A new hypothesis for the origin of the lager yeast Saccharomyces pastorianus.

Saccharomyces pastorianus, which is responsible for the production of bottom-fermented lager beer, is a hybrid species that arose from the mating of the top-fermenting ale yeast Saccharomyces cerevisiae and the cold-tolerant Saccharomyces eubayanus around the start of the 17th century. Based on detailed analysis of Central European brewing records, we propose that the critical event for the hybridization was the introduction of top-fermenting S. cerevisiae into an environment where S. eubayanus was present, rather than the other way around. Bottom fermentation in parts of Bavaria preceded the proposed hybridization date by a couple of hundred years and we suggest that this was carried out by mixtures of yeasts, which may have included S. eubayanus. A plausible case can be made that the S. cerevisiae parent came either from the Schwarzach wheat brewery or the city of Einbeck, and the formation of S. pastorianus happened in the Munich Hofbräuhaus between 1602 and 1615 when both wheat beer and lager were brewed contemporaneously. We also describe how the distribution of strains from the Munich Spaten brewery, and the development by Hansen and Linder of methods for producing pure starter cultures, facilitated the global spread of the Bavarian S. pastorianus lineages.

Identification by comparative transcriptomics of core regulatory genes for higher alcohol production in a top-fermenting yeast at different temperatures in beer fermentation.

Undesirable flavor caused by excessive higher alcohols restrains the development of the wheat beer industry. To clarify the regulation mechanism of the metabolism of higher alcohols in wheat beer brewing by the top-fermenting yeast Saccharomyces cerevisiae S17, the effect of temperature on the fermentation performance and transcriptional levels of relevant genes was investigated. The strain S17 produced 297.85 mg/L of higher alcohols at 20 °C, and the production did not increase at 25 °C, reaching about 297.43 mg/L. Metabolite analysis and transcriptome sequencing showed that the metabolic pathways of branched-chain amino acids, pyruvate, phenylalanine, and proline were the decisive factors that affected the formation of higher alcohols. Fourteen most promising genes were selected to evaluate the effects of single-gene deletions on the synthesis of higher alcohols. The total production of higher alcohols by the mutants Δtir1 and Δgap1 was reduced by 23.5 and 19.66% compared with the parent strain S17, respectively. The results confirmed that TIR1 and GAP1 are crucial regulatory genes in the metabolism of higher alcohols in the top-fermenting yeast. This study provides valuable knowledge on the metabolic pathways of higher alcohols and new strategies for reducing the amounts of higher alcohols in wheat beer.

Distinct Domestication Trajectories in Top-Fermenting Beer Yeasts and Wine Yeasts.

Beer is one of the oldest alcoholic beverages and is produced by the fermentation of sugars derived from starches present in cereal grains. Contrary to lager beers, made by bottom-fermenting strains of Saccharomyces pastorianus, a hybrid yeast, ale beers are closer to the ancient beer type and are fermented by S. cerevisiae, a top-fermenting yeast. Here, we use population genomics to investigate (1) the closest relatives of top-fermenting beer yeasts; (2) whether top-fermenting yeasts represent an independent domestication event separate from those already described; (3) whether single or multiple beer yeast domestication events can be inferred; and (4) whether top-fermenting yeasts represent non-recombinant or recombinant lineages. Our results revealed that top-fermenting beer yeasts are polyphyletic, with a main clade composed of at least three subgroups, dominantly represented by the German, British, and wheat beer strains. Other beer strains were phylogenetically close to sake, wine, or bread yeasts. We detected genetic signatures of beer yeast domestication by investigating genes previously linked to brewing and using genome-wide scans. We propose that the emergence of the main clade of beer yeasts is related with a domestication event distinct from the previously known cases of wine and sake yeast domestication. The nucleotide diversity of the main beer clade more than doubled that of wine yeasts, which might be a consequence of fundamental differences in the modes of beer and wine yeast domestication. The higher diversity of beer strains could be due to the more intense and different selection regimes associated to brewing.