Traditional Norwegian Kveik Are a Genetically Distinct Group of Domesticated Saccharomyces cerevisiae Brewing Yeasts

Overview of the Saccharomyces cerevisiae population structure through the lens of 3,034 genomes

With the rise of high-throughput sequencing technologies, a holistic view of genetic variation within populations-through population genomics studies-appears feasible, although it remains an ongoing effort. Genetic variation arises from a diverse range of evolutionary forces, with mutation and recombination being key drivers in shaping genomes. Studying genetic variation within a population represents a crucial first step in understanding the relationship between genotype and phenotype and the evolutionary history of species. In this context, the budding yeast Saccharomyces cerevisiae has been at the forefront of population genomic studies. In addition, it has a complex history that involves adaptation to a wide range of wild and human-related ecological niches. Although to date more than 3,000 diverse isolates have been sequenced, there is currently a lack of a resource bringing together sequencing data and associated metadata for all sequenced isolates. To perform a comprehensive analysis of the population structure of S. cerevisiae, we collected genome sequencing data from 3,034 natural isolates and processed the data uniformly. We determined ploidy levels, identified single nucleotide polymorphisms (SNPs), small insertion-deletions (InDels), copy number variations (CNVs), and aneuploidies across the population, creating a publicly accessible resource for the yeast research community. Interestingly, we showed that this population captures ∼93% of the species diversity. Using neighbor-joining and Bayesian methods, we redefined the populations, revealing clustering patterns primarily based on ecological origin. This work represents a valuable resource for the community and efforts have been made to make it evolvable and integrable to future yeast population studies.

Survival of the probiotic strain Lacticaseibacillus paracasei subsp. paracasei F19 in high-hopped beers

This study aims to enhance understanding of probiotic lactic acid bacteria (LAB) survival in high-hopped beer formulations and their interactions with different yeasts and highlights the fermentation processes, microbial metabolism, and production of distinctive beer flavors. For this, this research used Lacticaseibacillus paracasei F19 (F19), Saccharomycodes ludwigii, and Saccharomyces cerevisiae strains US-05 (US-05) and Kveik (Kveik) for brewing. Bacterial and yeast cultures were prepared, fermented in wort, and analyzed in different hop concentrations (International Bitterness Units - IBU 0, 20, 40). Methods included physicochemical analysis, yeast and bacterial counts, RT-qPCR for gene expression, statistical analysis, and sensory evaluation by sommeliers following BJCP guidelines. Physicochemical analysis showed efficient fermentation across all hop concentrations (IBU 0, 20, 40), with decreasing SG and pH over time due to lactic acid bacteria and yeast metabolism. Higher hop levels (IBU 20 and 40) resulted in less acidic beer, indicating hop interference with bacterial activity. Yeast populations remained stable regardless of hop content, with Saccharomyces cerevisiae and Saccharomycodes ludwigii performing well. Probiotic strain F19 exhibited robust viability in all formulations. Sensory analysis favored higher hop content beers, suggesting consumer acceptance and potential health benefits of probiotic, high-hop beers. Higher hop content hindered sour beer production as only hop-free beers reached low pH levels. Probiotic strain F19 remained viable under high IBU formulations (20 and 40), with these being preferred by sommeliers using BJCP methodology. All yeast strains supported F19 survival. Further studies are needed on gastrointestinal resistance and clinical benefits.

Aroma component analysis by HS-SPME/GC-MS to characterize Lager, Ale, and sour beer styles

The world of beer is a rich tapestry woven with diverse styles, each with its unique character. Lager, known for its crispness, ferments at lower temperatures, while ale, at warmer ones, boasts a wide spectrum of aromas. Belgian beers dazzle with their complexity, from fruity Trappist ales to sour lambics. German wheat beers, like hefeweizens, charm with their effervescence and fruity undertones. India Pale Ales (IPAs) showcase a hoppy burst, while sour ales tantalize with their tanginess. Craftsmanship, history, and regional ingredients intertwine in this world of brewing, offering aficionados an array of delightful experiences. Research on craft beer aromas is limited, and molecular fingerprint could be crucial. To date, there have been no studies focused on characterizing compound profiles to differentiate beer styles. The Headspace Solid Phase Microextraction (HS-SPME) method provides a rapid and solvent-free approach to volatile compound. The present study aims to characterize the aroma profile of a wide range of beers by using HS-SPME/GC-MS technique combined with multivariate data processing. A total of 120 beer samples were collected and divided into five categories: Pilsen (n = 28); Lager (n = 23); Ale (n = 32); Sour (n = 24); and Belgian Ales (n = 13). Among the Pilsen beers, 18 unique compounds were found for beers with hop extract and hops, and 2 for beers with hop extract (Octyl acetate; and alpha-Terpineol). When comparing the remaining groups to each other, Belgian beers exhibited 5 unique compounds, and Lagers had one (nonanal). Sours and Ales did not have unique compounds but shared 2 distinct compounds with the Belgian group each. We concluded that Belgian beers are the most complex in terms of various aroma-related compounds, and that it is possible to distinguish beers that use pure hops from hop extract.

