How Black Tea Turns Red: The Theaflavin Story

Tea Science｜Author：祥興茶行｜Published：2026-07-18

Colorless catechins, a pair of leaf enzymes, and a few hours of controlled oxidation build black tea's golden, luminous liquor. Here is the chemistry behind the color — and the dials tea makers use to steer it.

Leaves from the very same tea plant come out green as green tea and coppery red as black tea. The difference is not the cultivar. It is a chemical chain reaction that unfolds in the hours after the leaves are picked. Today we are taking apart black tea's alluring "golden ring" to see exactly where the red comes from.

The short version Black tea's red comes from the leaf's own colorless catechins being oxidized, discolored, and polymerized step by step by the leaf's own enzymes. The path first produces golden, luminous theaflavins, which then convert into red-brown thearubigins; push too far and you end up with dull theabrownins. The color deepens with every step — that is the through-line of this entire article.

A Leaf at the Fork in the Road

Every tea leaf cell already holds two things, ordinarily kept apart by cell membranes and minding their own business. On one side sits a large stock of catechins — the source of the liquor's bitterness, astringency, and biological activity, which you can think of as the leaf's natural pigment precursors. On the other side sits polyphenol oxidase (PPO), an enzyme whose specialty is hooking oxygen onto polyphenols — the same invisible hand that browns a cut apple in the kitchen.

Green tea making settles the matter at step one: the "kill-green" firing uses high heat to knock PPO out of commission, sealing off the oxidation pathway, so the catechins survive and the leaf stays green [1][8]. Black tea making does the opposite. Withering first softens the leaf, then rolling ruptures the cells, and enzyme, catechins, and air finally meet — oxidation is officially under way. Taiwan's Tea and Beverage Research Station puts this plainly: rolling exists to "break down the leaf tissue so the leaf's enzymes are released," after which temperature and humidity are controlled for slow oxidation, and "once black tea's characteristic aroma and color have formed, high heat halts the enzyme activity" [8].

For a fuller comparison of where all six tea categories fall on the oxidation spectrum, see the companion piece [Fermentation Is Really Oxidation: How the Six Tea Categories Divide](/articles?slug=teafermentaition). This article burrows into the chemistry of the black tea path alone.

First Stop, Golden: Theaflavins

The first star product of oxidation is theaflavin (TF). Its molecular core is a skeleton called benzotropolone, and its formation is choosy: it takes a catechin whose B-ring carries two hydroxyl groups, paired with a gallocatechin whose B-ring carries three, undergoing paired co-oxidation — lose either partner and nothing forms [1]. This is why not every catechin can become TF. It is a duet that demands a specific dance partner.

〔圖〕兒茶素 catechin 分子結構式

Catechin (B-ring bearing 2 hydroxyl groups)

〔圖〕EGCG 表沒食子兒茶素沒食子酸酯 分子結構式

EGCG (epigallocatechin gallate — B-ring bearing 3 hydroxyl groups)

↑ Only when these two kinds of catechin co-oxidize as a pair can theaflavin assemble its golden benzotropolone core (theaflavin itself is structurally complex; shown here are its two "dance partner" precursors)

Theaflavins are golden orange-red and — crucially — luminous. That is not just an impression. A Kenyan study measured a batch of black teas and found theaflavin content correlated with liquor brightness at r=0.87, the strongest of any single component [2]. A Japanese analysis of 12 commercial black teas likewise found that total theaflavins correlated positively with the colorimetric a value, which tracks redness (r≈0.79) — the more theaflavins, the further the liquor shifts toward red [4]. Another set of Kenyan samples showed the same pattern, with theaflavins tracking brightness (r≈0.72) [5].

💡 Worth noting: these are correlations, not causation, and they come from observational studies of different origins measured on different instruments — the numbers cannot be compared directly against one another. But each of the three points the same way: more theaflavins, and the liquor turns brighter and redder.

