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Fixing (kill-green) - how oxidation is stopped and why green tea stays green

The fact that green tea is green, and not brown like black tea, is no accident nor a matter of a different plant. It is the effect of one decisive step: fixing, that is the stopping of oxidation, in English kill-green. At this moment heat deactivates the enzymes in the leaf, before they can oxidise and brown it. It is precisely fixing that sets green tea apart from oolong or black. But there are two main ways to do it: frying the leaf in a hot wok or subjecting it to steam. Each gives a different flavour: Chinese pan-fired is roasty and nutty, Japanese steamed grassy and marine. Here is a guide to fixing: what it is, how it stops oxidation and how frying differs from steaming.

What fixing is

Fixing is a step in the production of green tea whose aim is to prevent oxidation, the process called kill-green, literally killing the green. It is what sets green tea apart from oolong and black. It consists of subjecting the leaf to heat, which deactivates, that is disables, the enzymes responsible for oxidation. These enzymes, woken earlier during withering, would normally begin to oxidise the leaf, browning it and changing its flavour. Fixing stops this process before it can develop, fixing the green, fresh character of the leaf. This is why green tea stays green. Understanding that fixing is the deliberate stopping of oxidation by heat is the starting point for all the rest: the two methods of doing it and their effect on flavour. We cover oxidation itself more in tea oxidation.

How heat stops oxidation

At the heart of fixing is the deactivation of the enzymes by heat. In the fresh, withered leaf there are enzymes which, in contact with oxygen, start oxidation, that is the reactions that brown the leaf and change its composition. These same enzymes are responsible for oolong and black tea being darker and having a different flavour. Fixing heats the leaf enough to disable these enzymes, that is to destroy their ability to act. When the enzymes are dead, oxidation can no longer occur, so the leaf stays green and fresh. The sense of timing is key here: fixing must come at the right moment, to stop oxidation where the maker wants. In green tea it is done early, so that oxidation does not start at all. This shows that fixing is a precise procedure, in which heat becomes a tool for stopping a natural process.

Fixing in the chain of processing

Fixing does not act in a vacuum but is part of a chain of processing. Before it the leaf undergoes withering, in which it loses water and its enzymes wake. After withering, in the case of green tea, comes precisely fixing, which disables these enzymes before they can start oxidation. Only then come the later steps, like rolling and drying. It is the order and the moment of fixing that decide what kind of tea is made: early fixing gives green, no fixing and full oxidation gives black, and intermediate approaches give oolong. We cover the first step more in tea withering. Fixing is therefore a turning point in processing, at which the maker decides whether and how much the leaf will oxidise. It is the moment that separates the roads leading to different kinds of tea, key especially for green.

Two ways: frying and steaming

Fixing can be done in two main ways, giving completely different flavours: hot frying or steam. In China the standard is pan-firing, that is frying the leaves in a hot wok, where the heat of the dry, heated metal disables the enzymes. In Japan steaming dominates, that is passing the freshly picked leaves through hot steam, usually for a dozen-odd to two hundred seconds depending on the style, where it is the hot steam that deactivates the enzymes. Both ways reach the same goal, the stopping of oxidation, but by a completely different road: one by dry heat, the other by moist steam. And it is precisely this difference of method that translates into two distinct styles of green tea. This is why Chinese and Japanese green taste so different, even though both are green. The way of fixing is one of the main differences between these two great traditions of tea.

A table: pan-fired versus steamed

Let us gather the two methods in one place:

The table shows the heart of the difference: the dry heat of the wok gives roasty and nutty notes, and the moist steam gives fresh, grassy and marine notes. They are two different faces of green tea.

The flavour of pan-fired

Chinese pan-firing, that is frying in a wok, gives the tea a characteristic roasty flavour. The dry heat of the heated metal not only disables the enzymes but also lightly roasts the leaf, giving nutty, toasty notes, sometimes resembling roasted chestnuts or grain. This is why many Chinese green teas, like the famous Longjing, have a warm, roasty, nutty character. Frying also affects the aromatic compounds of the leaf, increasing the share of certain terpenes, which enriches the aroma. Pan-fired green is usually less grassy and more warm and sweetish in profile. It is the signature of the Chinese school of green tea. The dry heat gives the leaf a character that steam would never give. The roasty, nutty flavour is the signature of wok fixing, far from the fresh grassiness of steamed teas.

The flavour of steamed

Japanese steaming gives a completely different flavour: fresh, grassy and marine. The hot steam quickly disables the enzymes, keeping the intensely green, vegetal character of the leaf. This is why Japanese green teas, like sencha, have a clear, grassy, almost marine profile, far from the roasty Chinese notes. Steaming also keeps a deeper green of the leaf and the infusion, giving a characteristic, juicy colour. Interestingly, steaming lowers the content of catechins and changes the profile of aromatic compounds differently from frying. This is why steamed teas tend to be a little more astringent and clearly fresher in flavour. The grassy, marine, intensely green character is the signature of the Japanese school. Steam keeps the raw freshness of the leaf, while the wok transforms it. They are two philosophies of fixing, giving two different ideals of green tea.

Why it matters for the flavour

The way of fixing is one of the main reasons Chinese and Japanese green tea taste so different, even though both are green and come from the same plant. It is not the variety or the origin, but precisely the method of stopping oxidation that largely divides these two styles. Frying gives warmth, roastiness and nuts, steaming freshness, grass and the sea. This shows how enormous an influence on the final flavour a single step of processing has. We cover Japanese green teas more in Japanese green tea. Fixing is not only a technical necessity to stop oxidation, but also the moment at which the maker gives the tea its character. Understanding this step is the key to grasping why two green teas can be so different.

How to sense it in the infusion

The way of fixing is easy to sense by comparing teas. Green tea of a warm, roasty, nutty, toasty character, often lighter in the infusion, is usually Chinese pan-fired. Green of a fresh, grassy, vegetal, almost marine flavour and an intensely green infusion is Japanese steamed. If a tea smells of roasted chestnuts and nuts, it is a sign of the wok; if of fresh grass, spinach and the sea, it is a sign of steam. It is worth brewing Chinese Longjing and Japanese sencha side by side, to feel the gulf created by fixing alone. We cover the whole process of making tea more in how tea is made. Over time you will start to recognise the method of fixing by flavour alone, which is a higher level of understanding tea. It is a step you can hear in every sip of green.

The essentials in brief

Let us gather it up. Fixing, that is kill-green, is the step that stops oxidation in the production of green tea. Heat deactivates the enzymes responsible for oxidation, before they can brown the leaf, so green tea stays green. It is what sets it apart from oolong and black. Fixing comes after withering, and its moment decides the kind of tea. It can be done in two ways: Chinese pan-firing, that is frying in a wok, gives a roasty, nutty flavour, and Japanese steaming gives a fresh, grassy, marine character and an intense green. This is why Chinese and Japanese green taste so different, even though both are green. Now you know how oxidation is stopped and why the way of fixing shapes the flavour of tea so much.

Note every tea in GustoNote - the kind, the method and the character you sense. Over time you will start to recognise pan-fired and steamed by flavour, and understand more deeply how fixing shapes green tea.