The acids in the leaf
A tea leaf contains a range of organic acids — gallic acid, quinic acid, citric and malic acids and their relatives among them — and their levels change through processing. Gallic acid in particular is closely tied to the polyphenol chemistry, since the galloylated catechins carry a gallate group and hydrolysis during processing and storage releases it. The result is a liquor that is mildly acidic, generally more so than plain water and far less so than a fruit juice or a soft drink. Two things are worth stating and neither is a number. TeaHQ does not publish a pH figure for any tea, because it varies with the leaf, the ratio, the steep and the water, and a value quoted without those is not a measurement. And the acidity of a normal tea liquor is not what a drinker perceives as sourness, which is a threshold phenomenon rather than a linear one — the acids are present in every cup and are noticed only when something has pushed them, or the perception of them, further forward than usual.
Sourness from storage, which is the commonest case
The most frequent source of unexpected sourness is moisture, and the signature is specific enough to be diagnostic. A lightly oxidised, unroasted oolong that has taken up atmospheric water turns sour, and so does a roasted oolong that has gone too long without re-firing, where reabsorbed moisture dulls the roast and pushes it in the same direction. This is the single most common storage fault with a sour signature, and it is why the traditional maintenance of aged oolong is a periodic light re-firing rather than tighter sealing. The distinguishing features are that the sourness is present in every cup regardless of preparation, that it is frequently detectable on the dry leaf, and that it is accompanied by dullness rather than by any gain. Where a tea has become sour since it was bought, the useful question is about the cupboard rather than the kettle, and no brewing adjustment addresses it. Where the sourness is sharp and accompanied by a musty, cellar-like smell on the dry leaf, that is a moisture failure of a more serious kind.
Sourness from brewing, which is easy to rule out
Brewing can produce a mild sourness in teas that had none, by two routes. A very long extraction — an over-long cold brew, or hot tea left standing for hours — develops a flat acidity from the slow chemistry of what is already in solution, and the change is one of the reasons a made pot does not keep. And additions supply acidity directly, lemon being the obvious case, which also shifts a green liquor towards a browner colour because the pigment chemistry responds to acidic conditions. The distinguishing test is trivial: a brewing sourness appears only under a particular preparation and disappears under another, while a storage sourness is present in every cup. Making one differently brewed cup settles which you have, and it takes five minutes. This is worth running before concluding anything, because the two conclusions lead to opposite actions — one means adjust the method, the other means look at where the tea has been kept and check whether the rest of the store is at risk.
Sourness as part of a style
Some teas are meant to be a little sour and the acidity is a feature rather than a fault. Certain aged white teas develop a plum-like acidity that drinkers value; some Liu Bao and other hei cha carry a genuine acidity from microbial activity, which is part of their character; and a number of post-fermented styles sit deliberately in that territory. There is also a small family of genuinely fermented tea products — the pickled and fermented leaf preparations of parts of Southeast Asia and of Japan among them — which are sour by construction and are a quite different category from anything else in this catalogue. Reading a sour note therefore requires knowing what was made: the same character is a storage failure in a high mountain oolong, a stage in a young shou, and a defining feature in an aged Liu Bao. This is the general pattern that runs through the whole sensory set, and sourness is the descriptor where it bites hardest, because the word carries a negative connotation in English that the underlying quality does not deserve.
What acidity does to everything else
Beyond being perceived as sourness, acidity acts on other things a drinker notices. It affects liquor colour, since the pigment chemistry of green tea responds to acidic conditions and shifts towards browner forms — which is why an iced green tea with lemon browns visibly and why a green liquor left standing dulls within the hour. It clears the haze that forms as a strong black tea cools, which is why a squeeze of lemon is a standard remedy for cloudy iced tea and why that remedy addresses an appearance rather than the drink. And it interacts perceptually with sweetness and bitterness in the ordinary way that tastes suppress and reinforce one another, so a tea with a noticeable acidity reads as less sweet at the same composition. None of these is a large effect in a normal cup and all of them become visible in the situations where acidity has moved, which makes them useful confirmation when trying to work out what has changed about a tea.
What this page does not say
No pH value, acid content or comparison between categories is given for any tea, and none should be inferred. Values depend on the leaf, the ratio, the temperature, the steep time, the infusion number, the water used and the analytical method, and a figure quoted without that basis cannot be checked, reproduced or compared — which is the same standard the catalogue applies to caffeine and to every other number it declines to publish. Nothing here describes any effect of acidity on a person, which is a different subject governed elsewhere under a different standard of evidence and which this page does not address. What is asserted is the direction of a small set of relationships: tea liquor is mildly acidic, sourness in a finished tea is most often a moisture signal, brewing can produce a mild sourness of its own that a differently brewed cup will distinguish, and in several post-fermented and aged styles acidity is a genuine part of the character rather than a defect.