Adaptive evolution of the lager brewing yeastSaccharomyces pastorianusfor improved growth under hyperosmotic conditions and its influence on fermentation performance

Directed Evolution of Microbial Communities in Fermented Foods: Strategies, Mechanisms, and Challenges

Directed Evolution of Microbial Communities (DEMC) offers a promising approach to enhance the functional attributes of microbial consortia in fermented foods by mimicking natural selection processes. This review details the application of DEMC in fermented foods, focusing on optimizing community traits to improve both fermentation efficiency and the sensory quality of the final products. We outline the core techniques used in DEMC, including the strategic construction of initial microbial communities, the systematic introduction of stress factors to induce desirable traits, and the use of artificial selection to cultivate superior communities. Additionally, we explore the integration of genomic tools and dynamic community analysis to understand and guide the evolutionary trajectories of these communities. While DEMC shows substantial potential for refining fermented food products, it faces challenges such as maintaining genetic diversity and functional stability of the communities. Looking ahead, the integration of advanced omics technologies and computational modeling is anticipated to significantly enhance the predictability and control of microbial community evolution in food fermentation processes. By systematically improving the selection and management of microbial traits, DEMC serves as a crucial tool for enhancing the quality and consistency of fermented foods, directly contributing to more robust and efficient food production systems.

Evaluation of Stress Tolerance and Fermentation Performance in Commercial Yeast Strains for Industrial Applications

This study evaluates the stress tolerance and metabolic adaptability of twelve yeast strains, including eleven commercial strains from Wyeast Laboratories and one prototrophic laboratory strain, under industrially relevant conditions. Yeast strains were assessed for their fermentation performance and stress responses under glucose limitation, osmotic stress, acid stress, elevated ethanol concentrations, and temperature fluctuations. Results revealed significant variability in glucose consumption, ethanol production, and stress tolerance across strains. ACY34 and ACY84 demonstrated the highest fermentation efficiency, while ACY19 exhibited exceptional stress resilience, excelling under multiple stress conditions such as osmotic and ethanol stress. The findings highlight strain-specific performance, with some strains suited for high-yield fermentation and others excelling under challenging environmental conditions. These results provide critical insights for selecting and optimizing yeast strains tailored to specific industrial fermentation processes, contributing to improved productivity and product quality in food and beverage production.

Recent Applications and Prospects of Enzymes in Quality and Safety Control of Fermented Foods

Fermented foods have gained global attention for their unique flavor and immense health benefits. These flavor compounds and nutrients result from the metabolic activities of microorganism during fermentation. However, some unpleasant sensory characteristics and biohazard substances could also be generated in fermentation process. These quality and safety issues in fermented foods could be addressed by endogenous enzymes. In this review, the applications of enzymes in quality control of fermented foods, including texture improvement, appearance stability, aroma enhancement, and debittering, are discussed. Furthermore, the enzymes employed in eliminating biohazard compounds such as ethyl carbamate, biogenic amines, and nitrites, formed during fermentation, are reviewed. Advanced biological methods used for enhancing the enzymatic activity and stability are also summarized. This review focused on the applications and future prospects of enzymes in the improvement quality and safety qualities of fermented foods.

Assess different fermentation characteristics of 54 lager yeasts based on group classification

Saccharomyces pastorianus, hybrids of Saccharomyces cerevisiae and Saccharomyces eubayanus, were generally regarded as authentic lager beer yeasts. In recent years, with more new findings of other Saccharomyces genus hybrids, yeasts used in lager beer brewing have been proved much more complicated than previous cognition. In this study, we analyzed the different fermentation characteristics of 54 yeast strains used for lager brewing in normal and very high gravity brewing based on group classification. The difference between Group Ⅰ and Group Ⅱ lager yeasts were more striking in very high gravity brewing. However, during our research progress, we realized that some yeasts used in this study were actually hybrids of S. cerevisiae and Saccharomyces kudriavzevii. Features of these hybrids could be beneficial to very high gravity brewing. We further discussed about the mechanism behind their outstanding characteristics and the reason why group classification methods of lager beer yeasts had limitations. Hybridization in yeasts is constantly getting richer. Lager yeasts could have more possibilities based on better understandings of their genetic background and roles of other Saccharomyces genus hybrids. Their heterosis shed light on innovation in brewing and other diverse fermentation industries.

Breeding of high-tolerance yeast by adaptive evolution and high-gravity brewing of mutant

BACKGROUND

Ethanol and osmotic stresses are the major limiting factors for brewing strong beer with high-gravity wort. Breeding of yeast strains with high osmotic and ethanol tolerance and studying very-high-gravity (VHG) brewing technology is of great significance for brewing strong beer.

