The extended aging allows residual yeast and bacteria to develop a sour flavor characteristic for this style.

[1] Usually, cultured yeast and bacteria are used, as stainless steel equipment does not harbor wild organisms as wood does.

[2] Historical examples tended to be more sour than modern commercial products.

Brouwerij De Brabandere in Bavikhove still produces a traditionally barrel-aged oud bruin under the Petrus brand.

[3] This style of beer is medium bodied, reddish-brown, and has a gentle malty flavor and no hop bitterness.