Second Stop, Red-Brown: Thearubigins

Theaflavin is really only a waypoint. The leaf carries another enzyme, peroxidase (POD), which — given hydrogen peroxide — takes the theaflavins already formed and oxidizes and polymerizes them onward into thearubigins (TR) [1][2]. Thearubigins are red-brown, larger in molecular size, and deeper in color; they supply the liquor's richness and weight on the palate. Oxidize past that point and you slide into dull theabrownins. The whole staircase looks like this:

```mermaid flowchart LR A[Catechins colorless] -->|PPO + oxygen paired co-oxidation| B[Theaflavins TF golden, luminous] B -->|POD + hydrogen peroxide polymerization| C[Thearubigins TR deep red-brown] C -->|over-oxidation| D[Theabrownins dull, dark] ```

Intriguingly, thearubigins pull liquor color in the opposite direction from theaflavins. In the same Kenyan work, the more thearubigins a tea held, the lower its brightness (r roughly −0.66 to −0.89) [2][5]. Nor are all thearubigins alike: Obanda's team in Kenya fractionated them and ran regressions, finding that the highly polymerized fraction (TRSII) was the main culprit darkening and clouding the liquor — in the model predicting sensory brightness, TRSII carried a regression coefficient B of −0.980, far outweighing the theaflavins' +0.162 [7].

Two reminders, so nothing gets muddled: ① The −0.980 and +0.162 above are unstandardized regression coefficients (B), not standardized ones — their magnitudes depend on each component's units and cannot be read as direct "multiples of influence." ② "Lightness L" and "brightness" are two different metrics. In the Japanese study, more theaflavins actually lowered lightness L — because the red-yellow color deepened and the liquor read as darker. That does not contradict "theaflavins brighten"; the two studies are measuring different color dimensions [4].

The Tea Maker's Dial: pH

If POD keeps shoving theaflavins onward into thearubigins, can you make it shove a little less and keep a little more? You can — and one of the key dials is the pH during fermentation.

The leaf's PPO is happiest at around pH 5.5, and as it oxidizes catechins it throws off hydrogen peroxide as a byproduct — precisely the fuel POD needs to convert theaflavins into thearubigins. When researchers held the fermentation down at pH 4.5, less hydrogen peroxide formed and less theaflavin got converted away, so theaflavins accumulated roughly 30 to 40 percent higher while thearubigins actually declined [2].

Beyond pH, temperature, time, and humidity all help tune this oxidation staircase. The fermentation range most often cited in the literature runs about 24–29°C, 95–98% relative humidity, for 2–4 hours [1]. Stretch the fermentation longer and the trihydroxy catechins burn off quickly — nearly undetectable past about 120 minutes — while theaflavins peak and then recede and thearubigins keep climbing [5]. So "ferment longer for a redder, deeper color" is true, but push past the peak and you walk out of bright golden-red into dark red-brown.

Correcting a Myth While We Are Here: The Astringency Is Not All Their Fault

Many people assume black tea's astringency comes from theaflavins and thearubigins. Recent taste analysis offers a more refined answer: the principal contributors to black tea's puckering astringency are in fact a group of flavonol glycosides — not the high-molecular thearubigins, and not the catechins or theaflavins themselves [6].

Just how potent are they? One of these glycosides has a taste threshold roughly 16,000 times lower than theaflavin's and about 190,000 times lower than that of EGCG (a catechin) [6] — perceptible at vanishingly small amounts. And the astringency they deliver is a silky, mouth-coating grip, different in character from the classic puckering, drying kind. Theaflavins are astringent in their own right (taste threshold around 125–800 mg/L); they are simply not the lead actor in black tea's astringency [6]. To dig into how roasting reshapes flavor through non-enzymatic heat, read on with [Catechins and the Roast: How Heat Rewrites a Cup](/articles?slug=catechin-roasting-basics).

Landing in Taiwan: Why Hong Yu Runs Golden-Red

Back to home ground. Taiwan Tea No. 18, "Hong Yu" (紅玉), is the flagship black-tea cultivar bred by the Tea and Beverage Research Station. The official description notes that it is "high in tea polyphenols, which after full fermentation convert into theaflavins and thearubigins," with a liquor of "vivid golden red" [8] — this article's entire chemical staircase, played out in a single cup of Taiwanese black tea: catechin-rich raw leaf, complete enzymatic oxidation, stacking up that handsome golden theaflavin ring over a thearubigin base.

The Takeaway in One Cup

1. Black tea's red is oxidized into being: rolling ruptures the cell walls so enzymes and catechins finally meet, setting off the color staircase from catechins → theaflavins → thearubigins → theabrownins. 2. Theaflavins brighten, thearubigins darken: the golden luminosity comes from theaflavins, the red-brown depth from thearubigins — they pull in opposite directions, and their ratio decides the liquor. 3. Astringency has a different lead actor: black tea's grip comes mainly from trace flavonol glycosides — do not pin the whole bill on theaflavins and thearubigins.

References

  1. Abudureheman, B., et al. (2022). Enzymatic Oxidation of Tea Catechins and Its Mechanism. _Molecules_, 27(3), 942. https://doi.org/10.3390/molecules27030942 （綜述；文中「POD 熱鈍化後殘活性 >5×PPO」一句為此綜述轉述、未見一手實測，本文未採用為定量事實）
  2. Subramanian, N., Venkatesh, P., Ganguli, S., & Sinkar, V. P. (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/jf981042y
  3. Bhuyan, L. P., et al. (2015). Spatial variability of theaflavins and thearubigins fractions and their impact on black tea quality. _Journal of Food Science and Technology_, 52(11), 7194–7204. https://doi.org/10.1007/s13197-015-1809-0
  4. 坂本 彬・井上博之・中川致之 (2012). 12 種類の紅茶の化學成分. 日本食品科學工學會誌, 59(7), 326–330. https://doi.org/10.3136/nskkk.59.326
  5. Ngure, F. M., Wanyoko, J. K., Mahungu, S. M., & Shitandi, A. A. (2009). Catechins depletion patterns in relation to theaflavin and thearubigins formation. _Food Chemistry_, 115(1), 8–14. https://doi.org/10.1016/j.foodchem.2008.10.006
  6. Scharbert, S., Holzmann, N., & Hofmann, T. (2004). Identification of the Astringent Taste Compounds in Black Tea Infusions by Combining Instrumental Analysis and Human Bioresponse. _Journal of Agricultural and Food Chemistry_, 52(11), 3498–3508. https://doi.org/10.1021/jf049802u
  7. Obanda, M., Owuor, P. O., Mang'oka, R., & Kavoi, M. M. (2004). Changes in thearubigin fractions and theaflavin levels due to variations in processing conditions and their influence on black tea liquor brightness and total colour. _Food Chemistry_, 85(2), 163–173. https://doi.org/10.1016/S0308-8146(02)00183-8
  8. 農業部茶及飲料作物改良場. 臺灣特色茶分類及加工製程簡介／臺茶 18 號. https://www.tbrs.gov.tw/ws.php?id=4748 ；https://www.tbrs.gov.tw/ws.php?id=4689