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STRESS TOLERANCE IN GROUP 1 AND 2 LAGER BREWING STRAINS Pauline Chabert, Tobias Fischborn and Chris Powell ABSTRACT Saccharomyces pastorianus strains are natural hybrids of S. cerevisiae and S. bayanus yeast and display properties particularly suited to the production of lager type beers. While it is accepted that lager strains arose approximately 200 years ago following a mating reaction between these two species, recent evidence has suggested that such an event may have occurred more than once in the evolution of this yeast species. Recently it has been suggested that the S. pastorianus species may contain two subgroups, designated Saaz (Group 1) and Frohberg (Group 2), as a result of separate mating events. These groups have been broadly differentiated according to a number of criteria including genome rearrangements, gene copy number, DNA sequence polymorphisms and differences in ploidy. Despite detailed genetic analysis, only limited studies have previously been performed to characterize their phenotypic properties. The purpose of this study was to investigate the similarities and differences between the physiological characteristics of Saaz (Group 1) and Frohberg (Group 2) yeast. A number of strains belonging to each group were selected and assessed for their growth characteristics and their capacity to assimilate sugars. Furthermore, each strain was analyzed for its ability to withstand stress factors associated with the production of alcoholic beverages, including temperature, osmotic and oxidative stress. The data obtained and presented here indicates that there are some fundamental differences between the capacity of each yeast group to respond to their immediate environment. While this is particularly interesting for the understanding of lager brewing yeast strain variation, it may also be significant for associated industries, such as the biofuel industry, where investigation into the properties of industrial yeast strains is of particular importance. Furthermore, this analysis is significant for the active dried yeast industry, where robust brewing yeast strains are required to withstand stress factors associated with desiccation, in addition to favourable properties during fermentation. Top