Krausen is the foamy and bubbly head that forms on top of beer during primary fermentation. As yeast ferments the sugars in a beer, it creates a great deal of CO2. The Krausen is formed as the CO2 rises to the top of the beer, mixing with proteins, yeast and residues and forming a tall layer of yeast saturated bubbles.
Krausening is also a term for when a measured amount of actively fermenting beer and/or krausen is added to a more thoroughly fermented beer as a means of conditioning or naturally carbonating the beer. Krausening is typically done as a means of carbonating a bottled beer with out violating the German beer purity laws.
Below is a video showing krausen in a 6.5 gallon carboy during primary fermentation.
Carbonation or carbonating is the process of dissolving carbon dioxide in beer. There are different methods of carbonating beer, but the end effect is basically the same from a CO2 standpoint. Carbon dioxide is built up under pressure, which carbonates the beer; when the pressure is reduced, the carbon dioxide is released as bubbles into the beer. Carbonation helps form the head of the beer and makes the beer effervescent. Carbonation has a significant impact on many aspects of a beer, from the body and mouthfeel to the aroma delivery and appearance.
Some of the different methods of carbonating beer include:
Krausening, which is the process of adding a small amount of young fermenting beer (about 10-20%) to a finished beer in order to carbonate it. You then seal the beer to allow the pressure to build and carbonate the beer. Krausening is typically a little less predictable then other forms of carbonation since it is more difficult to control the exact amount of carbonation that will occur. One of the benefits is that there is typically minimal impact to the flavor profile of the beer.
Force carbonating a beer is done by placing (preferably chilled) beer into a sealed vessel that is connected to a pressurize CO2 tank. You pressurize the sealed vessel via the CO2 tank, and the CO2 is rapidly absorbed into the beer. The benefits of forced carbonation are that it is quick, and, since you are not fermenting in the bottle to build the CO2, the beer is typically cleaner with far less bottle sediment and fewer flavors imparted by the yeast, if that is desired.
Another method is by starting to carbonate your beer towards the tail end of your fermentation. To do this, you can remove your air lock and seal the fermenter; this will pressurize it and allow it to carbonate naturally.
Lastly, you can bottle condition and carbonate your beer by priming it at the time of bottling with a specific amount of sugar. You should use approximately .5 teaspoons (½ tsp) of priming sugar per 12oz bottle. Typically you will want to prime your beer with corn sugar (dextrose). It is critical that your beer has completed its fermentation prior to priming and bottling, as residual fermentable sugars from the primary fermentation can create excessive pressure in the bottles and cause them to explode. It is also critical that the yeast is still viable, so that the priming sugar is converted to CO2 in the bottle, and you do not end up with a flat\sweet beer.