Figueroa Mountain Brewing Co.

What Is a Kettle Sour?

Posted by Figueroa Mountain Brewing Co. on October 03, 2016


After the release of our newest Berliner Weisse last Friday, you may be asking yourself..."what is a kettle sour?" Here's Buellton's Head Brewer, David, with some beer knowledge.

What's the difference between a normal sour and a kettle sour beer?

TRADITIONAL SOURS are created through mixed fermentation of the beer post-boil. This mixed fermentation can be simultaneous (traditional yeast, wild yeast, and bacteria added all at the same time), or, more commonly, sequential in a separate vessel. In this case, a beer will be fermented traditionally with common brewers yeast until it is at or near completion. This beer will then be transferred into barrels (usually neutral oak, and sometimes stainless steel tanks are used) where a complex community of wild yeast and bacteria will begin to re-ferment the beer, consuming sugars that traditional yeast are unable to eat. One byproduct of this secondary fermentation by bacteria is lactic acid, which, along with acetic acid and other organic acids, is responsible for souring the liquid. This process often takes a long time, months to years, but yields a complex final product that can be served as-is, or mixed with young beer to temper the sourness and add complexity (the Belgian style Geuze uses this technique). These beers will continue to develop and sour with time, including in the bottle. This process of post-boil mixed fermentation differs from the kettle souring process, in which the lactic acid is generated by a primary fermentation of a bacterial culture for a several hours to a few days before a final boil is conducted to kill the bacterial community, halt the production of lactic acid, and continue with a secondary fermentation by traditional yeast.

How do we Kettle Sour? 

The process of KETTLE SOURING is no more complicated than any other batch of beer we brew. As with other brews we run off wort into the kettle and boil 15-20 minutes to sterilize that liquid. We than cool the wort to temperatures that promote lactobacillus bacteria growth, 110°F or so. Once cooled we pitch lactobacillus directly into the kettle and let it sour your wort. The lactobacillus could come from a lab or even store bought yogurt. In fact we did use yogurt to sour this beer. This process may take a couple days to get to the desired ph or level of "sourness" that the brewer wants. Once the ph is at the desired level we than boil and hop the wort as you normally would. The boiling kills any remaining bacteria that could contaminate any future batches. Finally we ferment the beer as usual with either an Ale or Lager yeast strain.