There is perhaps nothing quite so inconsequential to ingest as foam, and yet foam – gas trapped in a liquid matrix – inspires fiery passions. Baristas strive for the smoothest microfoam, and a creamy, long-lasting head is the stuff of brewers’ dreams. Meringues, whipped cream, and souffles are foams too; edible foam exists in numerous, disparate spaces.
And of course, it can be found in haute cuisine. The godlike Ferran Adrià, Spanish champion of three precious Michelin stars, is credited for sparking the foam revolution in the mid-1990s. His experimental style turned to foams, a refreshing way of delicately incorporating savory flavors into dishes, and he did so with a rainbow of foods – olive oil, mushrooms, cod, and beets.
Foam construction requires two major components: a gaseous phase, and an aqueous phase. In the case of culinary foam, using an espuma or thermo whip, the two are forcefully combined with pressurized gas (often nitrous oxide) to generate a quivering protein film surrounding evanescent gas bubbles. Hundreds of bubbles settle into a dynamic structure stabilized by surface tension and the hydrophobic effect.
A key factor in edible foam is lifespan; a poorly constructed cod foam soon reduces to a piddly mess of cod-juice. Stability of the foam dictates whether the foam stands tall or melts away, and in 1873, Belgian physicist Joseph Plateau described three universal rules for the molecular organization of foam. Should a rogue bubble fail to adopt these rules, it pops.
The first rule is that when bubbles meet at an edge, it will be at the intersection of three surfaces. Second, each pair of surfaces meets at an angle of 120 degrees. Three surfaces therefore meet at a sum of 360 degrees, forming a full circle. Finally, the last rule is that when bubbles meet at a point, it will be at the intersection of four bubbles, at an angle of 109.5 degrees.
These rules reflect the adoption of the most stable conformation. The most stable bubbles will be polyhedral in nature, a three-dimensional space with straight edges. Liquids destabilize the structure, causing bubbles to round out and flow freely. In the image of foam structure above, water destabilizes the bottom of the foam, and the bubbles are rounded. The top half is dry, and the bubbles are clearly polyhedral as a result of their increased stability.
Foams are unique culinary phenomena, ethereal and fleeting structures capable of delivering heady flavors in light and unexpected ways. I read about foams and saw a photo of a thermo whip, imediately recognizing it as a gadget I saw a woman buy at a thrift store earlier this week. I hadn’t recognized it at the time, but she bought the strange two-necked bottle for just 50 cents, and the cashier told her it was a steal.
“Yeah,” the woman agreed. “You know what it is, right?” I didn’t.
“Oh yes! Go forth and culinate!”