Sour ales are one of the biggest things in craft beer right now. The style that started out as a niche Belgian import not too long ago has spread like wildfire across American bars and breweries. Producing sour beer at home can be difficult, but with some experimentation and education there's nothing stopping a homebrewer from creating a tart and funky ale just like the best of the commercial brewers.
The word "sour" in the beer lexicon covers a broad spectrum of flavors. It can mean a puckering tartness in a Geuze such as Lindemans Cuvee Renee, or a mild sweet and sour combination in a beer like Monk's Cafe Flemish Sour Ale. The term is even used to refer to beers that may not be sour in the traditional sense, but instead have rustic flavor qualities that might remind you of wet hay or leather.
The common thread in all sour ales is the ingredient used during fermentation. While all beer ferments using varieties of yeast known as Saccharomyces cerevisiae, a sour ale fermentation includes different species of yeasts and even some types of bacteria. The yeast that can be used in sour ales is called Brettanomyces, and the most common bacterias used are Lactobacillus and Pediococcus. These fermenting critters, affectionately called "bugs" in the brewing world, eat sugar just like regular yeast. The difference is that they produce lactic and acetic acids that cannot be made by the usual yeast. These acids, along with other flavor compounds, provide the tart and rustic flavors associated with sour ales.
Fermenting beer with sour bugs can be very different than fermenting non-sour beer. The most notable difference is the time commitment. While a standard homebrew typically ferments in one to three weeks with a few more weeks of aging, a sour ale can take months to ferment and is typically aged for one to three years. This aging process allows the sour flavors to develop, and smooths out the harsher flavors that can be produced by the exotic bacterias.
Historically, most beer produced before the use of stainless steel probably had at least some level of sourness. Sour bugs live quite comfortably in the staves of wooden barrels that were used to store and age ales. When a barrel was filled with a new brew, the bacteria would transfer to the liquid, eat the leftover sugars and then wait for the next batch of beer to be made. If a beer was consumed quickly, the sourness may not have been noticeable. Brews with a higher alcohol or hop content would also hold off the bugs for a longer period of time.
Since Brettanomyces and sour bacteria live so comfortably in wood, modern commercial brewers who are intentionally making sour ales will often age them in oak barrels. This creates a natural habitat for the bugs to work. Some of the bacteria will produce more acid in the presence of oxygen, which can permeate an oak barrel much easier than a stainless steel fermentor. Homebrewers who are brewing sour ales can mimic the commercial oak barrel techniques by adding oak chips in the carboy. The bacteria will find its way into the oak chips in one batch, and they can be transferred to a second carboy to pass on the souring bacteria from one batch to the next.
Brettanomyces, or Brett for short, is the most common yeast used when making a sour ale. It can produce some very funky flavors that some homebrewers describe as "horse blanket", but I prefer to call rustic. These flavors pair quite well with the sour characteristics you will get from the bacteria used to produce sour ales. Using Brett on its own will not make a beer sour, but this yeast will always make a very dry beer with a very low final gravity and funky flavors.
Brett is actually able to eat more complex sugar than standard yeast, but it tends to be a little slower. A common technique is to add a standard beer yeast and Brett to the fermentor at the same time. The regular yeast will ferment as much as it can at the beginning, and as it slows down the Brett will take over and finish the fermentation. In some cases, Brett is added after the primary fermentation is complete. The Belgian Trappist beer Orval is known for this technique, where the brewers add Brett to the beer just as it's being bottled. In this case, the yeast actually continues to change the flavor of the beer in the bottle for years, which is why a 5 year old bottle of Orval will taste very different from a new bottle if you try them side by side. Brettanomyces is available to homebrewers from both White Labs and Wyeast.
Lactobacillus is a bacteria that produces a distinct tartness in the form of lactic acid. The variety used in brewing is closely related to the Lactobacillus used in making cheese, but it's not exactly the same species. This sour bug is prominently exhibited in the sour German wheat styles Gose and Berliner Weiss. It's a very temperamental bacteria that does not propagate in beers with even a mild hop content or alcohol above 4% ABV. Homebrewers can buy Lactobacillus in most homebrew shops, but the use should be limited to styles that call for the bacteria specifically.
Pediococcus is a tenacious souring bacteria that also produces lactic acid and tends to make a lot of the buttery flavored compound called diacetyl. It is typically used in conjunction with funky and rustic flavored Brettanomyces. You can find Pediococcus in the bottom of bottles of the classic Belgian sour ales made by Cantillon. This bug is less common in homebrewing stores, but it is produced by Wyeast and it's available from White Labs in a blend of sour bacteria. Homebrewers should take extreme caution when using Pediococcus, since it can easily infect equipment and wreck other brews that are not supposed to turn out to be sour.
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