Sour Beer

Sour 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 Koelschip 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.