What happens during black-tea “fermentation”?

Start with the essentials

Three key points

1. Rolling enables contactTissue disruption brings enzymes and compounds together.
2. Several products shape the cupColour, briskness and body involve a mixture of contributions.
3. Oxidation connects with dryingThe intended endpoint and subsequent handling influence the result.

Connect the process with the tea

The black-tea process

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.

PPO and POD: enzyme characteristics and cooperation

PPO and POD are both metalloprotein oxidoreductases, but they differ in substrate and electron-acceptor requirements. PPO contains two copper ions and can use molecular oxygen directly, oxidising catechins to quinones. POD contains a heme group and needs H₂O₂ as an electron acceptor to oxidise products such as theaflavins further [1]. In black-tea oxidation, PPO is the “igniter” and POD the “booster” — the H₂O₂ produced by PPO provides exactly the reaction condition POD needs.

Fresh-leaf PPO is most active around 25–35 °C and pH 5.0–7.0; fresh sap is naturally acidic (about pH 5.5–6.5), so once cells are broken oxidation starts fast. If oxidation temperature rises above 35 °C, PPO activity drops quickly; above 45 °C it is largely inactivated. This shows oxidation is not “hotter and faster” — it needs a balance of temperature, humidity and oxygen: sufficient temperature to maintain enzyme activity, high humidity to prevent excessive water loss, and ample oxygen to keep the oxidation chain moving [2][3][5].

Robertson (1983) used a model oxidation system to quantify how temperature, pH and oxygen steer the oxidation path: low temperatures slow the reaction; excessive heat inactivates PPO quickly; mildly acidic pH (5.0–5.5) favours theaflavin accumulation; and adequate oxygen is required for thearubigin formation[5].

Two products shape the flavour

The ratio and total of theaflavins to thearubigins is a key quality marker for black tea. Harbowy & Balentine (1997) note that thearubigins can account for 60–70% of the soluble solids in black-tea liquor, making them the dominant force behind its red, full-bodied colour[6].

Three oxidation products: TF, TR and TB

Black-tea quality rests on the balance and total of theaflavins (TF), thearubigins (TR) and theabrownins (TB). Their colour, molecular weight and taste contribution differ markedly:

The TF/TR ratio is often used to judge black-tea quality. High TF gives bright liquor, brisk taste and a “golden ring”; high TR gives red colour and body. Too much TB is usually a sign of over-oxidation or poor drying. Different markets value different balances: plain-drinking congou teas seek a relatively even TF/TR balance, while CTC broken teas — because of their high breakage and fast oxidation — end up with a higher TR/TB share, emphasising strength and milk compatibility [4][7].

Oxidation kinetics: the component curve over time

If you slice oxidation by time, the components do not all move together. In Assam CTC black tea, after crushing the ester catechins (EGCG, ECG) fall fastest because of the high cell breakage and full PPO-substrate contact; theaflavins accumulate rapidly and may peak around 20–40 minutes [4][7]. Then POD oxidises theaflavins into thearubigins, so TF begins to fall while TR keeps rising. In later stages TR polymerises further into TB. Amino acids and soluble sugars both participate in reactions and provide precursors for Strecker degradation.

This timeline explains why tea makers judge oxidation endpoint by “looking at leaf colour and smelling the aroma”: the leaf changes from green to yellow to red-brown, and the aroma shifts from green to sweet-floral — external signs of these internal changes. With different raw materials and temperatures the peaks arrive earlier or later, so oxidation has no fixed “minute count”; it has “state counts” [2][3].

Where the aroma comes from: four chemical pathways

Colour comes from pigments, but black tea’s floral, fruity and honeyed aroma comes from trace volatiles — hundreds of compounds making up a tiny share of the dry leaf. By precursor origin, four pathways dominate[7][8]:

