How to Make Cold Brew Coffee, and What the Research Says About It

Steep 100 g of coarsely ground coffee in 500 g of cold water for 12 to 18 hours. Filter it twice — once through a mesh or cloth, then through paper — and you have about 300 g of concentrate. Dilute it roughly 1:1 with water or milk, over ice.

That is the whole method. What is more interesting is how much of the folklore around cold brew turns out to be wrong when someone measures it, and two papers in Scientific Reports did exactly that.

The recipe

The concentrate route is more flexible: you decide the strength glass by glass, and the jar takes up less fridge space. The ready-to-drink route is simpler if you always drink it the same way.

Step by step

1. Grind coarse. Roughly the texture of coarse sea salt, as for a French press.
2. Combine and stir. Add the coffee to the water, not the other way round, and stir until every particle is wet. Dry clumps float and never extract.
3. Cover and wait. Twelve to eighteen hours. Room temperature extracts faster than the fridge; if your kitchen is warm, steep it in the fridge.
4. Filter twice. Pour through a fine mesh sieve or a cloth to catch the grounds, then through a paper coffee filter to catch the fines. The second pass is slow. It is also what makes the difference between clean cold brew and murky cold brew.
5. Refrigerate the concentrate in a sealed jar, and dilute each glass as you pour it.

What the research found

It is not lower in pH

The claim that cold brew is dramatically less acidic is the main reason many people drink it. In 2018 Niny Rao and Megan Fuller compared cold and hot brews made from the same light roasts — coffees from Brazil, Ethiopia, Colombia, Myanmar and Mexico — at the same coffee-to-water ratio.

The pH was comparable: between 4.85 and 5.13 for both methods.

What did differ was titratable acidity — the total quantity of acids present, as opposed to how acidic the liquid is at a given moment. Hot brew extracted more total acid. That fits cold brew tasting smoother and less sharp, and cold drinks generally taste less sharp anyway. But "less acidic" in the sense of being gentler on the stomach is not something the pH supports.

The same study found hot brew had higher antioxidant activity, which the authors linked to the extra acids hot water pulls out. If antioxidants are part of why you drink coffee, hot brewing gets you more of them.

Caffeine is done in about seven hours

A year earlier, Fuller and Rao tracked how caffeine and 3-chlorogenic acid, coffee's main chlorogenic acid, came out of cold brew over time. Both reached equilibrium between six and seven hours, following first-order kinetics — fast at first, then levelling off.

Typical recipes say 10 to 24 hours. The research does not make those wrong, because caffeine and one chlorogenic acid are not the whole flavour of a cup, and many slower compounds keep developing. But it does reframe what a long steep is for. After about seven hours you are adjusting taste, not caffeine.

Grind size barely mattered — for those compounds

The same 2017 study found grind size did not significantly change the caffeine or chlorogenic acid concentrations in cold brew, which the authors read as meaning that surface area was not what limited their extraction.

That is the opposite of how hot brewing behaves, where grind size is the strongest lever you have. For cold brew it means there is little to gain by grinding fine and a lot to lose: fine grounds clog filters and leave the concentrate silty. Grind coarse.

It is not low in caffeine

Medium roasts produced higher caffeine and chlorogenic acid concentrations than dark roasts in that study, and coarse-ground cold brew measured substantially higher in caffeine than hot brew made at the same ratio.

Cold brew's reputation as a gentle drink comes from its smooth taste, not its chemistry. A glass of concentrate diluted 1:1 can easily carry more caffeine than a mug of filter coffee.

Getting the taste right

Because cold brew is diluted before drinking, the strength is mostly decided in the glass. Change the ratio of concentrate to water or milk first. Only change the brew once you are sure the problem is not dilution.

Medium roasts are the usual choice, and the 2017 results give a reason beyond taste: they extracted more of the measured compounds than dark roasts. Very light roasts often taste thin and grassy as cold brew; their character tends to show better when brewed hot.

Cold brew is not iced coffee

Cold brew is coffee that never meets hot water. Iced coffee, in the Japanese style, is hot coffee brewed directly onto ice so it chills the moment it is made.

They taste different for a reason. Hot water pulls out more of coffee's acids — the 2018 study measured that directly — along with more of its volatile aromatics, so flash-chilled hot coffee keeps the bright, fruity character of the beans, while cold brew is rounder, heavier and more chocolatey. Neither is better; they are different drinks, and a coffee you love hot may well suit the iced method better than the cold one.

Keeping it fresh and safe

Refrigerate the concentrate as soon as it is filtered, in a clean sealed jar. It keeps its flavour for about a week, then turns flat and papery. If you steep at room temperature, do not leave it out much past 18 hours, and steep in the fridge when your kitchen is warm. Wash the brewing vessel and filters thoroughly between batches: coffee oils go rancid, and old residue taints the next batch.