Saccharomyces pastorianus is a yeast used industrially for the production of lager beer, and was named in honour of Louis Pasteur by the German Max Reess in 1870. This yeast's complicated genome appears to be the result of hybridisation between two pure species in the Saccharomyces species complex, a factor that led to difficulty in establishing a proper taxonomy of the species.
The now-defunct synonym Saccharomyces carlsbergensis was and continues to be used in scientific literature, but is invalid, as the name Saccharomyces pastorianus (Reess 1870) has taxonomic precedence. The name S. carlsbergensis is typically attributed to Emil Christian Hansen from the era when he worked for the Danish brewery Carlsberg in 1883, but in actuality it was not officially described by Hansen as a distinct species until 1908, along with another synonym, Saccharomyces monacensis. The type strains of both synonyms are currently stored in yeast banks under the taxonomic name S. pastorianus.
So-called bottom fermenting strains of brewing yeast were described as early as the 14th century in Nuremberg and have remained an indispensable part of both Franconian and Bavarian brewing culture in southern Germany through modern times. During the explosion of scientific mycological studies in the 19th century, the yeast responsible for producing these so-called “bottom fermentations” was finally given a taxonomical classification, Saccharomyces pastorianus, by the German Max Reess in 1870.
In 1883 the Dane Emil Hansen published the findings of his research at the Carlsberg brewery in Copenhagen and described the isolation of a favourable pure yeast culture that he labeled “Unterhefe Nr. I” (bottom-fermenting yeast no. 1), a culture that he identified as identical to the sample originally donated to Carlsberg in 1845 by the Spaten Brewery of Munich. This yeast soon went into industrial production in Copenhagen in 1884 as Carlsberg yeast no. 1.
In 1904 Hansen published an important body of work where he reclassified the separate yeasts he worked with in terms of species, rather than as races or strains of the same species as he had previously done. Here Hansen classified a separate species of yeast isolated from the Carlsberg brewery as S. pastorianus, a name derived from and attributed to Reess 1870. This strain was admitted to the Centraalbureau voor Schimmelcultures (CBS) in 1935 as strain CBS 1538, Saccharomyces pastorianus Reess ex Hansen 1904. In a further publication in 1908, Hansen reclassified the original “Unterhefe Nr. I” as the new species Saccharomyces carlsbergensis and another yeast “Unterhefe Nr. II” as the new species Saccharomyces monacensis. The taxonomy was attributed to Hansen 1908 and the yeasts entered into the Centraalbureau voor Schimmelcultures in 1947 as CBS 1513 and CBS 1503 respectively.