Top fermentation, or top fermenting, describes the tendency of ale yeast cells to conduct the majority of fermentation on the surface of the fermentation vessel as opposed to the bottom, as is common with lager yeast. Top fermenting ale yeast is typically fermented at a temperature range between 65° F and 75° F; the lower the temperature, the slower the fermentation is carried out.
Excessive fermentation temperatures have been known to generate off flavors in beer, and that is why a temperature range of 65° F to 75° F is typically recommended. When a top fermenting ale is most active, a thick head of foam known as a krausen forms on the top of the fermentation vessel and will subside as the fermentation draws to an end. The length of fermentation is dependent on the health of the yeast, the original gravity of the wort, the temperature of the fermentation and the amount of yeast pitched, but typically takes anywhere from one week to three weeks for the majority of fermentation activity to complete. A secondary fermentation is oftentimes conducted so that any remaining fermentable sugars can be converted to alcohol, and the beer can condition and allow the yeast to precipitate to the bottom of the fermenter in preparation for bottling or kegging.
Below is a photo of a top fermenting ale that was recently transferred to a secondary fermentation carboy.