What is krausen in beer?

Krausen is a German term used to refer to the “blanket” of foam generated by fermentation at its peak activity, when the liquid is bubbling due to CO2 production. It should not be confused with the foam produced when we pour beer into a glass, which in Spanish we call “giste” (head).

Beyond the origin of the word, krausen is a German technique used to re-ferment beer without adding sugar. In 1516 the famous purity law (Reinheitsgebot) came into force, limiting beer production to three ingredients, not counting yeast, which was not yet known: water, barley malt and hops. Although today we are used to adding sugar to give beer a second fermentation, German brewers had to find ways to finish rounding out their beers without breaking the law.

To do this, they added wort that was still fermenting to the almost finished beer. This brings several advantages. The first is that the new yeast is active compared to the yeast that has already settled. In addition, we add sugars from the wort that provide extra carbonation to the beer. This way, undesirable by-products such as diacetyl, acetaldehyde, and other sulfur compounds are eliminated. In a way, this refermentation helps clean up the beer’s flavor and aroma.

Another interesting use of krausen is to enhance or fine-tune beers. For example, the Schlenkerla brewery applies this technique to its Aecht Schlenkerla. That is, it adds a portion of smoked Märzen to its Helles. This way they manage to imbue their pale lager with smoky nuances. This blending can be done between different beer styles, but also between different batches of the same beer to achieve the desired balance.

Some homebrewers choose to experiment with this technique, although they must be especially careful about contamination and not overdoing it. Otherwise, we risk over‑carbonating the beers and having the caps fly off. In the end, a krausen is similar to preparing a yeast starter for an already finished beer.

In conclusion, krausening is an interesting technique for those who want to give their beers a second fermentation (especially in high‑gravity, very strong styles) because we don’t dilute the beer with a kind of syrup; instead, we add wort directly. In this way we don’t adulterate the whole, but rather we keep the dense body we’re aiming for without sacrificing carbonation or leaving the beer unfinished.