The first five minutes: temperature and aroma
The fastest change is the one nobody counts as a change. A cup cools, and the same liquid tastes different at three temperatures — bitterness generally reads as less prominent as a drink cools and sweetness as more, which is why letting a harsh cup stand for a few minutes is such a reliable correction and why professional evaluation standardises the temperature at which liquors are assessed. Alongside that, the aromatic fraction is leaving. Volatile compounds escape from a hot open surface continuously, which is what a cup of tea smells like from across a room, and what is smelled in the room is no longer in the cup. A fragrant oolong or a good sencha is therefore at its aromatic best immediately and measurably less so after ten minutes, which is the practical argument for small cups in the gongfu styles: a small volume is drunk before it has time to cool or to disperse, and the next one is poured fresh. The same reasoning explains why a large pot poured out slowly across a morning is a different proposition from the same tea in a small vessel, and why a lidded cup or a cosy changes more than it appears to.
Tea cream, and why iced tea goes cloudy
A strong black tea liquor goes hazy as it cools, and the mechanism is worth knowing because it is so widely misread as a fault. Caffeine and the larger oxidation products of a fully oxidised tea form complexes that remain in solution while hot and come out of it as the liquor cools, producing the haze the trade calls cream. It is associated with a liquor rich in exactly the compounds a good black tea should have, so it is closer to a quality signal than a defect. It is also the direct cause of cloudy iced tea, which is the same event in a glass. The remedies all address the cooling rather than the tea: brew hot and chill quickly, brew at double strength over ice so the drop is fast, or brew cold from the start so the complexes never form. A little acid, as lemon, clears an existing haze. None of these improves the drink, and it is worth knowing that the cloudy version is generally the better-extracted one. Whether a household regards the haze as a problem is an aesthetic decision rather than a technical one, and the catalogue keeps a separate troubleshooting record for it because the question comes up so often.
Colour, and why it deepens on its own
A liquor left standing darkens, and it does so faster than any change in flavour would suggest. Two things contribute. The compounds already in solution continue to react slowly with dissolved oxygen and with one another, in a much slower version of the chemistry that darkened the leaf during manufacture. And in green tea specifically, the chlorophyll-derived pigments shift under acidic conditions towards the olive-brown forms that make an hour-old green liquor look tired, which is also why an iced green tea with lemon browns visibly. Neither change tells you much about how the drink tastes, which is the useful point: colour is a poor guide to strength at the best of times, and it is an actively misleading guide in a cup that has been standing. Anyone judging a tea, comparing two teas, or photographing a liquor should do it on a freshly poured cup, and anyone deciding whether a cup left on a desk is still worth drinking should smell and taste it rather than looking at it. The visible change runs well ahead of the perceptible one, which is the opposite of the assumption most people make.
Leaf left in the water
Where the leaf has not been separated from the liquor, none of the above is the main event. Extraction simply continues, and it continues along the curve that governs every brewing decision in this catalogue: the fast, sweet, aromatic fraction has largely finished, and what keeps arriving is the material associated with astringency and bitterness. A pot left with the leaf in becomes progressively grippier rather than merely stronger, which is why a cup that was pleasant at three minutes is unpleasant at fifteen, and why the fix for it is decanting rather than dilution — a liquor that has gone that way does not come back with hot water, it becomes a weaker version of the same imbalance. This is the whole argument for separating leaf and liquor as a habit, whether by pouring off into a second vessel, lifting an infuser basket out, or using a vessel that does it by construction. The grandpa style is the deliberate exception rather than a counterexample: it works because the ratio is low and the leaf is a category chosen to tolerate it, and the same practice applied to a fine oolong at a normal dose produces exactly the cup this paragraph describes.
Hours later, and what cold brewing changes
Over several hours a standing liquor develops a flat acidity of its own, from the slow chemistry of the organic acids and other components already in solution, and it loses essentially all of its aromatic top. What remains is dull and slightly sour, which is a different thing from a tea that was brewed too long: one is an extraction problem and the other is what happens to a solution over time. This is the honest reason a made pot does not keep, and it applies to a flask as much as to a cup, since a vacuum flask holds temperature and does nothing about time. Cold brewing sits differently. A long extraction at refrigerator temperature collects a sweeter, much less astringent liquor from the same leaf, and the low temperature also slows the standing changes described here — which is why cold-brewed tea holds its character better in a jug than hot-brewed tea does. That is a property of the temperature rather than of the method, and it is the practical reason cold brew is the sensible route for iced tea in quantity: it avoids the haze, it avoids the astringent late fraction, and it degrades more slowly once made.
What this catalogue does and does not address
Everything above concerns how a brewed liquor changes as a drink: its temperature, its colour, its aroma, its texture and its balance. TeaHQ regards brewed tea as something to be made and drunk rather than stored, and the practical guidance that follows is simply to make less at a time, to separate the leaf from the liquor, and to brew cold from the start where a quantity is wanted for later. Questions beyond the sensory ones — what is or is not acceptable to keep, and for how long — belong to food handling guidance rather than to a tea catalogue, and this page does not answer them. What it does answer is the question that actually prompts most of them, which is whether a cup that looks different from the one that was poured has gone wrong. Usually it has changed rather than gone wrong: it has cooled, it has darkened, it may have hazed, and if the leaf was left in it has moved along the extraction curve. Each of those is a specific event that can be headed off rather than corrected, and knowing which one has happened is what turns an unsatisfying cup into an adjustment for the next one.