This summer, the Mission District gastropub the Monk's Kettle offered a tasting flight of 10 beers from Copenhagen brewery Mikkeller. What distinguished these beers from the 175 or so others in the cellar was not just their pedigree - Mikkeller is one of the most highly regarded breweries in Denmark - but the fact that each was brewed using just a single variety of hop.

Mikkel Borg Bjergsø, Mikkeller's founder, is something of an evangelist for single-hop beers, an interest that traces back to his days as a home brewer, when he brewed batches using individual hop varieties to better understand their character.

"I thought it was very educational," he says, "and I thought if I could do the same thing for drinkers, it would be helpful."

When added at different stages of the brewing process, the female flowers of the hop plant (Humulus lupulus) impart bitterness, flavor and aroma to beer. Brewers typically use a combination of several cultivars to achieve a particular "hop profile."

But recently, a growing number of craft breweries from the European Union to the United States have been producing beers that employ and highlight the characteristics of one hop at a time.

Nicole Erny, a Bay Area beer expert who works at the Trappist in Oakland, speculates that the trend may have sprung in part from a recent worldwide hop shortage. In 2007, poor weather in hop-producing regions coupled with farmers reducing their acreage of hops in favor of other more lucrative crops left brewers scrambling.

"I think something happened during the hop crisis," said Erny. "Brewers had to get more creative with sourcing."

So brewers went further afield, to different growers, and often found themselves filling in the gaps with new hop varieties. Brewing single-hop beers was an excellent way to familiarize themselves with the new hops.

For eight months, Rodger Davis, the head brewer at Berkeley's Triple Rock Brewery, has been brewing beers in a series dubbed the Single Hop Experience. According to Davis, the single-hop program serves two purposes: It allows him to "see what a hop can do" and determine whether it can single-handedly support a beer or should be blended, and it gives customers an opportunity to experience the distinctive characteristics of hops one at a time. For the sake of consistency, Davis uses a standard base recipe to let the featured hop's qualities shine through.

A variety on tap in August, Apollo, took its name from a relatively new hop strain grown in Washington state. The Apollo's bouquet was assertively herbal and resinous, with a hint of skunkiness - a possible reminder that Humulus lupulus is a close relative of Cannabis sativa. The Apollo was substantial and engaging, the hop's bitterness offset with a pleasantly sweet breadiness.

Bjergsø, who founded Mikkeller in 2006, recalls how single-hop beers helped hone his early efforts.

"I did 15 beers, and it taught me a lot about the different hops," he says. "If you're using several different hops, it's easier to play with the hop profile, to get the correct bitterness and aroma. When you use a single hop, it's of course harder."

Indeed, it's illuminating to learn that a lone hop, judiciously applied, can bless a beer with the unmistakable aroma of grapefruit (from Amarillo hops, grown in Toppenish, Wash.), a bracing piney taste (from Simcoe, another Washington state hop), or a flavor that calls to mind Sauvignon Blanc (thanks to a cultivar from New Zealand, appropriately named Nelson Sauvin).

Monk's Kettle quickly exhausted its supply of Mikkeller's initial single-hop series, and the beers have all but vanished from local stores. But Bjergsø plans to brew five more single-hop beers this fall, and anticipates they will arrive in the United States in January.

As Sayre Piotrkowski, beer sommelier for the Monk's Kettle, sees it, people come to the bar seeking refreshment but also to expand their knowledge about beer.

"If someone comes in, and it dawns on them what a Cascade hop tastes like," Piotrkowski says, "they're going to want to learn more, and they're going to come back."