Kräusen is the German word used to describe the infusion of a strongly fermenting young beer into a larger volume of beer that has undergone primary fermentation. Traditionally, the wort used for kräusening is obtained from the high kräusen stage of primary fermentation and added in small portions (5‒20% by volume) to the green beer to start a secondary fermentation (Coors, 1977). MacDonald suggests adding a volume of kräusen equal to 10 to 12 percent of the “green” beer, containing approximately 2 percent (w/w) residual extract with a cell count of between 10 and 15 million (MacDonald et al., 1984).
Kräusen is usually added at a temperature between 6 and 9 degrees C (43 and 48°F) and eventually reduced to 1 degree C (34°F) by the end of fermentation (Coors, 1977). Of course, the temperature will depend on the yeast strain, with cold-tolerant strains preferring a colder secondary fermentation temperature (5‒7°C, 41–45°F).
Click on the following topics for more information on secondary fermentation.