There’s nothing fast about the Slow Pour Pils from Bierstadt Lagerhaus. The Denver brewery spends 30 hours brewing the straw-colored pilsner, ferments it for weeks and, finally, makes its taproom customers wait at least five minutes for a taste.

Bartenders pour the pilsner straight into the glass to create foam, then let it subside. They repeat the ritual several times until a lustrous white head rises above the slender cylinder’s rim like a snow-capped mountain.

The foam is pomp with a purpose. This slow build releases the prickly, belly-bloating bubbles of carbon dioxide, leaving a smoother, less filling brew. “Beer is the only beverage that makes and keeps foam,” said Ashleigh Carter, the head brewer and an owner. “It speaks to the quality of the beer.”

Yet in bars and restaurants across America, servers routinely take care to lay the bottle or spigot against the glass before pouring — an unsanitary method sure to produce no head at all. It’s unclear why, though one theory is that many beer drinkers’ (and pourers’) earliest experiences were at keg parties where somebody pumped the tap too many times.

When he first started drinking beer, “I was like, ‘Foam is the enemy,’ ” said David Robert Wolcheck, 38, a nonprofit manager in Brooklyn, New York. “There’s nothing more frustrating than an ounce of beer and 10 ounces of foam in a red Solo cup.”

Ashleigh Carter, a founder and the head brewer of Bierstadt Lagerhaus, pours a Slow Pour Pils, which takes at least five minutes to pour, at the brewery in Denver, Sept. 19, 2019. (Benjamin Rasmussen/The New York Times)
Ashleigh Carter, a founder and the head brewer of Bierstadt Lagerhaus, pours a Slow Pour Pils, which takes at least five minutes to pour, at the brewery in Denver, Sept. 19, 2019. (Benjamin Rasmussen/The New York Times)

As the craft-beer business continues to mature, bars and breweries are placing greater emphasis on serving beer, particularly pilsner, with foam. Aardwolf Brewing, in Jacksonville, Florida, serves the slowly poured Adalwolf, a German-style pilsner, with a head billowing above the rim. AC Golden, a subsidiary of MillerCoors, suggests that bartenders spend seven minutes pouring its Barmen Pilsner into a tall, elegant glass. And Brouwerij West, in San Pedro, California, aims for a 1 1/2-inch head on every beer, including its unfiltered Popfuji Pilsner.

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There’s science behind all this. As bubbles rise to the surface in pouring, so do proteins and hop compounds, which generate and stabilize foam. The congregated bubbles capture a beer’s fleeting fragrances, slowly releasing scents with each pop.

“If a beer is poured to the brim with no foam, all of those aromatics are just kind of going away,” said Neil Witte, owner of Craft Quality Solutions, a Kansas City, Missouri, consulting firm focused on beer service. He generally recommends a 1-inch collar of foam, though some highly carbonated beers, such as German weissbier, can be poured with a larger head.

A lack of foam is less of an issue for mainstream American lagers, which have few nose-worthy aromas. But many modern craft beers emphasize pungent scents and taste. The right combination of ingredients, alcohol level, equipment and serving style can produce an impressive, fragrant head.

Glassware has long been a hurdle to serving beer with foam. The prime culprit is the shaker pint, the default glass at many bars and breweries — a sturdy, affordable 16-ounce vessel designed for mixing cocktails. “I call shaker pints the cockroaches of beer glasses,” said Randy Mosher, author of “Tasting Beer.”

The problem with shaker pints is twofold: Their straight-walled, wide-mouthed shape doesn’t concentrate the foam, and they have enough space for only 1 pint of liquid — 16 ounces, filled to the top.

Some beer drinkers perceive foam as stealing a few precious ounces of beer. Bushwick Country Club, a bar in Williamsburg, Brooklyn, has one particularly foam-averse regular. “If there’s more than an eighth-inch of foam, he complains,” said John Roberts, the owner.

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This confusion stems in part from America’s Wild West, no-rules approach to serving size. The federal government doesn’t mandate that a pint glass contain a minimum volume, and some may hold only 14 ounces of liquid. In Britain, regulations require that a pint glass hold 20 imperial fluid ounces, equivalent to about 19.2 fluid ounces in the United States.

“You’d be amazed by how many glasses are ripping you off,” said Carter, of Bierstadt Lagerhaus. “Bars are saying you’re getting 16 ounces of beer, and you’re not.”

Bierstadt is transparent about its pouring sizes, which makes it easier to educate customers about the merits of foam. Since opening in 2016, the brewery has served beer only in its own style-specific glasses and requires that other bars serving its beers do the same. The glasses have clear markings that denote a specific metric liquid measure, such as the half-liter for its golden helles, a German-style lager served in a large mug.

“I tell people, ‘If you pay for a half-liter, you get beer to the half-liter mark,’” said Carter, who touts foam as “a bonus. If you let the beer sit long enough, it’ll float past the mark.” (Foam is, on average, about 25% beer.)

Foam is integral to many of Europe’s traditional beer brands and styles. Guinness teaches bartenders to take about two minutes to create the inky stout’s creamy foam collar.

“If you don’t do the last 5%, which is clean glassware and a proper pour, you’re undoing a lot of a brewery’s hard work,” said Anthony Malone, a partner in Swift Hibernian Lounge, in the NoHo section of Manhattan.

Lingering fingerprints, lipstick and detergent on badly rinsed glasses can make a beer’s head deflate like a punctured balloon — a telltale sign is tiny bubbles clinging to the grime on the walls of a glass instead of rising to the top.

Chris Lohring, the head brewer at Notch Brewery, pours an amber lager using a Czech side-pull tap to create a large head, at the brewery in Salem, Mass., Sept. 20, 2019. (Kayana Szymczak/The New York Times)
Chris Lohring, the head brewer at Notch Brewery, pours an amber lager using a Czech side-pull tap to create a large head, at the brewery in Salem, Mass., Sept. 20, 2019. (Kayana Szymczak/The New York Times)

Some American taprooms have begun to emulate Central European beer culture by installing side-pull taps that work like a dimmer switch, allowing bartenders to create precise amounts of foam. Among the earliest adopters was Chris Lohring, who founded Notch Brewing in 2010.

He opened Notch’s lager-focused brewery in Salem, Massachusetts, in 2016, outfitting the taproom with side-pull taps that he bought directly from a Czech manufacturer.

“I wanted to do it exactly the way a Czech lager should be poured,” with a dense, creamy head, Lohring said, adding that, unlike the slow-pour technique, the special taps let bartenders pour a half-liter in about 15 seconds.

“People drink through the foam, and then they get this expression on their face like: ‘Wow, this is a different experience. This is creamy, but it’s still snappy and clean.’”

At Notch, foam has become both a conversation starter (“Why so much foam? I love that question,” Lohring said) and a social-media star. The brewery offers a milky, all-foam pour called mlíko — Czech for milk. “It’s Instagram gold,” Lohring said. “People see it and come in and say, ‘I’ll have the milk pour.’ ”

Even in its humblest form, foam elevates a beer’s aesthetics. It can be an object of desire rather than ire.

Foam is the “the whipped cream on a sundae,” said Charlie Devereux, a founder of Wayfinder Beer, in Portland, Oregon, which specializes in European-inspired lagers. “You can make a sundae without whipped cream on top, but it’s so much better with the whipped cream.”