Espresso Tonic: How to Make It Without the Foam Overflow

An espresso tonic is a double espresso poured slowly over a tall glass of ice and cold tonic water, so the coffee floats in a dark layer on top of the fizz. To make one, fill a glass with ice, add about 150 ml of well-chilled tonic, leaving room at the top, and pour a freshly pulled double shot over the back of a spoon onto the tonic.

It sounds like a dare and tastes like a cocktail without the alcohol: bitter on bitter, brightened by the tonic's citrus and sweetness. The one technical problem is foam, and the order of pouring solves it.

The recipe

1. Fill a tall glass with ice and pour in the cold tonic, leaving a good 3 cm of space at the top.
2. Add a little syrup at this point if you want it sweeter, and stir gently.
3. Pull a double espresso into a small jug.
4. Pour the espresso very slowly over the back of a spoon held just above the tonic. It will float as a dark band on top.
5. Twist a strip of citrus peel over the glass and serve with a straw.

The roughly 1:4 ratio of espresso to tonic is a starting point. Less tonic makes it more coffee-forward; more makes it lighter and sweeter.

Why it foams — and how to stop it overflowing

Pour espresso carelessly into tonic and the glass erupts. The reason is the same physics that makes a soda bottle foam over when a sweet is dropped in.

The tonic is supersaturated. Tonic water is carbonated under pressure. Once the bottle is opened, it holds more carbon dioxide than it can keep at normal pressure, and the gas is looking for a way out.

Espresso gives it a way out. Bubbles almost never form from nothing in a carbonated drink. They grow from tiny pre-existing pockets of gas on particles, fibres or scratches, where the energy needed to start a bubble is far lower. Espresso brings exactly that: a crema full of tiny bubbles and a haze of fine coffee particles. Pour it in and the carbon dioxide rushes into thousands of new sites at once.

Warmth helps it along. Liquids hold less dissolved gas as they warm, which is why cold sparkling water stays fizzier than warm. A hot shot warms the tonic it touches and pushes more gas out.

Coffee holds the bubbles. Once formed, the bubbles are stabilised by espresso's own foam-forming compounds — in their review of espresso foam, Illy and Navarini identified proteins, polysaccharides and melanoidins as what holds coffee bubbles together.

The fix follows from the physics. Keep the tonic as cold as possible, put it in the glass first, and pour the espresso slowly and gently on top, where much of it floats instead of plunging through. Wikipedia's account of the drink notes the same: espresso poured over iced tonic makes a layered drink, while tonic poured onto espresso creates a head of foam. If you want the foam, do it that way round — in a glass with plenty of room.

What is in the glass

Quinine. Tonic water's bitterness comes from quinine, once used against malaria. Modern tonic is far weaker than the medicinal original: the US limits quinine in carbonated drinks to 83 parts per million — 83 mg per litre — and Europe to 100 mg per litre. At the US limit, 150 ml of tonic contains at most about 12.5 mg of quinine. Wikipedia puts the therapeutic dose for malaria at 10 mg per kilogram every eight hours — about 2,100 mg a day for a 70 kg adult — so one drink carries well under 1% of that. A small number of people do react to quinine even at tonic-water levels, with a skin reaction known as a fixed drug eruption.

Sugar. Most tonic is sweetened. The USDA lists tonic water at 8.8 g of sugar and 34 kcal per 100 g, so 150 ml brings about 13 g of sugar and 50 kcal — before any added syrup. Diet tonics cut that to almost nothing.

Caffeine. Only the espresso has any: about 76 mg for a 36 g double at the USDA's average for restaurant espresso. The caffeine guide puts that in context.

A fun aside: quinine fluoresces under ultraviolet light, so a tonic-based drink glows blue under a black light.

Choosing the coffee and the tonic

Try a bright, fruity espresso, so its acidity meets the tonic's citrus, and a clean tonic that is not too sweet. If the drink tastes harsh, a sweeter tonic or a little syrup will soften it. Japanese baristas are credited with being the first to add other flavours — cherry, lime and honey versions all appear on menus — and a strip of orange peel is the simplest addition. The espresso dial-in guide covers getting the shot right.

Where it came from

Tonic water dates to 1858, when a London firm patented a carbonated "quinine tonic water". Espresso tonic is far younger. The earliest record is from 2007 in Oslo, where it was mixed by a barista working with Anne Lunell and Charles Nystrand, the future founders of the Swedish roaster Koppi. Later that year it appeared on the Koppi menu in Helsingborg as Kaffe & Tonic.

It spread through Scandinavia, reached North American cafés in the 2010s by way of international barista competitions, and had arrived in Japan by 2015. In 2019 the British chain Caffè Nero began selling versions made with Fever-Tree tonic.

For other ways to drink espresso cold, see the shaken espresso guide and the iced americano, and for the full menu, the types of coffee drinks guide.