A university ice cream program doesn’t sound like the type of place where the next great cocktail innovation would be spawned. Yet, that is precisely where bartender Iain McPherson began realizing that the outer limits of freezing might one day revolutionize the drinks world.

The Scotsman took 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 2015, McPherson launched Señor Scoop, a liquor-infused ice cream brand available throughout the U.K., including via a vending machine at his Edinburgh bar Hoot The Redeemer. But that was just the beginning.

“I’ve always been fascinated by the flavors that freezing creates,” he explains of the impetus behind his latest cocktail technique: “switching,” a process that involves freezing the water content of a given spirit so it can be separated from the alcohol, then replaced with something else entirely, like clarified juice. “My main catalyst was a real determination to create a new technique that was completely bar-focused,” he says, describing his frustration with how reliant the bar industry has become on co-opting techniques designed primarily for culinary use.

Late last year he began to experiment with fractional freezing (also known as freeze concentration or freeze distillation) at his speakeasy-style cocktail bar, Panda & Sons, also in Edinburgh. The practice of skimming off frozen water from alcohol wasn’t itself a new idea—German eisbock beer and American applejack have long used the process to create more intense, alcoholic products. But McPherson took it a step further when he translated the technique for a cocktail bar setting.

Realizing that the practice ran the risk of concentrating not just desirable flavors, but, potentially, impurities as well—namely methanol—he wondered if there might be a way to counteract this effect by replacing the water he removed with something more flavorful. “I started thinking about Legos,” he explains. “How you can swap heads or legs, switch them around, and create a whole new thing.”

Though still a new trend, switching has already rocked the cocktail world at its most elite levels.

His first experiment in 2018 began with freezing Bombay Sapphire’s Star of Bombay gin. At 47.5 percent alcohol by volume, the gin’s water content would solidify at -38 degrees Celsius, he calculated. Unfortunately, Panda & Sons’ freezer, like most commercial freezers, could only get down to around -20C. Even for a spirit with 40 percent alcohol, he would need to get to -27C at the bare minimum. So he purchased a Tefcold chest freezer, capable of reaching temperatures as low as -50C. He poured the gin into a small cooler with the lid removed—to aid in directional freezing—and, after 24 hours, he was able to separate the liquid gin from the water. To get the gin’s ABV back to 47.5 percent, McPherson diluted it with clarified pink grapefruit and orange juice in place of the water. The result was a more flavorful gin with an incredibly rich mouthfeel.

“I coined the term ‘switch-finish,’” McPherson explains, likening his newfound process to how certain whiskeys are “finished” in unique barrels, like those used with sherry or port, for the final year or so of their maturation.

McPherson put a Negroni variation using the switched gin on the menu at Panda & Sons, but that wasn’t the end of his experimentation. Next, he tried freezing an Islay Scotch: He separated the water content, which retained a peated flavor, and added that to a rum that had been stripped of its proofing water using the same technique. The result was a subtly smoky rum unlike anything he had ever tasted before. (For the moment, a hint of alcohol remains in these separated waters, but McPherson believes that once he perfects his technique, the water will be completely non-alcoholic.)

“The process is so very simple that when I explain it to other bartenders I get a lot of ‘Fuck you! I can’t believe how easy this was to think of,’” says McPherson, though he’s always sure to mention that it took him months of research to get to this point.

McPherson’s research has revealed that switching generally works better with clear spirits rather than aged ones. To date, he’s switched gin, vodka, several varieties of Scotch and rum, though he notes that freezing the latter often reveals the integrity—or lack thereof—of the product. (Since the discreet addition of sugar is rife in the rum industry, but often goes undisclosed, the additives act as an antifreeze, screwing up McPherson’s temperature calculations.)

Though still a new trend, switching has already rocked the cocktail world at its most elite levels. When I recently spoke to McPherson in early September, he had just returned from Russia where he presented his findings at the Moscow Bar Show. Though few bartenders aside from McPherson have put commercial “switches” on their cocktail menus, the process is currently being explored by industry leaders across the globe, like Alexis Tinoco Belton of The Aviary in Chicago, and Nico de Soto, of Mace in New York and Danico in Paris. Mario Farulla, of Rome’s Baccano, is currently working on a switched Manhattan in which the bourbon’s water is replaced with watermelon juice. “It’s an amazing way to bond ingredients,” Farulla explains. His first switched cocktail will appear on Baccano’s menu in early October.

McPherson notes that switching is within the reach of the home bartender dilettante, too, the only barrier to entry being a $600 chest freezer. For this reason, McPherson sees switching as a major step forward in enabling smaller bars, those that can’t afford “labs” with $5,000 RotoVaps and liquid nitrogen tanks, to participate in the current cocktail zeitgeist. “So many bars have felt left out of being able to do these cool techniques,” he notes, “but this is one that they can actually use.”

McPherson believes the true potential in switching will ultimately lie in its byproducts, like the removed waters, which he envisions bars employing as aromatic sprays or as components in complex non-alcoholic offerings. Or maybe some bartender will take this kernel of an idea and bring it to places McPherson hasn’t yet dreamed of.

“I may have created it, but I don’t know everything about it. There’s so much we don’t know about freezing out there and the bar industry could really corner the market on it,” he says. “Switching is just the tip of the iceberg.”