Bottom and top fermenting yeast

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The yeast that ferments beer is conventionally divided into two broad classes, top-fermenting and bottom-fermenting. Correspondingly, beer is also divided into two very broad categories according to which yeast is used, respectively ale and lager.

The two categories of yeast get their names because of different typical behaviours. In particular, some strains of top-fermenting yeast gather very obviously on top of the fermenting beer, as part of the foamy head. Bottom-fermenting yeasts do not. Nonetheless, for both types, yeast is fully distributed through the beer while it is fermenting, and both equally flocculate (clump together and precipitate to the bottom of the vessel) when it is finished. By no means all top-fermenting yeasts demonstrate this behaviour, but it features strongly in many English ale yeasts which may also exhibit chain forming (the failure of budded cells to break from the mother cell) which is technically different from true 'flocculation'.

In many respects, the difference which gives the yeast types their names is the least important of the differences between them. Bottom-fermenting yeast, Saccharomyces uvarum, ferments at quite cool temperatures, 7-12°C (45-54°F), and will continue to affect the beer while it is conditioning at temperatures as low as 0°C (32°F). It also tends to ferment "cleanly", with relatively less production of esters and other by-products, and is thus in part responsible for the "crisper" character of lagers. By contrast, top-fermenting yeast, Saccharomyces cerevisiae, usually works at rather warmer temperatures, 15-23°C (59-73°F), and will tend to produce flavour compounds such as esters which contribute to a fruitier and sometimes more complex character. Ale and lager beers are normally hopped differently with lagers normally having less hop character than ales.