Kettle Souring: An Overview

Kettle sours continue to be popular as the brewing industry has matured. Used to make sour beers quickly and efficiently, this technique is common among homebrewers and professional brewers alike. This brief guide is intended to give you a better understanding of the fundamentals of this bacteria as it is used to make sour beers, as well as some potential concerns when making kettle-soured beers.

Kettle souring is a technique brewers use to quickly create acidity in a beer, typically by using Lactobacillus cultures to sour the wort before finishing fermentation with a standard ale yeast strain. It is called ​“kettle” souring in particular because you can briefly bring unhopped wort to a boil, allow it to chill to Lacto-safe temperatures, and pitch Lactobacillus directly into the kettle. The Lactobacillus will get to work and begin acidifying the wort, typically reaching the target pH/titratable acidity within 24 hours.

One benefit of kettle souring is that it allows brewers to introduce souring cultures into the brewery without the risk of contamination. Kettle souring is a process that happens exclusively on the ​“hot side” of the brewery, meaning the equipment can be re-sterilized easily once the wort is boiled. After the wort has reached its ideal sourness, brewers once again boil it (or bring it to pasteurization temperatures), so the souring bacteria is killed. That hot wort is then chilled and transferred to a fermentor like any other ​“clean” beer, and the preferred yeast strain is pitched as normal.

Many traditional sour beer makers employ a two-stage fermentation process that includes an extended secondary fermentation with Lactobacillus and/or Pediococcus. These bacteria work slowly to create a beer with complex acidity and a dry finish, and are sometimes blended with other batches to get a perfect, final flavor. However, this process takes months, and in some cases brewers spend years tinkering with these beers and their bacteria blends to achieve their ideal balance.

Kettle souring, however, gives brewers the option to make tart beers that fit into their normal production schedule. Brewers have a great opportunity to create sour beers with endless opportunities for riffing on fruit flavors and acidity levels without dedicating an outsized portion of their production to extended secondary fermentations.

There are several common types of Lactobacillus, and some are more commonly used in brewing than others: L. plantarum, L. brevis, and L. delbrueckii. All of them consume sugars and produce lactic acid. Some produce small amounts of CO₂ and ethanol as well, but the primary product is lactic acid, which is what makes the beer taste sour.

Most species of Lactobacillus will sour wort in 12 – 48 hours. For certain species to work efficiently, kettle temperature will have to be maintained, whether through direct heating or insulation, as Lactobacillus generally likes it pretty warm. Some brewers swear by hotter temperatures — 110 – 120°F or so — but some species don’t work as well being held at those temperatures. L. plantarum, for example, can be pitched at 95°F and allowed to free fall to a warmer room temperature. The thermal mass will keep it warm enough to sour the wort in about 24 – 48 hours.

To make cleanly kettle-soured beer, we recommend a pitch rate of approximately 1L of Lactobacillus cultures for every 3bbl of wort. A five-gallon starter (about 19L) is appropriate for quickly souring up to 50bbl of wort, with the starter size increasing as the batch size increases.

Your regular brewing setup is all you need since the souring is taking place in your boil kettle. To start, mash in and prepare wort as you normally would, excluding any hop additions — remember that just a few IBUs are enough to inhibit Lacto growth. Boil the wort briefly to give yourself a ​“clean” start with none of the microbes that may have been present on the grains, then chill the wort to 100°F (38°C) or cooler before pitching Lacto.

Most brewers will reach their desired pH within about 24 – 48 hours, but remember that the best metric for sourness is taste. Consider the sourness present and compare it with the unfermented sugars still present in the wort. If the now-soured wort seems sour enough to you, then you’re done with the souring process. To finish up, continue with your regular boil schedule and hop additions before chilling and transferring to a fermentor to finish out the beer with the yeast strain of your choice. 

Making kettle sours carries the same contamination risks as brewing other, non-soured beer, so cleanliness and sanitation are the key as usual. However, kettle-soured beers have their own set of unique concerns to be aware of.

Some brewers swear by keeping a sealed blanket of CO₂ atop the wort as it sours. This should not be necessary with a healthy pitch of Lacto culture. Lactobacillus is a facultative anaerobe, meaning it doesn’t need an anaerobic environment in order to function. Additionally, the presence of oxygen shouldn’t cause most strains of Lactobacillus to produce major off flavors, though L. brevis and L. plantarum can produce some acetic acid. 

The presence of butyric and isovaleric acids has been noted in some kettle sour production, but these sometimes pop up in both homebrewed and commercial kettle sours when brewers have used raw grains to sour, or haven’t boiled (or pasteurized) their wort before pitching Lactobacillus. Using CO₂ can help prevent these flavors, as they are mostly produced by aerobic bacteria, but shouldn’t be necessary if you take the time to ensure a clean slate by boiling before pitching bacteria. 

Another instance of potential contamination can occur if the souring bacteria has taken too long to drop the pH, and in the meantime any microbes present were able to begin producing off flavors. This can be avoided by using a healthy Lacto culture and pitching at a temperature where it will thrive.

Yeast, however, is the primary contamination concern when making kettle sours. Regular brewer’s yeast can unwittingly become a formidable adversary with just a few cells present. Yeast has been domesticated to the point where it can consume glucose very efficiently, and those few cells can quickly grow and outcompete the Lactobacillus to begin consuming glucose and fermenting the beer before the Lacto can get a hold of it. Generally, if you start to see the formation of krausen or your gravity drops more than 0.5°P (or about 2 specific gravity points) while souring is taking place, you likely have yeast contamination.

While making sour beer may seem like a challenge at first, it can be an efficient and rewarding way to add some new flavors to your lineup without committing to lengthy fermentation schedules. While kettle souring does avoid the risk of introducing souring cultures into your cellar, maintaining cleanliness and sanitation remains crucial to prevent off flavors and contamination.

As always, we recommend diligent note taking as you experiment, keeping track of how your souring cultures perform at different temperature ranges, as well as checking pH and tasting for acidity along the way. Consider kettle-soured beer to be a blank canvas where you can express all kinds of interesting flavor combinations. Kettle souring is likely to remain a staple technique for producing tart and refreshing beers — see what you can come up with, and let us know how your experimenting works out.