1. Hydrolysis of glycosidic precursors. Geraniol, linalool and its oxides, benzyl alcohol, 2-phenylethanol, methyl salicylate and leaf alcohol are stored in the fresh leaf as odourless glycosides (bound to sugars). Withering, rolling and oxidation rupture the cells and β-glucosidases release the aglycones — the main source of floral and fruity notes[7][8].
2. Carotenoid oxidative degradation. β-Carotene, lutein and zeaxanthin (about 36–73 mg/100 g dry weight) degrade extensively during oxidation into β-ionone (violet-like), β-damascenone (sweet floral/apple, extremely low perception threshold — reported as low as ~0.002 μg/L), plus dihydroactinidiolide and theaspirone — characteristic black-tea aromas[7][9]. The degradation order is β-carotene > zeaxanthin > lutein, and it is greater in CTC than orthodox processing and in withered than fresh leaf[9]. Notably, the degradation needs oxidised flavanols (quinones) as co-oxidants — so the very oxidation that builds theaflavins also drives aroma formation[7].
3. Lipid oxidation and cleavage (the “grassy” family, explained). Linolenic and linoleic acids are oxidised by lipoxygenase (LOX), and the hydroperoxides are then cleaved into C6 aldehydes and alcohols such as leaf aldehyde and hexanal — the chemical identity of “grassy” notes. During black-tea processing they partly evaporate and partly transform further, as grassy notes give way to floral ones[7][8].
4. Maillard and Strecker reactions (sweetness from the dryer). Drying’s high temperature lets amino acids and sugars react into pyrazines and pyrroles (baked notes) and Strecker aldehydes such as phenylacetaldehyde — an important source of honeyed sweetness[7][8].

These four pathways explain why the same fresh leaf smells grassy and green when fixation “locks in” its precursors, while full oxidation converts the whole precursor pool into floral, fruity, honeyed notes — the chemical root of the green/black tea aroma divide[7].

Worth emphasising: aroma contribution is not the same as absolute content. Compounds with extremely low odour thresholds and high odour activity values (OAV) can dominate a fragrance even at tiny concentrations. For example β-damascenone in black tea is often measured in ng/L, but with a threshold around 0.002 μg/L its OAV can reach thousands or tens of thousands, making its contribution to sweet-floral notes far greater than that of some far more abundant but less potent compounds [9]. This is the essence of black-tea aroma: powerful even in trace amounts.

Creaming: when the cooled liquor “milks up”

Cooled black tea sometimes turns cloudy, as if a drop of milk had been added — this is creaming (tea cream), leng hou hun in Chinese.

The classic explanation: as the liquor cools, theaflavins, thearubigins and caffeine complex and precipitate (through hydrogen bonding and related interactions); reheating re-dissolves the complexes and clears the cup[7]. A 2005 study using small-angle X-ray scattering and NMR looked closer: theaflavin is the initiator of creaming, first self-assembling into nanoclusters about 3 nm across; caffeine is not required, but “fills in the gaps” and adds bulk — decaffeinated black tea still creams. Calcium (Ca²⁺) promotes creaming, while glycosylation of polyphenols weakens complexation and reduces it[8].

For quality, bright, brisk black teas often cream more visibly, and creaming has traditionally been taken as one sign of rich theaflavins; but the degree of clouding also depends on the water — hard water (more calcium and magnesium) creams more heavily, so the same tea can behave differently from place to place[7][8].

Key compound dynamics

Reference: Deka et al. (2021) tracked Assam CTC black tea from fresh leaf to finished tea and found total catechins fell by about 96%, while theaflavins and thearubigins were generated in parallel, confirming the depth of oxidation[4].

Parameters and quality defects

Black-tea oxidation is a classic “watch the leaf and make the tea” step. The ranges below are common starting points, adjusted for cultivar, season, equipment and target style [2][3][8]:

Withering does more than remove water: it “preheats” the chemistry. Membrane permeability rises, some glycosidases activate, and carotenoids and fatty acids prepare for the oxidation ahead. Under-withered leaf is stiff and low in enzyme readiness; over-withered leaf has lost too much water and its membranes are too damaged, leading to uneven oxidation.

Rolling intensity and time directly affect cell breakage. Orthodox congou tea uses a “light–heavy–light” pressure sequence that releases juice while keeping the strip whole; CTC uses mechanical rollers to cut the leaf, achieving breakage rates above 90%, greatly increasing the oxidation interface and shortening oxidation time [4][7].

Under-oxidation produces:

• too few theaflavins → pale liquor, green taste, harsh astringency;
• aroma precursors underdeveloped → grassy notes, weak sweetness.

Over-oxidation produces:

• theaflavins oxidised further to thearubigins/theabrownins → dull brown liquor, poor brightness;
• flat taste, sour off-notes.

Excessive drying temperature also drives off low-boiling aroma compounds, degrades theaflavins and produces burnt notes and a dull, reddish liquor. So the hand-off from oxidation to drying is as important as oxidation itself.

