Once a style relegated to the shadows and certain regions in Belgium, sour beer is now in the mainstream. So much so, we have to dedicate several of our taps specifically towards sour beer. We get asked all the time about what makes beer sour, so we thought we’d take a second to give a quick lesson in what makes beer sour and the different methods used to produce it. Please know that this will be a quick, barebones overview. 

The main cause of sourness comes from bacteria, typically two different species known as Lactobacillus or Pediococcus, and in some rare occasions, Acetobacter producing bacteria. 

All of these bacteria utilize the sugars and carbohydrates found in wort or beer to create lactic acid (or acetic acid), which in turn lowers the pH of the final product, which causes varying levels of perceived tartness on the tongue. 

While brewers go through painstaking lengths to ensure their ‘clean’ beers contain none of these bacteria, when producing sour beers, they can be added in a few different ways. Additionally, sour beer production can be broken down into 2 different macro-production methods, quick sours and aged sours.

Quick sours can be produced in generally the same amount of time as a normal beer (usually 3-6 weeks depending on other factors).  They are soured in one of two ways. Sour mashed or kettle soured. Both involve building up large amounts of lactic acid producing bacteria so they can consume some of the carbohydrates, thereby lowering the pH of the wort. 

Sour mash utilizes naturally occurring bacteria present on the grain, by heating the grain with water (roughly around 100F). At this temperature the bacteria begin to consume carbohydrates in the grain and multiply, a process that typically happens overnight. Once the mash has reached its desired pH, it is brought to sugar conversion temperatures, the liquid is run off, and then boiled just like a standard beer. The boiling kills any remaining bacteria and locks in the pH of the beer. 

Kettle souring is a similar process. The mash is heated to starch conversion temperatures and run off into another vessel. At this point, bacteria are added or ‘pitched’ into the un-boiled wort and the same process takes place overnight or until the desired pH is achieved. Once again, the wort is boiled, killing off any remaining bacteria. 

Both of these styles of ‘quick sour’ beer are fermented normally, typically with a clean yeast strain. These sours tend to be fairly simple with a basic tartness and little additional character. Typically these types of sour don’t benefit with any additional aging, as they don’t contain any living bacteria. Although, in some cases, wild yeast added in secondary fermentation may give additional character. 

One of the main drawbacks to quick sours is that they are prone to off flavors. Some common ones examples are THP (Tetrahydropyridine), which gives flavors of Cherrios or Cap’n Crunch. This typically disappears with extended aging. Another one is Butyric Acid, which gives aromas and flavors of vomit. Only the addition of a Brettanomyces yeast strain and extended aging will cause this flavor to turn into a compound with aromas of pineapple. 

Long aged sours are pretty self-explanatory. These are beers that go through roughly the same brewing process as clean beers until it comes time for fermentation.  Once the boil is complete, the beer is either cooled and wild yeast and bacteria are pitched into the finished wort and allowed to ferment in a variety of containers or, in the case of spontaneous fermentation, the wort is allowed to cool naturally overnight and yeast and bacteria present in the air find their way in. The wort is usually then pumped into oak barrels to ferment for 1-4 years, before being blended and/or packaged. 

This long-term aging allows for the beer to slowly develop more subtle, rounded, and intertwined flavors. Additionally, the presence of the living organisms in the beer allow for long-term aging like a wine. 

We’ll do some more in depth posts about the specific biology and chemistry that’s involved in sour beer at a later date so keep checking back.