In the world of German brewing, Kräusen refers to the intense proliferation of bubbles and foam formed during fermentation of a young or “green” beer. And Aufkräusen (literally to kräusen it up), refers to a historic brewing technique in which some green beer is added to a fully-matured lager. This is a very old and very typically German tradition, and today even English-speaking brewers refer to this technique as krausening.
Krausening brings the vibrant freshness of young beer to the more refined, austere flavors of a fully-matured lager. It's an amber coloured, unfiltered lager beer with beautiful malt flavors and a slight smokiness. It matures for months in the deep rock cellars underneath the Schlenkerla brewery and is krausened with Aecht Schlenkerla Rauchbier