Krausen

Krausen is the rocky, often tan-to-brown foam that builds up on the surface of fermenting beer during its most active phase. It forms as yeast multiplies rapidly and produces carbon dioxide, which carries proteins, hop particles, and yeast cells up to the surface in a dense, sometimes several-inch-thick layer. Seeing a strong krausen form is one of the clearest visual signs that fermentation has kicked off.

How high and how long krausen builds depends on the yeast strain, the wort's sugar content, and fermentation temperature, with high-gravity beers and certain vigorous strains producing a much taller krausen than a mild session beer. As fermentation slows, krausen gradually falls back into the beer, and a fermenter's headspace needs to account for this rise so beer does not blow out through the airlock.

A common mistake is filling a fermenter too close to the top, leaving no room for krausen to rise, which forces foam and beer out through the airlock or lid seal, sometimes called blowing off. Switching to a wider blow-off tube during vigorous fermentations, or simply leaving more headspace from the start, prevents the mess and the loss of beer and hop character that comes with it.