How does a Beer become Sour?

At Rivertown, we love ALL kinds of beers and have a special place in our hearts for those sour and wildly fermented (Sour is the New Hoppy!).  We often get asked, what is a sour beer?  How does it get sour?  Is it supposed to be sour?  In this informational blog with help from Twitch, our in house Beer Scientist extraordinaire and passionate cultivator of wild and tamed yeasts, we define the 3 main ways that souring occurs in beers.


Cultured souring takes patience and involves strains of bacteria and yeast cultivated over a number of months and years.  It is done with great intention, knowledge and precision since souring beer this way, when not done carefully, can result in a brewery wide contamination where non-sour beers can become sour.

There are two main types of inoculation in Intentional Souring:

    Unrestrained Inoculation:  Also commonly known as “Open Fermentation”.  Best demonstrated (though not exclusively) in        the use of a coolship, wherein wort/beer is transported to a large, shallow vessel and allowed to sit, left exposed to the air for some span of time.  Microbes will settle into the wort/beer and many will begin to grow, and change the beer, adding their own array of flavors and aromatics to the finished product.  Commonly seen as being the most risky, since it allows for almost no control over what species and strains will impact the finished product.  This is a process we proudly utilize in all of our Barrel Aged Sours.

    Restrained Inoculation: The more common method; wort is inoculated with a chosen microbial culture, which can contain anything from a single strain of bacteria which has been hand-selected for that specific batch of beer, to a culture containing an unknown number of strains of an unknown number of species. This inoculation can come in the form of a vial of microbes, to simply reusing the same wooden vessel (the porous wood allows most microbes somewhere to “hibernate” when the vessel is empty).  When the vessel is refilled with wort/beer, they emerge again.  This is a process we proudly use with Divergent, Soulless and our Barrel Aged Sours.  


This technique ignores any microbes, and is quite simply pouring lactic acid into the wort/beer to produce a beer which tastes sour.  While it is the same lactic acid produced by the microbes, it lacks the myriad of subtle flavors, funk and complexity one obtains when implementing live bacteria.  Some breweries utilize this synthetic process and reject the use of souring microbes to avoid possible contamination which could result in ruining numerous batches of beer as well as the need to replace equipment.  Rivertown does not sour our beers this way.


The beer became infected with bacteria (possibly wild yeast), and this was not part of the plan.  Possibly the result of a brewery employee being unlucky, careless, unscrupulous or drunk.  This commonly results in the smaller components of the vessel, such as gaskets and attached hoses being discarded and replaced in an attempt to prevent further contamination.  Rivertown does not sour our beers this way.


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