The foamy, dirty head of bubbles, yeast, and fermentation by-products floating on the top of an actively fermenting beer. Some believe that removal of the krausen improves the final beer's flavor. Since the krausen has a tendency to clog airlocks (sometimes leading to exploding fermentors), blow-off tubes can be substituted during primary fermentation. "Krausening" refers to the addition of freshly fermenting wort (that is, wort with a krausen) to a previously fermented batch just before it is bottled. Krausening will carbonate the packaged product, and produce a freshness of flavor not found in artificial carbonation methods or natural priming with residual yeast.