On this page
If green tea stops oxidation early and black tea allows it to proceed more fully, oolong takes a middle road — partial oxidation. The maker judges the appropriate leaf state for the cultivar, raw leaf and intended style.
The step that makes this possible is zuoqing (做青, “making up the leaf”). It is not one action but many repeated cycles of shaking the leaves and then resting them in a thin layer. Each round of shaking bruises the leaf edges; the broken cells meet oxygen and begin to oxidise and redden, while the centre stays relatively intact and greener. After enough cycles, this often gives oolong its signature green leaf with red edge. The traditional word “fermentation” here primarily refers to controlled enzymatic oxidation; it is not a percentage with one universal method of measurement.
Zuoqing: bruising, resting, and "green leaf with red edges"
Drag through the make-up process; watch the edges redden and aroma build in cycles ↓
Schematic. Oolong is partially oxidised — between green (none) and black (full); the exact degree varies widely by style.
Zuoqing does more than control oxidation. Repeated shaking and water loss impose several stresses at once — mechanical wounding, dehydration and cool night temperatures — which can affect biochemical reactions and the formation of floral- and fruity-associated volatiles. The magnitude of these responses varies with cultivar and process. Once the maker judges the leaf is ready, high-heat fixation stops the principal enzymatic reactions.
L2 · Deep DiveThe oolong process
Biochemistry
During zuoqing, shaking makes the leaves collide and rub, bruising mainly the edges. The damaged cells open up, oxygen enters, and polyphenol oxidase can more readily meet catechins and drive enzymatic oxidation. Changes are usually more visible at the edge than at the centre, producing the classic lǜ yè hóng xiāng biān [4]. Theaflavins and other oxidation products can form during this process; their amounts and sensory contributions vary with the material and manufacture.
Oolong’s floral-fruity aroma is also linked to secondary metabolism affected by stress. Repeated shaking, water loss during withering and cool resting temperatures can act as multiple stresses on the leaf. Research suggests that the following pathways participate:
-
The LOX / HPL pathway. When cell membranes are damaged, unsaturated fatty acids can be converted through lipoxygenase, hydroperoxide lyase and related reactions into C6 and C9 volatiles and their precursors; these may participate in subsequent aroma transformations.
-
Terpenoid synthesis. Under some experimental conditions, mechanical wounding and low temperature can affect expression of terpene-synthesis genes and coincide with changes in α-farnesene, jasmine lactone and nerolidol, volatiles associated with floral and fruity aroma [1][3].
Zeng et al. (2020) linked multiple stresses with enrichment of volatile terpenes and esters in Chinese oolong aroma formation [1]. Under their experimental conditions, Zhou et al. (2020) found that wounding and low temperature synergistically induce volatile indole [2], while Yang et al. (2025) reported that UV-B plus mechanical damage can markedly increase α-farnesene and jasmine lactone accumulation [3].
Key compound dynamics
| Compound | Dynamic during zuoqing | Flavour contribution | Ref. |
|---|---|---|---|
| Catechins | May decline; the extent varies by leaf region and process | Associated with changes in astringency | [4] |
| Theaflavins | May form and change dynamically | May affect liquor colour and taste | [4] |
| α-Farnesene | Increases in some studies | Associated with floral and fruity notes | [1][3] |
| Jasmine lactone | Increases in some studies | Associated with jasmine-like aroma | [1][3] |
| Nerolidol | May change | Associated with floral-fruity aroma | [1] |
| Volatile indole | May fluctuate with stress and processing stage | Associated with floral / orchid-like notes | [2] |
| Glycosidically bound aroma precursors | May undergo hydrolysis | Releases free aroma molecules | [5] |
Parameters and quality defects
The ranges below represent selected production practices, not universal specifications for every oolong. Actual control depends on cultivar, plucking maturity, weather and equipment.
