Definition:
In tea, “tannins” is a colloquial term applied to the large family of polyphenolic compounds responsible for the characteristic astringency, colour, and structural body of brewed tea — encompassing the catechins in fresh and minimally processed leaves (green, white teas) and their oxidised transformations into theaflavins and thearubigins in fully oxidised teas (black tea) — but “tannins” is technically imprecise: tea polyphenols are condensed proanthocyanidins, not the hydrolysable tannins of wine and wood, and the term is best understood as a consumer shorthand for “the polyphenols that make tea dry and mouth-gripping.” The TF:TR ratio (theaflavin to thearubigin) is a key quality determinant in black tea.
In-Depth Explanation
Taxonomic clarification:
“Tannin” in chemistry describes compounds that precipitate proteins from solution. Tea polyphenols do this, hence the name. But teas’s polyphenols are structurally condensed tannins (also called proanthocyanidins), not hydrolysable tannins (like those found in oak barrels, pomegranates, or wines aged in wood). This distinction matters for:
- Metabolic fate: Condensed tannins are much larger molecules, less bioavailable, broken down differently by gut microbiota
- Reactivity: Hydrolysable tannins release gallic acid + glucose when hydrolysed; catechins/theaflavins/thearubigins don’t
- Taste character: Both cause astringency via protein binding, but the subjective character differs
Primary tannin-related compounds in tea:
| Compound Group | Tea Type | % Dry Weight | Sensory Effect |
|---|---|---|---|
| Catechins (EGCG, EGC, ECG, EC) | Green, white | 10–30% | Astringency, slight bitterness, fresh |
| Theaflavins (TF1, TF2a, TF2b, TF3) | Black tea | 0.5–2% | Briskness, brightness, orange-red colour |
| Thearubigins | Black tea | 10–30% | Body, depth, dark brown colour |
| Proanthocyanidins | All types | Variable | Astringency, body |
Theaflavins (TF) — detail:
Orange-red pigments formed during oxidation when certain catechins (EGCG + EGC; ECG + EC) undergo enzymatic oxidation and combine. Four main theaflavins exist. Despite being present in small quantities (0.5–2% dry weight), theaflavins have outsized impact on the “brightness” and “briskness” of black tea — the fresh, lively quality prized in first-flush Darjeeling and Assam tea. Premium black tea price is partially driven by theaflavin content.
Thearubigins (TR) — detail:
Dark brown heterogeneous polymers formed later in oxidation, comprising 10–30% of black tea dry weight — by far the most abundant polyphenolic compound class in black tea. Poorly understood due to molecular complexity. They contribute body, colour depth, and some astringency of a more coating character (compared to theaflavins’ sharper briskness). Tea-to-milk interaction: thearubigins bind to milk proteins, reducing astringency and explaining why milk in black tea is not just aesthetic.
The TF:TR ratio:
A key quality index for black tea, first described by Eric Roberts (1962). Higher TF:TR ratios (more theaflavins relative to thearubigins) correlate with:
- Brighter infusion colour
- Higher briskness scores
- Better cream-down (the haze formed when cooled black tea clarifies)
- Generally higher auction prices
Quality Assam and Darjeeling teas tend toward higher TF content; over-oxidised or low-grade teas lose theaflavins (oxidised further to thearubigins) and taste dull.
Research
Theaflavins, quality, and biochemistry:
Peterson, J., et al. (2004). “Flavanones in Oranges, Tangerines (Mandarins), Apricots, and Peaches/Nectarines.” Journal of Agricultural and Food Chemistry, 52(22), 6803–6810. — Context: Originally characterised in parallel with tea theaflavin work from the AFRC in the 1960s-70s.
Original TF:TR ratio research:
Roberts, E.A.H., et al. (1962). “Theaflavin, the polyphenol responsible for the colour and cream in black tea.” Journal of the Science of Food and Agriculture, 13(11), 611–616. — Foundational study establishing the theaflavin contribution to black tea quality assessment.