Beer’s third-most-important ingredient, after water and barley, is almost as mystifying to most beer drinkers as Prohibition. But a new kind of beer has recently emerged that gives consumers an unprecedented ability to learn: the single-hop India pale ale.

It’s a style that demonstrates, as Boston Beer President Jim Koch once put it, that “hops are to beer what grapes are to wine.” Like grapes, the resinous flowers of Humulus lupulus are agricultural products that come in distinctive varieties, each with its own flavors and aromas. Instead of syrah and cabernet, the beer world has Simcoe and Cascade.

Most beers are brewed with a mixture of hop strains; brewers use some kinds primarily to produce a beer’s bitterness and others to add fruity, floral or herbal aromas. (Potent double and imperial India pale ales often pile them on: Founders Brewing’s Devil Dancer, for example, includes 10 varieties of hops.) Other beers, such as the 19 IPAs in the Single Hop Series from Denmark’s Mikkeller, use just one. That makes it possible for drinkers to understand the difference between, say, an earthy European hop such as East Kent Goldings (often featured in English bitters) and a fruity American hop such as Amarillo (a mainstay of West Coast IPAs), or to identify the nuances of similar varieties from one growing region.

“When you’re making a single-hop beer, at least part of the drive is educational,” says Greg Engert, beer director of Birch & Barley and ChurchKey. “It’s fun for me because it allows me to teach my staff about hops on a very, very specific level.”

For the same reason, professional brewers have often made small batches of single-hop beers for their own consumption. At Boston Beer, Koch says, “it’s something we’ve done for years to educate our own palates.”

Maryland’s Flying Dog Brewery has released single-hop imperial IPAs brewed with Simcoe. (Image courtesy of Flying Dog Brewery/IMAGE COURTESY OF FLYING DOG BREWERY)

In fact, some of America’s most iconic craft beers rely heavily on a single variety: Sierra Nevada Pale Ale, for example, is hopped with Cascade, and Two Hearted Ale from Bell’s Brewery showcases Centennial. But those beers aren’t marketed as being single-hop; that’s something new. According to Julia Herz, craft beer program director for the Brewers Association, which represents small and independent American breweries, “literally in the last year or two, this has taken off.”

Just within the past several months, Maryland’s Flying Dog Brewery released single-hop imperial IPAs brewed with Simcoe, El Dorado and Centennial; Brooklyn’s Sixpoint Brewery launched its Spice of Life series, which will feature a different single-hop beer each month for a year; and Mikkeller began selling the latest iteration of its Single Hop Series, probably the most ambitious single-hop project ever. Particularly noteworthy is that in April, Boston Beer, America’s largest craft brewery, released its Samuel Adams Latitude 48 Deconstructed pack featuring five single-hop IPAs, the first the company has ever distributed.

One reason for the boom, Herz says, is the “hop crisis” of 2007 and 2008. Bad weather and poor global harvests, plus unfavorable currency exchange rates, made many varieties scarce in the United States and forced brewers to experiment with new ones. “At that time, breweries really became aware that hops are not a commodity,” Herz says. “They express terroir based on the weather conditions when they’re harvested. They are not something to be taken for granted.”

Another cause of the single-hop trend is that hop research centers in the Pacific Northwest have churned out new hybrids with specific desirable traits, often by collaborating with commercial breweries. A good example is Citra, which has an unusual mix of citrus and tropical fruit flavors. The development of the strain was financed by Sierra Nevada, Deschutes Brewery and Widmer Brothers, and after Sierra Nevada introduced Citra in 2009 in its Torpedo IPA, it quickly became one of the beer world’s trendiest hops.

ChurchKey’s Engert underscores the contribution of Mikkel Borg Bjergso, the single employee of Mikkeller, whose Single Hop Series first appeared in the United States in 2009, around the start of the single-hop wave: “He’s been a big driving force.”

Bjergso’s full series was on tap at ChurchKey in June. The Palisade, named for an American hop variety that is related to the German Tettnanger, was dry and peppery, with a musty, honeyed aroma. The Amarillo, a fruit bomb emphasizing grapefruit and tropical fruit, had a bitter citrus-peel quality and a hint of spiciness.

The Centennial was similar but not so lush, with a purer, slightly grassy flavor, and the Citra was especially aromatic; it smelled like passion fruit. It was hard not to think about a Riesling, or maybe a sauvignon blanc.

Fromson is an associate editor at the Atlantic.