Sour beer

Sour beer is beer which has an intentionally acidic, tart or sour taste. The most common sour beer styles are Belgian: lambics, gueuze and Flanders red ale.

At one time, all beers were sour to some degree. As pure yeast cultures were not available, the starter used from one batch to another usually contained some wild yeast and bacteria. Unlike modern brewing, which is done in a sterile environment to guard against the intrusion of wild yeast, sour beers are made by intentionally allowing wild yeast strains or bacteria into the brew. Traditionally, Belgian brewers allowed wild yeast to enter the brew naturally through the barrels or during the cooling of the wort in a coolship open to the outside air – an unpredictable process that many modern brewers avoid.

The most common agents used to intentionally sour beer are Lactobacillus, Brettanomyces, and Pediococcus. Another method for achieving a tart flavor is adding fruit during the aging process to spur a secondary fermentation or contribute microbes present on the fruit's skin.

Because of the uncertainty involved in using wild yeast, the sour beer brewing process is extremely unpredictable. The beer takes months to ferment and can take years to mature.

Making sour beer is a risky and specialized form of beer brewing, and longstanding breweries which produce it and other lambics often specialize in this and other Belgian-style beers. Established in 1836, one of the oldest breweries still in operation that produces sour beer is the Rodenbach Brewery of Roeselare, Belgium. Today sour beer has spread outside Belgium to include other European breweries and some in the United States.

While any type of beer may be soured, most follow traditional or standardized guidelines.

Beers brewed in America utilizing yeast and bacteria strains instead of or in addition to standard brewers yeasts tend to fall under the catch-all term American wild ale. These microflora may be cultured or acquired spontaneously, and the beer may be fermented in a number of different types of brewing vessels. American wild ales tend not to have a specific parameters or guidelines stylistically, but instead simply refer to the use of unusual yeasts.

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Wikipedia