Kräusening is a German term for the addition of actively fermenting wort as an inoculant to induce fermentation in a different batch of wort or beer. If this method is used to start a new fermentation, a small proportion of a previously brewed, vigorously fermenting wort is removed and added to fresh wort to initiate its fermentation.
This is often a preferred means of inoculation because the kräusen will contain active yeast cells that require limited adaption to the conditions of the new wort. Fermentation begins more quickly and may be better guaranteed than a stored sample of yeast, which may contain many dead cells.
Another application is the use of actively fermenting wort to prime beer when it is being bottled. In this case the kräusen brings sugars and nutrients as well as active yeast to the beer. This is helpful because the beer may have limited sugar left after its fermentation. The addition of this and live yeast cells allows a limited fermentation to occur in the bottle, producing carbon dioxide and secondary conditioning.
Care is necessary when kräusening to minimize the potential for carry-over of contamination by spoilage microorganisms and, in bottle-conditioned beer, to judge the correct level of kräusen needed. Too high a level of kräusening will result in over- carbonation and possible gushing of the beer upon opening.
Kräusening is also traditionally used as a method of conditioning finished beer in a closed tank. In this method, a portion of fermenting beer (kräusen) is added to the finished beer, restarting fermentation. The tank is then “bunged” (closed, so that gas is prevented from escaping), and the carbon dioxide from the continuing fermentation is dissolved into the beer. Many brewers feel that this method can reduce levels of off-flavors such as diacetyl and acetylaldehyde.
In some cases kräusening may be used to revitalize a dormant fermentation. This may be necessary in producing a very strong beer where high levels of alcohol kill the initial yeast. Kräusening may thus help complete the fermentation.