Simply put, to make any beer sour, brewers must inoculate it (fermenting the beer by introducing one, or a combination, of fermenting agents - a genus of yeast called Brettanomyces, acid producing bacteria, or any type of non-conventional yeast). When making non-sour beers, brewers ferment the wort (brewed, but unfermented beer) with any number of different species of Saccharomyces yeast. This type of yeast consumes the sugars in the wort and produces alcohol, carbon dioxide, and a range of flavor characteristics. These are controlled fermentations so the beer has a consistent taste.
When it comes to brewing sour beer however, the acid producing bacteria and wild yeast like Brettanomyces are often incorporated in combination with a traditional Saccharomyces species. In the presence of oxygen, Brettanomyces can metabolize alcohol into the vinegary acidity known as acetic acid. In addition to the acidity, Brettanomyces lends a complex array of flavor components - anywhere from earthy and funky to tropical and fruity. These flavors continue to develop and change throughout the fermentation process and can yield some of the most complex beers in existence.
Our recently featured sour beer, Rollingstoner Elderflower Sour from Olvalde Farm & Brewing is a great example of this complicated and intricate brewing process.