A lot of people are a bit unsure of the exact role yeast plays in brewing beer so we thought we’d give you a short run down. Yeast is a little, single-cell organism, one of the simplest forms of plant life, which is responsible for the fermentation process in beer. Yeast essentially takes simple sugars like glucose and maltose, and produces carbon dioxide and alcohol.
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 the beer would turn out right (the sticks retained the family yeast culture). The German Beer Purity Law of 1516 - The Reinheitsgebot, listed the only allowable ingredients 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.
Since the late 1800s, numerous pure yeast strains, more than 500 types, have been isolated, identified, and cultured. Many brewers consider their yeast to be their most secret ingredient and often guard its identity jealously, calling it a proprietary ingredient, with sterile cultures often kept on hand for future brews.
Yeast can also take credit for the classification of the beer style. Brewmasters pick a yeast according to the recipe or the style of beer they want to make. Yeast is identified as either an ale yeast or a lager yeast. An Ale yeast, is a top-fermenting strain, working best at warm temperatures, while Lager yeast, in contrast is a bottom-fermenting strain, performing best at cooler temperatures.
Because of the temperature differential, each yeast strain produces vastly different flavour and aroma characteristics that, in turn, create the different beer styles. Yeast, in combination with different fermentation processes and ingredients, can also contribute fruitiness and other flavour characteristics to the beer.