Yeast (Saccharomyces cerevisiae) is a microorganism that consumes sugar, producing carbon dioxide as a by-product. This carbon dioxide is the source of bubbles and foam in beer. Yeast for brewing is categorized into either "top-fermenting" or "bottom-fermenting". "Top-fermenting" yeast rises to the top of the liquid at the end of the main fermenting process; "bottom-fermenting" yeast settles to the bottom of the vat. The flavored, clarified "wort" is cooled to an optimum temperature for fermentation. Fermentation is started by "pitching" or mixing the "wort" with air and cultured yeast. After a suitable fermentation period, the beer is filtered and bottled.

Yeast (Saccharomyces spp.) cells magnified under a microscope

Smart, Katherine. Brewing Yeast Fermentation Performance, 2nd ed., Blackwell Science, Oxford, 2003. Crerar TP580 .B74 2003.