RESULTS

This study used an optimized microbial microdroplet culture (MMC) system for adaptive laboratory evolution (ALE) of Saccharomyces cerevisiae YN81 to improve its tolerance to osmotic and ethanol stress. Meanwhile, we investigated the VHG and VHG with added ethanol (VHGAE) brewing processes for the evolved mutants in brewing strong beer. The results showed that three evolved mutants were obtained; among them, the growth performance of YN81mc-8.3 under 300, 340, 380, 420 and 460 g L-1 sucrose stresses was greater than that of the other strains. The ethanol tolerance of YN81mc-8.3 was 12%, which was 20% higher than that of YN81. During strong-beer brewing in a 100 L cylindrical cone-bottom tank, the sugar utilization and ethanol yield of YN81mc-8.3 outperformed those of YN81 in both the VHG and VHGAE brewing processes. Measurement of the diacetyl concentration showed that YN81mc-8.3 had a stronger diacetyl reduction ability; in particular, the real degree of fermentation of beers brewed by YN81mc-8.3 in VHG and VHGAE brewing processes was 75.35% and 66.71%, respectively - higher than those of the two samples brewed by YN81. Meanwhile, the visual, olfactive and gustative properties of the strong beer produced by YN81mc-8.3 were better than those of the other beers.

CONCLUSION

In this study, the mutant YN81mc-8.3 and the VHGAE brewing process were optimal and represented a better alternative strong-beer brewing process. © 2023 Society of Chemical Industry.

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.

Improving CRISPR-Cas9 mediated genome integration in interspecific hybrid yeasts

Saccharomyces pastorianus is not a classical taxon, it is an interspecific hybrid resulting from the cross of Saccharomyces cerevisiae and Saccharomyces eubayanus. Exhibiting heterosis for phenotypic traits such as wort α-oligosaccharide consumption and fermentation at low temperature, it has been domesticated to become the main workhorse of the brewing industry. Although CRISPR-Cas9 has been shown to be functional in S. pastorianus, repair of CRISPR- induced double strand break is unpredictable and preferentially uses the homoeologous chromosome as template, preventing targeted introduction of the desired repair construct. Here, we demonstrate that lager hybrids can be edited with near 100% efficiency at carefully selected landing sites on the chimeric SeScCHRIII. The landing sites were systematically selected and evaluated for (i) absence of loss of heterozygosity upon CRISPR-editing, (ii) efficiency of the gRNA, and (iii) absence of effect on strain physiology. Successful examples of highly efficient single and double gene integration illustrated that genome editing can be applied in interspecies hybrids, paving the way to a new impulse to lager yeast strain development. DATA AVAILABILITY: Data underlying graphs and figures found in this manuscript are deposited at the 4TU research dat center (https://data.4tu.nl/info/en/) and available through the doi: 10.4121/21648329.

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.

Screening and transcriptomic analysis of the ethanol-tolerant mutant Saccharomyces cerevisiae YN81 for high-gravity brewing

Ethanol stress is one of the major limiting factors for high-gravity brewing. Breeding of yeast strain with high ethanol tolerance, and revealing the ethanol tolerance mechanism of Saccharomyces cerevisiae is of great significance to the production of high-gravity beer. In this study, the mutant YN81 was obtained by ultraviolet-diethyl sulfate (UV-DES) cooperative mutagenesis from parental strain CS31 used in high-gravity craft beer brewing. The ethanol tolerance experiment results showed that cell growth and viability of YN81 were significantly greater than that of CS31 under ethanol stress. The ethanol tolerance mechanisms of YN81 were studied through observation of cell morphology, intracellular trehalose content, and transcriptomic analysis. Results from scanning electron microscope (SEM) showed alcohol toxicity caused significant changes in the cell morphology of CS31, while the cell morphology of YN81 changed slightly, indicating the cell morphology of CS31 got worse (the formation of hole and cell wrinkle). In addition, compared with ethanol-free stress, the trehalose content of YN81 and CS31 increased dramatically under ethanol stress, but there was no significant difference between YN81 and CS31, whether with or without ethanol stress. GO functional annotation analysis showed that under alcohol stress, the number of membrane-associated genes in YN81 was higher than that without alcohol stress, as well as CS31, while membrane-associated genes in YN81 were expressed more than CS31 under alcohol stress. KEGG functional enrichment analysis showed unsaturated fatty acid synthesis pathways and amino acid metabolic pathways were involved in ethanol tolerance of YN81. The mutant YN81 and its ethanol tolerance mechanism provide an optimal strain and theoretical basis for high-gravity craft beer brewing.

