Lager yeast, Saccharomyces pastorianus, is a low-fermented yeast used to make lager-type beers. Physiologically, it differs from S. Cerevisiae brewer's yeast, which is highly fermented (so called because it forms a thick foam on the top of the must during fermentation), because of its ability to ferment at colder temperatures and to ferment melibiose sugar. Lager yeast also tends to ferment more sugars than brewer's yeast, leading to a crisper flavor.
Brewer's yeast is classified into two main varieties. They are classified as ale yeast (high fermentation type, Saccharomyces cerevisiae) or lager yeast (lower fermentation type, Saccharomyces uvarum). These two varieties are in turn divided into specific strain categories. There are hundreds of strains of ale and lager-type yeasts.
The strain, called Saccharomyces eubayanus, is one of the two parent species that produced the hybrid yeast that brewers use to make beers. A group of undergraduate students from University College Dublin identified the elusive strain on the floor of their campus. They shared the details of their finding this month in the journal FEMS Yeast Research. Some brewer yeast strains show a tendency to flocculate on the surface of fermenting beer during the first few days of fermentation, hence the term high fermentation.
Lager beer, on the other hand, is fermented at the bottom, which means that the yeast works at the bottom of the wort, at a much lower temperature. Because the reaction takes longer, lower fermentation lagers ferment at low temperature for longer. Chances are you've been in a situation where a beer was called either ale or lager, and you were hoping that no one would ask you the difference. Pastorians have been harvested in breweries ever since Hansen pioneered the cultivation of pure yeast in the late 19th century, and recent work has shown that all the lager beer strains used today are probably descended from one of two hybridization events between S.
But, until now, they have never been identified in Europe, even though lagers probably originated in Bavaria in the late Middle Ages. Lager yeast is a variety of yeast that is best used at temperatures ranging from 45 to 55 °F, and certain beer strains perform well at temperatures as low as 32 °F. Wild fermentation is the third essential way to ferment a beer, since it is mainly based, you guessed it, on wild microbes, especially Brettanomyces, a yeast known (and often sought after) for its ability to give beers a decidedly original flavor profile. Brewer's yeasts are responsible for a wide range of beer styles, such as witbiers, stouts, ambers, tripels, saisons, IPAs and more.
All lager yeast strains will flocculate and then settle to the bottom of the fermentation vessel, hence the term lower fermentation. This selection of colder fermented yeast may have taken place during successive rounds of cold-temperature fermentations, as a result of a 16th century Bavarian law that prohibited brewing beer during the summer months due to the lower quality of beers brewed in summer. And while you won't have to memorize the Latin nomenclature or identify your favorite beer by the yeast that fermented it, it's actually useful to know two types of yeast directly related to the two main fermentation methods involved in beer production.