If there's one trend in craft beer that has fought hardest to beat out the hoppy-hoppier-hoppiest IPA arms race, it's the boom in popularity of sour beers. Small production, time-intensive sour brews offer an intriguing history (and hype-inducing rarity), but it's their unique flavor that seems to turn most drinkers into dedicated sour beer fans. The tart, puckering taste is often met with a shocked, love-it-or-hate-it type of reaction, and those with the former can't seem to get enough of the stuff. The secret ingredients that set these beers apart from the rest of the brews on the shelf are actually living creatures: yeast and bacteria.

Sour beers are typically made with a cocktail of little fermenting organisms, each one leaving a unique and identifiable fingerprint on your beer. Aside from Saccharomyces cerevisiae, the most common yeast used for brewing ales, sour beer brewers will often employ the wild yeast Brettanomyces, as well as bacteria such as Lactobacillus, Pediococcus, and Acetobacter.

Either through the process of spontaneous fermentation, in which brewers submit their freshly-brewed wort to the mercy of the yeast and bacteria that invisibly surround us in the air, or through controlled inoculation of lab-isolated yeast and bacteria cultures, these little bugs jump at the opportunity to munch away at the sugar that pervades beer before fermentation. And if you want to figure out which of these creatures had a hand in making your beer, it actually isn't that tough.

Brettanomyces

Known for inspiring such tasting notes as "barnyard," and "horse blanket," Brettanomyces is a funky beast that tends to freak people out a little bit. Its impact on beer varies dramatically based on fermentation temperature, the other fermentation organisms present in the beer, and at what point it's added, but Brett (as it is commonly known), is most easily identifiable by that barnyard funk associated with 4-ethyl phenol, a flavor compound it produces. Brett is also known for tearing through sugars that normal yeast cannot, leaving beers very, very dry. Producing very little acid, Brett is usually not responsible for creating sourness in beers, instead, you're looking for horsey, earthy funk.

Seek out Orval, which is bottled with a dose of Brett, for a good example of what the yeast can do—there's a funky dryness on the finish that is a great expression of the yeast. Anchorage Brewing Company also makes great beers that employ Brett to develop flavors, and Denver's Crooked Stave has a line of 100% Brettanomyces-fermented beers called Wild Wild Brett. Check 'em out!

Lactobacillus and Pediococcus

Lactobacillus and its cousin Pediococcus are bacteria that produce lactic acid, which provides the tartness you can find in yogurt, buttermilk, and in almost all sour beers. While Lactobacillus tends to gently produce lactic acid, Pedio tends to take a more aggressive approach, quickly generating tons of lactic acid. Unfortunately, Pediococcus also produces a significant amount of the undesirable buttery flavor compound known as diacetyl and can leave beer with a snot-like consistency. (Ew.) So Pediococcus is rarely used without Brettanomyces by its side, since Brett "cleans up" after Pedio and yields a more palatable final product.

If chugging buttermilk doesn't sound like a very enjoyable way of tasting the lactic acid calling card of Lactobacillus and Pediococcus, seek out the sour beers of Portland, Oregon's Cascade Brewing. Their tart beers are soured with just Lactobacillus, which leaves them with a uniquely full mouthfeel and clean lactic sourness. Pediococcus is present in traditional lambics and many of the beers attempting to emulate that style (like the sours of Santa Rosa's Russian River Brewing)—look for an intense lactic acidity paired with a funky, dry Brett character.

Acetobacter

Inappropriate in most sour beer styles, Acetobacter produces acetic acid. This stuff eats the ethanol in beer and turns it, essentially, into vinegar. That sounds pretty terrible until you try it. For a pretty intense taste of acetic acid, take a swig off that bottle of distilled white vin you've got in your pantry—then go down to the beer store and grab a Duchesse du Bourgogne. This beer has an obvious vinegar-like acetic acid twang to it, and you'll never forget the taste and the benefits it can provide to beer.

All Together Now

Interested in seeing what these bugs can do together? Grab a traditional Belgian lambic—these beers typically contain all of the above organisms except Acetobacter, and the best ones showcase the deep complexity made possible by these yeast and bacteria.

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