Characterization of Saccharomyces Strains Isolated from “Kéknyelű” Grape Must and Their Potential for Wine Production

Novel wine yeast strains have the potential to satisfy customer demand for new sensorial experiences and to ensure that wine producers have strains that can produce wine as efficiently as possible. In this respect, hybrid yeast strains have recently been the subject of intense research, as they are able to combine the favourable characteristics of both parental strains. In this study, two Saccharomyces “Kéknyelű” grape juice isolates were identified by species-specific PCR and PCR-RFLP methods and investigated with respect to their wine fermentation potential. Physiological characterization of the isolated strains was performed and included assessment of ethanol, sulphur dioxide, temperature and glucose (osmotic stress) tolerance, killer-toxin production, glucose fermentation ability at 16 °C and 24 °C, and laboratory-scale fermentation using sterile “Kéknyelű” must. Volatile components of the final product were studied by gas chromatography (GC) and mass spectrometry (MS). One isolate was identified as a S. cerevisiae × S. kudriavzevii hybrid and the other was S. cerevisiae. Both strains were characterized by high ethanol, sulphur dioxide and glucose tolerance, and the S. cerevisiae strain exhibited the killer phenotype. The hybrid isolate showed good glucose fermentation ability and achieved the lowest residual sugar content in wine. The ester production of the hybrid strain was high compared to the control S. cerevisiae starter strain, and this contributed to the fruity aroma of the wine. Both strains have good oenological characteristics, but only the hybrid yeast has the potential for use in wine fermentation.

Adaptive Laboratory Evolution of Yeasts for Aroma Compound Production

Aroma compounds are important in the food and beverage industry, as they contribute to the quality of fermented products. Yeasts produce several aroma compounds during fermentation. In recent decades, production of many aroma compounds by yeasts obtained through adaptive laboratory evolution has become prevalent, due to consumer demand for yeast strains in the industry. This review presents general aspects of yeast, aroma production and adaptive laboratory evolution and focuses on the recent advances of yeast strains obtained by adaptive laboratory evolution to enhance the production of aroma compounds.

Brewer's yeast polysaccharides - A review of their exquisite structural features and biomedical applications

Recent advances on brewer's yeast cell wall polysaccharides have unraveled exquisite structural features and diverse composition with (β1→3), (β1→6), (α1→4), (β1→4)-mix-linked glucans that are recognized to interact with different cell receptors and trigger specific biological responses. Herein, a comprehensive showcase of structure-biofunctional relationships between yeast polysaccharides and their biological targets is highlighted, with a focus on polysaccharide features that govern the biomedical activity. The insolubility of β-glucans is a crucial factor for binding and activation of Dectin-1 receptor, operating as adjuvants of immune responses. Contrarily, soluble low molecular weight β-glucans have a strong inhibition of reactive oxygen species production, acting as antagonists of Dectin-1 mediated signaling. Soluble glucan-protein moieties can also act as antitumoral agents. The balance between mannoproteins-TLR2 and β-glucans-Dectin-1 receptors-activation is crucial for osteogenesis. Biomedical applications value can also be obtained from yeast microcapsules as oral delivery systems, where highly branched (β1→6)-glucans lead to higher receptor affinity.

Potential for Lager Beer Production from Saccharomyces cerevisiae Strains Isolated from the Vineyard Environment

Saccharomyces pastorianus, genetic hybrids of Saccharomyces cerevisiae and the Saccharomyces eubayanus, is one of the most widely used lager yeasts in the brewing industry. In recent years, new strategies have been adopted and new lines of research have been outlined to create and expand the pool of lager brewing starters. The vineyard microbiome has received significant attention in the past few years due to many opportunities in terms of biotechnological applications in the winemaking processes. However, the characterization of S. cerevisiae strains isolated from winery environments as an approach to selecting starters for beer production has not been fully investigated, and little is currently available. Four wild cryotolerant S. cerevisiae strains isolated from vineyard environments were evaluated as potential starters for lager beer production at laboratory scale using a model beer wort (MBW). In all tests, the industrial lager brewing S. pastorianus Weihenstephan 34/70 was used as a reference strain. The results obtained, although preliminary, showed some good properties of these strains, such as antioxidant activity, flocculation capacity, efficient fermentation at 15 °C and low diacetyl production. Further studies will be carried out using these S. cerevisiae strains as starters for lager beer production on a pilot scale in order to verify the chemical and sensory characteristics of the beers produced.

