Primary fermentation in beer brewing is the initial fermentation process where yeast will convert most or all of the wort sugars to alcohol and CO2 (carbon dioxide). After the yeast has been pitched into the wort, there is typically between 2 and 24 hour yeast lag time where the yeast acclimates to the fermentation environment and begins to replicate consuming sugars and the available oxygen in the wort; there is little alcohol conversion and CO2 generated during the lag phase.
Once the lag phase completes, a foamy head called a krausen begins to form in the fermentation vessel. The krausen is composed mostly of proteins, yeast, and the carbon dioxide that the yeast is rapidly producing. During primary fermentation the yeast is producing approximately equal parts of both alcohol and CO2. Depending on the style of beer, original gravity, quantity of yeast pitched, and fermentation temperature, the primary fermentation for an ale will last between 5-14 days, then it will then be transferred to a secondary fermentation vessel to allow the beer to condition and finish out its fermentation. In some cases only a primary fermentation is completed, and the beer may spending additional time in the primary fermenter or condition in the bottle, keg, or holding vessel.
Primary fermentation occurring two days after the yeast was pitched into an American Wheat style Hefeweizen. The krausen has formed and a great deal of alcohol and CO2 is being produced.
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.
Bottle conditioning refers to the process by which the beer is naturally carbonated in the bottle as a result of fermentation as opposed to being carbonated prior to filling. Oftentimes additional sugar or krausen is added to the beer prior to bottling or directly to the bottle so that the yeast will have enough sugar available to properly carbonate the beer.
A suitable fermentation temperature must be maintained for the conditioning beer to allow the yeast to adequately carbonate the beer. Since viable yeast is present in a bottle-conditioned beer, this provides an additional component of flavor that develops further as the beer ages. A slight layer of yeast on the bottom of a bottle of beer may be a sign that the beer had been bottled conditioned, but may also be due to poor filling or residual clarification of a non-filtered beer. The bottle and cap should always be sanitized before bottling occurs.