What Is It?
There was a time when the role of yeast in brewing was unknown. In the days of the Vikings, each family had their own brewing stick that they used for stirring the wort. These brewing sticks were regarded as family heirlooms because it was the use of that stick that guaranteed that the beer would turn out right. Obviously, those sticks retained the family yeast culture. The German Beer Purity Law of 1516 - The Reinheitsgebot, listed the only allowable materials for brewing as malt, hops, and water. With the discovery of yeast and its function in the late 1860's by Louis Pasteur, the law had to be amended.
Brewer's Yeast (Saccharomyces cerevisiae) is considered to be a type of fungus. It reproduces asexually by budding- splitting off little daughter cells. Yeast are unusual in that they can live and grow both with or without oxygen. Most micro-organisms can only do one or the other. Yeast can live without oxygen by a process that we refer to as fermentation. The yeast cells take in simple sugars like glucose and maltose and produce carbon dioxide and alcohol as waste products.
Along with converting sugar to ethyl alcohol and carbon dioxide, yeast produce many other compounds, including esters, fusel alcohols, ketones, various phenolics and fatty acids. Esters are the molecular compound responsible for the fruity notes in beer, phenols cause the spicy notes, and in combination with chlorine, medicinal notes. Diacetyl is a ketone compound that can be beneficial in limited amounts. It gives a butter or butterscotch note to the flavor profile of a beer and is desired to a degree in heavier Pale Ales, Scotch Ales and Stouts. Unfortunately, Diacetyl tends to be unstable and can take on stale, raunchy tones due to oxidation as the beer ages. This is particularly true for light lagers, where the presence of diacetyl is considered to be a flaw. Fusel alcohols are heavier molecular weight alcohols and are thought to be a major contributor to hangovers. These alcohols also have low taste thresholds and are often readily apparent as "sharp" notes. Fatty acids, although they take part in the chemical reactions that produce the desired compounds, also tend to oxidize in old beers and produce off-flavors.