Adaptive laboratory evolution principles and applications in industrial biotechnology

Adaptive laboratory evolution (ALE) is an innovative approach for the generation of evolved microbial strains with desired characteristics, by implementing the rules of natural selection as presented in the Darwinian Theory, on the laboratory bench. New as it might be, it has already been used by several researchers for the amelioration of a variety of characteristics of widely used microorganisms in biotechnology. ALE is used as a tool for the deeper understanding of the genetic and/or metabolic pathways of evolution. Another important field targeted by ALE is the manufacturing of products of (high) added value, such as ethanol, butanol and lipids. In the current review, we discuss the basic principles and techniques of ALE, and then we focus on studies where it has been applied to bacteria, fungi and microalgae, aiming to improve their performance to biotechnological procedures and/or inspect the genetic background of evolution. We conclude that ALE is a promising and efficacious method that has already led to the acquisition of useful new microbiological strains in biotechnology and could possibly offer even more interesting results in the future.

Packing a punch: understanding how flavours are produced in lager fermentations

Beer is one of the most popular beverages in the world and it has an irreplaceable place in culture. Although invented later than ale, lager beers dominate the current market. Many factors relating to the appearance (colour, clarity and foam stability) and sensory characters (flavour, taste and aroma) of beer, and other psychological determinants affect consumers' perception of the product and defines its drinkability. This review takes a wholistic approach to scrutinise flavour generation in the brewing process, focusing particularly on the contribution of the raw ingredients and the yeasts to the final flavour profiles of lager beers. In addition, we examine current developments to improve lager beer flavour profiles for the modern consumers.

Potential Applicability of Cocoa Pulp (Theobroma cacao L) as an Adjunct for Beer Production

The aim of this study was to evaluate the application of cocoa pulp as an adjunct for malt in beer production. The cocoa pulp was analyzed for humidity, proteins, lipids, sugars, total soluble solids, organic acids, and minerals. A study was carried out to reduce the cocoa pulp viscosity by enzymatic depectinization, making its use viable in beer production. The cocoa pulp showed relevant quantities of compounds important in fermentation, such as sugars, acids, and minerals. In fermentation using the adjunct, the proportions of pulp used were 10, 30, and 49%. A significant difference was found between the adjunct and all-malt worts. The 30% cocoa pulp concentration as an adjunct for malt in the fermentation medium contributed the most to the fermentative performance of the yeasts at both 15 and 22°C based on the consumption of apparent extract (°Plato), ethanol production, and cellular growth.

Potential application of yeasts from Ecuadorian chichas in controlled beer and chicha production

The potential of yeasts isolated from traditional chichas as starter cultures, either for controlled production of the native beverage or for industrial beer production, has been investigated. Three S. cerevisiae strains and one T. delbrueckii strain isolated from four different Ecuadorian chichas were compared to ale and lager beer strains with respect to fermentation performance, sugar utilisation, phenolic off-flavour production, flocculation and growth at low temperature. Fermentations were performed in 15 °P all-malt wort and in a model chicha substrate at 12 °C and 20 °C. Tall-tube fermentations (1.5 L) were also performed with both substrates to assess yeast performance and beer quality. Among the strains tested, only one Ecuadorian S. cerevisiae strain was able to ferment the wort sugars maltose and maltotriose. Fermentations with all Ecuadorian strains were poor in wort at 12 °C relative to 20 °C, but were similar in model chicha substrate at both temperatures. The aromatic profile was different between species and strains. These results indicate the potential of yeasts derived from traditional Andean fermented beverages for commercial applications. One of the chicha strains demonstrated traits typical of domesticated brewery strains and could be suitable for ale fermentation, while the other strains may have potential for low-alcohol beer or chicha production.

Genomic Adaptation of Saccharomyces Species to Industrial Environments

The budding yeast has been extensively studied for its physiological performance in fermentative environments and, due to its remarkable plasticity, is used in numerous industrial applications like in brewing, baking and wine fermentations. Furthermore, thanks to its small and relatively simple eukaryotic genome, the molecular mechanisms behind its evolution and domestication are more easily explored. Considerable work has been directed into examining the industrial adaptation processes that shaped the genotypes of species and hybrids belonging to the Saccharomyces group, specifically in relation to beverage fermentation performances. A variety of genetic mechanisms are responsible for the yeast response to stress conditions, such as genome duplication, chromosomal re-arrangements, hybridization and horizontal gene transfer, and these genetic alterations are also contributing to the diversity in the Saccharomyces industrial strains. Here, we review the recent genetic and evolutionary studies exploring domestication and biodiversity of yeast strains.

