Organic acids
The sourness axis, and the reason a tea liquor is mildly acidic — which decides what colour it is as much as what it tastes like.
What a drinker notices
- Sourness — a taste
A thin sourness. Wanted in the pickled teas of Japan and mainland South-East Asia, where it is the point; a fault signal almost everywhere else, especially in a stored cake.
How much of it there is matters: Barely perceptible at ordinary levels and a clear defect above them. There is no register in which a moderately sour black tea is considered good.
What this gets credited for, wrongly
Sourness in aged pu-erh is often described as a sign of age. It is more often a sign of damp storage or of an unrested pile, and genuine age does not make a tea sour.
What it is
Tea liquor is mildly acidic, and the small organic acids of the leaf are why. In the cup they contribute a faint sourness that is rarely a feature and often a signal: a distinctly sour dark tea is usually telling you about its storage, and a sour green tea about its age. Their more consequential role is indirect. Acidity governs the colour of both the theaflavins in a black tea and the anthocyanins in a purple one, so the acid balance of the cup — and of the water it was made with — changes what a tea looks like without changing what is dissolved in it. The lemon-in-tea effect is this, and nothing else.
Chemically: Small carboxylic acids — citric, malic, succinic, quinic and others.
Where it occurs in the plant
Present throughout the leaf as part of ordinary plant metabolism, and generated further during fermentation and storage. Pickled teas made by lactic fermentation are the extreme case and are frankly sour by design.
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.
- Anaerobic pickling — Creates it
- Lactic fermentation under exclusion of air generates them in quantity, which is what makes goishicha, awa bancha and the pickled teas of mainland South-East Asia distinctly sour — a taste no other tea process produces.
- Wet piling (wo dui) — Raises it
- Raised during the pile and reduced again by resting. A shou that tastes sour has usually not been rested long enough, and time in the open genuinely settles it.
- Warehouse storage and airing (ru cang, tui cang) — Raises it
- Damp storage raises them, which is why sourness is one of the standard markers used to identify a badly kept cake.
- Drying the pressed cake — Raises it
- A freshly pressed cake is wet in the middle, and how that moisture is taken out over the following days is where a great many sour young cakes are made. Dried too slowly, or stacked before the core is dry, and the interior ferments on its own terms — the sourness a buyer finds two years later was decided in the first week and is not a storage fault.
- Steam softening before pressing (zheng ya) — Raises it
- The steam that makes leaf compressible also puts water into it, and everything that follows has to remove that water again. A heavier steam gives a cake that presses beautifully and carries more of this the following spring, which is one of the quieter trade-offs in dark tea manufacture.
How brewing brings it out
Extraction rate: Fast — among the first things out of the leaf
Small, highly soluble and out immediately. A sour tea tastes sour from the first sip and does not become less so with a gentler brew.
Across infusions: Front-loaded. Rinsing and discarding takes some of it away, which is standard practice with a young shou for exactly this reason.
Water: Their effect on colour is larger than their effect on taste. Adding acid lightens a black tea's liquor visibly and dulls a green tea's; alkaline water does the reverse to both. Nothing has been diluted in either case.
Leaf, ratio and agitation: Extract early and fully, so the brightness they contribute is a first-infusion characteristic that a longer steep dilutes rather than deepens.
What you can change
- Rinse and discard the first infusion of a young dark tea
- Rest a sour cake in open air for months rather than brewing around it
- Use acidity deliberately to lighten an iced black tea
Where this gets misdescribed
That tea is alkaline, or that brewing it changes the pH of anything meaningfully. Tea liquor is mildly acidic, which is a fact about the cup and is stated here for that reason alone.
Teas this compound defines
Read next
- Gallic acid — The gallate group after it has been separated from its catechin — small, sour, drying, and a marker of how far a dark tea has fermented.
- Anthocyanins — The purple of purple tea — water-soluble, unlike the other pigments here, and it changes colour with the acidity of the water.
- Theaflavins — The bright orange-red pigments oxidation builds from catechins, and the compounds the tea trade means by briskness.
- Pheophytin — What chlorophyll becomes when heat or acid knocks the magnesium out — and the whole explanation for green tea going olive.
What this page does not say
This record describes acids in the leaf and in the cup, their taste and their effect on colour. It makes no statement about acidity in a body, which is the ground most writing on this subject occupies and is not a question this page answers.
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.