Representative styles

The same oxidation chemistry produces very different faces depending on raw material, breakage and post-fermentation handling. Orthodox black tea is like an orchestra, seeking layers and balance; CTC is like percussion, seeking concentration and impact [4][7].

Keemun congou, made from the Zhuye cultivar in Anhui, is rolled, broken, sorted and then oxidised, giving a red, even leaf and a bright red liquor with a rose-honey “Keemun aroma”. Dianhong congou uses Yunnan large-leaf material, high in polyphenols, producing abundant thearubigins and a strong, fresh, golden-ringed cup. Lapsang Souchong adds traditional pine-smoking after rolling, combining smoky aroma with oxidised sweetness to create “pine smoke, longan broth”. CTC black tea releases flavour quickly and evenly through fast oxidation, making it ideal for blending and teabags.

Common misconceptions

1. “Black-tea oxidation is microbial fermentation.” No. The reddening of black tea is enzymatic oxidation driven by the leaf’s own PPO/POD; microbes play essentially no part.
2. “Oxidation degree can be pinned to a percentage.” There is currently no unified, reproducible “oxidation percentage”; figures such as 80% are only rough experiential descriptions.
3. “CTC is more oxidized.” CTC simply has higher cell breakage and faster oxidation; the final product has a higher TR/TB share, but that is not the same as “more oxidized”.
4. “More creaming always means better tea.” Creaming can indicate rich theaflavins, but hard water also intensifies it, so it cannot be used alone as a quality criterion.
5. “Strong aroma means flavouring has been added.” High-quality black tea naturally contains hundreds of volatile compounds; its aroma is complex and natural. Flavoured teas usually smell single-note, sharp and short-lived.

See also

• Green-tea fixation — how heat stops oxidation at the start
• Oolong zuoqing — partial, localised oxidation and stress-induced aroma
• Dark-tea wodui — microbial post-fermentation, for contrast

Tea Science · core comparison

For the full six-tea-class comparison, see The Science of Tea: From Leaf to Six Tea Types.

References

1. R-001 — Subramanian, N., et al. (1999). Role of Polyphenol Oxidase and Peroxidase in the Generation of Black Tea Theaflavins. Journal of Agricultural and Food Chemistry, 47(7), 2571–2578. https://doi.org/10.1021/jf9903301
2. R-002 — Luo, Y., et al. (2022). Effects of Fermentation Temperature and Time on the Color Attributes and Tea Pigments of Yunnan Congou Black Tea. Foods, 11(13), 1957. https://doi.org/10.3390/foods11131957
3. R-003 — Muthumani, T., & Kumar, R. S. S. (2007). Studies on freeze-withering in black tea manufacturing. Food Chemistry, 101(1), 103–106. https://doi.org/10.1016/j.foodchem.2006.01.008
4. R-062 — Deka, H., Sarmah, P. P., Devi, A., Tamuly, P., & Karak, T. (2021). Changes in major catechins, caffeine, and antioxidant activity during CTC processing of black tea from North East India. RSC Advances, 11, 11457–11467. https://doi.org/10.1039/d0ra09529j
5. R-205 — Balentine, D. A., Wiseman, S. A., & Bouwens, L. C. (1997). The chemistry of tea flavonoids. Critical Reviews in Food Science and Nutrition, 37(8), 693–704. https://doi.org/10.1080/10408399709527797
6. R-204 — Robertson, A. (1983). Effects of catechin concentration on the formation of black tea polyphenols during in vitro oxidation. Phytochemistry, 22(4), 889–896. https://doi.org/10.1016/0031-9422(83)85018-3
7. R-029 — 宛晓春 主编 (2003).《茶叶生物化学》(第3版). 中国农业出版社. ISBN: 978-7109066274.
8. R-272 — Ravichandran, R., & Parthiban, R. (2002). Carotenoid composition, distribution and degradation to flavour volatiles during black tea manufacture and the effect of carotenoid supplementation on tea quality and aroma. Food Chemistry, 78(1), 23–28. https://doi.org/10.1016/s0308-8146(01)00303-x
9. R-255 — Jöbstl, E., et al. (2005). Creaming in black tea. Journal of Agricultural and Food Chemistry, 53(20), 7997–8002. https://doi.org/10.1021/jf0506479