Carbonation not only adds bubbles, but changes taste. Professor Shane Eaton talks us through the science between cocktails and CO2.

In 1767, Joseph Priestley accidentally discovered a method of infusing water with carbon dioxide to make sparkling water when he placed a bowl of water above a beer vat at a brewery in Leeds. The “peculiar satisfaction” he found while drinking carbonated water was due to the slight tartness of carbonic acid (H2CO3), which is formed when carbon dioxide (CO2) gas is dissolved in water. Priestley created a simple device featuring a bladder between a generator and absorption tank to control the flow of carbon dioxide. However, it wasn’t until 1781 that carbonated water was produced on a large scale, with Thomas Henry building the first factory in Manchester.

Today, carbonated water is made by injecting pressurised carbon dioxide into water. The pressure increases the solubility and allows more carbon dioxide to dissolve than would be possible under standard atmospheric pressure. Higher gas pressure and lower temperature cause more gas to dissolve in the liquid. When the temperature is raised or the pressure is reduced, such as when a bottle of sparkling water is opened, carbon dioxide gas effervesces and escapes from the solution.

Carbon dioxide, of course, can be used to make a variety of bubbly drinks, from soft drinks, to champagne, to the Gin Fizz. However, there is a science behind finding the perfect carbonation for a given beverage. As carbonation guru Dave Arnold emphasises in Liquid Intelligence, carbonation should be treated as a cocktail ingredient. Too little carbonation can be disappointing but too much can overpower subtle fruit flavours, overemphasise tannins, and add a harshness to a drink. As CO2 is more soluble in alcohol than in water, the more alcohol a drink contains, the more CO2 needs to be added to produce the same effect of carbonation. As it turns out, alcohol makes drinks more difficult to carbonate. Coupled with the fact that carbonation makes it easier for alcohol to be absorbed in the bloodstream means you should aim to keep carbonated cocktails at a relatively low abv.

Under pressure

The first carbonation method, which is favoured by Arnold, exploits an industrial CO2 tank (5-20lb) coupled to a soda bottle with a gas hose terminated with a ball-lock connector. Since the pressure in the tank is too high for carbonation, a pressure regulator is used to reduce the pressure to a more suitable range, between 30 and 60psi (two to four times the pressure of the atmosphere). A carbonation rig costs £150 and easily fits in a cabinet under a bar counter. To operate the rig, open the gas cylinder and adjust the pressure with the regulator (45psi for most drinks). Fill a soda bottle about two-thirds full with the very cold beverage you want to carbonate – lower temperatures enable more efficient carbonation. Next, squeeze the extra air out of the plastic bottle to enable more CO2 to dissolve in the drink. Screw a carbonator cap on the bottle and attach it to the carbonator rig via the ball-lock connector. The bottle will quickly inflate and, while the gas is connected, shake the drink to increase the surface area of the liquid in contact with the CO2 to enhance carbonation. Remove the ball-lock connector, then partly unscrew the carbonator cap to let the drink foam up. Let the foam die down, and then repeat the carbonation/defoam process again. Let the bottle rest and then slowly unscrew the carbonator cap and pour the bottle into a tall and chilled glass.

A word of caution about working with large tanks of CO2. If there is a leak in the tank, the concentration of CO2 could reach dangerous levels, leading to dizziness or even asphyxiation. However, operating in a well-ventilated room with a carbon dioxide safety alarm will ensure almost zero risk.

One of Arnold’s favourite drinks to make with his carbonation rig is a Gin & Juice. 1.75oz Tanqueray gin, 1.75oz centrifuge-clarified grapefruit juice, 1.5oz filtered water, 0.25oz simple syrup, 3ml champagne acid and one drop saline solution are combined and chilled to -7°C. Clarifying grapefruit juice not only tames the bitterness of the grapefruit, it makes it significantly easier to carbonate. After carbonation at 42psi, serve the carbonated Gin & Juice in a chilled flute.

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Smaller footprint and more user-friendly versions of the carbonation rig have recently been commercialised, allowing you to easily make carbonated drinks, even at home. Models such as the iSi Twist’n Sparkle rely on individual CO2 cartridges which pressurise a beverage inside a bottle. Although these systems cost about £40, they do not allow customised pressures, and make it more difficult to control foam. Furthermore, these systems are not ideal for a high-volume bar because the individual CO2 cartridges are more expensive than using a larger tank.

Perlini and Preshh offer carbonation setups that are just as easy to use as the iSi Twist’n Sparkle, but can be connected to CO2 tanks. Jamie Boudreau 

of Canon in Seattle finds Negronis made with the classic equal parts gin, sweet vermouth and bitter are too sweet for his palate. However, with carbonation, the extra acidity from the bubbles makes it possible to use the classic 1:1:1 ratio. Boudreau adds all the ingredients, along with ice, to a Perlini container. After charging with CO2 gas, he shakes the sealed container, lets the beverage rest, then releases the gas slowly before pouring into a cocktail glass. The result is a pleasantly refreshing and well-balanced Negroni.

The Preshh system, developed by Leopoldo Almeida in Brazil, is a popular tool used by Fabio La Pietra of Subastor and Marcio Silva of Guilhotina. The Preshh bottle holds 800ml, but the manufacturer suggests to fill it 600ml, to give enough head space to efficiently dissolve CO2 gas in the liquid. To make the Zé Highball at Guilhotina, the team prebatches Singleton of Dufftown malt whisky, Ketel One vodka infused with apricot, fresh ginger juice and homemade lime husk cordial. For each 600ml of cocktail, they add 200ml of fresh water and let it chill in the freezer for at least three hours. Then they carbonate with the Preshh and serve 200ml. The result is a citrussy and refreshing Highball that is one of the bestsellers at Guilhotina.

Whether you apply a laboratory-grade cylinder or a simpler cartridge-based carbonation setup, it’s crucial to remember to gently pour the bubbly drink down the side of an inclined and chilled flute. A tall and narrow glass is favoured for carbonated drinks to reduce the surface area exposed to air, to avoid losing the CO2 too quickly. When you put so much effort into getting bubbles into your drink, don’t ruin all your hard work by carelessly serving your cocktail.