The average beer is about 95 percent water and 5 percent alcohol. Tucked into the tiny space between 5 percent and “about” 95 percent are traces of proteins (more in unfiltered beer), some aromatic and flavor compounds created during fermentation, dissolved carbon dioxide, and the various bittering resins and oils that come from hops.

That space is also home to organic beer’s “organicness,” a source of great debate and angst these days. It all has to do with hops. Strong growth and the entry of major players into the organic-beer market are straining the supply of hops. Changes in federal rules about what can and can’t be included in products labeled “U.S.D.A. Organic” could affect the beer landscape even more profoundly.

“Just from a sheer quantity standpoint, there are not enough organic hops available for every organic brewer to use 100 percent organic hops,” says Jon Cadoux, president of Peak Organic Brewing, in Portland, Maine. “This would end the organic beer category.”

What’s so special about hops? Hops – leafy, green conelike flowers that grow on tall, twining vines—are the spice of beer. They provide the baseline bitterness that makes beer thirst-quenching. Hops are as varied as the grapes used in wine, and certain strains—Cascade, Hallertauer, Fuggles, Saaz, and many others—are especially prized for the aroma and flavor they give to beer.

The problem is that hops are a fragile crop, highly susceptible to damage from a range of pests, fungi, and mildews. “A hundred years ago, the Northeast was the predominant growing region for hops,” says Morgan Wolaver, president of Wolaver’s Brewing, the country’s largest organic craft brewer, in Middlebury, Vermont. “It was wiped out, primarily by mildew.”

By the 1920s, production had moved to the Pacific Northwest. Washington’s Yakima Valley alone produces about 75 percent of the nation’s hops.

Hops producers have used an array of chemicals and quarantines to ward off damage. But a farmer growing organic hops has a limited arsenal with which to fight pests and disease. “It’s a big risk for the farmer,” says Doug Muhleman, Anheuser-Busch’s vice-president of brewing operations. “This is anecdotal, but a lot of these guys who have put in organic [hops] fields have failed miserably. They get rainfall [at the wrong time], and everything goes to hell in a handbasket within 24 hours. They’ve got mildew, and they can’t produce. It’s not an easy proposition.”

The small amount of organic hops on the market led the Department of Agriculture to place the crop on a list of nonorganically grown ingredients allowed in food products marked with the green “U.S.D.A. Organic” seal. (Other items on the list: sausage casings, gelatin, and fish oil.) If brewers could prove that they were unable to purchase sufficient organic hops of the type they needed, they would be permitted to use nonorganic hops.

Like most organic brewers, Muhleman and his team at Anheuser-Busch made the decision to make their two organic beers—Stone Mill pale ale and Wild Hop amber lager—based on hops’ presence on the exemption list. “When we first got into brewing organic beers last year, that weighed heavily,” Muhleman says. “The initial pass was that there’s not enough organic hops out there to really do this.”

But a successful lawsuit brought against the U.S.D.A. by an organic blueberry farmer from Vermont has put that whole arrangement in doubt. The suit reflected the concerns of a number of organic consumers and veteran organic producers, who worry that the U.S.D.A. is watering down the term organic. One major point of contention was the exemption list. Now, the U.S.D.A. is accepting public comment on the list and will decide this fall whether to alter it, leaving the future of organic beer up in the air.

Organic hardliners have no doubts: There should be no exemption list at all. The Organic Consumers Association labeled hops’ presence on the list the “Budweiser exemption,” presumably because the huge company was a better target than the small Vermont brewery of a longtime organic activist like Wolaver.

That’s somewhat amusing, because Anheuser-Busch was able to accommodate the zealots’ demands, reformulating its brews to use organic hops exclusively. The beers were made with some organic hops already; as of June 9, the company’s brewers started brewing with only the organic hops they had.

But that’s not going to be easy to keep up. “We don’t have enough hops right now to last the year,” Muhleman said in early July. They’re finding sources for more organic hops—Muhleman recently flew to New Zealand to meet with a producer—but this only puts more strain on the hops market for other brewers.

Wolaver acknowledges that Anheuser-Busch’s entry into his niche isn’t all bad. “If Anheuser-Busch buys up all the organic hops, that helps the market,” he says. “It encourages farmers to grow organic hops.” But new hop vines take at least three years before they begin to produce, and Wolaver says he knows of two farmers who have torn out unsuccessful fields of Cascades.

It’s not going to get better anytime soon. Sales of organic beers have grown approximately 40 percent annually since 2004. SABMiller has launched an organic beer under its Henry Weinhard’s label; a number of craft brewers have added organic beers to their lines; and business is booming for Butte Creek Brewing, an organic brewer in Chico, California. They all want more organic hops.

“The U.S.D.A. is looking for people to prove [that they will face] a serious economic burden if a product is delisted,” Wolaver says. “If you take hops off the list, you’ve cut my production to about a fifth of what it is now. So who do you want me to fire?”

Even if taking hops off the list didn’t end up being cataclysmic for the organic-beer category, it would leave it dramatically changed. The organic hops that remain available represent a small fraction of the full range of hops. “Say I need Perle hops for a recipe,” Cadoux says. “People are going to say, ‘Can’t you just use something else that’s available organically?’ No, I can’t.” Organic brewers could compensate by brewing beers that don’t need a lot of hops. So you may see a lot more spiced witbier, smooth brown ale, malt-heavy bock, and strong, sweet abbey styles, but you won’t see many organic I.P.A.’s.