Iain McPherson was sitting in his bar one frigid Edinburgh day, thinking about how the freezing water could end up bursting the bar’s pipes—when a lightbulb went off. “That’s a lot of energy we could be utilizing,” he thought.
The science of freezing was not unfamiliar to the cocktail bar owner (Panda & Sons, Hoot The Redeemer, Nauticus Bar), whose father was in the ice cream business. He himself has taken the famed Science of Ice Cream course at the University of Reading, and went on to advance his skills at Bologna’s Carpigiani Gelato University. In the years since, he’s applied his learnings to the cocktail world. In 2019, McPherson developed a technique known as “switching,” which involves freezing the water content of a given spirit so it can be separated from the alcohol, then replacing it with something else entirely, like clarified juice.
Noticing that most bars turn to heating techniques, such as sous vide, to soften ingredients or infuse flavors, McPherson wondered if he could create an inverse technique, a sort of sous vide in a parallel universe, to even greater effect. He calls it “sous pression,” or “under pressure.”
“In sous vide, the vacuum extracts the flavor from a fruit or herb,” explains McPherson, “but sous pression uses force to leach it out.” The process begins by building a two-liter batch of any given cocktail and pouring it into a stainless steel beer growler along with the fresh fruit or herbs to infuse the liquid. McPherson uses the same Tefcold chest freezer that he has long employed for switching; it’s capable of reaching temperatures as low as –50°C, but he typically only goes down to –30°C for this technique. After 36 hours, the cocktail growler is frozen solid. If there were any air space in the growler at the beginning of the process, the volume of the liquid would expand, on average, 9 percent under these conditions.
“But we’re not allowing it to expand by that 9 percent because it’s got nowhere to go apart from going into the fruit, finding any air bubble possible, and—like a vacuum chamber would do—bringing all those juices out,” explains McPherson.
The growler is left to defrost, coming back up to room temperature in a matter of hours. To be completely safe, and avoid any explosions, McPherson doesn’t reopen its swing-top closure until he’s sure the contents have liquified again. He then strains out the fruit or herbs using a Superbag sieve; the booze-soaked fruit is eventually repurposed as a garnish to help illustrate the “flavor journey” the fruit undergoes in making the cocktail. Bartenders at Panda & Sons then dole out a serving of the infused cocktail batch, and shake or stir it just as they normally would.
“Not only does it infuse the flavor beautifully, it also softens the alcohol,” says McPherson, recalling a quality he first noticed when switching spirits. “The [acidity] changes too, which results in a smoother finish.”
For now, McPherson is only using sous pression on classic cocktails as a way to ensure guests can understand what the technique is actually adding to the drinks they’re likely familiar with. On the current menu, four cocktails use the technique, including a Tuxedo infused with Sable grapes and a Bobby Burns infused with sliced cherries. McPherson says he and his staff are still in the early stages of understanding the potential and limitations of sous pression—Would dense coconut meat, with its lower water content, infuse spirits under pressure? What about juice-forward cocktails like Daiquiris?—before they apply it to more obscure drinks.
McPherson’s ultimate goal, however, is not to revolutionize the cocktail world with sous pression, but to achieve the opposite of what usually happens in his space.
“Bartenders always look at the best chefs in the world and go, ‘They’ve done this really cool technique, how do I adapt this into making drinks?’” says McPherson. “We’ve taken so many techniques from the kitchen. One day, my dream, my driving force, is that some top restaurant or top chef will finally take a bar technique and adapt it for food.”