What Makes That Beer a Sour?

Sours are as curious a category as you're likely to find in the world of beer. Over the past decade the style has seen its stature swing from relative obscurity to dependable darling of the craft movement. Yet it is technically among the oldest forms of ales on earth. Stranger still, despite its newfound celebrity status, sour beer remains difficult to comprehensively define — even if you ask some of its most imperious imbibers. All this is to say, if you're thirsting for some clarity on the subject you're not alone.

A beer by any other name would taste more sweet

In the most basic sense "a 'sour beer' is any beer where acidity forms the backbone of the beer's flavor structure," according to Garrett Oliver, legendary brewmaster at Brooklyn Brewery. "For most beers that role is occupied by hop bitterness, which acts like tannins in red wine. Sour beers have a structure more like white wine, with acid in the center."

This is the organoleptic way to define the style, based upon whether or not we perceive sourness on the tongue. This is ultimately a relative reflection of the amount of hydrogen ions swimming around in your suds; the more of those free-roaming particles, the more acidic,or sour, a beer will taste.

Remember the term pH from high school chemistry class? It actually stands for "potential hydrogen," and is a logarithmic gauge of how much of those respective ions are in solution. Water is considered neutral with a pH of 7, while a liquid with a pH of 6 is ten times more acidic — that is, it contains ten times more hydrogen ions. A standard ale or lager is typically in the range of 4-5 in pH. Whereas a sour will be closer to three. It doesn't sound like much of a shift, but remember, this is 10,000 times more acidic than water!

How does sour get its power?

There are multiple ways in which a beer can be sculpted to fit this general flavor profile. "Sour beers are [most often] made acidic by lactic acid-producing bacterial strains," adds Oliver. "Wild yeasts can also produce some acids, but lactobacilli — the same critters that sour yogurt — are usually the main actors here."

These microorganisms can be added at different phases of the brewing process for varying effect. In a so-called "simple sour," they are inoculated into the wort (the grain-steeped liquid that provides sugars necessary for fermentation) over a period of one to two days before everything is boiled and fermented as it would in a traditional beer. This produces a lighter variety of tartness typically more perceptible through aroma rather than palate. In "mixed fermentation", those aforementioned bacteria are introduced along with the yeast, lowering the pH of the beer as it picks up alcohol.

Feature or flaw? Depends on who you ask

Then there's wild ale, a sort of sour-adjacent style wherein ambient yeasts from the immediate environment of the brewhouse are allowed to spontaneously ferment the wort. The Belgians have been brewing beer this way since at least the 13th Century in a style known as lambic.Absent here are the domesticated strains of yeast that have been cultivated for generations to keep beer clean and consistent. Which is why their beer-making counterparts in Germany—a nation notorious for its beer purity law—generally abhor the style. They're more likely to view it as a flaw rather than a feature.

Indeed, many modern brewers are terrified of introducing wild yeast strains such as Brettanomyces into the brewhouse for fear that it could unintentionally infect production lines. For their part, the Belgians developed a way to maintain consistency from year to year by combining several vintages of lambic into a batched subcategory called "gueuze." It continues to ferment long after bottling, leading to an unofficial moniker: "The Champagne of Brussels."

When produced domestically, it's more commonly classified as American Wild Ale, as is the case at Russian River Brewing Company, a revered operation out of Santa Rosa, California that helped popularize the style in the United States. Though these sorts of beers can skew towards tartness, grouping them along with other sour beers helps confound consumer understanding of the category. "It's important to understand that Brettanomyces yeast does not make a whole lot of acid," explains Russian River co-founder and brewmaster Vinnie Cilurzo. "Instead it contributes more funky and wild qualities to the beer. It's also important to distinguish the difference between acidic and acetic. We want a light acid contribution which I call acidic, however, we do not want acetic which is more like balsamic vinegar."

In other words, not all sours are created equal. And not all sours are actually even sour — regardless of what the Titratable Acidity (a measurement of acid concentration in food or drink) on the label may read. As with any style of beer, a skilled brewmaster is one who mines intended flavors by design, leaving little to chance. "At Russian River, we age the beer for long periods in the barrel with brett, lacto, and pedio (lactobacillus and pediococcus, respectively; the most common types of bacteria used to sour beers) and over time these yeast and bacteria work slowly to develop the unique tart and wild notes that all of these microorganisms contribute," adds Cilurzo. "Sometimes these beers will be quite sour but we try for a more subtle contribution. I'd prefer to have more Brett contribution as compared to the sourness from the two bacterias."

Do sour beers have probiotics?

Certain sours on shelves today share a flavor profile with another tart, trendy liquid: kombucha. The commonality has led some health-conscious imbibers to wonder if sour beers, like their piquant cousins, can also be considered "probiotic." Here brewmaster Garrett Oliver spills the tea:

"In the 'simple sour' — also known as 'kettle sour' — technique, the wort is boiled after the souring, and no probiotic bacteria survive. But both mixed fermentation beers and wild-fermented sour beers will usually contain live bacteria unless they've been pasteurized after fermentation."

For those seeking something gluten-free, however, the news is a little less sweet. Oliver points out that if there's gluten in the grains that went into brewing it, the resulting beer will have gluten too, regardless of whether that beer is sour or bitter.