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 and/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.
Sour beer styles
While any type of beer may be soured, most follow traditional or standardized guidelines.
American wild ale
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.
At one time the most popular alcoholic beverage in Berlin, this is a somewhat weaker (usually around 3% abv) beer made sour by use of Lactobacillus bacteria. This type of beer is usually served with flavored syrups to balance the tart flavor.
Flanders red ale
Descendent from English porters of the 17th century, Flanders red ales are first fermented with usual brewers yeast, then placed in to oak barrels to age and mature. Usually, the mature beer is blended with younger beer to adjust the taste for consistency. The name comes from the usual color of these ales.
Gose is a top-fermenting beer that originated in Goslar, Germany. This style is characterized by the use of coriander and salt and is made sour by inoculating the wort with lactic acid bacteria before primary alcoholic fermentation.
Lambic beer is spontaneously fermented beer made in the Pajottenland region of Belgium and Brussels. Wort is left to cool overnight in the koelschip where it is exposed to the open air during the winter and spring, and then 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 exists.
Originating from the Flemish region of Belgium, oud bruins are differentiated from the Flanders red ale in that they are darker in color and not aged on wood. As such this style tends to use cultured yeasts to impart its sour notes.
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