The Oxford Companion to Beer definition of
Top Fermentation, generally associated with ales, is a mode of fermentation in which the flocculating yeast rises to the surface of the fermenting wort, rendering it possible to “skim” the crop of yeast from the surface of the vessel ready for transfer to the next batch of wort. The yeast concerned, “ale yeast” or Saccharomyces cerevisiae, is relatively hydrophobic, such that upon flocculating it tends to seek the surface of the liquid to escape the aqueous milieu of the fermenter contents. (In “bottom fermentation” on the other hand, the yeast, often described as a “lager yeast” or Saccharomyces pastorianus, drops to the bottom of the fermentation vessel.) See bottom fermentation. Top fermentation is carried out traditionally in open fermenters, which in the modern era are likely to be fabricated from stainless steel. They are usually either square or round in cross-section and typically 2–4 m (6–13 ft) deep. A classic example is the “Yorkshire square” fermentation vessel. See yorkshire square. An alternative approach is the Burton Union system, where the rising yeast passes from a series of oak barrels in which fermentation is effected into a “top trough.” See burton union system. Top fermentations tend to be carried out at higher temperatures (16°C and higher) than bottom fermentations, although some styles of beer using top fermentations also ferment at lower temperatures than this (kölsch beers, for example).
See also ale yeast, fermentation, kölsch, and open fermentation.
Briggs, D. E.C. A. Boulton, P. A.Brookes, and R. Stevens. Brewing: Science and practice Cambridge, UK: Woodhead, 2004.