| Parameter | Typical range / practice | Too low risk | Too high risk |
|---|---|---|---|
| Shaking intensity | Light to heavy, increasing each round | Under-oxidation, green off-notes | Excessive bruising, uncontrolled reddening |
| Number of shakes | Traditionally 3–6 rounds, adjusted to leaf state | Thin aroma, poor water movement | High leaf temperature, over-oxidation |
| Resting time | 0.5–2 h per round; total zuoqing 6–12 h | Oxidation stalls, greenness remains | Too-fast water loss, stems soften |
| Ambient temperature | 18–26 °C (cooler nights common) | Slow enzyme activity, weak aroma | Rapid oxidation, sour notes |
| Relative humidity | 70–85% | Dry edges, fast water loss | Waterlogging, stale off-notes |
| Leaf moisture after withering | About 65–70% | Hard to move water, stiff leaf | Too wet, prone to stewing, aroma trapped |
Under-oxidation. If shaking is too light, resting too short or the temperature too low, the leaf edges oxidise only weakly. The result is persistent greenness, obvious astringency, thin aroma and too much green in the infused leaf — the tea tastes unfinished.
Over-oxidation. If shaking is too heavy, leaf temperature rises too high or resting runs too long, the red margin spreads toward the centre and polyphenols oxidise excessively. The tea then shows cooked, flat notes, sourness, faded florals and a dull reddish liquor, losing the lively layers that define oolong.
Representative styles
| Style | Relative processing direction | Zuoqing character | Common aroma descriptions | Ref. |
|---|---|---|---|---|
| Fragrant Tieguanyin | Lighter, retaining a fresh profile | Lighter shaking, fewer rounds, often temperature-controlled | Orchid, fresh and lifted | [6] |
| Wuyi rock tea | Medium to relatively heavy; later roasting also matters | Heavier shaking; roasting contributes to the final profile | Floral-fruity and roasted notes | [7][8] |
| Fenghuang Dancong | Medium to relatively heavy | Relatively heavy zuoqing, single-bush picking, often night processing | Honey orchid, gardenia, ginger flower | [9] |
| Taiwan oolong (qingxiang / semi-ball) | Light to medium | Light shaking, cloth-wrapped rolling, often cooler conditions | Floral and fruity notes | [9] |
Flavour sketch
Oolong · flavour sketch
Tea Science · core comparison
| Feature | Oolong | Black tea | Green tea |
|---|---|---|---|
| Oxidation pattern | Localised, partial enzymatic oxidation; its extent varies by style and process and cannot be captured by one universal percentage | Usually more extensive enzymatic oxidation | Fixation usually occurs early, leaving relatively limited oxidation |
| Main driver | Zuoqing: mechanical wounding + dehydration + low-temperature stress | Rolling followed by continuous enzymatic oxidation | High-heat fixation rapidly deactivates enzymes |
| Key products | Catechins partly retained; small theaflavins; abundant terpene floral volatiles | Theaflavins, thearubigins, theabrownins | Catechins, chlorophyll, amino acids largely preserved |
| Liquor colour | Golden to orange (light oxidation) or orange-red (heavy oxidation) | Bright red | Pale green to yellow-green |
References
- Zeng, L., et al. (2020). Formation of aroma compounds in Chinese oolong tea under multiple stresses: A review. Trends in Food Science & Technology.
- Zhou, Y., et al. (2020). Wounding and low temperature synergistically induce volatile indole in tea leaves. Journal of Experimental Botany.
- Yang, X., et al. (2025). UV-B and mechanical wounding enhance α-farnesene and jasmine lactone accumulation in tea leaves. Plant Physiology and Biochemistry.
- Frontiers in Plant Science (2021). Architecture and dynamics of the wounding-induced gene regulatory network during the oolong tea manufacturing process.
- Analysis of glycosidically bound aroma precursors in tea leaves (3): Change during the oolong tea manufacturing process.
- Study on dynamic alterations of volatile organic compounds reveals aroma development over the enzymatic-catalyzed process of Tieguanyin oolong tea production.
- GB/T 18745—2006 Geographical indication product — Wuyi rock tea.
- Research on Wuyi rock tea processing and quality formation.
- Tea primary processing parameters.