Sour ales, or beers that feature acidic and tart qualities derived from one or more strains of lactic acid bacteria, can be made in many different ways. Generally speaking, there are traditional, long-aged sour ales and there are quick/wort-soured sour ales. Most of the commercial sour ales on the market are quick/kettle soured because they are quicker, easier, cheaper, more predictable, more controllable, and less risky to produce than traditional, aged sours.
Wort souring can be done in the mash (although sour mashing is waning in popularity), in the kettle (right after lautering), or in the primary fermenter (allowing the bacteria to get a head start before pitching brewer’s yeast).
On the other hand, traditional, long-aged sours are often made by pitching (or spontaneously inoculating) brewer’s yeast and bacteria together or at different stages of the fermentation. This method requires more time for the bacteria (lactobacillus and/or pediococcus) to work their magic, as they’re competing with yeast for sugars, but it results in a much more complex beer. Beyond the long fermentation time, aged sours can be less predictable and less consistent, which is why different batches are often blended.
I have made sour ales both ways, and each has its own set of pros and cons. As mentioned, kettle souring is much quicker, easier, more controllable, and more consistent, plus it alleviates concerns of contaminating cold-side equipment with bacteria. Some people, however, argue that these beers are one dimensional and that they lack the complexity of aged sour ales. Broadly speaking, I agree, but I also believe that kettle sours have their place, and they’re especially suited for certain low-gravity styles, such as gose, Berliner weisse, tart saisons, and various fruited and/or spiced sours. In fact, some people actually prefer these “clean” sour beers over “funky” sour beers.
If you ask 10 brewers how they kettle sour a beer, you’ll probably get 10 different responses. No one way is right, as many factors come into play, such as your equipment, the type of lactic-acid-producing bacteria you’re using, the type of beer you’re trying to make, and more. Below is just my method.
It should be noted that all sour ales can be challenging to make, and there’s a long learning curve (even more so for aged sour ales because the feedback comes slow). Even though I have been making sour ales for many years now, I definitely do not consider myself an expert. I’m continually learning, and new information, science, and techniques surface often.
When I formulate a recipe for a kettle sour, it usually features a simple malt bill containing pilsner malt and/or 2-row pale, along with wheat, maybe some oats, Vienna or Munich, and possibly even a touch of caramel/crystal malts. Layering in all sorts of specialty malts isn’t that important here, as nuances will only become obscured by acid (and fruit, if applicable). This is one reason why many sour beer breweries only use a few base recipes for all their different beers. In the end, the tart characteristics and any additional ingredients (e.g., fruit, spices, herbs, wood, dry hops, etc.) are the real stars of this show.
While many long-aged sours contain limited amounts of hop IBUs (IBUs inhibit – and sometimes kill – bacteria), it’s not a concern with kettle sours because the bacteria are killed in the boil before hops are added. IBUs still need to be kept to a minimum, however, because heavy bitterness clashes with sourness. Fortunately, late-addition hops and dry hops add very little bitterness, so they are often employed to add floral, spicy, and fruity characteristics to the beer.
With kettle sours, yeast selection isn’t so important. Delicate yeast flavors and esters rarely express themselves completely in kettle sours. I believe this is due to the reduced food supply and the low-pH environment, but the exact chemistry is above my paygrade. Of course, in traditional, long-aged sours, yeast contributes significantly to the beer’s profile, especially if Brettanomyces yeast is used. But if you ferment a kettle sour with a characterful saison yeast, a Belgian Abbey yeast, or even some strain of Brettanomyces, the yeast won’t be as pronounced as it would be in a long-aged sour. As an example, I fermented one kettle sour with Belgian Abbey yeast and one with Safale’s US-05 ale yeast, and if you tasted them blindly, you probably couldn’t tell which one was fermented with which yeast.
SOURING THE WORT
After I mash, just like I would with any “clean” beer, I vorlauf, sparge, lauter, and then transfer the wort to my kettle. I typically mash around 150 F (you can mash in the high 140s F for a drier, more attenuated beer or in the low 150s for more body and residual sweetness).
Since I want to control the source of bacteria in my beer, I pasteurize the wort by bringing it up to a boil briefly, just to kill any potential competing yeast or bacteria that might have taken hold. Because pasteurization time decreases at an exponential rate as temperature increases, some brewers feel this step is unnecessary, as the 1-hour-long, 150-degree mash should be more than sufficient to pasteurize the wort, but sometimes there can be pockets of grain that weren’t mixed in well enough, or there could be yeast/bacteria pickup during the transfer.
I then chill the wort back down to my Lactobacillus’ optimum temperature. I use Lactobacillus Plantarum, which has been reported to survive at surprisingly high temps, but most sources suggest its optimal temperature range is between 90 and 100. I shoot for 95 and just let it free fall to room temperature from there. Unlike many other strains of lacto, Plantarum still performs at room temperature.
Before pitching the Plantarum, I pre-acidify the wort using 88% food-grade lactic acid. Sometimes it takes quite a few teaspoons, with repeated stirs (using a sanitized stainless steel spoon) and pH readings between each addition. The goal is to drop the pH from whatever it is in the 5s down to 4.0-4.3. Apparently, anything below 4.4 pH helps to prevent “bad” bacteria, spoiling microbes, mold, etc. from being able to take hold. Doing this also aids in head retention.
One thing to keep in mind is that hops are Plantarum’s Kryptonite. Even a few IBUs will kill Plantarum, so do not add any hops to the wort until souring has completed and the boil has begun.
After chilling the wort to the appropriate temperature and then adjusting the pH to about 4.3, I pitch my Lactobacillus. There are many different strains out there, but I like to use Plantarum because it works well, it’s inexpensive, and it’s easy to procure. My favorite source for Plantarum is the plain, sugar-free GoodBelly StraightShot. A 4-pack costs about $4 (sold locally at Whole Foods), and each 2.7-ounce shot contains about 20 billion cells, which is sufficient for a 5-gallon batch of wort. I usually pitch 3 shots (sanitized first, of course), just to be sure, and then I drink the fourth one.
To further inhibit bad microbe growth, I purge the surface of the wort with CO2 (lactobacillus is an anaerobe, so it does not require oxygen), and then I seal the top well with plastic wrap. I have found that Glad’s Press’n Seal brand works best and keeps it air tight. From there, I leave it alone for 36 to 48 hours, occasionally taking pH readings (having a kettle with a ball valve / spigot is ideal for taking readings). Once it has dropped to the low 3s, typically 3.2 to 3.4, I’ll start the boil to stop any further souring.
Aside from limiting IBUs, I treat the wort like any other beer from this point on. Options are limitless. You can add late-addition hops or spices or whatever, and when you rack it into a secondary, you can age it on fruit, which is what I like to do.
A couple safety precautions:
Smell all you want to make sure things are on track, but do not taste the wort/beer until you have confirmed that the pH has dropped and/or the gravity has dropped (i.e., fermentation has taken place). Low-pH environments and/or an ABV of just a few percent should deem it safe to drink.
Do not use an aluminum kettle. Wort with a pH below 4.5 will strip the kettle’s protective oxide layer.