Unraveling Brazilian bioethanol yeasts as novel starters for high-gravity brewing

High-gravity (HG) brewing has broader application to succeed on beer differentiation and production optimization. However, such process imposes a handicap to yeasts, which must be able to deal with stressful conditions in fermentation. In this work, we assessed different physiological traits of 24 Saccharomyces cerevisiae strains isolated from Brazilian bioethanol distilleries for the selection of novel starters for HG brewing. Five yeast strains were selected with ability to overcome different stressors under HG beer fermentation, showing high fermentability rates, resilience to ethanol stress, low production of foam and hydrogen sulfide, as well as similar flocculation rates to brewer's yeasts. After five fermentation recycles, most strains sustained a viability rate higher than 90% and were able to efficiently accumulate trehalose and glycogen, besides presenting no detectable petite mutants at the final stage. In the sensory analysis, the beers obtained from the five selected strains showed greater aromatic complexity, with predominance of 'spicy', 'dried' and 'fresh fruits' descriptors. In conclusion, this study sheds light on the potential of yeast strains from Brazilian bioethanol process to produce distinctive specialty beers, aside from proposing an effective selection methodology based on relevant physiological attributes for HG brewing process.

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.

Adaptive Evolution of Industrial Brewer’s Yeast Strains towards a Snowflake Phenotype

Flocculation or cell aggregation is a well-appreciated characteristic of industrial brewer’s strains, since it allows removal of the cells from the beer in a cost-efficient and environmentally-friendly manner. However, many industrial strains are non-flocculent and genetic interference to increase the flocculation characteristics are not appreciated by the consumers. We applied adaptive laboratory evolution (ALE) to three non-flocculent, industrial Saccharomyces cerevisiae brewer’s strains using small continuous bioreactors (ministats) to obtain an aggregative phenotype, i.e., the “snowflake” phenotype. These aggregates could increase yeast sedimentation considerably. We evaluated the performance of these evolved strains and their produced flavor during lab scale beer fermentations. The small aggregates did not result in a premature sedimentation during the fermentation and did not result in major flavor changes of the produced beer. These results show that ALE could be used to increase the sedimentation behavior of non-flocculent brewer’s strains.

Designing New Yeasts for Craft Brewing: When Natural Biodiversity Meets Biotechnology

Beer is a fermented beverage with a history as old as human civilization. Ales and lagers are by far the most common beers; however, diversification is becoming increasingly important in the brewing market and the brewers are continuously interested in improving and extending the range of products, especially in the craft brewery sector. Fermentation is one of the widest spaces for innovation in the brewing process. Besides Saccharomyces cerevisiae ale and Saccharomyces pastorianus lager strains conventionally used in macro-breweries, there is an increasing demand for novel yeast starter cultures tailored for producing beer styles with diversified aroma profiles. Recently, four genetic engineering-free approaches expanded the genetic background and the phenotypic biodiversity of brewing yeasts and allowed novel costumed-designed starter cultures to be developed: (1) the research for new performant S. cerevisiae yeasts from fermented foods alternative to beer; (2) the creation of synthetic hybrids between S. cerevisiae and Saccharomyces non-cerevisiae in order to mimic lager yeasts; (3) the exploitation of evolutionary engineering approaches; (4) the usage of non-Saccharomyces yeasts. Here, we summarized the pro and contra of these approaches and provided an overview on the most recent advances on how brewing yeast genome evolved and domestication took place. The resulting correlation maps between genotypes and relevant brewing phenotypes can assist and further improve the search for novel craft beer starter yeasts, enhancing the portfolio of diversified products offered to the final customer.

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.

Diacetyl control during brewery fermentation via adaptive laboratory engineering of the lager yeast Saccharomyces pastorianus

Diacetyl contributes to the flavor profile of many fermented products. Its typical buttery flavor is considered as an off flavor in lager-style beers, and its removal has a major impact on time and energy expenditure in breweries. Here, we investigated the possibility of lowering beer diacetyl levels through evolutionary engineering of lager yeast for altered synthesis of α-acetolactate, the precursor of diacetyl. Cells were exposed repeatedly to a sub-lethal level of chlorsulfuron, which inhibits the acetohydroxy acid synthase responsible for α-acetolactate production. Initial screening of 7 adapted isolates showed a lower level of diacetyl during wort fermentation and no apparent negative influence on fermentation rate or alcohol yield. Pilot-scale fermentation was carried out with one isolate and results confirmed the positive effect of chlorsulfuron adaptation. Diacetyl levels were over 60% lower at the end of primary fermentation relative to the non-adapted lager yeast and no significant change in fermentation performance or volatile flavor profile was observed due to the adaptation. Whole-genome sequencing revealed a non-synonymous SNP in the ILV2 gene of the adapted isolate. This mutation is known to confer general tolerance to sulfonylurea compounds, and is the most likely cause of the improved tolerance. Adaptive laboratory evolution appears to be a natural, simple and cost-effective strategy for diacetyl control in brewing.

