A Tale of Two Yeasts

It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, and where there was one, now there are many, yeast that is. No this is not some Dickens’ novel but the real life tale of brewer’s yeast. Most homebrewers are aware that there are generally two types of yeast used to make beer, ale yeast (Saccharomyces cerevisiae) and lager yeast (Saccharomyces pastorianus aka S. carlsbergensis), but things are rarely as simple as they seem.

The domestication of barley in Sumeria 6,000 years ago likely led to the predecessor to modern day beer and the beginnings of yeast domestication. During the Middle Ages in Europe, ale-type beers (likely brewed with Saccharomyces cerevisiae) were beginning to be produced with lager-brewing arising in 15th century Bavaria⁽¹⁾. The first yeast strain (CBS1171) wasn’t isolated until 1883 by Emile Christian Hansen and was designated as the neo type (specimen a species is named for) of Saccharomyces cerevisiae. This isolation led to the first pure cultures of yeast strains used for commercial beer production in the late 1800’s⁽²⁾. It was generally recognized that ale and lager yeasts were different but it would take another 100 years to identify that difference (Figure 1).

S. cerevisiae 

S. pastorianus 

S. bayanus

Figure 1. See why yeast species identification is difficult. All three species are part of the Saccharomyces sensu stricto group.

In 1985, the Saccharomyces group known as Saccharomyces sensu stricto were split utilizing DNA analyses into four distinct species: ale style yeast, S. cerevisiae (neo type strain CBS1171NT); wine and cider yeast, S. bayanus (CBS380T); S. paradoxus (CBS432NT); and lager yeast, S. pastorianus (CBS1538NT)⁽³⁾. These experiments also revealed that S. pastorianus (lager yeast) was a hybrid species with S. cerevisiae and another species ⁽⁴⁾, but more on that coming up. Since 1985, this group has been divided into even more species. Many of these species are differentiated by only a few nucleotide base pairs at certain chromosomal loci known for sugar (maltose) and sulfite metabolism. Some species are allotetraploid hybrids (a duplication of each chromosome from the parents) or sterile haploids and diploids incapable of sexual reproduction, but yeast reproduction can be saved for another post.

As you can see, yeast genetics is complicated and contentious. Lager yeast (S. pastorianus) unlike ale yeasts, have never been isolated from the wild and depends on humans for its propagation ⁽⁴⁾. In 2011, scientists identified a cryotolerant species of yeast (S. eubayanus) in Patagonia that is likely the missing parent of lager yeast (Figure 2). These yeasts were isolated from beech trees and their associated fungi (Cyttaria sp.) in the Southern Hemisphere. These beech trees are equivalent to their northern cousins, oak trees ⁽⁵⁾. Saccharomyces spp. are often associated with oak trees in the Northern Hemisphere and are thought to be the origins of brewers and baker’s yeast in Europe. How South American yeast made their way to Europe is still under some debate. Some believe the early trade routes may have brought back exotic wood and the yeast hitched a ride to European breweries. Others believe that fruit flies stowed away on shipping vessels may have carried S. eubayanus in their guts until finding their way to Europe ⁽⁵⁾. We may never know how S. eubayanus found their way to Bavarian breweries but they have helped to create one of the most popular and commonly used yeast in the brewer’s arsenal. Prost!

Figure 2. Cyttaria from Chile growing in a tree branch where S. eubayanus was isolate   

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1. Corran HS (1975) A History of Brewing (David and Charles, London, UK).
2. Nguyen H-V, Legras J-L, Neuve´ glise C, Gaillardin C (2011) Deciphering the Hybridization History Leading to the Lager Lineage Based on the Mosaic Genomes of Saccharomyces bayanus Strains NBRC1948 and CBS380T. PLoS ONE 6(10): e25821. doi:10.1371/journal.pone.0025821
3. Vaughan-Martini A, Kurtzman CP (1985) Deoxyribonucleic Acid Relatedness among Species of the Genus Saccharomyces Sensu Stricto. International Journal of Systematic Bacteriology 35: 508–511.
4. Dunn B, Sherlock G (2008) Reconstruction of the genome origins and evolution of the hybrid lager yeast Saccharomyces pastorianus. Genome Res 18:1610–1623.
5. Libkind et al. (2011). Microbe domestication and the identification of the wild genetic stock of lager-brewing yeast. PNAS 108:14539-14544.