Become a Kettle Sour Expert

Meet your new friend, Lactobacillus, and learn the ins and outs of kettle souring.

Jester Goldman Aug 4, 2017 - 7 min read

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It seems like almost every craft brewery has something tart and tangy on its beer list. In the past, only a few places were willing to risk bringing microbial contamination into their brewhouse. The big change is due in part to kettle souring, which works the foreign beasties for a shorter period up front and then boils them away to keep them out of the fermentor. You can reap the same benefits at home to brew a refreshing Berliner weisse, a complex gose, or a balanced fruit sour. Or you can get really creative and open your imagination to find where a clean sour bite might fit in, like maybe a tripel with a just tinge of tart.

The linchpin of kettle souring is your new friend, Lactobacillus.

Controlled Infection with Lacto

It may seem counterintuitive. For beginning homebrewers, the lesson of proper sanitation is hammered home. It’s a good lesson. Most infected homebrew is spoiled with either Lactobacillus or Acetobacter. But the kettle souring process will reduce your risk of getting unwanted off-flavors. We want to get the cleaner sour flavor of Lactobacillus at the level we want, while avoiding the vinegar of Acetobacter.

Our controlled infection will let Lacto do its job, which is primarily creating lactic acid, but with the chemistry of acids and alcohols plus the nuanced differences between strains of Lacto, you can end up with low but noticeable levels of succinate, formate, and other chemicals, which can lead to interesting esters that come across as geranium, woody flavors, raspberry, and even sweet cream.


It’s also important to understand that Lactobacillus is a catch-all family, with a number of different strains and qualities. L. delbrueckii, for instance, tends to generate a cleaner lactic bite and may address some autolysis flavors. L. brevis can generate a bit of acetic acid as well, and it’s not as attenuative. L. plantarum, on the other hand, is fast and clean and will eat a bit more of the malt sugars. The best plan is to look at what strains are available and choose one that fits with your beer. I would go with L. plantarum in a gose, for example.

With that groundwork, let’s look at the process of brewing a kettle sour.

Prepare the Wort

Kettle sours start out like any normal batch of beer: either mash your grain and sparge to collect the wort or dissolve the extract into water. The only difference is that you should adjust the pH down between 4.0 and 4.3. You can use whatever acid is convenient: lactic or malic acid are both fine.

While you could proceed immediately to inoculation, it’s a good idea to boil the wort for a couple of minutes first to eliminate any competing bacteria or wild yeast. Then you can cool it down to the right temperature range for your Lacto culture (the vendor should provide that information). This will probably be 110–120°F (43–49°C), but may be as low as 85°F (29°C).


Inoculate your Wort

Once your wort is at the right temperature, it’s time to stir in your Lactobacillus culture. This is a lot like adding yeast; a larger, healthier culture will work more quickly. Just as with yeast, a starter culture of 300–500 ml is a good volume.

Wait and Test

Once you’ve inoculated your wort, you have to give the bacteria time to get busy and you have to protect them from interlopers. A blanket of CO2, the wort acidification you did earlier, and a good seal are all you need. You could put the wort into a keg or carboy, raising the philosophical question of whether it would still be a kettle sour, but it’s fine to just purge the headspace of the kettle with CO2 and then cover it with plastic wrap before sealing it with the lid.

Next, you need to hold your kettle in the target temperature range. People often use an electric blanket or heating pad, but a wand-type sous vide device is simple to use and won’t require as much attention. Just place the kettle in a tub with water, then set your desired temperature on the sous vide device. Otherwise you need to find the right setting to hold the wort in the right range.

The wort souring can take anywhere from a few hours to a few days. Pull a sample at 12 hours in and see where you are, either by pH (aim for 3.0–3.7) or by taste. How close you are should give you an idea of when to check next. Remember to reapply the CO2 blanket and seal the kettle after each sample.


Boil as Usual

Once you’ve achieved the right tartness, you can resume your normal process. Go ahead and run your boil to kill off the Lacto, following whatever hopping schedule your recipe calls for. Then chill it down and pitch your yeast. Your fermentation may be a bit slower; the low pH provides adverse conditions for yeast health. As a result, choose a fairly strong yeast strain and be sure to use a starter culture to give them a boost.

Words to the Wise

Kettle souring is fairly straightforward, but there are some good things to keep in mind:

  • Don’t use any hops until the final boil. Hops will severely inhibit Lactobacillus.
  • Protect the kettle from unwanted competition. The pre-souring boil will give you a clean arena, lower wort pH should inhibit wild yeast, and keeping out oxygen will limit Acetobacter.
  • Pitch plenty of Lacto—use a starter culture, if necessary.
  • Pitch plenty of yeast when you’re ready for fermentation. The yeast cells will have to fight a bit against the acidity.

From Berliner weisse to gose and points in between, quick souring is rapidly becoming the time-constrained brewer’s choice for building pleasant tartness on a schedule. In Craft Beer & Brewing Magazine®’s online course, Quick Souring Methods, Funkwerks Cofounder Gordon Schuck explains how to use Lactobacillus bacteria, experiment with sour mashing, test acidity levels, and more. Sign up today!