kräusen

Kräusen is a secondary fermentation technique used in lagering where actively fermenting wort is added to nearly completed beer to revive yeast activity and generate carbonation. The term comes from German, referring to the dense foam or head (Krausen) that forms during vigorous fermentation. This method predates modern force carbonation and remains standard in traditional and some craft breweries.

The process involves introducing young, vigorously fermenting wort, typically at the same gravity and from the same brew, into lagering tanks containing beer that has finished primary fermentation but still contains residual sugars and viable yeast. The fresh yeast population and available fermentable sugars restart fermentation, which produces CO2 naturally and raises the conditioning temperature slightly. The volume added is usually 5 to 15 percent of the total beer volume, enough to achieve desired carbonation without excessive re-fermentation.

Timing and Beer Composition

Timing is critical. Kräusen works best when the base beer has dropped to approximately 1 to 2 degrees Plato residual gravity but still contains living yeast. If added too early, the beer may ferment too vigorously and produce off-flavors; if too late, insufficient carbonation may develop. The technique requires reliable temperature control, typically holding the tank at 8 to 12 degrees Celsius during the conditioning phase, which can last one to three weeks depending on target carbonation levels.

The alternative to kräusen is force carbonation, where CO2 gas is directly injected into chilled beer under pressure. This is faster and more controllable but lacks the subtle sensory effects some brewers attribute to kräusen conditioning: slightly softer carbonation, integrated flavor development, and the reduced oxidation risk from minimal headspace exposure. Bottle-conditioned beers use a similar principle with added yeast and sugar in the bottle rather than the main vessel.

Practical challenges include ensuring the fermenting wort is the correct age and vigor, maintaining sanitation during the addition, and preventing over-carbonation or gushing. Breweries using kräusen must maintain parallel fermentation schedules and careful record-keeping. The method is less forgiving than force carbonation but remains valued in lager production, particularly in Bavarian and Czech brewing traditions.