The American love affair with sour beer is still in its infancy. Along with the seemingly unslakable thirst for IPA, the recent penchant for tartness seems to controvert a fundamental quality of the American palate: namely, that we love the sweet and unchallenging above all else. Perhaps the decade-plus of IPA’s dominance has reconfigured things a little. Certainly it must have been hard to envisage ten or fifteen years ago that the style leading the craft category – unshakably, undeniably – is one that runs the gamut from somewhat to extraordinarily bitter, when most American consumers can’t even abide black coffee. So it seems especially striking that Americans are so enamored of sours beers. But are they really? Add up all the checkins for sours on the social beer app Untappd, and they make up 5% of the whole. Still, more new sours are being added to Untappd at an increasing rate, and that added availability means more consumers are facing sours on the aisles, with questions.

Mike Duesenberg / Good Beer Hunting. Data: BeerGraphs The number of new sour beers added to the Untappd database, grouped by month since January 2014.

What constitutes a sour beer? How are they made? What connotes sourness to the consumer, trying to navigate choices in the local bottle shop or beer aisle? All fine, straightforward questions with complicated, somewhat tortuous answers. Let’s take up the first two, as they will help us answer the third. When you go to purchase a sour beer in today’s beer world, I would argue that you’re picking from two broad and very different categories. Generally, you’re either buying a kettle sour or a traditional sour/wild ale. “Kettle sour” is a term that many (if not most) craft consumers will have heard by now; they are inexpensive, quick and relatively easy to make, are often canned, and are responsible for the vast majority of the impressive growth in the sour beer category. It is, itself, not a style. Instead it refers to the production method used in brewing the beer. As the name might imply, souring takes place not in a barrel, but in the kettle or the mash tun: large, stainless steel vessels used for holding crushed barley malt and other brewing grains at set temperatures in order to convert starch to sugar before separating (or lautering) the sweet wort (i.e., unhopped, unfermented beer) from the spent grains and continuing on with the brewing process. In kettle souring, an extra step is added between mashing and lautering: kettle souring (or sour mashing) involves, typically, the addition of a lactic acid bacteria culture to the sweet wort. The wort is then held at a warm temperature while a short lactic fermentation takes place: the bacteria produce a sharp, lemony tartness that one perceives at the front of one’s palate, akin to the tartness in unsweetened yogurt. When the desired level of tartness is reached, lautering continues as normal and the beer is brewed, fermented with “clean” brewer’s yeast, and conditioned and packaged conventionally. Perhaps, as is exceedingly common in kettle sours, fruit is added at some point in the conditioning process. For a more detailed look at many different American breweries’ souring methods, both quick and traditional, pick up Michael Tonsmeire’s absolutely excellent American Sour Beers.

If your experience of sour beer is limited to what you can buy in a six pack of cans, you’re getting less than half the story. Much less.”

So kettle sours aren’t a style unto themselves, but the process is used to create two styles more than any other: Berliner Weiss and Gose. Gose (from the Goslar region of Germany) is a low-alcohol wheat beer with light-to-moderate amounts of lactic acid tartness, traditionally flavored with sea salt and coriander. Emblematic examples are Brauerei Reichenbrand’s Ritterguts Gose and Bayerischer Banhof’s Original Leipziger Gose. Both are more refined and less tart than many modern American interpretations of the style. Berliner Weiss, another low-alcohol wheat beer that was historically less tart than its American contemporaries, also derives its sourness from lactic acid-producing bacteria – the sour mashing technique so widely employed in American breweries today arguably originated with this style, as many Berliners were traditionally soured pre-boil, or not boiled at all. While the Berliner Weiss has become ubiquitous in American craft brewing, there are virtually no German exemplars of the style available to American consumers; perhaps the most readily found (although somewhat untraditional) example is Professor Fritz Briem’s 1809 Berliner Weiss, notable for being one of the very few unfiltered and unpasteurized versions of the style available. Most American Berliners, and certainly *all* of them in cans, are either filtered, pasteurized, or both. There are a plethora of Goses and Berliner Weisses available in cans in American craft brewing. In the past two years, I’ve seen these low alcohol, quick turnover sours explode in popularity; they’ve become some Ohio breweries’ stock-in-trade. Many of these beers are perfectly refreshing; one might even, if one were pressed, call them “crushable.” The vast majority of them also perfectly demonstrate how limited and simple the flavors derived from kettle souring are: lactic sourness, not much more. That’s why so many of them employ adjuncts, flavorings, even food grade acids to up the complexity and try to give the beers some amount of balance. If your experience of sour beer is limited to what you can buy in a six pack of cans, you’re getting less than half the story. Much less. What is a traditional wild or sour ale, then? Put simply, it’s a beer produced through what we call a “mixed fermentation,” that is, a fermentation that uses more than simply traditional brewer’s yeast to convert starches and sugars into alcohol and flavor/aroma compounds. Typically, a mixed fermentation will include a “clean” ale or lager yeast, either or both of the lactic acid bacteria lactobacillus and pediococcus, and some strain of the “wild” (although these days almost certainly lab-cultured) yeast, Brettanomyces. These ales have a long and rich history, and many are produced with traditional brewing methods that long predate pasteurization. They are also some of the most complex and nuanced beers in the world.

So why go to all this trouble if kettle souring is so much faster, easier and less expensive?”

The wonderful variety of wild ale brewing methods is beyond the scope of this article, but there are a few key things to understand about mixed fermentation sours: * They contain a mix of microbiota to achieve their fermentation, usually some combination of the bugs listed above. * The fermentation agents (lacto, pedio, etc.) may come from a lab culture, may live in oak barrels previously used to age wild/sour beer, or may come partially or totally from the air in the brewery itself ("spontaneous fermentation”). * Most spend some amount of time on or in oak, usually wine or spirit barrels. These provide an ideal environment for the bugs to do their thing, because… * Most mixed fermentation beers take a long time to finish. The mix of bugs fermenting the beer can take months, sometimes years, to achieve what the brewer intended or hoped for – if ever. These fermentations are slow and unpredictable, which in turn is why… * The beers tend to be expensive, much more expensive than “comparable” kettle soured beers. While a kettle sour Berliner might be ready in a week, a true mixed fermentation sour ale conditioned in oak may take literally years. The time, equipment and storage costs pile up quickly, while the beer just sits. So why go to all this trouble if kettle souring is so much faster, easier and less expensive? A masterfully made kettle sour will never achieve the complexity and nuance of a true mixed fermentation sour ale. The various yeast and bacteria used in traditional sour ale production create something larger than the sum of their various fermentation products; one need only try a Rodenbach Grand Cru or Boon Oude Geuze Mariage Parfait to get a sense of the complexity inherent in these beers. They're literally more alive. While kettle souring usually employs lactobacillus in order to drop the beer’s pH and achieve the desired sourness, the lactic fermentation is separate and apart from the main event, that is, the fermentation with clean ale yeast. To keep the pH stable, kettle sours are pasteurized after souring (via the boil), and perhaps again before packaging in order to make them bottle- or can-stable. Traditional mixed fermentation sour ales are usually living beers, packaged with their bugs, and may develop and change in their bottles wondrously over time.

New Belgium’s La Folie and Le Terroir both see wide distribution and are generally affordable introductions to American sours.”