Leave a few tea leaves in a jug of cold water overnight and you get something that tastes quite unlike the same leaves made hot: sweeter, rounder, with almost none of the dry, puckering edge. No kettle, no timer, and it is very hard to ruin. The only thing to get right is how much leaf and how long, and that is what the calculator below works out.
Tea leaves contain compounds that dissolve into water at different speeds. Catechins (such as EGCG) and caffeine are the bitter, astringent part. Amino acids, mainly theanine, taste savoury and gently sweet. Heat pulls out all of them fast. Cold water is slow and picky: a Japanese review of cold-water extraction (looking at sencha) reports that water at about 10 °C extracted only around a fifth of the EGCG and about half of the caffeine that hot water did, while the amino acids came out far more readily. In iced water (about 0.5 °C), caffeine fell below a fifth of the hot-water level, but amino acids still reached roughly 80 percent.
| Compound | Tastes | Cold water vs hot |
|---|---|---|
| EGCG (catechin) | Bitter, astringent | About 20% at 10 °C |
| Caffeine | Bitter | About 50% at 10 °C, under 20% at 0.5 °C |
| Theanine and other amino acids | Savoury, sweet | About 80% at 0.5 °C |
The result is a cup where the sweet part is intact and the harsh part is turned down. That is also why a cold brew is forgiving: leave it an extra hour or two and it gets stronger, not suddenly bitter. It still contains caffeine, just less of it than a hot cup made from the same leaf.
Choose a tea, set the size of your jug and how strong you like it, and read off the leaf and the steeping window. Then drag the hours to see where you are. The numbers are rules of thumb from published cold-brew guides, not a lab model.
A hot cup is usually about 2 g of leaf per 150 ml, roughly 13 g per litre. Cold water extracts less, but the long steep makes up for much of it, so cold brew guides land on 6 to 12 g per litre, with one popular recipe using 8 g in 750 ml. Start at 9 g per litre and adjust: if it tastes thin, add leaf next time rather than steeping for days.
Time depends on the leaf. Green and white teas are delicate and open up in about 6 to 8 hours. Black and oolong teas are denser and usually want 8 to 12 hours, so overnight suits them. Herbal infusions follow the black tea window. Rolled or broken leaf (and tea bags) gives up flavour faster than whole leaf, so lean to the short end of the window.
Cold brewing swaps speed for forgiveness: the longer you wait, the stronger it gets, but it rarely turns harsh.
Steep it in the fridge, not on the counter, then strain out the leaves so it stops getting stronger. Keep it covered and cold, and drink it within two to three days, since there is no hot water to kill anything off. Flavour fades before safety does, so a jug that smells off or looks cloudy and slimy goes down the sink. Use a clean jug and clean water.
A second steep of the same leaves works: add fresh water and give them a longer steep than the first time. For a fizzy version, pour a third of a glass of cold brew over ice and top up with chilled sparkling water; fruit slices or a mint sprig add aroma without sugar. Sencha, jasmine green and a plain Darjeeling are friendly starting points. Cold brew black tea is also a good base for an alcohol-free iced tea.
You need very little kit. A clean, lidded jug that fits in your fridge door is the main thing. A fine mesh or a bottle with a built-in infuser makes straining easy, and whole or loose leaf tea usually gives a sweeter result than dust-fine bag tea. Good water matters because it is most of the drink, so use filtered water if your tap water is hard or tastes of chlorine.