Kill-Green
The heat step that arrests enzymatic oxidation in oolong. What denatures the enzymes and why it cannot be undone, the wok and the drum with their real numbers, how a maker judges it done, the two ways it fails, and the later defect that only resembles one.
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Kill-green (shaqing, 杀青, literally "killing the green") is the burst of applied heat that denatures the leaf's oxidizing enzymes and so arrests enzymatic oxidation at the point the maker chooses. It is the one mechanical event that fixes how far an oolong is oxidized.
The enzyme, and why the step cannot be undone
Oxidation in tea is enzymatic. Two enzymes drive it: polyphenol oxidase (PPO) and peroxidase, which act on the leaf's catechins once bruising has let enzyme and substrate meet, per the mechanism documented for Camellia sinensis2. Kill-green ends that reaction by denaturing both enzymes with heat. Tea's own polyphenol oxidase dies in two stages, a heat-labile fraction going first and a more heat-resistant fraction following (International Journal of Tea Science6).
Once a protein denatures under that kind of heat, it does not refold. Its structure is broken, so it cannot restart when the leaf cools. This is what makes kill-green a one-way decision: the moment the enzymes are dead, the oxidation level is fixed for the life of the tea, and no later step returns it. The Specialty Tea Alliance names this arrest as the purpose of the step1.
The leaf must reach a floor of roughly 65C (150F) for the enzymes to denature and oxidation to stop, per the Specialty Tea Alliance1. That figure is the minimum for arrest, not the working temperature. Practical fixation runs far hotter, because the leaf must be brought through that threshold fast and evenly, not merely nudged past it. Reach the floor too slowly and the enzymes keep working right up until they die.
The wok and the drum
Two methods do the same job. The older one is hand pan-firing (chaoqing, 炒青): an iron wok heated over a fire to about 210 to 230C (about 410 to 445F), into which the withered, shaken leaf is poured all at once and turned continuously by hand or a wooden paddle. It is now rarely used in production. The leaf loses about 8 to 10 percent of its weight in water over the pass (tguanyin.com3).
The modern standard is the mechanized rotary drum, the "110-type" machine, which most production now runs. It reaches a wall temperature higher than a hand can work at, about 260 to 300C (about 500 to 570F), takes roughly 5 kg (about 11 lb) of leaf per batch, turns at about 20 rpm, and runs 10 to 15 minutes (tguanyin.com3). Both bring the leaf through the arrest threshold fast; they differ in labor, load, and control.
| Hand wok (chaoqing) | Rotary drum (110-type) | |
|---|---|---|
| Temperature | 210 to 230C (about 410 to 445F) | 260 to 300C (about 500 to 570F) |
| Duration | judged by the leaf | 10 to 15 minutes |
| Leaf load | one small hand-turned batch | about 5 kg (about 11 lb) per batch |
| Labor | continuous hand or paddle turning | one operator, drum at about 20 rpm |
Both methods reach the same endpoint. Judge that endpoint by the leaf, not the clock: the edges go somewhat dry and stiff, and a handful of leaf no longer clumps together when squeezed (tguanyin.com3).
Two ways to get it wrong
The dial has two ends, and the tea fails differently at each.
Run the vessel too hot and the leaf over-fixes. Its edges scorch (jiaobian, 焦边), and that scorching is named a leading cause of the heavy smoky taint and the excess broken fragments found in some finished teas. Chlorophyll breaks down heavily, so the leaf color turns yellow. Caffeine, theanine, and soluble sugars all fall, and the tea's quality shifts with them (sohu.com4).
Run it too cool and the leaf under-fixes. The enzymes are never fully killed, so oxidation continues where it should have stopped: the result is hong geng hong ye (红梗红叶, "red stem, red leaf"), and the finished tea carries a raw grassy note, sometimes a sour one. Hold that low heat too long, and the leaf darkens and blackens right there in the kill-green pan (sohu.com4).
What kill-green is not: fan qing
Fan qing (返青, "returning to green") appears after a finished Wuyi rock tea's roast has faded (褪火), as a green aroma in the dry leaf and a green, astringent taste in the cup (fx361.com5). Under-fixation and fan qing both leave a green taste (青味) in the cup, but they arise at opposite ends of the tea's life. Under-fixation is a kill-green failure baked into the leaf and permanent.