While technically a sour beer is any beer that is intentionally (or unintentionally) tart, sour or acidic flavor. The most common sour beers are the wildly fermented Belgian lambics, gueuze and flanders red ale.
The traditional process for brewing these style beers is almost unrecognizable when compared with the sterile sanitized stainless steel factory that produces beer today. Sour beers are made by intentionally allowing wild yeast strains or bacteria into the brew. Historically this was done by introducing the beer to barrels which contained the wild yeast, or during the cooling of the wort (wort is the term used for the liquid that is produced after the boil before the yeast is added) in a coolship open to the outside air.
In fact, modern breweries go to great lengths to prevent wild yeast from getting into the wort when brewing, so it seems counterintuitive to see many of these sterile breweries intentionally introducing Brettanomyces, Lactobacillus, or Pediococcus into the brewing process to create sour beer. Another method for creating a tart flavor is to add fruit during fermentation, this can spur a secondary fermentation.
Using wild yeast is risky business, it’s unpredictable. A beer that could be ready in 7-10 days with yeast being pitched normally could take several months to ferment and even longer to age; aging could even take years!
Ten years ago the only breweries that would dabble in making a sour beer are those who were established as brewery that specialized in lambics or other Belgian-style beers. Perhaps the oldest brewery still in operation that produces sour beer is the Rodenbach Brewery of Roeselare, Belgium. While the Belgians may have started the trend, brewing sour beer has spread outside Belgium, to other European countries, the United States and Canada.
You can technically sour any beer however, most follow traditional style guidelines. Here are the most common in alphabetical order.
American Wild Ale
Well obviously, it has to be brewed in the United States… This is perhaps the most undefined of the ales. It essentially is any beer that is brewed with unusual yeasts. They will use yeast and bacteria strains instead of or in addition to standard brewers yeasts. The yeasts/bacteria may be cultured or acquired spontaneously, and the beer may be fermented in a number of different types of brewing vessels.
If you are looking for a nice low alcohol (around 3%) but flavorful beer, look no further than the Berliner Weisse. Once the most popular alcoholic beverage in Berlin, the beer is made sour by the addition of Lactobacillus bacteria. In Berlin this beer is usually served with flavored syrups to balance the tart flavor. I’ve had the privilege of trying these in Berlin and I recall a red and a greenish color syrup. They are both delicious.
Flanders Red Ale
These are unique because they are often brewed traditionally with brewers yeast, then placed into oak barrels to age and mature and this is where the acquire the flavors. Once mature, this beer is then often blended with younger beer to adjust the taste to add consistency from brew to brew.
Originally from Goslar, Germany, this style of Ale is characterized by the use of coriander and salt and is made sour by inoculating the wort with lactic acid bacteria before primary fermentation.
Lambic is perhaps the most risky sour beer to brew. The debate over where a lambic CAN be brewed is EPIC! In essence, any beer that is spontaneously-fermented beer is made in the “Lambic Style”. But like fine wine, most brewers and beer snobs agree that the beer has to also be brewed in the Pajottenland region around Brussels, Belgium to truly be called a “Lambic”.
The process is extremely unique and old fashioned. Rather than speeding up the process of cooling the wort by using a wort chiller and keeping the beer completely sealed and sanitary the wort is left to cool overnight in the koelschip where it is exposed to the open air during the winter and spring, and placed into barrels to ferment and mature. Most lambics are blends of several season's batches, such as gueuze, or are secondarily fermented with fruits, such as kriek and framboise. As such, pure unblended lambic is quite rare, and few bottled examples exist.
Originating from the Flemish region of Belgium, oud bruins are similar but different from the Flanders red ale in that they are darker in color (hence the “Bruin” which means brown in French) and they are also not aged on wood. This style tends to use cultured yeasts to impart its sour notes.
As with any beer, they all benefit from being served in the proper glass.