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.

CatechinsA family, not a single compoundAlso called flavan-3-olsAlso called tea polyphenolsAlso called 儿茶素Also called カテキン

What a drinker notices

Astringency — a tactile sensation
Bitterness — a taste

Astringency above all — a tactile sensation, felt as the mouth drying and the surfaces of the tongue and cheek losing their slip. It comes from polyphenols associating with the proline-rich proteins in saliva and taking them out of solution, so the lubrication fails. Some bitterness comes with it, but the two are separable and a well-made green tea can be markedly drying and barely bitter.

How much of it there is matters: Astringency builds with contact time in a way bitterness does not: a cup left on the leaf keeps getting drier long after it has stopped getting much more bitter, which is why the standard advice to lower the temperature only half-fixes a hard cup.

What this gets credited for, wrongly

This is a GROUP and its members disagree. Attributing a tea's character to 'catechins' says roughly as much as attributing a wine's to 'phenolics'. The galloylated ones and the non-galloylated ones give different sensations, and the balance between them is what processing actually moves.

What it is

Catechins are the reason a fresh tea leaf is worth processing. They are the biggest soluble group in green tea by a wide margin, they produce the drying sensation the whole category is judged on, and they are the raw material for oxidation: theaflavins, thearubigins and theasinensins are all catechins that have been rearranged. Six matter in tea — EGCG, ECG, EGC, EC, and smaller amounts of gallocatechin and catechin itself — and they do not behave alike. The two carrying a gallate group are markedly the most drying; the two without it read as bitter more than as drying. A tea's character depends less on how many catechins it has than on which ones.

Chemically: Flavan-3-ols — the dominant polyphenol group of the fresh tea leaf.

Where it occurs in the plant

Highest in young leaf grown in full sun and lowest in shaded or early-spring leaf, which is the mirror image of the amino acid gradient and the reason the two are treated as a ratio. Light drives their formation, so shading suppresses them directly.

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.

Shading — Lowers it: Shading holds catechin formation down while the amino acids accumulate. It is the single most effective agricultural lever on astringency, and it is the reason gyokuro tastes the way it does.
Oxidation — Turns it into something else: Oxidation does not destroy catechins, it rearranges them. Ruptured cells release polyphenol oxidase, the catechins are oxidised to reactive quinones, and those combine into theaflavins and then thearubigins. The same carbon, a different arrangement, an entirely different cup — and 'black tea has fewer catechins' and 'black tea's catechins became theaflavins' are the same measurement with only the second explaining anything.
Kill-green (fixation) — Locks it in place: Fixation denatures the leaf's oxidase and freezes the catechin profile as it stands. Everything that makes green tea green tea follows from this step happening early rather than late.
Wet piling (wo dui) — Turns it into something else: In shou pu-erh the microbial pile takes the catechins somewhere oxidation alone does not, breaking them down and building the large brown polymers that make a ripe pu-erh dark, thick and almost free of astringency within weeks.
Ageing and storage — Lowers it: Slow change over years in a sheng pu-erh or an aged white tea, without a pile and without enzymes. Astringency falls and the liquor darkens — the reason aged tea is described as softening rather than as improving.
Drum fixation — Locks it in place: Most Chinese green tea is now fixed in a heated rotating cylinder that leaf passes through continuously, and the dwell time is what decides whether this profile is frozen or merely slowed. A drum run slightly cool leaves enzyme activity alive in the thickest pieces, which is why an under-fixed green tea keeps changing in the bag.
Sealed yellowing (men huang) — Lowers it: The smothering step works on the polyphenols as well as on the pigment. Hours of warm damp confinement after fixation soften this pool by slow non-enzymatic change, and the mellowness that separates a yellow tea from the green tea it would otherwise have been is largely that.
Heaped rest before fixation (dui qing) — Turns it into something else: Heaping the leaf at the end of the shaking cycles raises its own temperature and pushes the last of the oxidation through quickly, just before the heat goes on. It is a short, deliberate, hard-to-reverse acceleration, and the difference between an oolong that reads as green and one that reads as ripe can be this step alone.
Repeat rolling and roll-breaking (fu rou) — Turns it into something else: One long rolling pass tightens the outside of a ball of leaf and never reaches the middle. Rolling in two or three passes with the lumps broken up between them exposes fresh cell contents each time, so oxidase and substrate keep meeting — which makes the conversion even rather than merely thorough.
Wetting the pile (chao shui) — Turns it into something else: Adding a measured amount of water to dry maocha is what starts a shou fermentation at all, and this pool is the substrate the microbes work on. Too little water and the pile never heats; too much and it runs sour. It is the single decision that most decides what the finished tea tastes like, and it is made in the first hour.
Turning the pile (fan dui) — Turns it into something else: The scheduled turns are how a pile is steered. Turning redistributes heat and moisture so the conversion runs at a similar rate through the heap instead of racing at the core and stalling at the edge — which is the difference between an even shou and one with three teas in it.
Stone pressing — Lowers it: Pressing does not change this pool and the density it sets decides how fast everything afterwards does. A stone-pressed cake, weighted by a person standing on it, comes out looser than a hydraulic one, and a looser cake breathes and ages faster — so the same maocha pressed two ways is two teas ten years later.
Log packing (Qian Liang) — Turns it into something else: Ramming steamed dark tea into a woven casing and compressing the whole column by treading it produces something with a genuine interior: the middle of a Qian Liang log stays warm and damp for a long time after the outside has dried, and this pool goes on converting in there. The shape is a fermentation vessel, not a package.
Basket ageing — Lowers it: Woven bamboo regulates airflow rather than excluding it, so a dark tea aged in basket keeps a slow supply of the air and moisture this change needs. It is why basket storage is treated as part of manufacture in Liu Bao rather than as warehousing.

How brewing brings it out

Extraction rate: Slow — needs heat and time

Catechins need heat in a way the amino acids and caffeine do not, and the galloylated ones need it most. That single difference in rate is the mechanism behind almost every temperature recommendation on this site: hot water pulls the drying fraction, cool water leaves much of it behind.

Across infusions: Back-loaded relative to caffeine and amino acids. It is why a first infusion tastes sweeter and a second tastes drier — not because anything was 'used up', but because the fast compounds came out first and the slow ones are still arriving. That observation is repeated everywhere in gongfu writing and this is its whole explanation.

Water: Extraction rises with temperature and falls in very hard water, where calcium and magnesium interfere with the polyphenols and dull both the colour and the drying edge. Alkaline water darkens the liquor and flattens it.

Leaf, ratio and agitation: Agitation and grade matter as much as time. Broken leaf and stirring raise astringency sharply at the same nominal steep, which is why a stirred mug of CTC is harsher than an unstirred pot of orthodox at the same strength.

What you can change

Lower the water temperature
Shorten the contact time rather than diluting afterwards
Stop stirring, and pour off completely
Use whole leaf rather than broken

Where this gets misdescribed

That catechins are tannins. In wine and oak, tannins are a specific class of large polyphenols; catechins are smaller, chemically distinct, and true condensed tannins are a minor part of what tea contains. The word survives because it names a sensation people recognise, and it names the wrong chemistry.

Teas this compound defines

What this page does not say

Catechins reach most readers through a health heading rather than a sensory one, and this record is the sensory one. It describes where they occur, what processing does to them, how they extract and what they feel like in the mouth. It does not say what they do in a body — not in either direction, and not hedged. That surface is held on TeaHQ and stays held.

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.

How TeaHQ handles sources