Imagine a place where all beers available for purchase are at least a little bit sour. Now take a step back and think because that place was the entire world of beer until the last 100 years or so. This sourness was largely due to lack of understanding of bacterias as well as the absence of proper cleaning and sanitization practices. The critical gaps in understanding and practices would leave naturally occurring unpredictable yeasts in the batches of brew. Wild yeast then causes unintentional fermentation and flavoring, as well as a citric acid buildup causing the sour taste.
Brewers these days have access to cheap and plentiful means of sanitization and all wild yeasts and unpredictability are generally taken out of the process. With this resurgence we are seeing in the popularity of sour beers, brewers are taking their properly sanitized equipment and adding unusual or wild yeast or one or more varieties to create the sour effect. Often specific acid producing microbes are added to the batches such as the varieties discussed below.
Some modern brewers are using the classical method of simply leaving parts of the brewing process exposed to wild yeast. A gentle breeze or perhaps the wood of the building can be home to wild yeast and will provide the microbes needed for souring to occur. This approach is generally too unpredictable for the standard brewery but is being implemented by those going for a classic approach.
Sours get their trademark tartness and sourness from bacteria and wild yeasts—Lactobacillus, Acetobacter, Brettanomyces, and other critters—that you wouldn’t find in other styles of beer. Each type of bacteria gives its own trademark flavor and aroma: Lactobacillus has a yogurt tang, Acetobacter has the sourness of vinegar, and Brettanomyces has a barnyard, earthy, or farmhouse aroma. via Paste Magazine
One of the biggest sources of joy for a drinker of sour beer is the complexity endowed to each sour. The variety of wild yeast from batch to batch creates unique and distinct tastes based on the mix of yeasts and can be hard to replicate. Some breweries such as Brasserie Cantillon in Belgium believes it is so important to replicate the yeasts naturally occurring in the brewery that they are spraying down new locations with their beer to pass on the yeast! “The brewery allows the specific yeasts found in their building to ferment their beers in similar ways through the ages (Cantillon was founded in 1900), resulting in the consistent, aforementioned world-renowned product.” via Food and Wine
Now go try a “contaminated” beer!