Oud Bruin, also known as Flanders Brown, is a style of beer originating from the Flemish region of Belgium. The name literally translates as "old brown", referring to the long aging process which can take up to a year. It undergoes a secondary fermentation, which takes several weeks to a month, and is followed by bottle aging for several more months. The extended aging allows residual yeast and bacteria to develop a sour flavor characteristic for this style. While some examples of an Oud Bruin may be aged in oak, typical beers in this style are not, and this is what helps Flanders Brown ales distinguish themselves from the more sour Flanders Red ales.
This style of beer is medium bodied, reddish-brown, and has a gentle malty flavor and no hop bitterness. The aroma is a complex mixture of fruits and estery smells of plum, raisin and cherry. The flavor is sweet, caramel, fruity, tart, and slightly acidic, caused by various bacteria in the maturation process. In a good example, the tartness is often balanced with a malty character, tasting of toffee and a giving a "sweet-and-sour" impression. Commercial versions may mix aged beer with younger, sweeter beer to temper the acidity and create a more complex flavor.