Bartenders, meet acid phosphate, invigorating health tonic of yesteryear and once-beloved fixture of soda pop counters everywhere. Presumably abandoned along with sock hops, innocence, and our national sense of humor in the cultural ice age of the early Cold War, acid phosphate is making a comeback on the American drinks menu. And this time around, it’s not here to adjust imbalanced humors or satisfy pre-adolescent soda-pop lust; it’s here to bestow creative liberty on the mixologist’s acid arsenal.
The resurgence of acid phosphate (which has yet to slide down the slippery slope of trendiness) comes courtesy of Canadian industrial chemist-turned-professional bartender Darcy O’Neill, who discovered the stuff as part of a Tales-inspired investigation into the forgotten culture of the soda fountain. The majority of his research made its way into his Tales of the Cocktail "Spirited Awards" nominee, Fix the Pumps. But O’Neill also shares some of his findings on his website, artofdrink.com. Pay attention, non-scientists: “Acid phosphate is more than just diluted phosphoric acid," O'Neill explains. “It's actually a partially neutralized solution made with salts of calcium, magnesium, and potassium.” (Um, we knew that...) The result is more shelf-stable than citric acid, and has a pH between 2 and 2.2, “about the same as fresh squeezed lime juice.”
The key is, it’s not lime juice—and that’s where the all the fun freedom begins. While lemon and lime juice go a long way in acidifying and balancing cocktails, they have one nagging drawback: fruitiness. There are certainly exceptions, but generally speaking, wherever there is lemon or lime juice, there is the flavor of lemon or lime. Not so for acid phosphate. It’s neutrally tart, the blank slate of sour, which actually makes it tough to quantify. “It’s hard to define what it actually does to a drink,” says Paul Gustings, no-nonsense NOLA mixologist behind the bar at Tujague’s (that’s pronounced “two-Jacks”). “It’s like the MSG of drinks. MSG gives Chinese food that shiny color, that glaze over it. This is what the acid phosphate does. It ties the drink together.”
Acid phosphate is chemically different from MSG, the once reviled umami-imparter mono-sodium glutamate. But it does have a similar “je ne sais quoi” magic. More than just souring or balancing a drink, acid phosphate enhances its flavors, owing largely to the mineral salts, which work in drinks the way salt does in the kitchen. “It brings the flavors out,” Gustings confirms, which is no doubt why he doses four drinks on the Tujague’s menu with the stuff. “I’m looking to add one more.” The feeling’s mutual at Austin's Second Bar + Kitchen, where William “Bad Billy” Hankey uses acid phosphate to magnify flavors. “The more I work with acid phosphate,” says Hankey, “I realize what great potential the rebirth of this ingredient has.”
It sounds like a mixologist’s parlor trick (“just a dash of acid phosphate and—presto!”) but we actually got to taste the results, and by gum, they’re right. Gusting’s Angostura Rum Phosphate—based on an old acid phosphate-angostura hangover cure—combines (ideally) Angostura rum, a one-ounce pour of Angostura bitters, lemon juice, St. Elizabeth’s Allspice Dram, and two tasting spoons of acid phosphate. The resulting drink is a symphony of bitterness, acid, and umami-rich spice, with a round sweetness and a frothy head (a common sight where acid phosphate is concerned).
Indeed, unlike lemon and lime juice—which shine more brightly in lighter drinks—acid phosphate likes to play in the flavor deep end, bringing out nuances of the darker, spicier, and more challenging flavors in the mixologist’s mise-en-place. Hankey uses acid phosphate to accentuate the molasses of Cruzan rum and tart up the fruit of sloe gin in his Acid Brothers cocktail (the other brother, we assume, is the quarter-ounce of lime juice). And Gustings uses it in The Sitchel, an everything-but-the-kitchen-sink of a drink with Captain Morgan’s Spiced Rum (we loved it, and Gustings defies any haters), Laird’s applejack, lemon juice, rice wine vinegar, acid phosphate, Angostura and Peychaud’s bitters, powdered sugar, and—oh yes—a barspoon of molasses. This might seem like a whirling flavor-storm of a drink, but it’s actually incredible, a palate-stretching masterpiece owing a lot of its genius to the acid phosphate.
You might notice that both drinks combine acid phosphate and citrus (and, in Gustings’ case, acetic acid via rice vinegar). But doubling or tripling up on the souring agents “was actually a very common practice,” says Hankey, “for instance, using lemon oil and acid phosphate combined with soda [to make] our first lemon phosphate sodas.” The same is true behind the bar, where mixologists can apply a lifting neutral sour with acid phosphate and/or a flavoring sour like lemon, lime juice, or vinegar, as in a shrub. As with anything that encourages experimentation behind the bar, we’re guessing this is gonna catch on. “The only thing holding back acceptance is hesitation due to its formal sounding name, and the big, sour flavor,” says Hankey. “When we move past these minor points, it will spread rapidly.” (The key is dosing judiciously; and really, what’s in a name?)
It seems like it is beginning to spread rapidly, at least in Austin. "We sell a lot more of it than I thought we would," says Dustin Popken, who handles the more esoteric mixology wholesale orders for The Austin Wine Merchant. "There's a bit of a soda fountain trend going on now." Of course, not everyone is a fan—which is lucky, since sourcing can prove a bit of a problem. O’Neill is a key, and lonesome, purveyor, and high demand could quickly outpace supply. "I think it would be great for Darcy," says Gustings. "But the problem with it becoming a trend is it would be hard to get." Meaning the acid phosphate haters (acid phosphaters?) are a welcome dissenting voice in the acid debate.
“I don’t like the final flavor profile it gives cocktails,” says Jason Stevens, Congress Austin mixologist and one of the flavor-obsessed founders of Bad Dog Bar Craft bitters company. Not surprisingly, Stevens prefers his own house-made malic or tartaric acid. “With malic acid, making sour cherry, peach, pineapple juice—I can go on forever—to use in place of traditional lemon-lime, opens up a huge world of new flavors and sour style cocktails,” he says. “A few dashes of malic acid solution added to a Cognac Manhattan is absolutely incredible,” he says. Stevens, of course, “always encourages experimentation” among his fellow bartending brethren. Or, as Bad Billy Hankey puts it, “just find that sweet spot you like and share with others.” Which is to say, the mere fact of having a souring agent preference is fairly new (at least in the rediscovered sense). And that, we can all agree, is nothing to be sour over.