http://www.whitelabs.com/beer/homebrew/ ... efinitions
Brewers have created their own unique vocabulary. Words such as pitching, attenuation, and flocculation take on special meaning to brewers. Pitching is adding yeast to wort to start fermentation. Attenuation is the percentage of sugars yeast consume during fermentation. The magical art of yeast coming together, dropping to the bottom of a fermentor, is called flocculation.
Flocculation is a desirable and important characteristic that is unique to brewers yeast. When brewers yeast nears the end of fermentation, single cells aggregate into clumps of thousands of cells, and drop to the bottom of the fermentor, leaving clear beer behind. If yeast flocculate too early, the beer will be underattenuated and sweet. If yeast do not flocculate, the beer will be cloudy and have a yeasty taste.
Most strains of yeast, which brewers call “wild” yeast, do not flocculate well, and remain in suspension for extended periods of time. The ability to flocculate is a product of natural selection. Brewers have continually collected yeast either from the bottom or top of a fermentor and in doing so, selected for increasingly flocculent stains. The chemistry of flocculation is complex, and will be the subject of a future article.
Yeast flocculation can be classified as high, medium, or low. Ale yeast strains are found in each category, while lager yeast are predominantly medium flocculators. An English/London Ale strain would be a high flocculator, while an California/American Ale strain a medium flocculator. A Hefeweizen strain is an example of a low flocculator. It is difficult to tell which category of flocculator is used to produce individual commercial beers, because most commercial beers are filtered before being bottled or kegged.[/quote]