Definition:
Kill-green (杀青, sha qing, literally “kill green”) is the heat-application step — executed by pan-firing, steaming, baking, or hot-air tumbling — that rapidly denatures (destroys the activity of) polyphenol oxidase (PPO) and related enzymes in freshly harvested tea leaves, halting oxidation and preserving the leaf’s green character. It is the defining step that separates green teas from black teas, and for oolong is applied after a controlled period of partial oxidation.
In-Depth Explanation
Why kill-green is necessary: Fresh tea leaves contain polyphenol oxidase in quantities sufficient to fully brown the leaf within hours if cell walls are broken. Without kill-green, any processing (rolling, stacking, transporting) would trigger full oxidation — producing black tea. Kill-green halts this by raising the leaf temperature above the enzyme’s denaturation threshold (~70°C for PPO).
Methods of kill-green and their effects:
- Pan-firing (炒青): Wok or drum-based; the standard Chinese method; the dry heat produces toasty-nutty-mellow flavour notes that are the standard for Chinese green tea character (longjing, biluochun).
- Steaming (蒸青): Water vapour; standard Japanese method; faster heat transfer; preserves bright green colour and fresh-grassy notes; produces the characteristic Japanese green tea flavour profile (sencha, gyokuro).
- Hot air tumbling (烘青): Used for some Chinese greens; produces a gentler heat; preserves delicate flavour but can result in slightly less vivid colour.
- Baking (烘焙): Lower temperature baking applied later in oolong and some Chinese greens; not the initial kill-green but sometimes a secondary heat step.
Completeness matters: Incomplete kill-green — where leaf temperature didn’t reach threshold in all parts of the leaf — leaves residual enzyme activity that causes uneven browning and off-flavours during storage. Skilled kill-green is a critical quality control step.
Japanese steaming duration: In Japanese green tea, steaming time during kill-green is a primary quality variable:
- Standard: 20–40 seconds (futsu-mushi) — sencha
- Deep steam: 60–120 seconds (fukamushi) — fukamushi sencha
- Super deep steam: 120+ seconds — very fine particles, brighter colour, bolder flavour
See Also
Related Terms
Research
- Liang, Y., et al. (2015). Effect of kill-green method on enzymatic activities, chlorophyll content, and sensory quality in Chinese green tea. Journal of Food Processing and Preservation, 39(6), 2482–2490.
[Compared pan-firing vs. steaming vs. hot-air kill-green; confirmed pan-firing produces measurably higher pyrazine and furanic compound levels (responsible for toasty notes), while steaming preserves higher chlorophyll and green volatile concentrations.]
- Nishida, R., et al. (2018). Steam duration effects on catechin and amino acid profiles in Japanese green tea. Bioscience, Biotechnology, and Biochemistry, 82(8), 1421–1430.
[Quantified that extended steaming in fukamushi-style reduced overall catechin content (via oxidative leaching of leaf structure) and increased relative theanine concentration in the infusion.]