“Nitro” coffees and beers are the coolest things to sip this summer. What makes them so creamy?

Nitrogen gives beers like Guinness a creamy, smooth foam.
Nitrogen gives beers like Guinness a creamy, smooth foam.
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Guinness has been doing it for decades, and now a slew of high-end coffee shops and craft brewers, as well as Sam Adams and Starbucks, are getting in on the practice. They’re infusing their beers and coffees with nitrogen, giving the drinks that mesmerizing, Guinness-like cascade of foamy bubbles and smooth mouthfeel. But what exactly makes “nitro” beers and coffees so creamy?

Carbonated drinks are typically made with carbon dioxide (CO2) dissolved into the liquid, which creates a fizzy sensation and releases aromatics, the molecules responsible for the smell of a drink. (The fermentation process to make beer also produces CO2, but many beers are then force-carbonated to make them more fizzy.) When the gas infused into the drink is nitrogen instead of CO2, a much denser foam of tiny bubbles forms. These small bubbles create a ”silky, creamy, tactile sensation,” Thomas Shellhammer, a brewing chemist at Oregon State University, explained.

It all comes down to solubility. CO2 is about 50 times more soluble in water (or beer or coffee) than nitrogen, and it produces larger bubbles as the gas comes out of solution. In contrast, nitrogen comes out of solution quickly, forming a fine fizz.

There’s a bit of chemistry involved, too. CO2 forms carbonic acid when it comes into contact with water. For some beers, this acidity isn’t very noticeable because lighter beers tend to be a little sour anyway. But for coffee, or darker beers like stouts, acidity doesn’t fit with other flavors in the drink. Nitrogen doesn’t create any acidity, which is why using it saves coffees and stouts from that sourness.

Nitrogen also gives beer or cold brew coffee that distinctive falling foam pattern that’s familiar to Guinness stout drinkers, as the tiny bubbles are easily pushed around the glass by the circulating liquid, according to a 2011 paper (paywall) on the physics of foam in stout beers. (Yes, really.)

While Guinness has been using nitrogen for its signature stout for about 60 years, the Irish company’s latest offering, a nitro India pale ale, is one of many new beers getting the nitro treatment:

Guinness joins Sam Adams, as well as a handful of smaller craft brewers offering nitro beers, including Oregon’s Deschutes Brewery and Colorado’s Left Hand Brewing Company. Nitro beer can be served from a keg, and some breweries have, like Guinness, developed gadgets to facilitate nitrogen infusion in a can or bottle.

The expansion of the nitro technique to non-stout beers is somewhat unexpected, Shellhammer said.

“It’s a little bit counterproductive,” he said. “IPAs are all about aroma,” he explained, and the foam on a nitro beer keeps aromatics from leaving the beer. But for those IPAs that are particularly bitter, nitro “is potentially a way to soften that.” It’s also, of course, a way for breweries to stand out in a crowded craft beer market.

When it comes to nitro coffee, this summer’s coolest way to get wired, those tiny nitrogen bubbles change how your mouth interprets the flavor. Nitro coffee tastes sweeter and creamier, even without milk or sugar, thanks to the way nitrogen bubbles interact with your taste buds. “It makes the cold brew seem smoother, denser,” said Diane Aylsworth, vice president of cold brew at Stumptown Coffee Roasters. “It tricks your palate.”