Cold Brew Tonic: Eighteen Hours, Two Ingredients, One Glass
Cold brew tonic is the easiest drink on a brunch menu and the one most often made badly, because the two things that matter — grind size and dilution — are invisible. Get the concentrate right once and it keeps ten days in the fridge, so the drink itself becomes a fifteen-second assembly job during service.
Ingredient card
| Item | Amount |
|---|---|
| Coarse-ground coffee, medium roast | 150 g |
| Cold filtered water | 1 litre |
| Indian tonic water, chilled | 200 ml per drink |
| Orange peel, expressed | 1 strip per drink |
| Ice | a full glass per drink |
Scaling
for 2: 150 g coffee to 1 litre water for the concentrate, then 60 ml per drink. Two drinks need 120 ml and one tonic bottle.
for 4: The full concentrate as written below is enough for six drinks at 60 ml. For four, use 240 ml concentrate and two 200 ml tonic bottles.
for 8: Make a double concentrate in two jars rather than one — a single jar with 300 g of coffee will not fully saturate and you lose about 15 percent of the yield.
Time budget and burner map
Make ahead
- Steep for 18 hours at room temperature. Under 12 hours it tastes thin and grassy; past 24 hours it picks up a woody, bitter note that no amount of tonic hides.
- Strain twice: a metal sieve first, then a paper filter. The paper pass takes about 8 minutes and is the difference between a clear drink and a cloudy one.
- Store in a sealed bottle in the fridge for up to 10 days. Do not store it at room temperature — it ferments visibly by day four.
To the pass
- Ice first, all the way to the top. A half-full glass of ice dilutes the drink twice as fast because there is more liquid per ice cube.
- Pour the concentrate over the ice, then the tonic down the side of the glass to keep the carbonation.
- Peel last, and express it cut-side down. The oils sit on the surface and you smell them before you taste them.
Where it goes wrong
- Using a fine grind. Espresso-ground coffee in cold water clogs the filter, takes 40 minutes to strain, and comes out muddy with a gritty texture. Coarse, like raw sugar.
- Shaking the tonic. Any agitation after the tonic goes in drives off the carbon dioxide. Stir once with a bar spoon, at most.
- Squeezing the orange peel into the drink. You want the oils, not the juice. Hold the peel skin-side down over the glass and snap it — the spray is the point.
The method, in full
Why cold extraction tastes different
Hot water extracts acids and aromatics fast, and bitter compounds faster still as temperature climbs. Cold water extracts the same compounds but in a different order and much more slowly — after 18 hours you have most of the aromatics and roughly half the bitterness of the same coffee brewed hot.
The practical result is a concentrate that tastes sweet without sugar and does not fight the quinine in tonic water. That is why the pairing works: both are bitter in different registers, and the coffee provides the aroma the tonic lacks.
The concentrate ratio and how to adjust it
One hundred and fifty grams to one litre is a 1:6.7 concentrate. Used at 60 ml per drink topped with 200 ml of tonic, it lands near a 1:10 final dilution, which is roughly the strength of a strong filter coffee.
If your first batch tastes weak, do not add more coffee — steep longer, up to 24 hours, and check again at 20. If it tastes woody, you went past the window; cut the next batch at 16 hours. Two variables, one of which you can control on the second attempt.
Serving four at once
Build in a pitcher rather than glass by glass: 240 ml concentrate, 800 ml tonic, ice, and stir once. It holds for about ten minutes before the ice dilutes it noticeably, so pour it into pre-chilled glasses straight away.
For a non-alcoholic brunch alternative, swap 50 ml of the tonic for a light ginger beer. The ginger adds a heat that reads as more complex and it is what a lot of people actually want at eleven in the morning.
Kitchen questions
Can I make cold brew in a French press?
Yes, but do not press it. Steep in the carafe for 18 hours, then plunge very slowly and decant immediately through a paper filter. Pressing forces fine particles through and clouds the concentrate.
How much caffeine is in one drink?
Roughly 90 to 110 mg per 60 ml of this concentrate, similar to a double espresso. Cold brew is not lower in caffeine than hot coffee — it is often higher per millilitre because of the ratio.
Can I use a lighter roast?
Yes, and it works well with tonic because lighter roasts carry more fruit acidity. Add two hours to the steep and taste at 16 hours, since lighter roasts extract more slowly.
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