Skip to main content
Support

Browse by category

All categories
Home

Tea leaf from China, India, Sri Lanka, Kenya and more

Black and oolong tea "fermentation" (enzymatic oxidation)发酵 (fājiào, trade usage)

In black tea, and partly in oolong, the "fermentation" step is not done by microbes. Rolling breaks the leaf cells, and the leaf's own enzymes, polyphenol oxidase and peroxidase, oxidise the catechins into theaflavins and thearubigins while the leaf darkens. This is a boundary entry; only the dark teas, such as ripe pu-erh and Liu Bao, are truly fermented by microbes.

Fully documentedTea leaf9.8 h – 19 h

Recipe

No fixed amounts recordedFactory process with no fixed batch size. The leaf is the whole recipe.
Fresh tea shoots (Camellia sinensis), one bud and two leavesNothing else is added; the leaf's own enzymes do the work. The Yunnan study used one cultivar, Mengku dayezhong.[dang2026][samanta2015]
Air (atmospheric oxygen)The step is enzymatic oxidation of the polyphenols in the presence of atmospheric oxygen, so the rolled leaf is kept in air[samanta2015]
Starternone (the leaf's own enzymes)

Total time across the recorded steps: 9.8 h – 19 h

Start
Drag the bar or press Play

What changes: Spread out indoors, the fresh leaves lose water and go limp

Step 1 of 4
Wither the leaves
Time
8–12 h
Reported
Temperature
Not documented
Salt
None given
Acidity
Not documented

Output: withered leaf.

Flavour
Not documented
Texture
leaves go limp
Colour
Not documented

Microbes at work in this step

Not documented for this step.

Method

Measured measured in a studySet fixed by a specification or tested recipeReported one source reports itTypical a general value, not sourced for this fermentUnconfirmed not yet checked against a source

At a glance

In black tea, and partly in oolong, the "fermentation" step is not done by microbes. Rolling breaks the leaf cells, and the leaf's own enzymes, polyphenol oxidase and peroxidase, oxidise the catechins into theaflavins and thearubigins while the leaf darkens. This is a boundary entry; only the dark teas, such as ripe pu-erh and Liu Bao, are truly fermented by microbes. Made in 4 steps over 9.8 h – 19 h. The longest step, wither the leaves, takes 8–12 h (Dang X 2026). None. The leaf's own polyphenol oxidase and peroxidase oxidise the catechins; no microbe is needed does most of the fermenting.

Key numbers
Main step8–12 hReported[dang2026]Wither the leaves
Temperature20–35 °CReported[samanta2015][hua2022]Let the leaf oxidise in the air

What makes this one different

No microbe takes part: the change is the leaf's own enzymes working in the air, which is why tea makers call it fermentation and scientists call it oxidation. In the dark teas the green leaf is first heated to kill those enzymes, and then moulds and bacteria change it during pile fermentation, for about 35–40 days in one Yunnan study.

Related ferments

Variants

  • Oolong (semi-fermented tea)Only partly oxidised before firing.
  • White tea (lightly fermented tea)Long withering with slight natural oxidation; it keeps over ten times the catechins of the black tea in one Yunnan study (Dang 2026).
  • Ripe pu-erh and other dark teas (post-fermented tea)Truly microbial. Sun-dried green tea is pile-fermented at about 45 % moisture with Aspergillus, Thermomyces and Bacillus; one Yunnan study piled it for about 35–40 days with 6–8 turnings.
  • Pickled tea (anaerobic fermentation)Anaerobic fermentation after the leaf is fixed (de-enzymed), driven mainly by lactic acid bacteria and yeast, which gives a mildly sour taste; made by ethnic groups such as the De'ang, Bulang and Jingpo in southwestern Yunnan (Dang 2026).

Safety

Black tea is dried after oxidation, and dried tea of any kind is kept at 4–6 % moisture for storage. A 2026 review of tea storage says moisture above 7 % substantially raises the risk of microbial growth and mould spoilage. These are general tea figures, not limits set for black tea (Guo 2026).

History and sources

Every number is labelled with how it is known. How this atlas is made

Chinese tea classes still use the trade terms non-fermented (green), lightly fermented (white, yellow), semi-fermented (oolong), fully fermented (black) and post-fermented (dark) tea (Dang 2026).

  1. [dang2026] Dang X., Li Y., Lv Z. et al. 2026, Impact of processing techniques on chemical compositions and sensory attributes across five types of Yunnan large-leaf teas, Food Chemistry X 35:103825 (full text via Europe PMC)paper
  2. [hua2022] Hua J., Wang H., Yuan H., Yin P., Wang J., Guo G., Jiang Y. 2022, New insights into the effect of fermentation temperature and duration on catechins conversion and formation of tea pigments and theasinensins in black tea, Journal of the Science of Food and Agriculture 102:2750–2760 (abstract)paper
  3. [samanta2015] Samanta T., Cheeni V., Das S., Roy A.B., Ghosh B.C., Mitra A. 2015, Assessing biochemical changes during standardization of fermentation time and temperature for manufacturing quality black tea, Journal of Food Science and Technology 52:2387–2393 (abstract)paper
  4. [guo2026] Guo X. 2026, Postharvest quality deterioration mechanisms and preservation technologies of tea (Camellia sinensis), a review, Foods 15(20):3766, doi:10.3390/foods15203766paper
  5. [s1] Wikipedia, Tea processing (outline only)wiki