Kettle Souring

It seems that many sour beers are being brewed these days that are clean, tart, and a great base on which to layer other flavors. At the 2015 Great American Beer Festival held in Denver, Colorado, I had some really nice, clean sours flavored with a wide range of fruits, herbs, and spices. I know for sure that some of these beers were brewed using the kettle sour method that you succinctly describe. That is, wort inoculated with Lactobacillus, held warm for a period of time to sour and then boiled. One of the alluring facets of this method is that the bacteria used to sour the wort are killed by boiling, and before the wort is cooled and transferred to fermentation. This is especially appealing to commercial brewers who want to brew sour beers, yet are not keen on turning their entire cellar operations into a funk factory.

At a very basic sense, kettle souring has a lot in common with making yogurt because both methods benefit from an incubator of sorts to keep things in the 85–110 °F (29–43 °C) range. I suggest reading about home yogurt making to get ideas on cost-effective incubators. My experience with kettle souring is on a larger scale and I will pass on some of the techniques being used by commercial breweries.

I know of three craft breweries using the same basic method to produce kettle sour-type beers. These brewers all have a special fermenter equipped with tank heating using hot water/glycol flowing through the tank jacket, much like a normal fermenter being chilled with glycol. The technique is to produce unhopped wort as if the plan was to boil the wort and add hops. But instead of bringing the unhopped wort to a boil, it is cooled to about 100 °F (38 °C), inoculated with Lactobacillus and maintained at the ideal temperature for the strain being used (usually in the 100–110 °F/38–43 °C range) for 24–72 hours. By the way, hops inhibit the growth of lactic acid bacteria and it is important to sour unhopped wort.

During this time period lactic acid is produced and the pH of the culture drops. Since the principle product of the reaction is lactic acid, pH is a useful method to monitor progress. Tasting is also very useful to monitor progress. The soured wort can be used as the sole source of fermentables in a beer or it can be blended in with normal wort. A commercial example that explains this process on their website is New Belgium’s Snapshot Wheat.

There are a couple of things about Lactobacilli that help explain how brewers use these organisms in brewing. The first is that Lactobacillus species are anaerobic bacteria that are not harmed by oxygen and are considered aerotolerant anaerobes. In contrast, facultative anaerobes, such as yeast, are able to use aerobic metabolic pathways in the presence of oxygen and anaerobic metabolic pathways in the absence of oxygen. The fact that Lactobacilli are aerotolerant anaerobes means that soured food products, such as sauerkraut, kimchi, and cultured dairy products are easy to ferment in an open container. When wort is soured by adding malt to wort in the 86–113 °F (30–40 °C) range, aerobic spoilage bacteria can also grow and produce volatile fatty acids, such as butyric acid, along with other unpleasant aromas. The easiest way to suppress the growth of aerobic wort spoilers is to create an anaerobic environment.

Another important thing to know about Lactobacilli is that some are homofermentative, meaning that lactic acid is the sole product of glucose fermentation, and some are heterofermentative, meaning that lactic acid is not the only product of fermentation. Heterofermentative species produce lactic acid, alcohol, and carbon dioxide from glucose and oftentimes kick out acetic acid. Lactobacillus delbrueckii is a homofermentative strain and Lactobacillus brevis is a heterofermentative strain.

In the German brewing tradition, Lactobacillus delbrueckii is the primary lactic acid producer in Berliner weisse and is also used to produce sour mash to adjust mash pH in the biological acidification process and to produce lactic acid for sauermalz (sour malt or acidified malt). The latter two uses specifically relate to mash pH adjustment and the Reinheitsgebot. The traditional way of brewing Berliner weisse is to make sour wort in a dedicated vessel and then blend with regular wort to a target pH prior to boiling, so that the final beer pH is between 3.2 and 3.4. Not all brewing yeast strains tolerate this low pH and it is important to consider this when selecting yeast.

Lactobacillus delbrueckii will also produce that clean canvas to paint your Gose atop. If you make sour wort using the method described above and add the spices and salt to the kettle boil you have laid the groundwork for a solid Gose. Yeast selection with this style is important and all of the normal rules apply downstream of wort production.

Heterofermentative species are more commonly found in funkier styles as these bacteria produce more than just lactic acid. I am not sure what lactics traditionally grow in Gose wort, but using a heterofermentative bacteria could make an interesting contribution to your Gose. It seems that heterofermentative species are more popular than species that only produce lactic acid.

Response by Ashton Lewis.