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.

This study describes a screening system for future brewing yeasts focusing on nonSaccharomyces yeasts. The aim was to find new yeast strains that can ferment beer wort into a respectable beer. Ten Torulaspora delbrueckii strains were put through the screening system, which included sugar utilization tests, hop resistance tests, ethanol resistance tests, polymerase chain reaction fingerprinting, propagation tests, amino acid catabolism and anabolism, phenolic off-flavour tests and trial fermentations. Trial fermentations were analysed for extract reduction, pH drop, yeast concentration in bulk fluid and fermentation by-products. All investigated strains were able to partly ferment wort sugars and showed high tolerance to hop compounds and ethanol. One of the investigated yeast strains fermented all the wort sugars and produced a respectable fruity flavour and a beer of average ethanol content with a high volatile flavour compound concentration. Two other strains could possibly be used for pre-fermentation as a bio-flavouring agent for beers that have been postfermented by Saccharomyces strains as a consequence of their low sugar utilization but good flavour-forming properties.

Hybridisation between species often leads to inviable or infertile offspring, yet examples of evolutionary successful interspecific hybrids have been reported in all kingdoms of life. However, many questions on the ecological circumstances and evolutionary aftermath of interspecific hybridisation remain unanswered. In this study, we sequenced and phenotyped a large set of interspecific yeast hybrids isolated from the brewing environments to uncover the influence of interspecific hybridisation in yeast adaptation and domestication. Our analyses demonstrate that several hybrids between Saccharomyces species originated and diversified in industrial environments by combining key traits of each parental species. Furthermore, post-hybridisation evolution within each hybrid lineage reflects sub-specialisation and adaptation to specific beer styles, a process that was accompanied by extensive chimerisation between subgenomes. Our results reveal how interspecific hybridisation provides an important evolutionary route that allows swift adaptation to novel environments.

Recently there has been increased interest in using non‐Saccharomyces yeasts to ferment beer. The worldwide growth of craft beer and microbreweries has revitalised the use of different yeast strains with a pronounced impact on aroma and flavour. Using non‐conventional yeast gives brewers a unique selling point to differentiate themselves. Belgian brewers have been very successful in using wild yeasts and mixed fermentations that often contain non‐Saccharomyces yeasts. Historically, ancient beers and beers produced before the domestication of commonly used Saccharomyces strains most likely included non‐Saccharomyces species. Given the renewed interest in using non‐Saccharomyces yeasts to brew traditional beers and their potential application to produce low‐alcohol or alcohol‐free beer, the fermentation and flavour characteristics of different species of non‐Saccharomyces pure culture yeast were screened for brewing potential (Brettanomyces anomalus and bruxellensis, Candida tropicalis and shehatae, Saccharomycodes ludwigii, Torulaspora delbrueckii, Pichia kluyveri, Zygosaccharomyces rouxii). Alcohol‐free beer is already industrially produced using S. ludwigii, a maltose‐negative species, which is a good example of the introduction of non‐Saccharomyces yeast to breweries. Overall, non‐Saccharomyces yeasts represent a large resource of biodiversity for the production of new beers and have the potential for wider application to other beverage and industrial applications. Almost all of the trials reviewed were conducted with varying fermentation parameters, which plays an important role in the outcome of the studies. To understand these impacts all trials were described with their major fermentation parameters. Copyright © 2016 The Institute of Brewing & Distilling