Flanders Red, Oud Bruin, Gueuze, Gose, Berliner Weisse, Lambic, Lichtenhainer. These are just several of the handful of historic Belgian and German sour beer styles that have seen a massive surge in popularity in recent years among the American craft beer consumer. This newly acquired and widespread appreciation for sour beer in the United States has spawned handfuls of breweries that exclusively produce sour beer, as well as novel and innately American methods of brewing them.
Among these methods is a process known as kettle souring, a term that has proved to be rather divisive among sour beer enthusiasts. Some sour beer purists believe that kettle souring has no place in craft beer, and others laud it for its efficiency and affordability.
But what is kettle souring, exactly?
By kettle souring a beer, brewers lower the pH of a beer by pitching a culture of a lactic acid-producing bacteria (namely lactobacillus) directly into the boil kettle and allow it to sour the sugar-rich precursor to beer, known as wort. The brewer’s target pH is generally attained after one to two days, and then wort is boiled, effectively killing the microorganisms and boiling off the unpleasant byproducts of lactic acid fermentation. From here, the brewing process continues as it would for any other brew. (For a brief overview of the brewing process, click HERE).
Souring by Barrel Aging & Spontaneous Fermentation
In contrast, traditionally soured beer was (and still is) produced by spontaneous fermentation or by aging beer in large oak barrels, also known as foudres. These barrels are home to a community of microorganisms that add both acidity and complexity to the beer over time, often upwards of one to two years.
Making sour beer by these methods isn’t cheap, or quick, or even safe when you consider the potential havoc that a rogue wild yeast or bacteria could wreak on a production brewery. These beers are, however, objectively more complex than kettle soured beers. With proper aging and conditioning, these beers naturally develop a wide range of pleasant flavors and aromas over time, from black cherry and plum to vanilla and tart apple.
Does this mean that mean that kettle soured beers are definitively inferior to traditionally soured ones? Not necessarily.
Don’t get me wrong... If I order a Flanders red ale at the bar and I’m served a very one-note, kettle soured red ale that just happens to also be tart, you can bet your ass I’ll be less than thrilled.
When Kettle Souring is Appropriate
Under some circumstances, however, a clean lactic tartness is exactly what you want. For instance, remarkably clean sour beer styles such as Berliner Weisse and Gose are perfectly suited for kettle souring. While both Gose & Berliner Weisse are traditionally inoculated with a lactobacillus culture after the boil (or in the absence of any boil), kettle souring yields a beer acidic enough to make it refreshing, yet simple enough to let other flavors take center stage.
As Matt Miller of Sourbeerblog.com states, “[kettle sours] are less complex, but they are very sessionable and make a perfect base for the addition of other ingredients like fruit or dry hops.”
In reference to Gose and Berliner Weisse produced traditionally by a post-boil addition of lactic acid bacteria, he explains, “They are undoubtedly more complex… But to some palates that additional complexity may be a blend of low-level off-flavors. Additionally, from a production perspective, these beers may take longer to age to the point where they are tasting good,” and this extra time incurs significant production costs.
He continues, “To my palate, the sulfur/vegetal/sauerkraut aromas/flavors [that result from a lactobacillus fermentation] aren't pleasant in the final beer.” He explains, “they can be removed by re-boiling the wort in the kettle souring process.”
In Defense of the Kettle Sour
While no one will deny that kettle sours are cheaper, easier to produce, and less complex than traditional barrel-aged and spontaneously fermented sour beers, to say that they have no place in craft beer would be an extreme and uninformed claim. Well established and highly acclaimed breweries all over the country have had great success producing both Gose and Berliner Weisse by this method. In the right setting, and for the appropriate style, a kettle soured beer can be a bright, refreshing alternative to generic light lagers, as well as an affordable, approachable entrée into the strange and beautiful landscape of traditionally produced sour beer.