European farmhouse brewing yeasts form a distinct genetic group

The brewing industry is constantly evolving, driven by the quest for novel flavours and fermentation characteristics that cater to evolving consumer preferences. This study explores the genetic and phenotypic diversity of European farmhouse yeasts, traditionally used in rural brewing practices and maintained outside of pure culture industrial yeast selection. We isolated landrace brewing yeast strains from diverse geographical locations across Europe, including Norway, Lithuania, Latvia, and Russia, and also included African farmhouse brewing strains from Ghana. Our genomic analysis using long-read and short-read whole genome sequencing uncovered a genetically distinct group that diverges from industrial brewing yeasts. This group, which is closely related to ale brewing strains, is preliminarily named the 'European Farmhouse' group and shows greater predicted admixture from Asian fermentation strains. Through genomic and phenotypic analyses, including flavour metabolite analysis via headspace gas chromatography-mass spectrometry, sugar metabolite analysis via high-performance liquid chromatography, and wort fermentation analysis, we found a broad spectrum of fermentation capabilities, from rapid and efficient fermentation to unique aroma and flavour compound profiles, potentially offering novel traits for brewing applications. This study highlights the importance of preservation of brewing cultural heritage knowledge and resources including yeast cultures. KEY POINTS: • A large set of geographically diverse farmhouse brewing strains were characterized • Norwegian and Baltic farmhouse brewing strains form a distinct genetic group • Farmhouse strains show considerable diversity in fermentation and flavour formation.

The origins of the Guinness stout yeast

Beer is made via the fermentation of an aqueous extract predominantly composed of malted barley flavoured with hops. The transforming microorganism is typically a single strain of Saccharomyces cerevisiae, and for the majority of major beer brands the yeast strain is a unique component. The present yeast used to make Guinness stout brewed in Dublin, Ireland, can be traced back to 1903, but its origins are unknown. To that end, we used Illumina and Nanopore sequencing to generate whole-genome sequencing data for a total of 22 S. cerevisiae yeast strains: 16 from the Guinness collection and 6 other historical Irish brewing. The origins of the Guinness yeast were determined with a SNP-based analysis, demonstrating that the Guinness strains occupy a distinct group separate from other historical Irish brewing yeasts. Assessment of chromosome number, copy number variation and phenotypic evaluation of key brewing attributes established Guinness yeast-specific SNPs but no specific chromosomal amplifications. Our analysis also demonstrated the effects of yeast storage on phylogeny. Altogether, our results suggest that the Guinness yeast used today is related to the first deposited Guinness yeast; the 1903 Watling Laboratory Guinness yeast.

The potential for Scotch Malt Whisky flavour diversification by yeast

Scotch Whisky, a product of high importance to Scotland, has gained global approval for its distinctive qualities derived from the traditional production process, which is defined in law. However, ongoing research continuously enhances Scotch Whisky production and is fostering a diversification of flavour profiles. To be classified as Scotch Whisky, the final spirit needs to retain the aroma and taste of 'Scotch'. While each production step contributes significantly to whisky flavour-from malt preparation and mashing to fermentation, distillation, and maturation-the impact of yeast during fermentation is crucially important. Not only does the yeast convert the sugar to alcohol, it also produces important volatile compounds, e.g. esters and higher alcohols, that contribute to the final flavour profile of whisky. The yeast chosen for whisky fermentations can significantly influence whisky flavour, so the yeast strain employed is of high importance. This review explores the role of yeast in Scotch Whisky production and its influence on flavour diversification. Furthermore, an extensive examination of nonconventional yeasts employed in brewing and winemaking is undertaken to assess their potential suitability for adoption as Scotch Whisky yeast strains, followed by a review of methods for evaluating new yeast strains.

Sour Beer as Bioreservoir of Novel Craft Ale Yeast Cultures

The increasing demand for craft beer is driving the search for novel ale yeast cultures from brewing-related wild environments. The focus of bioprospecting for craft cultures is to identify feral yeasts suitable to imprint unique sensorial attributes onto the final product. Here, we integrated phylogenetic, genotypic, genetic, and metabolomic techniques to demonstrate that sour beer during aging in wooden barrels is a source of suitable craft ale yeast candidates. In contrast to the traditional lambic beer maturation phase, during the aging of sour-matured production-style beer, different biotypes of Saccharomyces cerevisiae dominated the cultivable in-house mycobiota, which were followed by Pichia membranifaciens, Brettanomyces bruxellensis, and Brettanomyces anomalus. In addition, three putative S. cerevisiae × Saccharomyces uvarum hybrids were identified. S. cerevisiae feral strains sporulated, produced viable monosporic progenies, and had the STA1 gene downstream as a full-length promoter. During hopped wort fermentation, four S. cerevisiae strains and the S. cerevisiae × S. uvarum hybrid WY213 exceeded non-Saccharomyces strains in fermentative rate and ethanol production except for P. membranifaciens WY122. This strain consumed maltose after a long lag phase, in contrast to the phenotypic profile described for the species. According to the STA1+ genotype, S. cerevisiae partially consumed dextrin. Among the volatile organic compounds (VOCs) produced by S. cerevisiae and the S. cerevisiae × S. uvarum hybrid, phenylethyl alcohol, which has a fruit-like aroma, was the most prevalent. In conclusion, the strains characterized here have relevant brewing properties and are exploitable as indigenous craft beer starters.

