Underjäsning

Underjäsning, or bottom fermentation, is a method used in beer production in which yeast ferments at the bottom of the fermentation vessel. The term is commonly used in Swedish brewing to describe the process that yields lager-style beers. It contrasts with överjäsning (top fermentation), where yeast remains suspended near the surface. Bottom fermentation is associated with clean, crisp flavors and relatively low fruity esters.

During underjäsning, a bottom-fermenting yeast such as Saccharomyces pastorianus is used. Fermentation occurs at cooler temperatures, typically about 7–13°C (45–55°F). The process is slower than top fermentation, allowing proteins and residual sugars to settle and the yeast to flocculate and drop to the vessel bottom. After active fermentation, the beer is often conditioned at cold temperatures (lagering) for several weeks to months to mature and clarify the beer.

Common lager styles produced with underjäsning include pilsner, Helles, Märzen and other pale or amber lagers. Darker lagers such as dunkels and Schwarzbiers can also use bottom-fermenting yeast. The result is a beer with a clean malt and hop balance, reduced fruity notes, and a smoother mouthfeel. The method is widely used in Central and Northern Europe and remains standard in modern lager production.