Kill-green

Kill-green is the heat that stops oxidation. The Chinese name is shaqing (杀青), which translates as killing the green and it describes what the heat does to the enzymes rather than to the colour: the colour is what gets preserved. English also calls it fixing, de-enzyming, denaturing or firing. When it happens, or whether it happens at all, is the hinge the six tea types turn on.

What it is

Denaturing two enzymes with heat. Polyphenol oxidase and peroxidase are proteins folded into a particular shape and heat unpicks that shape until they no longer work, in the same way that heat turns a clear egg white into a white solid. The leaf has to reach roughly 66°C (150°F) before oxidation is brought to a near-halt. Equipment runs far hotter than that, because the job is to carry the whole leaf past the threshold quickly and evenly rather than to hold it at the minimum. Firing generally starts above 85°C and runs from thirty seconds to five minutes. Oolong goes far hotter, and the reported drum temperatures spread rather than settle: 204-260°C for one to five minutes at the cooler end and 300°C for five to seven at the hotter.

What it does to the leaf

It freezes the chemistry where the maker left it. Catechins that have not converted stay catechins, so the leaf keeps its green colour and its vegetal character and everything downstream becomes shaping and drying rather than change. The domestic version of this is apple pie: a cut apple browns and a baked one does not, because the oven denatured the same family of enzymes. The heat does a second thing on the way past. Sugars and proteins transform under it, which is where a pan-fired green tea gets its roasted, nutty edge, and it is the reason the same operation can both stop a flavour and start one.

How it's done

Four common methods and three less common ones. Pan firing heats the leaf in a metal pan or wok over gas or wood, in small batches, pressed down and constantly stirred so nothing scorches. Steaming forces steam through the mass of leaf and is the Japanese default, at 20 to 80 seconds: a short steam gives the asamushi style, a long one the fukamushi. Tumblers heat the leaf in a rotating drum, which is what most modern oolong passes through. Baking uses an oven-type machine. The three less common ones are sun drying, where dehydration does the denaturing; microwaving, mostly a commercial technique; and plunging the leaf into boiling water.

What it tastes like

The method is audible in the cup. Steaming gives a bright, vivid green colour and a marine, grassy flavour. Pan firing gives a duller green and a nuttier, toastier one. It is the cleanest single fork in green tea and worth tasting side by side once, because a sencha and a Long Jing never sound alike afterwards. Timing matters as much as method: called too early the tea stays raw and grassy, called too late it has already moved toward oolong and makers judge the moment by smell rather than by instrument.

What people get wrong

Reading fixed as sealed. Heat brings oxidation nearly to a stop and does not end it: above the halting temperature the reaction continues at a crawl and drying slows it further without closing it. The second mistake is hearing kill-green as an equipment temperature. What matters is the temperature inside the leaf, which is why thick leaf, thin leaf and a large batch each need a different time, and why this is the operation most sensitive to the maker's judgment.

Where the boundary blurs

For a fully oxidised tea the drying does this job and there is no separate kill-green at all. Most black tea and much dark tea never gets a shaqing. Once the maker is satisfied with the oxidation the leaf goes to the dryer, the heat denatures the enzymes on the way to shelf stability and the halt happens inside a step named for something else. That is why a black tea's chain has no kill-green row: not because nothing stopped the reaction, but because drying stopped it. Hand-made Chinese green blurs the boundary from the other side, where the fixing and the shaping happen in the same pan at the same time.

Styles where this step is distinctive

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teabert, the tealytics teapot, keeper of the kettle

Buy a sencha and a Long Jing and brew them side by side once. Steamed against pan-fired is the loudest fork in green tea and you never stop hearing it afterwards.

Sources

kill-green derives from the Chinese shaqing (杀青), 'killing the green'; the four common methods are pan firing, steaming, tumblers and baking, and the less common ones are sun drying, microwaving and plunging in boiling water Gebely, Tea: A User's Guide (2016), ch. 4
the leaf must reach roughly 150°F (66°C) to halt oxidation, and above that temperature oxidation continues at an extremely slow pace Gebely, Tea: A User's Guide (2016), ch. 4
firing usually exposes the leaf to temperatures above 185°F (85°C) for thirty seconds to five minutes, and oolong is often fired at 400-500°F (204-260°C) in a revolving drum Uhl, The Art and Craft of Tea (2016), 'Processing'
many black teas and most dark teas get no shaqing at all, because the oxidation is halted during the drying stage Uhl, The Art and Craft of Tea (2016), 'Processing'
enzymes are denatured by heat, losing the three-dimensional structure they need to work; steaming deactivates them at 100°C and pans or drums go higher Francl, Steeped: The Chemistry of Tea (2024), ch. 2
Japanese steaming lasts 20 to 80 seconds, with 20-40 seconds giving the asamushi style and 40-80 seconds the fukamushi style Gascoyne et al., Tea: History, Terroirs, Varieties (2014), 'Processing Japanese Green Teas'
green wulong is fired in a heated rotating cylinder at about 572°F (300°C) for five to seven minutes Gascoyne et al., Tea: History, Terroirs, Varieties (2014), 'Processing Green Wulong Teas'