Sierra Nevada Brewing is introducing two IPAs made with hop strains that are so new, they’re identified only by numbers. The Harvest Single Hop IPA is brewed with Yakima #291. (Sierra Nevada Brewing Co.)

“Just the kiss of the hops.” In its World War II-era advertising, Schlitz tried to entice drinkers by promising the barest hint of the bitter herb in its beer.

A slogan like that would leave modern craft beer drinkers cold. They don’t want a peck on the cheek; they crave intimacy. Heck, some devotees would walk a hop vine down the aisle if the law allowed.

To no one’s great surprise, the market research group IRI recently announced that India pale ale has become the top-selling craft beer style. Ordinary pale ale used to hold that distinction. In another 10 years, one can imagine the even more hop-forward imperial IPA unseating its progenitor.

But love doesn’t always come at first sight . . . or first sip. This romance took more than 1,000 years to kindle.

Hops, the seed cones of a perennial vine related to marijuana (they both belong to the family Cannabinaceae), have been cultivated since the Middle Ages. Brewers have long recognized the utility of hops: They add a little kick to the bland sweetness of the malt, and their antibacterial properties help preserve beer from spoilage. But for centuries, it was more a marriage of convenience than an affair of the heart.

The Abbess Hildegard von Bingen, whose 12th-century pharmacopeia “Physica Sacra” was perhaps the first document to record the use of hops in brewing, didn’t exactly give them a ringing endorsement. She claimed that they induce melancholy and weigh down one’s insides. Nor were hops welcomed with open arms when they crossed the Channel. Henry VIII is reported to have given the royal ale brewer strict orders to use neither hops nor brimstone in the king’s libations. As late as the 1990s, centuries after the hop had become universal in commercial brewing, one major American brand proclaimed its lack of hops to be a virtue, using the tagline, “No more bitter beer face!”

So how did the hop — essentially a seasoning, like parsley or black pepper — nudge barley off center stage and grab the limelight? How did Jim Koch, chairman of Boston Beer, come to the conclusion, solemnly intoned in his commercials, that “hops are the soul of beer”?

 Koch markets a “Hop-ology” six-pack containing a half-dozen variations on the IPA style. He’s rolling out still another: Rebel IPA. In terms of international bitterness units, this is the weakling of the bunch, registering only 45 IBUs. (Those units measure alpha acids, the main bittering chemical in hops. Claims of 100 IBUs and above are common in the industry.)

And yet, this is Boston Beer’s most successful attempt to date to mimic the no-holds-barred hop character of West Coast IPAs, an explosion of pine resin, grapefruit, orange and peppercorns.

The genius of American craft brewers — and hop growers — lies in the realization that “bitterness is just one component of hop character,” suggests Koch. Newer varieties are being bred to minimize that bitterness and to eliminate a defect called “cattiness,” the waft of unchanged litter box that emanates from some ales aggressively hopped with high-alpha American strains. At the same time, they’re emphasizing the volatile oils that give different hop varieties their distinctive perfumes.

A little mystery can spice up a relationship. Sierra Nevada Brewing in Chico, Calif., as part of its new Harvest series, plans to introduce two IPAs this year hopped with strains so new and experimental that they’re identified only by numbers. The first, which is shipping in 24-ounce bottles and kegs even as you read this, employs a hop dubbed 291.

Tom Nielsen, Sierra Nevada’s R&D man for raw ingredients, flirted with more than 70 new cultivars before consensus was reached on this strain, says communications manager Ryan Arnold. “It really stood out for what we picked up as strong blueberry and blackberry flavors,” he elaborates. What’s more, “it imparts a really mousselike smooth, white head.”

Toward the end of the year, Sierra Nevada will release an IPA hopped with a wild subspecies of hop called Humulus lupulus neomexicanus. Drinkers should pick up a fiesta of “fresh dill, as well as papaya, orange rind and citrus,” comments Arnold.

Brewers are constantly seeking new ways to romance the hop. Atlanta-based SweetWater Brewing — no relation to the Sweetwater Tavern brewpub near Merrifield — plans to release a “hop hash” IPA, salvaging the green gunk that accumulates in the machinery used to process raw hops into pellets. SweetWater contracted with a hop supplier to scrape off the residue, press it into bricks and send it to the brewery. Months after the autumnal harvest, it’s still “very concentrated stuff,” states SweetWater’s “minister of propaganda,” Steve Farace. “It’s got a very resiny mouth feel, a potent aroma and a unique, almost silky, bitterness, if that doesn’t sound like a contradiction in terms.” Look for the yet-unnamed IPA to debut in April as part of the brewery’s “Dank Tank” series of limited releases.

Far from being a solitary curmudgeon, the hop is surprisingly social, meshing well with other ingredients. SweetWater, which just began marketing in the Washington area, also offers a year-around IPA called LowRyeder with 25 percent rye in the grist. Harpoon Brewing in Boston just introduced the Long Thaw, an IPA mixing hops with coriander and orange peel. (Called a “white IPA,” this American take on Belgian witbier has become a style in its own right.) Magic Hat Brewing teamed up with neighboring beermaker Vermont Pub & Brewery to release Steven Sour, an IPA blended with tart passion fruit juice. California’s Stone Brewing has dabbled with coffee and coconut IPAs.

That’s a lot of love for an herb that makes up far less than 1 percent of the brew, volume-wise. My Bitter Valentine? “Hops Are a Many-Splendored Thing” might be a better anthem to sing its praises.

Kitsock is the editor of Mid-Atlantic Brewing News.