Special to The Washington Post To most beermakers, it's the enemy: a wild yeast that lurks on unsterilized equipment and inside wooden tanks, capable of spoiling a fine brew if allowed to grow unchecked. But Brettanomyces -- Brett, for short -- has long been welcomed by Belgian brewers for the extra dimension of flavor it adds to their ales. And now a few pioneering U.S. beermakers are courting the bug. Brett reaches its full flavor potential in Belgian lambics, beers exposed to the atmosphere to spark a spontaneous fermentation. After contact with Brett they often display intense, gamy flavors that are described as 'horse blanket,' 'barnyard' and 'old leather.' (My favorite descriptor, from a tasting 10 years ago: 'saddle-ridden hard and put away wet.') Other brewers limit Brett's influence by using a standard Saccharomyces yeast for the primary fermentation, then adding a pinch of Brett for a secondary fermentation in the bottle. Brett is also capable of imparting more-subtle flavors: cidery, earthy, spicy, clove-y, citric and phenolic. It's slow but methodical, gradually chewing up all sugars (including complex varieties that ordinary yeast can't handle) to produce a beer that's dry and quenching. The term 'wild ale' is often used for American Brett beers, but it's something of a misnomer. Most American brewers don't expose their beers to ambient yeast; they obtain their yeast cultures from suppliers, such as White Labs and Wyeast. However, their beers tend to have extremely lively carbonation and dense, billowy heads of foam that will cascade over the lip of the glass if the pourer is careless. In that sense, they are wild. Orval Trappist Ale, from the Abbey of Notre Dame d'Orval in Belgium, probably is the most admired and imitated Brett beer. It's unique for a Trappist beer, not fitting into the usual categories of single, double or triple. Packaged in distinctive, bowling-pin-shape bottles, Orval pours a burnt orange with head as thick as a milkshake. The brewery allows the beer to sit for nine weeks after bottling. After shipping, Orval probably takes at least six months to find its way onto store shelves, which gives the Brett plenty of time to work its magic. The beer arrives exceedingly dry, with a cidery tartness and a peppery, herbal character that comes from suspending sacks of hops in the fermenting beer. (The process, called dry-hopping, is common here but rare in Belgium.) Matilda, from Goose Island Beer in Chicago, is one of the better Orval tribute beers. It comes in a plain package: a 22-ounce bottle with an unadorned, black-and-white label. This Belgian-style strong ale is brilliant orange in color with an earthy aroma, a honeylike sweetness tempered with an herbal hoppy flavor (think of honey menthol cough drops) and a lingering, dry aftertaste. Saison Rue hails from the Bruery, a tiny microbrewery about three miles from Disneyland in Placentia, Calif. The name of the beer and brewery hark to the Rue family, which runs the business. 'When I was in law school, my wife told me I needed a hobby,' says chief executive Patrick Rue, who took up home-brewing and found a new vocation. Saison Rue has a tart, fruity aroma and a limitless ability to foam over. Even after I left the bottle in the fridge overnight with the cap half off, the beer still produced a huge head when poured. Saison Rue is a pale orange color and has an intense, bittersweet spiciness. I thought I picked up notes of clove, coriander, maybe a little cinnamon, but Rue assured me that he adds no condiments. It all comes from the yeast, plus the 30 percent rye malt in the recipe. The beer also has a citric character that Rue likened to that of pineapple. Wild Devil, from Victory Brewing in Downingtown, Pa., is not for the timid. Based on the brewery's mainstay HopDevil, an India pale ale, it has an aggressive, resiny hop bitterness and an equally powerful 'horsy' aroma and flavor, with notes of fresh earth and smoke. It's unusual in that it undergoes a primary fermentation solely with a Brett strain cultured from an Orval bottle, after which it's bottle-conditioned with both Brett and normal yeasts. Brett beers pair well with a variety of foods, including shellfish, barbecue and sharp cheeses, but this one calls out for wild game such as venison. If you have the willpower to cellar them, Brett beers will evolve, becoming drier and more complex. A bottle of Orval packaged Nov. 13, 2008, promises it will remain drinkable until Nov. 13, 2013. Likewise, the label for Matilda asserts that it will develop in the bottle 'for up to five years.' Some of these beers, however, are so new on the market that their evolution is an uncharted course. Saison Rue, for instance, has been in existence for only a year and a half. A 2007 bottle of Ommegeddon Funkhouse, from Brewery Ommegang in Cooperstown, N.Y., had so much pent-up carbonation that the cork exploding out of the bottle could have signaled the start of Armageddon. The pale orange liquid within was bone-dry, acidic and citrusy, with a funky, earthy aroma. Steven Cardello, market manager for Duvel Moortgat USA (the company that distributes Ommegang brands), says the beer would be great with mussels or scallops. A recently released bottle of Biere de Mars, another of Brewery Ommegang's experiments with Brett, was the polar opposite: darker, maltier, with a caramel-apple sweetness and only a hint of the Brett flavor that surely will manifest itself in months and years to come. Phil Leinhart, brew master for Brewery Ommegang, says he's experimenting with other strains of Brett besides the B. bruxellensis commonly used by U.S. craft brewers. 'I love the effect the Brett gives the beer: a real floral, fruity note,' he says. But Brett can create a disaster if it makes its way into the brewery's more conventional beers. Consequently, the tank that holds the Brett-impregnated beer just before bottling gets a thorough cleansing with a caustic solution and iodine before it's used for any other brew. With a little elbow grease, wild yeast can be tamed. 