Increased Rate of Yeast Cultivation from Packaged Beer with Environmentally Relevant Anaerobic Handling

Beer production necessitates oxygen exclusion for the proper packaging and aging of the beer. Standard operating procedures, including those for quality testing, involve culturing microbes from packaged beer exposed to atmospheric oxygen, despite the generalized fact that packaged beer is an anaerobic environment. Our research goal was to apply an environmentally relevant culturing approach to improve yeast cultivation from bottled beer by attempting to ameliorate transplant shock. This is applicable to uniquely scrutinous quality assurance/control objectives and/or to grand cultivation goals, such as ancient beer samples. Although yeasts have the genetic capacity of oxygen protection, their epigenetic/biochemical states within anaerobic packaging may not adequately protect all cells from reactive oxygen species (ROS) at the moment of opening. Soon after opening, beer yeasts were found to be catalase negative, indicating deficient protection from at least one ROS. The general reduction/inhibition of growth was observed when the beer yeast was exposed to ROS in media, and atmospheric bottle opening was found to expose beer yeast to significantly increased levels of ROS. Our primary finding is that different oxygen handling methodologies (aerobic/microaerophilic/anaerobic) significantly impact the viable Saccharomyces yeast recovery rates of Bamberger's Mahr's Bräu Unfiltered Lager. Immediate anaerobic handling improved cultivation success rates, with significantly higher colony forming units (CFU)/mL being cultured, and reduced the volume of beer required to recover viable yeast. Aerobic standard operating procedures have mainly been developed to harvest yeast on large volumetric samples and/or samples with high viable cell numbers, but these procedures may be suboptimal and may underrepresent potential viable cell numbers. IMPORTANCE Procedures of beer production and packaging exclude oxygen to create a shelf-stable anaerobic environment, within which any viable organisms are stored. However, standard methodologies to cultivate microbes from such environments generally include opening in an oxygenated atmosphere. This study applies environmentally relevant culturing methods and compares the yeast recovery rates of beers handled in various oxygen conditions. When beer bottles were opened in anoxic conditions, higher colony counts were obtained, so a smaller volume of beer was required to recover viable cells. The yeast in beer, stored anaerobically, may not be biochemically prepared to fully protect cells from oxygen at the moment of opening. Negative catalase activity showed beer yeasts' vulnerabilities to reactive oxygen. Atmospheric opening may reduce viability, causing the underreporting of viable cells. Anaerobic opening could increase the odds of successfully detecting/cultivating viable cell(s) that are present, which is pertinent to uniquely stringent quality screens and ambitious culturing attempts from rare samples.

Schizosaccharomyces pombe in the Brewing Process: Mixed-Culture Fermentation for More Complete Attenuation of High-Gravity Wort

High-gravity brewing is a method that maximises brewhouse capacity and reduces energy consumption per unit of beer produced. The fermentation of wort with high sugar content is known to impact the fermentation characteristics and production of aroma-active volatiles, and as such, cultures that are adapted to this method are industrially valuable. Mixed-culture fermentation offers brewers the opportunity to combine desirable features from multiple strains of yeast and to take advantage of the interactions between those strains. In this study, a highly attenuative strain of Schizosaccharomyces pombe is paired with a fast-fermenting brewing strain of Saccharomyces cerevisiae in the fermentation of wort at both standard and high gravity at centilitre scale. Mixed cultures were found to produce several esters and higher alcohols in higher concentration than in either of the parent monocultures at both standard and high gravity. The mixed culture also represented a compromise between fermentation length (modelled by the logistic equation), which was extended by the inclusion of S. pombe, and ethanol yield, which was increased. The application of mixed-culture strategies to high-gravity brewing practices may allow brewers greater flexibility in achieving desired flavour profiles whilst increasing brewhouse efficiency.

Microbes of traditional fermentation processes as synthetic biology chassis to tackle future food challenges

Microbial diversity is magnificent and essential to almost all life on Earth. Microbes are an essential part of every human, allowing us to utilize otherwise inaccessible resources. It is no surprise that humans started, initially unconsciously, domesticating microbes for food production: one may call this microbial domestication 1.0. Sourdough bread is just one of the miracles performed by microbial fermentation, allowing extraction of more nutrients from flour and at the same time creating a fluffy and delicious loaf. There are a broad range of products the production of which requires fermentation such as chocolate, cheese, coffee and vinegar. Eventually, with the rise of microscopy, humans became aware of microbial life. Today our knowledge and technological advances allow us to genetically engineer microbes - one may call this microbial domestication 2.0. Synthetic biology and microbial chassis adaptation allow us to tackle current and future food challenges. One of the most apparent challenges is the limited space on Earth available for agriculture and its major tolls on the environment through use of pesticides and the replacement of ecosystems with monocultures. Further challenges include transport and packaging, exacerbated by the 24/7 on-demand mentality of many customers. Synthetic biology already tackles multiple food challenges and will be able to tackle many future food challenges. In this perspective article, we highlight recent microbial synthetic biology research to address future food challenges. We further give a perspective on how synthetic biology tools may teach old microbes new tricks, and what standardized microbial domestication could look like.

