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 storage defect that only resembles one.
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, and everything the finished tea will be depends on that one setting. This page documents the step itself: the enzymes it kills and why the kill is permanent, the two methods and their numbers, the two execution defects and their distinct symptoms, and one storage defect that wears the same grassy vocabulary but is not a kill-green fault at all. For where fixation sits between the withering, shaking, and roasting steps, see the oxidation guide.
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. Tracked against heat, tea's own polyphenol oxidase dies in two stages, a heat-labile fraction going first and a more heat-resistant fraction following as the temperature climbs (International Journal of Tea Science7). Once a protein denatures under that kind of heat, it does not refold. Its structure is broken, not merely paused, 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 65.5C (150F) for the enzymes to denature and oxidation to stop, per the Specialty Tea Alliance1. This guide is the canonical home for that threshold; the oxidation guide glosses it and links back here. 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, which is the first of the two defects below.
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. Anxi's processing record puts no clock on the hand method; the pass is judged by the leaf, not timed. It gives a different number instead: the leaf sheds about 8 to 10 percent of its moisture 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 | not timed in the record; 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 as well as too cool, and the leaf darkens and blackens right there in the kill-green pan (sohu.com4).
What kill-green is not: fan qing
There is a third grassy fault, and it is not a kill-green fault. Fan qing (返青, "returning to green") happens long after kill-green and roasting are finished, in storage. A roasted oolong reabsorbs ambient moisture over time, and its flavor regresses toward a green, grassy character the tea does not want, per Orientaleaf6. It afflicts medium to heavily roasted oolongs most, and a gentle re-bake can often correct it. Fan qing is a documented, named phenomenon in Wuyi rock-tea practice (fx361.com5), which is why it is worth telling apart from a heat-step error. Both faults speak of "grass," but they arise at opposite ends of the tea's life. Under-fixation is a kill-green failure baked into the leaf and permanent. Fan qing is a storage failure that arrives months later and can be reversed by fire. One is the maker's error, the other is the cellar's.
Of all the numbers in an oolong's making, these leave the least room for error. Reach the enzyme too slowly and the leaf reddens; reach it too hard and the leaf scorches. Between those two errors sits a narrow window, closed by heat and judged by eye rather than a clock. Once it closes, the tea's oxidation level cannot change again.