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What is a kettle sour?

Allagash Brewing Company brewing vessel

If you’ve started to search out beers with a sour flavor profile, you’ve likely come across the term “kettle sour.” It’s probably one of the most widely available forms of sour beer at craft breweries here in the U.S. To help you understand the term, we’re going to break down what kettle souring is, in particular, and various other ways to get sour flavor into a beer.

Quickly, before we dive into the process of kettle souring itself, we need to talk about one souring bacteria that’s key in sour beer: lactobacillus. You might recognize lactobacillus as the same bacteria that gives yogurt its tartness. While yeast produces alcohol and carbonation, lactobacillus produces lactic acid, creating clean tartness. Alright, on to kettle souring.


In a kettle-soured beer, the brewer adds lactobacillus to the wort (unfermented beer) before fermentation. More specifically, they add it while the wort is in a brewing vessel called the “kettle.” Thus the name “kettle souring.” By adding the bacteria at that point, the brewer is able to sour the beer in a matter of hours rather than months. 

In a kettle-soured beer, you’ll find a simple, one-note acidity. What you’re not able to get with kettle souring are many of the subtle and complex flavors beyond just tartness. So how do you get a more complex sourness into your beer?


The ways to fill beer with a sour profile are various. In essence, adding sourness to beer involves adding souring bacteria after brewing, and then to give that bacteria the time to work its magic. We actually go into detail on four ways we sour beer here at Allagash in our blog right here. For a quick look—to give you an idea of how they compare to kettle souring—here are the four we tend to use most at Allagash:

  • Putting beer in a barrel that once held sour beer. The souring bacteria still residing in the oak barrel staves works to sour the new batch.
  • Adding fresh fruit to our beer—souring bacteria like lactobacillus and pediococcus naturally live on the skin of fresh fruit. 
  • Spontaneously fermenting our beer. Basically we allow microbiota from the air to inoculate our beer and ferment it—rather than physically adding yeast or bacteria. Think of this as the “sourdough” method of fermentation.
  • Physically adding lactobacillus and pediococcus (another souring bacteria) to our fermenting beer. This differs from “kettle souring” in that these two bacteria are added after brewing is complete. Remember, in kettle souring, you’re adding lactobacillus during the actual brewing process. When added later, the lacto and pedio take more time to do their job, but also create a more well-rounded (we think) tartness and flavor to the beer.

Like everything else in brewing, no one souring method is “better” than any other. They’re more like tools in the brewing toolbox—depending on the end flavor profile you’re looking to create, one method of souring will be more appropriate than another.

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