The intuition, and why it fails
If heat makes extraction faster, it seems to follow that cold water simply needs longer to reach the same place. It does not, and the reason is that temperature does not scale every component’s extraction equally. The compounds most associated with bitterness and astringency are considerably more reluctant to leave the leaf in cold water than the amino acids and sugars are, so a long cold steep does not slowly accumulate the same profile — it accumulates a different one and then largely stops. Cold brewing is selective in a way that no adjustment of hot brewing reproduces.
Why cold brewing is so forgiving
Because the harsh fraction is barely participating, the usual penalty for over-steeping is mostly absent. An extra two hours in the refrigerator changes the result far less than an extra two minutes changes a hot steep, which is why cold brewing is the most reliable method for someone who does not want to pay attention. That forgiveness comes with a corollary: it is also the method with the least room to steer. There is very little to adjust and very little that adjustment does, so cold brewing rewards good leaf and offers no technique to hide behind.
What cold water does not get out
Compounds that need heat to become available stay in the leaf. That is most obvious with heavily roasted and compressed teas, which give thin, characterless cold brews because the material that makes them interesting is not accessible at low temperature. Caffeine extracts more slowly in cold water than in hot; the widely repeated claim that cold brew is therefore a low-caffeine drink runs well past what follows from that, because a cold brew usually runs for many hours and often uses a generous quantity of leaf. TeaHQ states the extraction behaviour and declines to convert it into a figure for the finished drink.
Sequence, not just endpoint
There is a second difference that matters for anyone reasoning about the two. In hot water the process is largely finished within minutes and the danger zone is at the end. In cold water the process is still meaningfully incomplete after hours, so the practical question is not when to stop but whether more time is still adding anything worth having. In practice most cold brews reach the useful part of their range within a working night and continue changing slowly afterwards, gaining body and losing freshness in roughly equal measure.
Temperature and how long the water is standing
The one genuine practical caution about cold methods is not about flavour. Refrigerated cold brewing keeps the water cold throughout. Steeping at room temperature or in sunlight sits the water in the range where microbial growth in a nutrient-rich liquid is fastest, for hours, which is the reason several public-health bodies advise against sun tea and the reason refrigerator cold brew is the sensible default. It is also better tasting, which makes the choice an easy one — there is no trade-off here to weigh up.
What the evidence supports and what it does not
The direction of the effect is well established and easily demonstrated at home: brew the same leaf hot and cold side by side and the difference is unmistakable. What is not settled, and what confident writing frequently asserts, is the precise composition of a cold brew relative to a hot one for a given tea. Published comparisons exist, they cover a limited range of teas and conditions, and they do not generalise cleanly to whatever is in your cupboard. Cold brew is different, reliably and substantially; the specific numbers people attach to that difference deserve more scepticism than they usually get.