Brewing

Steep coffee in room-temperature water, then filter. Measurements show caffeine levels off at about 6–7 hours. Ratio, time, grind and storage.

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Coffee Rue Journal
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Drawing from above. A glass jar of coffee steeping in cold water, its screw lid beside it and a dial run round to seven hours.

To make cold brew, mix ground coffee with cool or room-temperature water in a jar, leave it for several hours and then filter it. There is no heat; time does the extracting. Two peer-reviewed studies used one part coffee to ten parts water, 21–25 °C and about seven hours of steeping. Here is what they found, and how to use it at home.

How long should it steep?

Most recipes give 12 to 24 hours. A 2017 study in Scientific Reports by Megan Fuller and Niny Rao of Thomas Jefferson University measured instead of assuming. They steeped 35 g of coffee in 350 ml of water at room temperature and sampled from 15 minutes up to 24 hours.

Caffeine and 3-chlorogenic acid (3-CGA) reached equilibrium at roughly 400 minutes — around six to seven hours. After that point there was no further significant extraction of 3-CGA.

It's worth reading this carefully. The study measured two compounds, not everything that makes up flavour, and a longer steep may still change the taste. But the data don't support the idea that cold brew needs at least 12 hours. Starting at seven or eight hours and adjusting by taste is a reasonable approach.

Does grind size matter?

The same study compared medium and coarse grinds. In cold brew, grind size didn't significantly change the amount of caffeine or 3-CGA. The researchers read this as a sign that extraction of these compounds isn't limited by surface area.

In practice, a coarse grind is easier to filter and leaves less sludge. If you are using a paper filter, it also helps keep the filter from clogging.

Roast level

Overall, the medium roast in the study gave more 3-CGA and caffeine than the dark roast. It used a single coffee (a Kona Typica arabica), so treat this as a tendency rather than a rule.

Is cold brew less acidic?

It's a common claim. In a second study in 2018, Rao and Fuller brewed six light roast coffees both hot and cold. They found:

  • pH values were very close. All samples fell between 4.85 and 5.13.
  • Titratable acidity was higher in the hot brews. Hot brewing extracts more acid without changing pH much.
  • Antioxidant activity was also higher in the hot brews.

The researchers describe any health implications as speculative and call for more work. In short, these studies offer no clear evidence that cold brew is easier on the stomach.

Storage

With hot coffee, the brewing heat gives a safety margin; with cold brew, there isn't one. The US National Coffee Association (NCA) has published a separate guide for businesses, noting that keeping cold brew "cool" requires special attention to food safety.

At home, that translates into simple habits: brew in a clean container, refrigerate after filtering and don't keep it for long. We couldn't find reliable shelf-life data for home use, so we don't give a number of days; if in doubt, pour it away.

Quick recipe

  1. Grind the coffee coarsely.
  2. Put one part coffee to ten parts water in a jar (for example 35 g coffee to 350 ml water).
  3. Stir, close the lid and leave at room temperature for 7–8 hours.
  4. Filter through paper or a fine sieve.
  5. Keep it in the fridge. Dilute with water or milk if it's too strong.

Frequently asked questions

How long does cold brew take?

Recipes usually say 12–24 hours. In published measurements, caffeine and chlorogenic acid levelled off at around 6–7 hours. Start at 7–8 hours and adjust to taste.

Does cold brew have more caffeine?

In Fuller and Rao's study, cold brew made at the same ratio had more caffeine than hot French press coffee, especially with a coarse grind. How much ends up in your glass also depends on how much you dilute it.

Should cold brew steep in the fridge or at room temperature?

The studies cited here were done at room temperature (21–25 °C). Steeping in the fridge works too; it's reasonable to expect extraction to be slower in the cold, so it may need longer.

Is cold brew the same as iced coffee?

No. Iced coffee is brewed hot and then cooled. Cold brew is never heated and steeps for hours.

Sources