Krausen

Krausen is the foamy layer that forms on the surface of beer during active fermentation. The term comes from the German krausen, meaning froth, head, or crust, and it is most commonly described in the context of ale and other top-fermenting fermentations.

The foam forms as carbon dioxide produced by yeast carries wort proteins, sugars, hop resins, and other solids to the surface. This creates a foam cap known as the krausen. The thickness and duration of krausen depend on factors such as the yeast strain, wort gravity, and temperature. A vigorous fermentation can produce a thick krausen that may overflow the fermenter if not vented or if a blow-off tube is not used.

During fermentation, the krausen is not confined to the top; some foam may cling to the sides of the vessel, producing a krausen ring. As fermentation subsides, the krausen collapses and falls back into the beer, along with yeast and solids, and the remaining material typically settles as part of the trub.

Practical notes: observing krausen height and duration can provide a rough indication of fermentation activity, though it is not a precise measure of completion. A thick krausen can hinder airlock function or cause overflow, so brewers may use a wide-mouth fermenter, venting, or a blow-off setup. After fermentation ends, krausen components generally dissolve or settle, and the beer can be transferred or conditioned as part of the maturation process.