Kettle Souring

By Carli Severson
Microtechnologist
The Brewing Science Institute

If you have been to a liquor store or brewery recently, chances are you have come across sour beers. Sour beer is a beer with an intentional tart or acidic taste. They come in a variety of styles including Berliner Weisse, Gose, sour IPAs, fruited sours, Saison and farmhouse ales (3). Each style is made by either barrel aging or kettle souring. The sour taste is due to the embrace of spoilage microorganisms, such as Brettanomyces, Lactobacillus, and Pediococcus, during the brewing process. These can spoil beer through acidification, but this characteristic is sought after when brewing a sour beer (5).

The key additive in sour beer is Lactobacillus. Lactobacillus consumes a portion of the sugar in wort to produce lactic acid, which gives it a clean tartness (7). It is the same bacteria that give yogurt its sharp flavor. Lactobacillus reproduces quickly, ferments, and thrives in unhopped wort (6). Throughout history, acidification by Lactobacillus helped prevent the growth of other microbes in beer and increased the availability of starch by softening protein sheaths surrounding the starch granules (5).

Traditional Method

Traditionally, sour beers utilize a long aging period that uses various bacterial yeast cultures to slowly transform the fermented beer, resulting in signature complex and tart flavors. In this technique, microbes are added after the beer ferments, meaning there are already hop oils and alcohol present. Alcohol slows down the brewing yeast, while hops can hinder bacteria, resulting in an extended maturing process that can take anywhere from a month to years (2). In addition, it takes more than one wave of microbes to complete fermentation, drawing out the process even longer (6). The time commitment needed for this method presents a challenge to brewers, who must prevent the souring microbes from contaminating other beers in their brewhouse (2).

Kettle Souring: The Efficient Way

An alternative to the time-consuming, traditional method is kettle souring. This technique, performed in a stainless-steel mash tun, produces sour beers in a quick, cost-effective, and scalable way (4). Nicknamed fast souring, the kettle souring process can be completed within forty-eight hours and produces a brightly colored, and unique flavor profile (8). By adding microbes that produce lactic acid, before fermentation, brewers significantly shortened the aging period needed to make a sour beer (8). Although this time frame can vary based on the buffering capacity of the wort, the microbe load, or the attemperation, completion is generally reached when the desired pH has been acquired. The pH of a subtle sour beverage is around 3.6 (3). Kettle souring is a better choice compared to traditional mixed fermentation if you want sweet and tart or strong and sour, which is sought after by most brewers in their sour profile (6).

In the present day, it is easier to acidify the wort separately, purify by boiling, cool, and then inoculate. Lactobacillus thrives in warm, consistent temperatures; therefore kettle souring is generally done in brew kettles (1). When lactic fermentation is done on an entire batch of wort, it produces a more desirable acidic mash with no funky flavors and a stable, predictable pH that only needs to be boiled to kill the souring agent in the end. This protects the microbes from escaping and unintentionally fouling other beers in the same production facility (2). The acidified wort is blended with fresh wort to the desired pH and pitched with regular yeast to then go through standard alcohol fermentation and hopped (1).

There are many benefits to kettle souring. There is a reduced risk of contamination due to the souring bacteria being killed during the boil prior to the heat exchanger and other equipment (8). Other than the overnight souring with bacteria, the process of producing a sour is the same as a standard batch ale. Kettle souring allows brewers to produce a mouth-puckering sour in the same amount of time as any other beer, and around the same price point as a standard pint of beer (2). A drawback of kettle souring has to do with the complexity of flavors. Although clean and refreshing, most kettle-soured beers will not have the same flavor profile of one that has spent months or years in a microbe-laden barrel (2).

Compared to the traditional process, kettle souring can make sour beers attainable in a short time frame, for any brewer, whether they are professional or homebrewers. First, wort is produced with little or no hops and heated to kill off any wild microbes. This provides a clean slate to start. The wort is chilled to an incubation temperature of 95 to 115°F (35 to 46°C), depending on the recommended souring temperature of the Lactobacillus strain that is being used (3). Refined lactic acid is added to lower the pH of the wort to about 4.5. Pre-acidification inhibits reproduction by enteric bacteria, which can create off flavors and cause allergic reactions for some drinkers (6). The lower pH also reduces the activity of Lactobacillus’s proteolytic enzyme, thus preserving head retention and body (6). The wort is inoculated with lactic acid bacteria. If pitching a pure culture, the presence of some oxygen is not a major concern because Lactobacillus is aerotolerant. That is why a mash tun or stainless-steel kettle can suffice. Aluminum kettles are not stable at a low pH, and should not be used (3). Lactobacillus works best at 110 to 115°F (43 to 46°C), but most ferment well at room temperature (6). Once the target acidity or sourness is achieved, the wort is brought up to 170°F (77°C) (6) to kill the lactic acid bacteria and stop the souring process. Wort can be hopped during or after the second boil to contribute to bitterness and microbial protective effects (5). It can then be treated as clean wort with a low risk of cross-contamination. After, the wort is cooled and then inoculated with brewing yeast to initiate primary fermentation as done in a traditional mixed culture souring process.

Kettle souring is a mixture of art and science. The sour beer trend continues to grow as more breweries are producing sour styles with this fast and cost-effective method. Once the basic kettle sour technique is mastered, one can experiment with hop combinations and fruit additions. The opportunities are endless.

References
1. Alworth, Jeff. “Quirks of Brewing: Kettle Souring.” All About Beer, 5 Apr. 2016, allaboutbeer.com/kettle-souring/.
2. Bland, Alastair. “How Brewers Are Churning Out Tangy Sours Without The Hefty Price Tag.” NPR, NPR, 28 Aug. 2015, www.npr.org/sections/thesalt/2015/08/28/434960369/how-brewers-are-churning-out-tangy-sours-without-the-hefty-price-tag.
3. Green, Tom. “How to Kettle Sour: Homebrew Sour Beer Without the Wait.” Bison Brew, 1 July 2020, bisonbrew.com/how-to-kettle-sour/.
4. Segall, Brad. “How to Kettle Sour Beer.” Northern Brewer, Northern Brewer, 2 July 2019, www.northernbrewer.com/blogs/brewing-techniques/kettle-souring-made-easy.
5. Sipple, Lauren. “The Science of Sour Beer.” Science Meets Food, 26 Sept. 2015, sciencemeetsfood.org/science-sour-beer/.
6. Tonsmeire, Michael. “Overnight Acidification.” Brew Your Own, 2019, byo.com/article/overnight-acidification/.
7. “What Is a Kettle Sour?” Allagash Brewing Company, 10 Feb. 2020, www.allagash.com/blog/what-is-a-kettle-sour/.
8. Windhausen, Alan B. “The Basics of Kettle Souring.” Colorado Brewers Guild, 17 May 2018, coloradobeer.org/tech-safety-post/the-basics-of-kettle-souring/.