explicitly cool the leaves before this step. But other processes deliberately work the leaves while they are still hot.
A scientific review dedicated to thermal transformations in tea thus reminds us that green tea rolling is frequently performed while the fixed leaves are still hot.
The Essential Point:
the producer is not always trying to get the leaf as cold as possible. They are trying to get the leaf state appropriate for the next step.
There Is Therefore No Single Good Temperature
The temptation would be to turn this operation into a simple rule:
"After fixation, the leaves must be cooled."
That would be too categorical.
Manufacturing methods vary depending on the type of tea, the cultivar, the machines, the water content of the leaf, and the desired style.
In some cases, a prolonged rest before rolling is planned.
In others, rolling follows fixation almost immediately.
What matters is the mastery of the sequence and not the application of a universal recipe.
Cooling and Moisture Are Closely Linked
A hot leaf still contains a lot of water.
Depending on the fixation technique, this moisture can be distributed differently in the tissues or on their surface.
Resting and air circulation can therefore contribute to simultaneously changing temperature, moisture, and suppleness.
A study dedicated to different Chinese green tea cultivars, for example, describes a sequence combining fixation, cooling, and "moisturizing" before rolling, then other cooling phases during shaping and drying.
This alternation reminds us of one essential thing: tea manufacturing is rarely a succession of completely independent blocks.
Each step prepares the next.
Heat Can Also Shape the Taste
Temperature is not only used to deactivate enzymes.
Thermal transformations also contribute to the formation of taste and aromas.
Studies show that the different fixation methods change the composition of non-volatile and volatile compounds in green tea, and thus its sensory profile.
The subsequent steps of drying and roasting then continue these transformations. In some green teas with roasted aromas or reminiscent of chestnuts, the thermal treatments of drying play a major role, in particular, in the formation of characteristic aromatic compounds.
Controlling the heat after fixation therefore also means controlling the sensory trajectory of the leaf.
A Discreet but Revealing Step of Expertise
When observing tea manufacturing, certain operations immediately catch the eye.
The hot wok.
The steam.
The spinning drum.
The rolling of the leaves.
The drying.
But a leaf simply spread out on a mat seems almost inactive.
Yet, this moment can be technically important.
The producer is then observing a material that has just undergone a brutal transformation and must be brought to the desired state before the rest of the journey.
In tea, waiting is not always an absence of action.
It can be a decision.
The Tea Tribunal
"After fixation, the leaves must always be completely cooled before rolling."
VERDICT: FALSE.
Some protocols indeed provide for a cooling phase before rolling. Others work the leaves while they are still hot.
The determining factor is therefore not a universal temperature, but the state of the leaf desired for the next manufacturing step.
The Breath Ritual — Watch the Heat Leave the Leaf
During your next green tea infusion, collect a few leaves immediately after the infusion.
Place them flat on a small plate.
Slowly bring your hand close, without touching them.
You will feel the heat they release for a while longer.
Then observe their texture as they cool.
On another scale, the producer must also manage this transition: the heat source has disappeared, but the leaf is still hot.
What to Remember
After fixation, tea leaves can still retain a significant amount of heat.
Depending on the process, they can be spread out, cooled, or left to rest before rolling.
This step allows, in particular, to control their temperature, moisture, and mechanical behavior.
But complete cooling is not a universal rule: some methods deliberately roll the leaves while they are still hot.
Tea manufacturing therefore relies less on a succession of fixed recipes than on the precise mastery of the state of the leaf at every moment.
Because knowing how to heat a leaf requires expertise.
Knowing what to do with this heat next requires just as much.
To Continue the Journey
→ Tsuru No. 038 — Why Are Tea Leaves Heated?
→ Why Are Tea Leaves Rolled?
→ The Withering of Tea
Sources & Verifications
This article is based on scientific publications dedicated to fixation, cooling, rolling, and thermal transformations occurring during green tea manufacturing.
- Effects of electromagnetic roller-hot-air-steam triple-coupled fixation on green tea — protocol describing cooling of fixed leaves before rolling.
- Insights into the effects of fixation methods on the sensory quality of green tea — comparison of fixation methods with cooling before rolling.
- Non-volatile and volatile metabolite analyses and the effect of fixation methods on green tea — study of the impact of fixation methods and the processing protocol.
- Comprehensive applications of metabolomics on tea science and technology — scientific review of thermal transformations of tea, specifying in particular that rolling can be performed on leaves that are still hot.
- Influence mechanism of fixation on the development of green tea flavor — work on the effect of different fixation methods on the compounds responsible for taste and aromas.
