What Is Krausen?
The word krausen (pronounced kroy-ZEN) describes the foamy head that develops on top of fermenting beer. It is used by brewers to gauge when the fermentation process is going strong and when it is complete. Krausen is both a verb — used to describe the fermentation stage of brewing beer, and a noun — the foamy, rocky head of yeast that forms at the peak of fermentation.
German lager brewers traditionally took high-krausen wort from a newly fermenting batch and added it to a fully-fermented batch of the same recipe. This process, known as krausening, introduces healthy, new yeast to pick up where the primary yeast, which went dormant due to the layering temperatures, left off.
Due to the German beer purity law (the Reinheitsgebot), the use of sugars not derived from malt was not allowed. So alternative means were needed to supply a small dose of sugar for the yeast to convert and ultimately carbonate beer. The solution was to add high-krausen wort of the same recipe to an already fermented batch to provide the sugar needed for bottle carbonation. The same technique can also be implemented when naturally carbonating in casks and firkins. Some brewers will add non-fermented wort called speise as the priming sugar, but the addition of active yeast.
While all instances of krausening are technically activating fermentation, the process can be used for instances of either starting or restarting fermentation. Using the krausening technique to initiate fermentation is essentially the same as making a starter, where the entire contents of the starter would be added to the wort at the height of its activity. If you are experiencing stuck fermentation or need to finish off a high-gravity beer, krausening adds more healthy yeast that are up to the task. And, as was the case in traditional brewing, krausening can be used to add more yeast to a finished lager ready for conditioning and carbonation where the initial yeast has gone dormant due to the chilly fermentation temperatures.
While carbonation was the primary goal of krausening in traditional German lager brewing, it was also recognized to clean up some of the “green” qualities found in young beer by aiding the maturation process. It can also be used to help with flaws such as diacetyl and acetylaldehyde. Adding more yeast through krausening is also said to help reduce oxidation in batches that may have been unintentionally aerated.
Once both batches reach fermentation temperature, the reserve will undergo krausening once again. Subsequently, the brewer can introduce krausening beer into the primary batch before it undergoes kegging, canning, or bottling.