Pseudo-Lager—Brewing with Lutra® Kveik Yeast

Brewers commonly produce ales since the ale yeast is more resilient, ferments quicker and requires higher temperatures, which are easier to ensure as opposed to lager and pilsner, which require lower temperatures and longer lagering time. However, Kveik yeasts are also resilient, ferment at fairly high temperatures (up to 35 °C), and can provide light, lager-like beers, but more quickly, in shorter lagering time, and with reduced off flavors. Diacetyl rest is not needed. The intention of this paper was to assess the possibility of producing pseudo-lager by using Lutra® Kveik. A batch (120 L) was divided into six fermenting vessels and inoculated with Lutra® yeast. To test its possibility to result in lager-like beer at higher temperature, we conducted fermentation at two temperatures (21 and 35 °C). Fermentation subjected to 21 °C lasted for 9 days, while at 35 °C, fermentation was finished in 2 days. After fermentation, both beers were stored in cold temperatures (4 °C) and then kegged, carbonized, and analyzed (pH, ethanol, polyphenols, color, bitterness, clarity). Alongside the sensory evaluation, a GC-MS analysis was also conducted in order to determine if there are any difference between the samples.

Kveik Brewing Yeasts Demonstrate Wide Flexibility in Beer Fermentation Temperature Tolerance and Exhibit Enhanced Trehalose Accumulation

Traditional Norwegian Farmhouse ale yeasts, also known as kveik, have captured the attention of the brewing community in recent years. Kveik were recently reported as fast fermenting thermo- and ethanol tolerant yeasts with the capacity to produce a variety of interesting flavor metabolites. They are a genetically distinct group of domesticated beer yeasts of admixed origin with one parent from the “Beer 1” clade and the other unknown. While kveik are known to ferment wort efficiently at warmer temperatures, their range of fermentation temperatures and corresponding fermentation efficiencies, remain uncharacterized. In addition, the characteristics responsible for their increased thermotolerance remain largely unknown. Here we demonstrate variation in kveik strains at a wide range of fermentation temperatures and show not all kveik strains are equal in fermentation performance and stress tolerance. Furthermore, we uncovered an increased capacity of kveik strains to accumulate intracellular trehalose, which likely contributes to their increased thermo- and ethanol tolerances. Taken together our results present a clearer picture of the future opportunities presented by Norwegian kveik yeasts and offer further insight into their applications in brewing.

Characteristics of New England India Pale Ale Beer Produced with the Use of Norwegian KVEIK Yeast

The aim of this research was to determine the potential of four unconventional Norwegian yeasts of the KVEIK type to produce NEIPA beer. The influence of yeast strains on fermentation process, physicochemical properties, antioxidant potential, volatile compounds, and sensory properties was investigated. The KVEIK-fermented beer did not differ in terms of physicochemical parameters from the beer produced with the commercial variants of US-05 yeast. The yeast strain influenced the sensory quality (taste and aroma) of the beers, with KVEIK-fermented beer rating significantly higher. The antioxidant activity of the tested beers also significantly depended on the yeast strain applied. The beers fermented with KVEIK had a significantly higher antioxidant potential (ABTS^•+) than those fermented with US-05. The strongest antioxidant activity was found in the beer brewed with the Lida KVEIK yeast. The use of KVEIK to produce NEIPA beer allowed enrichment of the finished products with volatile compounds isobutanol, 2-pentanol, 3-methylobutanol, ethyl octanoate, and ethyl decanoate.

Yeast Hybrids in Brewing

Microbiology has long been a keystone in fermentation, and innovative yeast molecular biotechnology continues to represent a fruitful frontier in brewing science. Consequently, modern understanding of brewer’s yeast has undergone significant refinement over the last few decades. This publication presents a condensed summation of Saccharomyces species dynamics with an emphasis on the relationship between; traditional Saccharomyces cerevisiae ale yeast, S. pastorianus interspecific hybrids used in lager production, and novel hybrid yeast progress. Moreover, introgression from other Saccharomyces species is briefly addressed. The unique history of Saccharomyces cerevisiae and Saccharomyces hybrids is exemplified by recent genomic sequencing studies aimed at categorizing brewing strains through phylogeny and redefining Saccharomyces species boundaries. Phylogenetic investigations highlight the genomic diversity of Saccharomyces cerevisiae ale strains long known to brewers for their fermentation characteristics and phenotypes. The discovery of genomic contributions from interspecific Saccharomyces species into the genome of S. cerevisiae strains is ever more apparent with increasing research investigating the hybrid nature of modern industrial and historical fermentation yeast.

