The examples and perspective in this article or section might have an extensive bias or disproportional coverage towards Flanders/Belgium. Please improve this article or discuss the issue on the talk page.(August 2013)
Oud Bruin (Du: "old brown"), also known as Flanders Brown, is a style of beer originating from the Flemish region of Belgium. The Dutch name refers to the long aging process, 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. Usually, cultured yeast and bacterias are used, as stainless does not harbor wild organisms as wood does.
These beers were kept as so called provision beers, to be stored and allow the flavor to develop. Liefmans Brewery has been brewing the style since the 17th century. Historical examples tended to be more sour and acidic than modern commercial products.
This style of beer is medium bodied, reddish-brown, and has a gentle malty flavor and no hop bitterness. Commercial versions may mix aged beer with younger, sweeter beer to temper the acidity and allow for further fermentation.