Why your cold brew tastes flat

Cold water is a weak solvent

Extraction is driven by temperature. At 4 °C or at room temperature, water pulls compounds out of coffee slowly, and it pulls them out selectively: acids and most of the bitterness come out reluctantly, sugars and caffeine come out readily. That selectivity is exactly why cold brew tastes sweet and smooth.

It also means the margin for under extraction is wide. A cup that tastes watery, papery, of nothing much, is almost always a cup that did not extract enough.

The three flat cold brews

Under extracted. Thin, faintly sour, no sweetness, no finish. Causes: too short a steep, too coarse a grind, or too little coffee. Fix one at a time, in that order.

Over diluted. Sweet but weak, tasting correct in the first sip and empty after it. You made a concentrate and treated it like a ready to drink brew. Weigh both the concentrate and the water going into the glass, and remember that ice adds 20 to 40 per cent more water as it melts.

Stale. Flat in a different way: no aroma at all, a slight cardboard note. Cold brew oxidises in the fridge like everything else. Concentrate keeps about two weeks, ready to drink three days, and coffee that was already old when it went in was never going to give much.

The recipe that works

1. Weigh. 1:5 coffee to water for concentrate, 1:12 to 1:15 for ready to drink.
2. Grind coarse, around a French press setting. Coarser than that and 16 hours will not be enough; finer and the filtering afterwards becomes a chore.
3. Wet everything. Stir once so no dry pockets float on top. Dry grounds extract nothing.
4. Steep 14 to 18 hours at room temperature, or 18 to 24 in the fridge. Room temperature extracts faster and gives more body; the fridge gives a cleaner cup.
5. Filter through a fine mesh and then through paper if you want it clear.
6. Taste before diluting, and dilute to taste rather than to a rule.

Two upgrades worth the effort

Use water with some mineral content. Cold brew made with distilled or heavily softened water is the flattest of all, because there is nothing in the water to carry extraction.

And try a light roast. The usual advice to use dark roast exists to avoid bitterness that cold water was never going to extract anyway. A washed Kenyan steeped cold gives blackcurrant and sugar that no dark roast will ever produce.