A near-infrared optical nanosensor for measuring aerobic respiration in microbial systems

We developed a ratiometric oxygen-sensitive nanosensor and demonstrated application in monitoring metabolic oxygen consumption in microbial samples over time. Based on a near-infrared (NIR) emitting oxygen-quenched luminophore, platinum(II) octaethylporphine ketone (PtOEPK), along with a stable dioctadecyl dicarbocyanine reference dye (DiD), this nanosensor system provides an advantageous approach for overcoming imaging issues in biological systems, such as autofluorescence and optical scattering in the visible wavelength region. The dyes are encapsulated within a polymer-based nanoparticle matrix to maintain them at a constant ratio in biological samples, precluding the need for complex synthetic approaches. With this constant ratio of the two dyes, the nanosensor response can be measured as a ratio of their two signals, accounting for nanosensor concentration artifacts in measurements. The nanosensors are reversible, which enabled us to temporally monitor systems in which dissolved oxygen concentrations both increase and decrease. These sensors were applied for the monitoring of oxygen in samples of Saccharomyces cerevisiae (brewing yeast) in a 96-well optical fluorescence plate reader format over 60 h. By mixing the nanosensors directly into the sample well with the yeast, we were able to dynamically track metabolic activity changes over time due to varying cell concentration and exposure to an antimicrobial agent. This system could be a potential platform for high-throughput screening of various species or variants of microbes with unknown metabolic rates in response to external stimuli (antimicrobials, metabolites, etc.).

The Potential of Traditional Norwegian KVEIK Yeast for Brewing Novel Beer on the Example of Foreign Extra Stout

The development of craft brewing has spurred huge interest in unusual and traditional technologies and ingredients allowing the production of beers that would fulfil consumers' growing demands. In this study, we evaluated the brewing performance of traditional Norwegian KVEIK yeast during the production of Foreign Extra Stout beer. The content of alcohol of the KVEIK-fermented beer was 5.11-5.58% v/v, the extract content was 5.05-6.66% w/w, and the pH value was 4.53-4.83. The KVEIK yeast was able to completely consume maltose and maltotriose. The mean concentration of glycerol in KVEIK-fermented beers was higher than in the control sample (1.51 g/L vs. 1.12 g/L, respectively). The use of KVEIK-type yeast can offer a viable method for increasing the concentration of phenolic compounds in beer and for boosting its antioxidative potential. The beers produced with KVEIK-type yeast had a total phenol content of 446.9-598.7 mg GAE/L, exhibited antioxidative potential of 0.63-1.08 mM TE/L in the DPPH^• assay and 3.85-5.16 mM TE/L in the ABTS^•+ assay, and showed a ferric ion reducing capacity (FRAP) of 3.54-4.14 mM TE/L. The KVEIK-fermented bears contained various levels of volatile compounds (lower or higher depending on the yeast strain) and especially of higher alcohols, such as 3-metylobutanol, 2-metylobutanol, and 1-propanol, or ethyl esters, such as ethyl acetate or decanoate, compared to the control beers. In addition, they featured a richer fruity aroma (apricot, dried fruit, apples) than the control beers fermented with a commercial US-05 strain.

Characterization of the Fermentation and Sensory Profiles of Novel Yeast-Fermented Acid Whey Beverages

Acid whey is a by-product generated in large quantities during dairy processing, and is characterized by its low pH and high chemical oxygen demand. Due to a lack of reliable disposal pathways, acid whey currently presents a major sustainability challenge to the dairy industry. The study presented in this paper proposes a solution to this issue by transforming yogurt acid whey (YAW) into potentially palatable and marketable beverages through yeast fermentation. In this study, five prototypes were developed and fermented by Kluyveromyces marxianus, Brettanomyces bruxellensis, Brettanomyces claussenii, Saccharomyces cerevisiae (strain: Hornindal kveik), and IOC Be Fruits (IOCBF) S. cerevisiae, respectively. Their fermentation profiles were characterized by changes in density, pH, cell count, and concentrations of ethanol and organic acids. The prototypes were also evaluated on 26 sensory attributes, which were generated through a training session with 14 participants. While S. cerevisiae (IOCBF) underwent the fastest fermentation (8 days) and B. claussenii the slowest (21 days), K. marxianus and S. cerevisiae (Hornindal kveik) showed similar fermentation rates, finishing on day 20. The change in pH of the fermentate was similar for all five strains (from around 4.45 to between 4.25 and 4.31). Cell counts remained stable throughout the fermentation for all five strains (at around 6 log colony-forming units (CFU)/mL) except in the case of S. cerevisiae (Hornindal kveik), which ultimately decreased by 1.63 log CFU/mL. B. bruxellensis was the only strain unable to utilize all of the sugars in the substrate, with residual galactose remaining after fermentation. While both S. cerevisiae (IOCBF)- and B. claussenii-fermented samples were characterized by a fruity apple aroma, the former also had an aroma characteristic of lactic acid, dairy products, bakeries and yeast. A chemical odor characteristic of petroleum, gasoline or solvents, was perceived in samples fermented by B. bruxellensis and K. marxianus. An aroma of poorly aged or rancid cheese or milk also resulted from B. bruxellensis fermentation. In terms of appearance and mouthfeel, the S. cerevisiae (IOCBF)-fermented sample was rated the cloudiest, with the heaviest body. This study provides a toolkit for product development in a potential dairy-based category of fermented alcoholic beverages, which can increase revenue for the dairy industry by upcycling the common waste product YAW.