Selection of Pof-Saccharomyces eubayanus Variants for the Construction of S. cerevisiae × S. eubayanus Hybrids With Reduced 4-Vinyl Guaiacol Formation

Saccharomyces pastorianus is an interspecies hybrid between S. cerevisiae and S. eubayanus. The identification of the parental species of S. pastorianus enabled the de novo reconstruction of hybrids that could potentially combine a wide array of phenotypic traits. Lager yeasts are characterized by their inability to decarboxylate ferulic acid present in wort, a phenotype also known as Pof ^- (phenolic off-flavor). However, all known S. eubayanus strains characterized so far produce clove-like aroma specific of 4-vinyl guaiacol, a decarboxylated form of ferulic acid. This study explored a non-GMO approach to construct Pof ^-S. eubayanus variants derived from the parental strain S. eubayanus CBS 12357. To rapidly screen a population of UV-mutagenized cells two complementary assays were developed. The first assay was based on the difference of light absorption spectra of ferulic acid and 4-vinyl guaiacol, while the second was based on the difference of sensitivity of Pof ^- and Pof⁺ strains to cinnamic acid. The S. eubayanus variant HTSE042 was selected and was confirmed not to produce 4-vinyl guaiacol. Whole genome sequencing revealed that this variant lost the subtelomeric region of the CHRXIII right arm that carried the two clustered genes SePAD1- SeFDC1 whose deletion in a naïve S. eubayanus strain (CBS 12357/FM1318) resulted in an identical phenotype. Subsequently, the Pof ^- variant was crossed with a Pof^-S. cerevisiae partner. The resulting hybrid was not able to convert ferulic acid demonstrating the undisputable value of the mutagenized variant HTSE042 to eventually construct S. cerevisiae × S. eubayanus hybrids with phenotypic characteristics of S. pastorianus.

Incorporating comparative genomics into the design-test-learn cycle of microbial strain engineering

Engineering microbes with new properties is an important goal in industrial engineering, to establish biological factories for production of biofuels, commodity chemicals and pharmaceutics. But engineering microbes to produce new compounds with high yield remains a major challenge toward economically viable production. Incorporating several modern approaches, including synthetic and systems biology, metabolic modeling and regulatory rewiring, has proven to significantly advance industrial strain engineering. This review highlights how comparative genomics can also facilitate strain engineering, by identifying novel genes and pathways, regulatory mechanisms and genetic background effects for engineering. We discuss how incorporating comparative genomics into the design-test-learn cycle of strain engineering can provide novel information that complements other engineering strategies.

Enhanced Wort Fermentation with De Novo Lager Hybrids Adapted to High-Ethanol Environments

Interspecific hybridization is a valuable tool for developing and improving brewing yeast in a number of industry-relevant aspects. However, the genomes of newly formed hybrids can be unstable. Here, we exploited this trait by adapting four brewing yeast strains, three of which were de novo interspecific lager hybrids with different ploidy levels, to high ethanol concentrations in an attempt to generate variant strains with improved fermentation performance in high-gravity wort. Through a batch fermentation-based adaptation process and selection based on a two-step screening process, we obtained eight variant strains which we compared to the wild-type strains in 2-liter-scale wort fermentations replicating industrial conditions. The results revealed that the adapted variants outperformed the strains from which they were derived, and the majority also possessed several desirable brewing-relevant traits, such as increased ester formation and ethanol tolerance, as well as decreased diacetyl formation. The variants obtained from the polyploid hybrids appeared to show greater improvements in fermentation performance than those derived from diploid strains. Interestingly, it was not only the hybrid strains, but also the Saccharomyces cerevisiae parent strain, that appeared to adapt and showed considerable changes in genome size. Genome sequencing and ploidy analysis revealed that changes had occurred at both the chromosome and single nucleotide levels in all variants. Our study demonstrates the possibility of improving de novo lager yeast hybrids through adaptive evolution by generating stable and superior variants that possess traits relevant to industrial lager beer fermentation.IMPORTANCE Recent studies have shown that hybridization is a valuable tool for creating new and diverse strains of lager yeast. Adaptive evolution is another strain development tool that can be applied in order to improve upon desirable traits. Here, we apply adaptive evolution to newly created lager yeast hybrids by subjecting them to environments containing high ethanol levels. We isolated and characterized a number of adapted variants which possess improved fermentation properties and ethanol tolerance. Genome analysis revealed substantial changes in the variants compared to the original strains. These improved variant strains were produced without any genetic modification and are suitable for industrial lager beer fermentations.

