No longer the elusive Belgian mystery, sour beer is beginning to make its way into mainstream American beer culture. From light, luscious tartness to pucker-inducing acidity, sour beer is a fascinating genre that is altogether different from other beer styles.

There are six subcategories of Sour Ale in the BJCP Guidelines — Berliner Weisse, Flanders Red Ale, Flanders Brown Ale/Oud Bruin, Straight (Unblended) Lambic, Gueuze, and Fruit Lambic. Some of these styles can be dry and light, others are dark, rich, and fruity. But the common flavor throughout them all is a disctinctive sour acidity.

So what makes these beers sour? As we all know, beer is fermented with brewers yeast, Saccharomyces. Of course, there are a bajillion different strains of Saccharomyces, but generally, they all produce pretty clean, pleasant flavors in beer.

Another genus of yeast, Brettanomyces, is sort of like Saccharomyces’ evil cousin. Lovingly referred to by brewers as “Brett,” this yeast gives beer a uniquely sour, horsey, barnyard flavor to beer. Sounds delicious, right? Believe it or not, these flavors are suprisingly appealing in farmhouse beers.

Another souring critter commonly used is Lactobacillus, a genus of lactic acid bacteria. That’s right, people actually add bacteria to their beer intentionally! These bugs eat up lactose and other sugars to produce lactic acid, which gives the beer a good, clean sourness. Pediococcus is another common lactic acid bacteria used in sour beers.

There’s also Acetobacter, a genus of acetic acid bacteria, which gives off flavors of cider and vinegar. The different species of bacteria are often used in conjunction with Saccharomyces and Brettanomyces.

So how does one go about getting these critters in their beer? If you want full control over what’s going in your wort, you can buy strains of the different yeast and bacteria, just like you would with regular brewers yeast.

If you’re a little more experimental and free-spirited, you can just open up your fermentation vessel and let nature take over. There are plenty of naturally-occurring yeast and bacteria floating around in the air, and often times, darn good beer can come about from them. Some brewers will also age their beer in unsanitized oak that has previously been the home to desirable bugs.

At home, it’s easy enough to innoculate your wort with Lactobacillus. Grain husks are a natural home for L. delbruckii, so all you have to do is throw in a handful of grains to a mash that has been cooled to below 120° F and let it “cook” for a few days. For more detailed information on sour mashing, check out this article from BYO Magazine. Tim and I plan to try this method when we brew a Berliner Weisse in the coming weeks. I’ll be sure to report back!

If you want to take the easy route and avoid using bugs to sour your beer, you could just add straight-up lactic acid or acidulated malt, which contains a small amount of lactic acid. But where’s the fun in that?

I don’t want to overwhelm anyone with too much info at once (I’m sure I’ve already eeked out a few of you with the microscopic pictures of our friends Brett and Lacto), so I’m going to save the sour beer reviews for my next post. Stay tuned to find out just what these sour anomalies can taste like!