[Non-conventional yeasts as tools for innovation and differentiation in brewing]

Yeasts play a crucial role in brewing. During fermentation, besides ethanol and carbon dioxide, yeasts produce a considerable number of organic compounds, which are essential for beer flavor. In particular, Saccharomyces cerevisiae and Saccharomyces pastorianus are traditionally used in the production of ale and lager beers, respectively. Nowadays, the continuous growth of the craft beer market motivates the production of differential and innovative beers; leading specialists and brewers focus on non-conventional yeasts as tools for new product development. In this work, we describe the potential application of non-conventional yeast species such as those of the genera Brettanomyces, Torulaspora, Lachancea, Wickerhamomyces, Pichia and Mrakia in the craft brewing industry, as well as non-traditional brewing yeasts of the Saccharomyces genus. Furthermore, the fermentation conditions of these non-conventional yeasts are discussed, along with their abilities to assimilate and metabolize diverse wort components providing differential characteristics to the final product. In summary, we present a comprehensive review of the state-of-the-art of non-conventional yeasts, which is highly relevant for their application in the production of novel craft beers including flavored beers, non-alcoholic beers, low-calorie beers and functional beers.

Never Change a Brewing Yeast? Why Not, There Are Plenty to Choose From

Fermented foods and particularly beer have accompanied the development of human civilization for thousands of years. Saccharomyces cerevisiae, the dominant yeast in the production of alcoholic beverages, probably co-evolved with human activity. Considering that alcoholic fermentations emerged worldwide, the number of strains used in beer production nowadays is surprisingly low. Thus, the genetic diversity is often limited. This is among others related to the switch from a household brewing style to a more artisan brewing regime during the sixteenth century and latterly the development of single yeast isolation techniques at the Carlsberg Research Laboratory in 1883, resulting in process optimizations in the brewing industry. However, due to fierce competition within the beer market and the increasing demand for novel beer styles, diversification is becoming increasingly important. Moreover, the emergence of craft brewing has influenced big breweries to rediscover yeast as a significant contributor to a beer's aroma profile and realize that there is still room for innovation in the fermentation process. Here, we aim at giving a brief overview on how currently used S. cerevisiae brewing yeasts emerged and comment on the rationale behind replacing them with novel strains. We will present potential sources of yeasts that have not only been used in beer brewing before, including natural sources and sources linked to human activity but also an overlooked source, such as yeast culture collections. We will briefly comment on common yeast isolation techniques and finally touch on additional challenges for the brewing industry in replacing their current brewer's yeasts.

Wild Yeast for the Future: Exploring the Use of Wild Strains for Wine and Beer Fermentation

The continuous usage of single Saccharomyces cerevisiae strains as starter cultures in fermentation led to the domestication and propagation of highly specialized strains in fermentation, resulting in the standardization of wines and beers. In this way, hundreds of commercial strains have been developed to satisfy producers’ and consumers’ demands, including beverages with high/low ethanol content, nutrient deprivation tolerance, diverse aromatic profiles, and fast fermentations. However, studies in the last 20 years have demonstrated that the genetic and phenotypic diversity in commercial S. cerevisiae strains is low. This lack of diversity limits alternative wines and beers, stressing the need to explore new genetic resources to differentiate each fermentation product. In this sense, wild strains harbor a higher than thought genetic and phenotypic diversity, representing a feasible option to generate new fermentative beverages. Numerous recent studies have identified alleles in wild strains that could favor phenotypes of interest, such as nitrogen consumption, tolerance to cold or high temperatures, and the production of metabolites, such as glycerol and aroma compounds. Here, we review the recent literature on the use of commercial and wild S. cerevisiae strains in wine and beer fermentation, providing molecular evidence of the advantages of using wild strains for the generation of improved genetic stocks for the industry according to the product style.

Genetic Engineering and Synthetic Genomics in Yeast to Understand Life and Boost Biotechnology

The field of genetic engineering was born in 1973 with the "construction of biologically functional bacterial plasmids in vitro". Since then, a vast number of technologies have been developed allowing large-scale reading and writing of DNA, as well as tools for complex modifications and alterations of the genetic code. Natural genomes can be seen as software version 1.0; synthetic genomics aims to rewrite this software with "build to understand" and "build to apply" philosophies. One of the predominant model organisms is the baker's yeast Saccharomyces cerevisiae. Its importance ranges from ancient biotechnologies such as baking and brewing, to high-end valuable compound synthesis on industrial scales. This tiny sugar fungus contributed greatly to enabling humankind to reach its current development status. This review discusses recent developments in the field of genetic engineering for budding yeast S. cerevisiae, and its application in biotechnology. The article highlights advances from Sc1.0 to the developments in synthetic genomics paving the way towards Sc2.0. With the synthetic genome of Sc2.0 nearing completion, the article also aims to propose perspectives for potential Sc3.0 and subsequent versions as well as its implications for basic and applied research.

