Yeast

Saccharomyces cerevisiae is alternatively known as brewers or bakers yeast. That’s no accident as brewing and baking were, in may parts of the world, intimately connected. Prior to the 20th Century bakers often relied on a supply of yeast from brewers in order to make their bread. Based on genetic research, Saccharomyces cerevisiae is closely related to the wild yeast Saccharomyces Paradoxus which can be isolated from tree bark and fruit skins, and often coexists in the wild with Saccharomyces cerevisiae.

Like any organism, yeast needs to absorb and digest food in order to survive and fuel growth and reproduction. During the process of fermentation, Saccharomyces cerevisiae digests any available fermentable sugars and creates the by products carbon dioxide gas, ethyl alcohol and other compounds that contribute to the flavour profiel and mouth feel of beer.

Ale yeast, Saccharomyces cerevisiae is a top fermenting yeast. This means that the most vigorous aspects of fermentation occur at the top of the beer as it ferments. Saccharomyces cerevisiae ferments quickly causing particles in the beer to flocculate or clump together at the surface of the fermenting wort, often combining with the carbon dioxide bubbles to create a layer of particles and foam that can be upto several inches thick (see photo to the right). 

Lager yeast Saccharomyces pastorianus is a slower, bottom fermenting yeast. Saccharomyces pastorianus ferments slowly and at lower temperatures (around 5 °C (41 °F)) than Saccharomyces cerevisiae, causing the particles produced during fermentation to accumulate at the bottom of the fermentation vessel.