Saccharomyces pastorianus - the odd one out of the Saccharomyces genus
Lagers currently account for more than 90% of the global beer market (www.statista.com/statis- tics/270275/worldwide-beer-production/). They are typically fermented at a lower temperature (8°C to 15°C), after which a period of cold storage (i.e. lagering, a traditional practice vital for sensorial quality) is performed. It is believed that the lager beer production process originally was introduced in the fifteenth century in Bavaria (Germany), when brewing became legally restricted to wintertime (at colder temperatures) to minimize the microbial spoilage of Bavarian beers. Later, the advent of refrigeration in the nineteenth century enabled lager brewing throughout the whole year (Gibson and Liti, 2014; Kodama et al., 2006; Querol and Bond, 2009). Because of the high appreciation of this type of beer, it quickly spread around the globe. The yeasts used in this practice typically sink to the bottom (and do not rise to the top) of the fermentation vessel towards the end of the fermentation, and are therefore often called ‘bottom-fermenting yeasts'. Interestingly, this bottom-fermenting phenotype was described very quickly after the dawn of lager beer brewing in Nuremberg, a town in the state of Bavaria (Bond, 2009; Gibson and Liti, 2014; Hornsey, 2003; Kodama et al., 2006; Smart, 2007). In the nineteenth century, groundbreaking work by Louis Pasteur established that this phenomenon was caused by yeasts (Barnett, 2000; Meussdoerffer, 2009; Pasteur, 1873, 1876; Rees, 1870), and the species name S. pastorianus was first coined by the German scientist Max Rees in 1870 as a tribute to Pasteur's work in the field. Emil Christian Hansen isolated shortly thereafter the first pure yeast cultures from lager beer fermentation during his work in the Carlsberg laboratory in Copenhagen. He classified the three isolated pure yeast lineages as separate species; one as S. pastorianus, one as Saccharomyces carlsbergenesis (‘Unterhefe Nr. 1'), and one as Saccharomyces monacensis (‘Unterhefe Nr. 2') (Barnett, 2000; Meussdoerffer, 2009; Regenberg and Hansen, 2001). After Hansen's findings, starter cultures became general practice in the breweries and bottom-fermenting yeast was classified as S. carlsbergensis, disregarding the earlier classification of bottom fermenting yeast as S. pastorianus by Max Rees. However, genetic analysis of the various isolates in 1985 showed that the type strains of S. carlsbergensis, S. monacensis, and S. pastorianus (the original isolates from Hansen) were almost identical, resulting in the reclassification of all bottom-fermenting yeasts to S. pastorianus (Bond, 2009; Gibson and Liti, 2014; Kodama et al., 2006; Polaina, 2002; Smart, 2007; Wendland, 2014). Furthermore, recent research has revealed that S. pastorianus is not a true species at all, but instead an interspecific hybrid of S. cerevisiae x Saccharomyces eubayanus (see following section and Chapter 4); nevertheless, in practice the species name S. pastorianus is still used to denote this lineage of interspecific hybrids. Because of its industrial importance, much research has been dedicated to the characterization of the lager yeast genome. This led to novel insights in the peculiar genome of this species, provided clues about its origin and shed light on some evolutionary processes that enabled this species to thrive in lager beer fermentation.