Wooden barrels give depth and complexity to beer. The many different yeasts and bacteria in our beer react with the lingering microbes from previous batches, creating new flavours. Tiny amounts of oxygen that seep through wood also form chemical reactions with microbes, to the same effect. Brettanomyces is capable of eating sugars naturally present in wood, excreting aroma's in the process. Any lingering taste of the liquid that was previously in the barrel (wine, whisky, port, bourbon…) can rub off on the beer, for an extra dimension. The type of wood used for the barrel can lend subtle aromas like vanilla, and its tannins give varying degrees of bitterness.

Barrel aging opens up a world of flavours, but it's an exercise in patience too: it can take years for a beer to fully reach its potential inside a barrel.