People are calling sour beers “all the rage” right now. Major news outlets are writing articles about them—about what they are and why they’re so suddenly popular. Sour beers encompass some of my favorite styles when done correctly, and they’re also some of my favorites to brew because they require an entirely different skill set than traditional, “clean” beers. A note, though: a “sour beer” is generally referred to as any beer with tartness derived from a lowered pH. However, sour beers are not necessarily a style unto themselves; rather, there are several beer styles that might be traditionally sour.
One of those sour styles is a gose (pronounced go-zuh), a German wheat beer that was all but extinct five years ago but is now being hailed as the “beer of the summer.” Goses are traditionally brewed with coriander and salt, but you may find some out there with a few fun additions (like, you know, watermelon…).
We can clarify a few things up front:
1) Brettanomyces—that is, the house yeast at Central State for example—is not directly responsible for souring beer. Bacteria is. Brett can produce acetic acid given the right circumstances, but this is not the kind of acid normally found in pleasantly sour beers.
2) The term “souring” refers to dropping the pH of a beer, often with the use of bacteria, with the production of lactic acid.
3) More acid does not necessarily mean a better beer. Traditionally, many sour beers were created and/or blended with balance in mind: enough acidity to be pleasantly tart, balanced with enough fruit or malt backbone to offer complexity.
4) Brettanomyces is not directly responsible for souring beer.
One of the techniques sometimes employed by brewers is referred to as “kettle souring.” (I assume I’ll have ample opportunity to talk about other methods in the future, so hold your horses.) If you’ve seen Ottermelon Gose, you can now tell your friends that you know how it was soured, and wow them with the process.
The kettle souring process is fairly easy (says the guy who didn’t lift a finger to make the beer). It’s referred to as a “quick souring” technique. Souring a beer during fermentation can take anywhere from a couple of months to over a year to sour to a desired point. Kettle souring takes a fraction of the time.
The process begins like other brewing processes: mashing to collect a kettle of wort. It’s here, though, that things change up. In a normal beer, the wort would be heated to a boil, boiled for an hour or more, hops added, and then rapidly cooled to fermentation temperatures and transferred to a fermentation vessel.
Instead, a pitch of lactobacillus is added to the wort while it sits in the kettle. Lactobacillus is a bacterium that (like yeast) eats sugar and (unlike yeast) converts it to lactic acid. In a good majority of the sour beers you’ve ever drunk, lactobacillus is the party responsible for souring the beer.
The wort in the kettle is heated to and held at a temperature usually between 100 and 120 degrees F. Lactobacillus thrives in hot (but not boiling) temperatures. This warm temp allows the lactobacillus to grow rapidly, creating lactic acid and dropping the wort’s pH. With a healthy pitch of lacto, the pH of a kettle of wort can drop to desired ranges within 12-18 hours. During this time, a blanket of CO2 is usually laid over the top of the wort to prevent oxygen from spoiling the beer. Another option is to bubble CO2 up through the wort as it sits and acidifies. Once the pH hits the acidity level the brewer is looking for, the wort is boiled to kill off any undesirables in the beer—and in the process, the lactobacillus is killed, which prevents further acidification.
(If the brewer’s feeling spunky, he or she can instead boil the wort briefly before pitching the lactobacillus, and let the lacto continue living after acidifying the beer to allow further development. However, if the souring takes place after the initial boil, the living lactobacillus will infect all equipment downstream. Lacto is notoriously difficult to kill, and it loves to hide in little cracks that sanitizers can’t get into, which leads to unintentionally sour beers down the line. That is to say: probably don’t kettle sour after the boil, unless you’re also boiling post-sour.)
After that, the beer is cooled, transferred from the kettle, allowed to ferment, and then carbonated.
Feeling grossed out about the idea of bacteria in your beer? Well, don’t be. If you’ve ever eaten yogurt, you’ve eaten some of the same bacteria that’s in most sour beer. And nothing that can survive in alcohol can harm you. Though I can’t promise anything about acid reflux or heartburn.
Kettle souring allows brewers to employ more techniques not usually available with traditional souring methods. For example, you’ll never find a traditionally soured IPA. Hops inhibit the production of lactic acid. That means that after loading hops into the boil of an IPA, pitching lactobacillus won’t have much of an effect in lowering the pH. However, if you lower the pH of the wort before boiling, you’re free to add as many hops as you want.
This innovative method allow for a lot of fun variance in sour beers. And best of all: it gets them into our stomachs faster.