Scientifically, there are two types of yeast used in home brewing beer (or the brewing of any beer..); Saccharomyces Cerevisiae (ale yeast) and Saccharomyces uvarum (lager yeast).

Yeast is a member of the fungus family, and is a single celled organism capable of asexual reproduction. Yeast is responsible for the fermentation of the sweet wort into beer. It feeds on the sugar molecules from the wort and converts them into alcohol. It also excretes co2, which can actually be seen with the use of an airlock on the fermentation container. The yeast multiply by splitting in two. This asexual reproduction creates enough yeast cells to fully ferment wort into beer.

Malt, hops, and water composition all affect the flavor of the finished beer, but many people don’t realize that yeast also has a major impact on flavor. In the two species of beer yeast, there are many sub-species which all contribute different flavor compounds.

Saccharomyces Cerevisiae (ale yeast) is top fermenting and prefers temperatures of 60-70 degrees Fahrenheit. Because of the warmer temperatures, this yeast provides a greater range of flavor compounds (esters) than the lager yeast strain.

Saccharomyces uvarum (lager yeast) is bottom fermenting and prefers much colder temperatures than ale yeast strains; around 38-50 degrees Fahrenheit. Because of the colder temperatures, this yeast strain ferments without as many flavor compounds (esters) and makes for a cleaner, crisper beer flavor.

Ale yeast has been used since the beginning of beer brewing, but lager yeast has only been used recently. The use of modern refrigeration helped to promote this strain. It was actually isolated and developed by Danish brewers in the late 1800’s.