Sourdough cultures as reservoirs of maltose-negative yeasts for low-alcohol beer brewing

De novo sourdough cultures were here assessed for their potential as sources of yeast strains for low-alcohol beer brewing. NGS analysis revealed an abundance of ascomycete yeasts, with some influence of grain type on fungal community composition. Ten different ascomycete yeast species were isolated from different sourdough types (including wheat, rye, and barley) and seven of these were screened for a number of brewing-relevant phenotypes. All seven were maltose-negative and produced less than 1% (v/v) alcohol from a 12 °Plato wort in initial fermentation trials. Strains were further screened for their bioflavouring potential (production of volatile aromas and phenolic notes, reduction of wort aldehydes), stress tolerance (temperature extremes, osmotic stress and ethanol tolerance) and flocculence. Based on these criteria, two species (Kazachstania servazzii and Pichia fermentans) were selected for 10 L-scale fermentation trials and sensory analysis of beers. The latter species was considered particularly suitable for production of low-alcohol wheat beers due to its production of the spice/clove aroma 4-vinylguaiacol, while the former showed potential for lager-style beers due to its clean flavour profile and tolerance to low temperature conditions.

Effect of the Ala234Asp replacement in mitochondrial branched-chain amino acid aminotransferase on the production of BCAAs and fusel alcohols in yeast

In the yeast Saccharomyces cerevisiae, the mitochondrial branched-chain amino acid (BCAA) aminotransferase Bat1 plays an important role in the synthesis of BCAAs (valine, leucine, and isoleucine). Our upcoming study (Large et al. bioRχiv. 10.1101/2020.06.26.166157, Large et al. 2020) will show that the heterozygous tetraploid beer yeast strain, Wyeast 1056, which natively has a variant causing one amino acid substitution of Ala234Asp in Bat1 on one of the four chromosomes, produced higher levels of BCAA-derived fusel alcohols in the brewer's wort medium than a derived strain lacking this mutation. Here, we investigated the physiological role of the A234D variant Bat1 in S. cerevisiae. Both bat1∆ and bat1^A234D cells exhibited the same phenotypes relative to the wild-type Bat1 strain-namely, a repressive growth rate in the logarithmic phase; decreases in intracellular valine and leucine content in the logarithmic and stationary growth phases, respectively; an increase in fusel alcohol content in culture medium; and a decrease in the carbon dioxide productivity. These results indicate that amino acid change from Ala to Asp at position 234 led to a functional impairment of Bat1, although homology modeling suggests that Asp234 in the variant Bat1 did not inhibit enzymatic activity directly. KEY POINTS: • Yeast cells expressing Bat1^A234D exhibited a slower growth phenotype. • The Val and Leu levels were decreased in yeast cells expressing Bat1^A234D. • The A234D substitution causes a loss-of-function in Bat1. • The A234D substitution in Bat1 increased fusel alcohol production in yeast cells.

QTL analysis of natural Saccharomyces cerevisiae isolates reveals unique alleles involved in lignocellulosic inhibitor tolerance

Decoding the genetic basis of lignocellulosic inhibitor tolerance in Saccharomyces cerevisiae is crucial for rational engineering of bioethanol strains with enhanced robustness. The genetic diversity of natural strains present an invaluable resource for the exploration of complex traits of industrial importance from a pan-genomic perspective to complement the limited range of specialised, tolerant industrial strains. Natural S. cerevisiae isolates have lately garnered interest as a promising toolbox for engineering novel, genetically encoded tolerance phenotypes into commercial strains. To this end, we investigated the genetic basis for lignocellulosic inhibitor tolerance of natural S. cerevisiae isolates. A total of 12 quantitative trait loci underpinning tolerance were identified by next-generation sequencing linked bulk-segregant analysis of superior interbred pools. Our findings corroborate the current perspective of lignocellulosic inhibitor tolerance as a multigenic, complex trait. Apart from a core set of genetic variants required for inhibitor tolerance, an additional genetic background-specific response was observed. Functional analyses of the identified genetic loci revealed the uncharacterised ORF, YGL176C and the bud-site selection XRN1/BUD13 as potentially beneficial alleles contributing to tolerance to a complex lignocellulosic inhibitor mixture. We present evidence for the consideration of both regulatory and coding sequence variants for strain improvement.

Adaptive Laboratory Evolution of Ale and Lager Yeasts for Improved Brewing Efficiency and Beer Quality

Yeasts directly impact the efficiency of brewery fermentations as well as the character of the beers produced. In recent years, there has been renewed interest in yeast selection and development inspired by the demand to utilize resources more efficiently and the need to differentiate beers in a competitive market. Reviewed here are the different, non-genetically modified (GM) approaches that have been considered, including bioprospecting, hybridization, and adaptive laboratory evolution (ALE). Particular emphasis is placed on the latter, which represents an extension of the processes that have led to the domestication of strains already used in commercial breweries. ALE can be used to accentuate the positive traits of brewing yeast as well as temper some of the traits that are less desirable from a modern brewer's perspective. This method has the added advantage of being non-GM and therefore suitable for food and beverage production.

Histone chaperones and the Rrm3p helicase regulate flocculation in S. cerevisiae

Background

Biofilm formation or flocculation is a major phenotype in wild type budding yeasts but rarely seen in laboratory yeast strains. Here, we analysed flocculation phenotypes and the expression of FLO genes in laboratory strains with various genetic backgrounds.

