Oolong Polyphenols
The compound family behind color and astringency. What catechins, theaflavins, and thearubigins actually are, structurally, and what partial oxidation does to each before the leaf ever reaches a cup.
On this page
"Tea polyphenol" is not one compound. It is a family: flavanols (the catechins, the dominant share), flavonols and their glycosides, phenolic acids, and, once oxidation has acted on them, theaflavins and thearubigins.
The catechin family, and what each member is
Catechins belong to the flavan-3-ol subclass and share a common two-ring, oxygen-bridged skeleton. Six of them account for nearly all of a fresh leaf's catechin content: catechin (C), epicatechin (EC), gallocatechin (GC), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG). Pooled across cultivars, EGCG is usually the largest single share of a fresh, minimally oxidized leaf, and its galloyl group gives it aggressive salivary-protein binding, among the strongest of the six alongside ECG.
A review6 found EGCG the predominant catechin in Chinese oolong tea, ahead of EGC then ECG, though the same review found oolong carries the highest EGC concentration of any tea category it compared, ahead of white, green, and black tea.
The six sort into two functional camps that matter in the next section: catechol-type catechins (EC, ECG) and pyrogallol-type catechins (EGC, EGCG), named for the ring structure that decides how each pairs up once oxidation begins.
What kill-green freezes, and what runs past it
Oxidation is enzymatic, catalyzed by polyphenol oxidase (PPO)1 the moment bruising ruptures the leaf's cells and lets the enzyme reach the catechins. PPO first turns a catechin into its reactive quinone form. From there, a catechol-type quinone pairs with the electron-rich B-ring of a pyrogallol-type catechin, and the joined product cyclizes into a theaflavin: a molecule built on a fused benzotropolone skeleton2.
| Catechin pair | Product | Short name |
|---|---|---|
| EC + EGC | theaflavin | TF1 |
| EC + EGCG | theaflavin-3-gallate | TF2A |
| ECG + EGC | theaflavin-3-prime-gallate | TF2B |
| ECG + EGCG | theaflavin-3,3-prime-digallate | TFDG |
Push the reaction further and theaflavins polymerize again into thearubigins, the brown compounds that give a fully oxidized liquor its depth. One analysis, resolving the class by LC/MS/MS1, found the thearubigins it could identify were built entirely from comparatively low-molecular-weight compounds, under 2,100 grams per mole, not the sprawling giants the older literature assumed. The class is still only partly mapped.
Run the reaction to its end and roughly three-quarters of a leaf's original catechin content converts into theaflavins, thearubigins, and related oxidation products, which is what a fully fermented black tea does. Oolong's kill-green cuts in mid-reaction, so a finished oolong always carries all three: unconverted catechins, first-generation theaflavins, and some thearubigins, in a ratio that shifts continuously with how far you let kill-green wait. A Bao zhong fixed near the 8 to 12 percent floor is still mostly catechins. A Wuyi yancha fixed near 60 to 80 percent has traded much of that catechin pool for theaflavins and thearubigins, which is also why its liquor runs amber to red rather than pale gold.
Where oolong sits
Published catechin percentages for oolong disagree with each other: methodology5, cultivar, season, and roast all move the number, and no two labs report quite the same range. What holds across every study, regardless of the exact numbers each one reports, is the order: green tea highest, oolong in the middle, black tea lowest, tracking the oxidation percentage directly rather than any one cultivar's chemistry.
Astringency is a protein reaction
Astringency is a tactile sensation, not one of the five basic tastes: catechins, the gallate-bearing ECG and EGCG most aggressively7, bind salivary proline-rich proteins and cross-link them into a precipitate3 large enough to strip your mouth's normal lubrication. Because theaflavins and thearubigins bind less aggressively than the catechins they were built from, a more heavily oxidized oolong is typically rounder and less astringent than a lightly oxidized one at the same leaf-to-water ratio. How far the leaf was oxidized, not steep time alone, is the lever worth learning if astringency is what you are managing.
The cup that clouds when it cools, and clears again when you reheat it
Cool a strong, heavily oxidized cup and it turns faintly cloudy. Reheat it, and the cloud vanishes. The haze is a reversible, non-covalent complex forming between theaflavins and caffeine4 as the liquid cools. The more theaflavin a liquor carries, the more readily this "cream down" appears, so a heavily oxidized yancha will haze on cooling far more visibly than a jade Tieguanyin barely past the green edge.
The health trials
Oolong and Health reads the cardiovascular, bone, and inflammation record against this chemistry in full. The Oolong Weight-Loss Trials, The Oolong Diabetes Trial Everyone Cites, and The 1987 Paper That Invented GABA Oolong each test one specific claim built on it.