Digital Image Analysis of Yeast Single Cells Growing in Two Different Oxygen Concentrations to Analyze the Population Growth and to Assist Individual-Based Modeling

Nowadays control of the growth of Saccharomyces to obtain biomass or cellular wall components is crucial for specific industrial applications. The general aim of this contribution is to deal with experimental data obtained from yeast cells and from yeast cultures to attempt the integration of the two levels of information, individual and population, to progress in the control of yeast biotechnological processes by means of the overall analysis of this set of experimental data, and to assist in the improvement of an individual-based model, namely, INDISIM-Saccha. Populations of S. cerevisiae growing in liquid batch culture, in aerobic and microaerophilic conditions, were studied. A set of digital images was taken during the population growth, and a protocol for the treatment and analyses of the images obtained was established. The piecewise linear model of Buchanan was adjusted to the temporal evolutions of the yeast populations to determine the kinetic parameters and changes of growth phases. In parallel, for all the yeast cells analyzed, values of direct morphological parameters, such as area, perimeter, major diameter, minor diameter, and derived ones, such as circularity and elongation, were obtained. Graphical and numerical methods from descriptive statistics were applied to these data to characterize the growth phases and the budding state of the yeast cells in both experimental conditions, and inferential statistical methods were used to compare the diverse groups of data achieved. Oxidative metabolism of yeast in a medium with oxygen available and low initial sugar concentration can be taken into account in order to obtain a greater number of cells or larger cells. Morphological parameters were analyzed statistically to identify which were the most useful for the discrimination of the different states, according to budding and/or growth phase, in aerobic and microaerophilic conditions. The use of the experimental data for subsequent modeling work was then discussed and compared to simulation results generated with INDISIM-Saccha, which allowed us to advance in the development of this yeast model, and illustrated the utility of data at different levels of observation and the needs and logic behind the development of a microbial individual-based model.

Enhancing the performance of brewing yeasts

Beer production is one of the oldest known traditional biotechnological processes, but is nowadays facing increasing demands not only for enhanced product quality, but also for improved production economics. Targeted genetic modification of a yeast strain is one way to increase beer quality and to improve the economics of beer production. In this review we will present current knowledge on traditional approaches for improving brewing strains and for rational metabolic engineering. These research efforts will, in the near future, lead to the development of a wider range of industrial strains that should increase the diversity of commercial beers.

Evolutionary Engineering in Chemostat Cultures for Improved Maltotriose Fermentation Kinetics in Saccharomyces pastorianus Lager Brewing Yeast

The lager brewing yeast Saccharomyces pastorianus, an interspecies hybrid of S. eubayanus and S. cerevisiae, ferments maltotriose, maltose, sucrose, glucose and fructose in wort to ethanol and carbon dioxide. Complete and timely conversion ("attenuation") of maltotriose by industrial S. pastorianus strains is a key requirement for process intensification. This study explores a new evolutionary engineering strategy for improving maltotriose fermentation kinetics. Prolonged carbon-limited, anaerobic chemostat cultivation of the reference strain S. pastorianus CBS1483 on a maltotriose-enriched sugar mixture was used to select for spontaneous mutants with improved affinity for maltotriose. Evolved populations exhibited an up to 5-fold lower residual maltotriose concentration and a higher ethanol concentration than the parental strain. Uptake studies with ¹⁴C-labeled sugars revealed an up to 4.75-fold higher transport capacity for maltotriose in evolved strains. In laboratory batch cultures on wort, evolved strains showed improved attenuation and higher ethanol concentrations. These improvements were also observed in pilot fermentations at 1,000-L scale with high-gravity wort. Although the evolved strain exhibited multiple chromosomal copy number changes, analysis of beer made from pilot fermentations showed no negative effects on flavor compound profiles. These results demonstrate the potential of evolutionary engineering for strain improvement of hybrid, alloploid brewing strains.

