pastorianus

Pastorianus is the yeast most commonly used to ferment lager beers. Its scientific name is Saccharomyces pastorianus, a natural hybrid of Saccharomyces cerevisiae and Saccharomyces eubayanus. This hybrid background gives lager yeast a combination of traits from both parents, including the ability to ferment at lower temperatures and to produce a clean, balanced flavor profile. The organism was historically described as Saccharomyces carlsbergensis, a name linked to its isolation at the Carlsberg Laboratory.

Historically, pastorianus strains were isolated in the late 19th century by Emil Christian Hansen during studies of beer fermentation at Carlsberg. The strains were found to be distinct from top-fermenting ale yeasts and were later recognized as a separate species. In modern taxonomy, many commercial strains are referred to as Saccharomyces pastorianus, with Saccharomyces carlsbergensis treated as a synonym or historical name. Genetic analyses have identified two major lineages within S. pastorianus, commonly described as group 1 and group 2, reflecting differences in genetics, ploidy, and fermentation behavior.

In fermentation, pastorianus strains are usually bottom-fermenting and thrive at cooler temperatures, typically in the range of about 7–13°C (45–55°F). They tend to produce fewer fruity esters than ale yeasts, yielding the clean, crisp flavors characteristic of most lagers. Maltose and maltotriose are fermentable by many strains, though maltotriose utilization can vary; flocculation and attenuation levels also differ among strains, influencing fermentor performance and beer profile.

As the backbone of modern lager brewing, pastorianus remains central to many beer styles worldwide, with ongoing strain development and hybridization aimed at improving flavor, attenuation, and cold-fermentation stability.