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Black tea’s “fermentation” is a long-standing misnomer. Unlike the microbial fermentation of yoghurt or wine, what actually happens is an oxidation driven by the leaf’s own enzymes.
Green tea uses heat to switch enzymes off before oxidation can occur; black tea does the opposite — it encourages that oxidation. Rolling ruptures the leaf cells so that polyphenol oxidase (PPO) and catechins, normally kept apart, mix freely. With oxygen, the colourless catechins are oxidised step by step into golden theaflavins and red-brown thearubigins — the source of black tea’s red liquor and full body.
Oxidation: from catechins to theaflavins to thearubigins
Drag to advance oxidation; watch the compounds and the liquor colour change ↓
Schematic, not exact values. "Fermentation" here is enzymatic oxidation, not microbial.
Once oxidation reaches the desired level, drying at around 90 °C deactivates the enzymes and “freezes” the tea at its best — and black tea is born.
L2 · Deep DiveThe four steps of black tea
The chemical mechanism
After rolling breaks the cell structure, PPO catalyses the formation of quinones from catechins in the presence of oxygen. Two such quinones then couple — a dihydroxy B-ring flavanol with a trihydroxy B-ring flavanol — to form theaflavins [1].
While oxidising catechins, PPO also generates hydrogen peroxide (H₂O₂); peroxidase (POD) then uses that H₂O₂ to further oxidise theaflavins into the larger, more complex thearubigins [1]. So theaflavins can be seen as the “intermediate” product of oxidation, thearubigins the “deep” product.
Two products shape the flavour
| Product | Colour | Flavour contribution |
|---|---|---|
| Theaflavins | yellow, bright | briskness, astringency, liquor brightness [1] |
| Thearubigins | red-brown | body, mouthfeel, red-brown colour [1] |
The ratio and total of theaflavins to thearubigins is a key quality marker for black tea — which is why controlling the degree of “fermentation” matters so much.
Key processing parameters
| Step | Typical conditions | Purpose |
|---|---|---|
| Withering | moisture ~70–80% → 55–70%, ~18–20 h [2] | water loss, prepares enzymes & compounds |
| Rolling | ~75 min, alternating pressure [3] | ruptures cells, starts oxidation |
| Oxidation | ~20–30 °C, ~90% RH, ~1–3 h [2][3] | the main stage of enzymatic oxidation |
| Drying | ~90 °C, to 3–4% moisture [2] | deactivates enzymes, halts oxidation |
Parameters vary widely by region, cultivar and method (orthodox vs. CTC). The table shows representative ranges from research literature, for understanding the mechanism only.
Typical flavour
Orthodox black tea · flavour sketch
See also
- The Science of Green-Tea Fixation — the same enzyme, the opposite treatment
The full process & parameters
| Step | Parameters | Role |
|---|---|---|
| Withering | natural / warm / sun, ~55–65% water loss | softens, raises enzyme activity, sheds grassy notes |
| Rolling (or CTC) | pressure, 30–90 min | ruptures cells, PPO meets catechins, creates the oxidation interface |
| Oxidation ⭐ | ~22–28 °C, 95%+ RH, 2–4 h | enzymatic oxidation: catechins→quinones→theaflavins→thearubigins→theabrownins; Maillard/Strecker add sweet-floral |
| Drying | 110–120 °C (two firings) | halts enzymes, fixes the red liquor and leaf |
Representative ranges; they vary widely by cultivar, origin and process.
References
- Subramanian N., et al. Role of Polyphenol Oxidase and Peroxidase in the Generation of Black Tea Theaflavins. Journal of Agricultural and Food Chemistry, 1999. https://pubmed.ncbi.nlm.nih.gov/10552528/
- Effects of Fermentation Temperature and Time on the Color Attributes and Tea Pigments of Yunnan Congou Black Tea. PMC, 2022. https://pmc.ncbi.nlm.nih.gov/articles/PMC9265920/
- Optimization of the factors affecting black tea fermentation using Response Surface Methodology. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC8857412/