Cocoa pulp in beer production: Applicability and fermentative process performance

This work evaluated the effect of cocoa pulp as a malt adjunct on the parameters of fermentation for beer production on a pilot scale. For this purpose, yeast isolated from the spontaneous fermentation of cachaça (SC52), belonging to the strain bank of the State University of Feira de Santana-Ba (Brazil), and a commercial strain of ale yeast (Safale S-04 Belgium) were used. The beer produced was subjected to acceptance and purchase intention tests for sensorial analysis. At the beginning of fermentation, 30% cocoa pulp (adjunct) was added to the wort at 12°P concentration. The production of beer on a pilot scale was carried out in a bioreactor with a 100-liter capacity, a usable volume of 60 liters, a temperature of 22°C and a fermentation time of 96 hours. The fermentation parameters evaluated were consumption of fermentable sugars and production of ethanol, glycerol and esters. The beer produced using the adjunct and yeast SC52 showed better fermentation performance and better acceptance according to sensorial analysis.

Novel brewing yeast hybrids: creation and application

The natural interspecies Saccharomyces cerevisiae × Saccharomyces eubayanus hybrid yeast is responsible for global lager beer production and is one of the most important industrial microorganisms. Its success in the lager brewing environment is due to a combination of traits not commonly found in pure yeast species, principally low-temperature tolerance, and maltotriose utilization. Parental transgression is typical of hybrid organisms and has been exploited previously for, e.g., the production of wine yeast with beneficial properties. The parental strain S. eubayanus has only been discovered recently and newly created lager yeast strains have not yet been applied industrially. A number of reports attest to the feasibility of this approach and artificially created hybrids are likely to have a significant impact on the future of lager brewing. De novo S. cerevisiae × S. eubayanus hybrids outperform their parent strains in a number of respects, including, but not restricted to, fermentation rate, sugar utilization, stress tolerance, and aroma formation. Hybrid genome function and stability, as well as different techniques for generating hybrids and their relative merits are discussed. Hybridization not only offers the possibility of generating novel non-GM brewing yeast strains with unique properties, but is expected to aid in unraveling the complex evolutionary history of industrial lager yeast.

A large set of newly created interspecific Saccharomyces hybrids increases aromatic diversity in lager beers

Lager beer is the most consumed alcoholic beverage in the world. Its production process is marked by a fermentation conducted at low (8 to 15°C) temperatures and by the use of Saccharomyces pastorianus, an interspecific hybrid between Saccharomyces cerevisiae and the cold-tolerant Saccharomyces eubayanus. Recent whole-genome-sequencing efforts revealed that the currently available lager yeasts belong to one of only two archetypes, "Saaz" and "Frohberg." This limited genetic variation likely reflects that all lager yeasts descend from only two separate interspecific hybridization events, which may also explain the relatively limited aromatic diversity between the available lager beer yeasts compared to, for example, wine and ale beer yeasts. In this study, 31 novel interspecific yeast hybrids were developed, resulting from large-scale robot-assisted selection and breeding between carefully selected strains of S. cerevisiae (six strains) and S. eubayanus (two strains). Interestingly, many of the resulting hybrids showed a broader temperature tolerance than their parental strains and reference S. pastorianus yeasts. Moreover, they combined a high fermentation capacity with a desirable aroma profile in laboratory-scale lager beer fermentations, thereby successfully enriching the currently available lager yeast biodiversity. Pilot-scale trials further confirmed the industrial potential of these hybrids and identified one strain, hybrid H29, which combines a fast fermentation, high attenuation, and the production of a complex, desirable fruity aroma.

Effect of aerobic and microaerophilic culture in the growth dynamics of Saccharomyces cerevisiae and in training of quiescent and non-quiescent subpopulations

Saccharomyces cerevisiae is industrially the most important yeast, and its growth in different concentrations of oxygen can be used to improve various application processes. The aims of this work were to study in aerobic and microaerophilic growth conditions the cell size and tendency of morphological changes in S. cerevisiae in different stages of growth and to assess the effect of the two growth conditions in the differentiation of quiescent and non-quiescent subpopulations in the stationary phase. Dissolved oxygen levels in the culture medium for aerobic and microaerophilic conditions were 6.6 and 5.2 mg L(-1), respectively. In both growth conditions, similar viable cell populations were obtained, although in aerobic conditions the stationary phase was reached and the quiescent and non-quiescent subpopulations were also differentiated. The microaerophilic growth produced a significant reduction in the specific growth rate and consequently also in glucose and oxygen consumption. The most notable changes in cellular size and morphology occurred with the depletion of glucose and oxygen. The concentration of dissolved oxygen in the culture medium significantly modulated the growth kinetics of S. cerevisiae and their development and differentiation to quiescent cells. This could justify the need to readjust small variations in oxygen levels during yeast cultures in biotechnological processes.