At Silver Lyan in Washington, D.C., the Project Apollo, a spruced-up Gin Sour, is the picture of foamy egg white perfection. A crimson crescent of cranberry dust sits atop a sturdy peak of egg whites, which appears like a well-coiffed, breeze-proof pompadour. The key to sending those egg whites soaring above the rim of the glass is a deceivingly simple ingredient: soda water. When added to an egg white cocktail during the shaking process, soda water creates the puffiest, fluffiest egg white peaks.

While the team at Silver Lyan was hesitant to divulge their secrets on the record, the technique has nonetheless begun making its way across the larger D.C. bar scene. “Shaking an egg white cocktail with a bit of soda water in the tin is a technique I recently discovered via my friends over at Silver Lyan,” says Deke Dunne, head bartender at Allegory in the Eaton DC hotel.

Soda water is one of the secrets to a supercharged Ramos Gin Fizz, but whereas in that recipe it’s added at the end to heighten foam, in Dunne’s Pisco Sour, a half-ounce is added to the second round of his reverse dry shake. “What we discovered is that you do get a larger, smoother head with a higher peak, but what really drew me into the technique was how balanced the final product ended up being,” he says. To account for dilution, Dunne shortens his first shake, with ice, to seven seconds, and the second shake, without ice, to 10 to 15 seconds. “It really [rounds] out the cocktail and [leaves] you with the silkiest aftertaste.” (Dunne prefers Topo Chico for its sharp bubble.)

At barmini, the cocktail lounge adjoining José Andrés’ molecular gastronomy restaurant minibar, cocktail director Miguel Lancha takes a precise, scientific approach to recipes and technique. At first, he was skeptical of the soda water technique. “Just on paper and theoretically, I didn’t quite see how it would benefit,” Lancha says, citing as concerns both dilution and the egg whites’ potential failure to support soda water’s carbon dioxide.

While Lancha’s tests “yielded a fluffier foamy head that also lasted for longer,” he’s not yet a full convert. “[The fluffier egg whites] came at the cost of diluting the drink more than desired,” he says. He’s still testing his shake times and ice amounts, and considers the matter a work in progress.

Meanwhile, at Cantina & Pisco Bar and Popular at the Public Hotel in New York, Peruvian chef Diego Muñoz—who previously helmed Lima’s world-renowned Astrid y Gastón—has given the technique his seal of approval. His bar team deploys the method in their Pisco Sour Brûlée, which is dusted with caster sugar and torched after it’s strained and served.

“The version of our traditional Pisco Sour featuring this technique turned out to be a total success,” says Muñoz. He loves the resulting texture of the egg white foam, which also provides a base sturdy enough to allow for a brûléed surface and a temperature differential between liquid and crust. At Muñoz’s establishments, rather than shaking sours manually, bartenders blitz them in a blender, a common technique deployed at high-volume bars and restaurants across Peru. Muñoz suggests trying a small hand whisk as an alternative, if a blender isn’t in your toolkit.

No matter the method of emulsification, the simple addition of soda water seems to promise drinks with luscious mouthfeel and high-altitude peaks with surprising staying power. To test the technique, Dunne even conducted several rounds of blind taste tests with his team. The results were clear, says Dunne: “The Pisco Sour with soda water added to our reverse dry shake was the winner every time.”