Theasinensins
The dimers formed when two catechins of the same type couple — the oxidation route oolong takes and black tea largely does not.
What a drinker notices
- Astringency — a tactile sensation
- Body and texture — a tactile sensation
A soft, rounded drying sensation and a thickening of the texture, without the bright grip theaflavins give. It is part of what makes a well-oxidised Oriental Beauty feel dense and honeyed rather than brisk.
What this gets credited for, wrongly
The honeyed depth of leafhopper-bitten oolong is usually credited entirely to the aroma compounds the insect damage provokes. The aroma is genuinely the headline; this fraction is a large part of why the texture matches it.
What it is
A theaflavin needs one catechin-type unit and one gallocatechin-type unit. When two gallocatechin-type units couple with each other instead, the product is a theasinensin, and the tea gets darker and heavier without getting brighter. That route is favoured by the partial, uneven, bruise-and-rest oxidation of oolong manufacture rather than by the full, even oxidation of black tea, and it is part of why a heavily oxidised oolong does not simply look and taste like a weak black tea. They also form in withering white tea, where nothing is rolled and oxidation happens slowly at the leaf edges.
Chemically: Catechin dimers formed by oxidative coupling of two gallocatechin-type units.
Where it occurs in the plant
Absent from the fresh leaf. Formed during partial oxidation, particularly where leaf is bruised at the edges and rested repeatedly rather than macerated all at once.
What processing does to it
In tea, processing is not a modifier applied to a fixed composition — it is what decides the composition. These are the steps that move this compound, and each links to the step itself.
- Bruising (shaking) — Creates it
- Repeated gentle bruising of the leaf edge with rests between is the oolong maker's method, and it favours this coupling route over the theaflavin one. The bruise-and-rest cycle is the reason oolong chemistry is not simply black tea chemistry stopped early.
- Standing rest between shakes (jing zhi) — Raises it
- The rests between bruisings are where the coupling actually happens. The step that looks like doing nothing is doing most of the work.
- Withering — Creates it
- Formed slowly in a long white tea wither, where there is no rolling at all and the leaf oxidises gently from its own edges over days.
- Sieve combining (bing shai) — Raises it
- Tipping two or three white-tea sieves into one part-way through the wither thickens the bed, and a thicker bed holds its own heat and gives up moisture more slowly. Both favour this slow edge-oxidation route, which is why the last hours of a combined wither decide how much body a Bai Mudan finishes with.
- Heaped rest before fixation (dui qing) — Raises it
- Heaping the leaf at the end of the shaking cycles pushes the last of the oxidation quickly, just before the heat goes on. It is a short deliberate acceleration on the coupling route rather than the theaflavin one, and it is much of what separates a ripe-tasting oolong from a green-tasting one made from the same leaf.
- Solar withering — Raises it
- A short spell in direct sun opens both the oolong and the white tea route by warming the leaf well above ambient. The maker is starting a reaction with the weather, which is why solar withering is watched by the minute and abandoned when the cloud comes over.
How brewing brings it out
Extraction rate: Moderate
Soluble in hot water and slow in cool, like the other oxidation products. Oolong's standard high brewing temperature suits them.
Across infusions: Sustained rather than front-loaded, which is part of why a good oolong keeps giving body over many infusions.
Water: Formed from the same galloylated catechins hard water binds, and behaving similarly once made: a hard-water cup of oolong reads flatter and throws more surface scum than a soft-water one.
What you can change
- Brew hot, as oolong convention already recommends
- Expect body to persist across infusions after brightness has gone
Where this gets misdescribed
That oolong is 'partially oxidised black tea'. The percentage framing hides a difference in kind: bruise-and-rest oxidation favours a different coupling route from full maceration, so a 60% oxidised oolong is not a black tea that was stopped at 60%.
Teas this compound defines
Read next
- Theaflavins — The bright orange-red pigments oxidation builds from catechins, and the compounds the tea trade means by briskness.
- EGCG (epigallocatechin gallate) — The most abundant catechin in green tea, and the clearest single demonstration that astringency and bitterness are different things.
- Catechins — The largest soluble fraction of a green tea leaf, the source of astringency, and the compounds oxidation turns into everything a black tea is.
What this page does not say
This record describes a class of oxidation products and their contribution to texture. It makes no statement about what they do in a body.
Sources
- TeaHQ editorial synthesis — TeaHQ
This page is TeaHQ’s own synthesis of well-established tea chemistry. No external technical document stands behind it, and that constraint is why there are almost no numbers here: directions, rankings and mechanisms are stated plainly, quantities are not, because a figure nobody can check would look more authoritative than it deserves.