The base of beers of mixed origin is usually a top-fermenting beer. After the main fermentation, part of the brew is pumped and kept in oak barrels for 18 months or more. During fermentation a lactic acid yeast is produced, while the microorganism and tannins in the wood also act on the beer, forming a fruity taste. After that the beer is filtered and blended with younger beer. Examples of mixed fermentation beers are Flemish red-brown beers.