Anaerobic fermentation (coffee)

Instead of fermenting in open air as usual, the coffee ferments in sealed tanks or barrels, often with a valve through which the gases produced can escape. Microorganisms break down the sugars in the fruit without oxygen. Duration, temperature and any added starter cultures steer the result.

Anaerobically fermented coffees often taste intensely fruity, sometimes wine-like or liqueur-like. A related variant is carbonic maceration, borrowed from winemaking. The method is common mainly in specialty coffee and its flavour is not without controversy.