When Americans fell in love with the eccentric beers of Belgium, we started with the styles that are easiest to like: the rich, strong brews made in the monastic tradition.

It seems we’re now ready to look beyond the cozy nuclear family of Belgian beer and embrace the crazy cousins. It’s time to get acquainted with the antique beer styles whose dominant accent is sour, not bitter. And although the term “sour beer” sounds off-putting at first, some exciting flavors await the adventurous drinker.

Sourness in beer can come from a number of sources and be expressed in an array of intensities, from lightly tangy to puckering. Souring bacteria predominantly contribute acidic notes. Archaic yeast strains can play a role, and all of these organisms produce different effects depending on their succession in fermenting and aging beer. Fruits, spices and other unusual additions can contribute to a sour profile.

Our tongues register sourness when they detect levels of acidity. The primary sources of acidity are the closely related bacteria Lactobacillus (present in yogurt and sauerkraut) and Pediococcus, both of which produce lactic acid; and Acetobacter, the source of acetic acid, or vinegar. Lactic acid has a sweet-sour quality; acetic acid is sharper. Both acids can interact with alcohol to form chemical compounds known as esters. When these compounds combine, they can produce great complexity in a beer.

When brewers talk about sour beers, they may also be referring to two other trends. One is “wild yeast,” meaning microorganisms besides purified brewers’ yeast. And wood or barrel aging is often associated with sour beers. Wood is permeable to oxygen, which allows communities of organisms to live on and in its surface, where they contribute to beer character. But wood can promote many effects: Beers labeled “barrel-aged” may or may not be sour.

West Flanders Red Ale: These brilliant, tart beers rely on the action of bacteria in huge wooden vessels to add lactic and acetic (vinegar) notes to the aging beer, as well as related esters. Rodenbach is the classic example, deep red and wine-like.

East Flanders Brown Ale/ Oud Bruin: In East Flanders, the region’s sour beer is based on a brown ale, with more caramel, toffee and dark fruit notes than the red beers of its western neighbor. The oud bruin (old brown) beers are matured in stainless steel, a more controllable medium than wood. One notable example of oud bruin, Liefmans Goudenband, is an almost purely lactic beer, with little acetic or wild character. The final beer has a rich, malty character, almost sherry-like.

White Ale/ Wit Beer: This spicy Belgian wheat ale teetered on the edge of extinction early in the 20th century and was revived by the late Pierre Celis, who brought the style to the U.S. Cloudy, spiced with coriander and orange peel, it also features a refreshing lactic jolt. If you’ve recently enjoyed Blue Moon from brewing giant Coors, or White Rascal from the Avery microbrewery in Colorado, you’ve already discovered sour beer.

Berliner Weisse: This cleanly acidic German wheat beer, dubbed “the champagne of the North,” underscores the fact that sour beer was once common across Europe. It’s now a rare style, Kindl Weisse being the sole example still brewed in Berlin. However, American innovators at Dogfish Head Craft Brewery in Delaware base their peach-tinged Festina Peche on a Berliner Weisse, and Colorado’s New Belgium has recently released an imperial version for the Lips of Faith series.

Lambic: These wild-fermented beers from the area near Brussels are the oldest beer style still made. The brewing process exposes the beer to the open air, to be colonized by ambient wild yeast and bacteria. After one to three years of maturation, a master blender combines lambics of various ages to create a final beverage that is layered, complex and challengingan acquired taste.

American experiments: New Belgium is the clear trailblazer, though its extraordinary La Folie, a blended sour brown ale aged in French oak, is not available locally.

Look for the sophisticated beers from Michigan’s Jolly Pumpkin, or the exclusiveand expensiveselection from Cascade in Oregon, as well as the occasional sour treat from The Bruery, based in Los Angeles.

Locally brewed sours, apart from the popular wit style, are usually one-off efforts, although both Natty Greene’s and Fullsteam have poured examples in the past.

Sour beers certainly challenge the brewer; man more of nature’s variables are at play during fermentation and maturation. But beer lovers who embrace these new interpretations are finding flavors that stretch our modern definition of “beer”and remind us of its origins.

This is an excerpt from a column appearing in the May edition of All About Beer, of which Johnson is the editor.