Results

We show that mutations in histone chaperones, the helicase RRM3 and the Histone Deacetylase HDA1 de-repress the FLO genes and partially reconstitute flocculation. We demonstrate that the loss of repression correlates to elevated expression of several FLO genes, to increased acetylation of histones at the promoter of FLO1 and to variegated expression of FLO11. We show that these effects are related to the activity of CAF-1 at the replication forks. We also demonstrate that nitrogen starvation or inhibition of histone deacetylases do not produce flocculation in W303 and BY4742 strains but do so in strains compromised for chromatin maintenance. Finally, we correlate the de-repression of FLO genes to the loss of silencing at the subtelomeric and mating type gene loci.

Conclusions

We conclude that the deregulation of chromatin maintenance and transmission is sufficient to reconstitute flocculation in laboratory yeast strains. Consequently, we propose that a gain in epigenetic silencing is a major contributing factor for the loss of flocculation phenotypes in these strains. We suggest that flocculation in yeasts provides an excellent model for addressing the challenging issue of how epigenetic mechanisms contribute to evolution.

A Quasi-Domesticate Relic Hybrid Population of Saccharomyces cerevisiae × S. paradoxus Adapted to Olive Brine

The adaptation of the yeast Saccharomyces cerevisiae to man-made environments for the fermentation of foodstuffs and beverages illustrates the scientific, social, and economic relevance of microbe domestication. Here we address a yet unexplored aspect of S. cerevisiae domestication, that of the emergence of lineages harboring some domestication signatures but that do not fit completely in the archetype of a domesticated yeast, by studying S. cerevisiae strains associated with processed olives, namely table olives, olive brine, olive oil, and alpechin. We confirmed earlier observations that reported that the Olives population results from a hybridization between S. cerevisiae and S. paradoxus. We concluded that the olive hybrids form a monophyletic lineage and that the S. cerevisiae progenitor belonged to the wine population of this species. We propose that homoploid hybridization gave rise to a diploid hybrid genome, which subsequently underwent the loss of most of the S. paradoxus sub-genome. Such a massive loss of heterozygosity was probably driven by adaptation to the new niche. We observed that olive strains are more fit to grow and survive in olive brine than control S. cerevisiae wine strains and that they appear to be adapted to cope with the presence of NaCl in olive brine through expansion of copy number of ENA genes. We also investigated whether the S. paradoxus HXT alleles retained by the Olives population were likely to contribute to the observed superior ability of these strains to consume sugars in brine. Our experiments indicate that sugar consumption profiles in the presence of NaCl are different between members of the Olives and Wine populations and only when cells are cultivated in nutritional conditions that support adaptation of their proteome to the high salt environment, which suggests that the observed differences are due to a better overall fitness of olives strains in the presence of high NaCl concentrations. Although relic olive hybrids exhibit several characteristics of a domesticated lineage, tangible benefits to humans cannot be associated with their phenotypes. These strains can be seen as a case of adaptation without positive or negative consequences to humans, that we define as a quasi-domestication.

Bioprospecting for brewers: Exploiting natural diversity for naturally diverse beers

The burgeoning interest in archaic, traditional, and novel beer styles has coincided with a growing appreciation of the role of yeasts in determining beer character as well as a better understanding of the ecology and biogeography of yeasts. Multiple studies in recent years have highlighted the potential of wild Saccharomyces and non-Saccharomyces yeasts for production of beers with novel flavour profiles and other desirable properties. Yeasts isolated from spontaneously fermented beers as well as from other food systems (wine, bread, and kombucha) have shown promise for brewing application, and there is evidence that such cross-system transfers have occurred naturally in the past. We review here the available literature pertaining to the use of nonconventional yeasts in brewing, with a focus on the origins of these yeasts, including methods of isolation. Practical aspects of utilizing nondomesticated yeasts are discussed, and modern methods to facilitate discovery of yeasts with brewing potential are highlighted.

A Unique Saccharomyces cerevisiae × Saccharomyces uvarum Hybrid Isolated From Norwegian Farmhouse Beer: Characterization and Reconstruction

An unknown interspecies Saccharomyces hybrid, "Muri," was recently isolated from a "kveik" culture, a traditional Norwegian farmhouse brewing yeast culture (Preiss et al., 2018). Here we used whole genome sequencing to reveal the strain as an allodiploid Saccharomyces cerevisiae × Saccharomyces uvarum hybrid. Phylogenetic analysis of its sub-genomes revealed that the S. cerevisiae and S. uvarum parent strains of Muri appear to be most closely related to English ale and Central European cider and wine strains, respectively. We then performed phenotypic analysis on a number of brewing-relevant traits in a range of S. cerevisiae, S. uvarum and hybrid strains closely related to the Muri hybrid. The Muri strain possesses a range of industrially desirable phenotypic properties, including broad temperature tolerance, good ethanol tolerance, and efficient carbohydrate use, therefore making it an interesting candidate for not only brewing applications, but potentially various other industrial fermentations, such as biofuel production and distilling. We identified the two S. cerevisiae and S. uvarum strains that were genetically and phenotypically most similar to the Muri hybrid, and then attempted to reconstruct the Muri hybrid by generating de novo interspecific hybrids between these two strains. The de novo hybrids were compared with the original Muri hybrid, and many appeared phenotypically more similar to Muri than either of the parent strains. This study introduces a novel approach to studying hybrid strains and strain development by combining genomic and phenotypic analysis to identify closely related parent strains for construction of de novo hybrids.