Enological characterization of Spanish Saccharomyces kudriavzevii strains, one of the closest relatives to parental strains of winemaking and brewing Saccharomyces cerevisiae × S. kudriavzevii hybrids

Adaptive evolution in the Saccharomyces kudriavzevii Aro4p promoted a reduced production of higher alcohols

The use of unconventional yeast species in human-driven fermentations has attracted a lot of attention in the last few years. This tool allows the alcoholic beverage industries to solve problems related to climate change or the consumer demand for newer high-quality products. In this sense, one of the most attractive species is Saccharomyces kudriavzevii, which shows interesting fermentative traits such as the increased and diverse aroma compound production in wines. Specifically, it has been observed that different isolates of this species can produce higher amounts of higher alcohols such as phenylethanol compared with Saccharomyces cerevisiae. In this work, we have shed light on this feature relating it to the S. kudriavzevii aromatic amino acid anabolic pathway in which the enzyme Aro4p plays an essential role. Unexpectedly, we observed that the presence of the S. kudriavzevii ARO4 variant reduces phenylethanol production compared with the S. cerevisiae ARO4 allele. Our experiments suggest that this can be explained by increased feedback inhibition, which might be a consequence of the changes detected in the Aro4p amino end such as L₂₆ Q₂₄ that have been under positive selection in the S. kudriavzevii specie.

Wine Yeasts Selection: Laboratory Characterization and Protocol Review

Wine reflects the specificity of a terroir, including the native microbiota. In contrast to the use of Saccharomyces cerevisiae commercial starters, a way to maintain wines' microbial terroir identities, guaranteeing at the same time the predictability and reproducibility of the wines, is the selection of autochthonous Saccharomyces and non-Saccharomyces strains towards optimal enological characteristics for the chosen area of isolation. This field has been explored but there is a lack of a compendium covering the main methods to use. Autochthonous wine yeasts from different areas of Slovakia were identified and tested, in the form of colonies grown either on nutrient agar plates or in grape must micro-fermentations, for technological and qualitative enological characteristics. Based on the combined results, Saccharomyces cerevisiae PDA W 10, Lachancea thermotolerans 5-1-1 and Metschnikowia pulcherrima 125/14 were selected as potential wine starters. This paper, as a mixture of experimental and review contributions, provides a compendium of methods used to select autochthonous wine yeasts. Thanks to the presence of images, this compendium could guide other researchers in screening their own yeast strains for wine production.

An update on the diversity, ecology and biogeography of the Saccharomyces genus

Saccharomyces cerevisiae is the most extensively studied yeast and, over the last century, provided insights on the physiology, genetics, cellular biology and molecular mechanisms of eukaryotes. More recently, the increase in the discovery of wild strains, species and hybrids of the genus Saccharomyces has shifted the attention towards studies on genome evolution, ecology and biogeography, with the yeast becoming a model system for population genomic studies. The genus currently comprises eight species, some of clear industrial importance, while others are confined to natural environments, such as wild forests devoid from human domestication activities. To date, numerous studies showed that some Saccharomyces species form genetically diverged populations that are structured by geography, ecology or domestication activity and that the yeast species can also hybridize readily both in natural and domesticated environments. Much emphasis is now placed on the evolutionary process that drives phenotypic diversity between species, hybrids and populations to allow adaptation to different niches. Here, we provide an update of the biodiversity, ecology and population structure of the Saccharomyces species, and recapitulate the current knowledge on the natural history of Saccharomyces genus.

100 Years Later, What Is New in Glycerol Bioproduction?

Industrial production of glycerol by yeast, which began during WWI in the so-called Neuberg fermentation, was the first example of metabolic engineering. However, this process, based on bisulfite addition to fermentation liquid, has many drawbacks and was replaced by other methods of glycerol production. Osmotolerant yeasts and other microorganisms that do not require addition of bisulfite to steer cellular metabolism towards glycerol synthesis have been discovered or engineered. Because the glycerol market is expected to reach 5 billion US$ by 2024, microbial fermentation may again become a promising way to produce glycerol. This review summarizes some problems and perspectives on the production of glycerol by natural or engineered eukaryotic and prokaryotic microorganisms.

Dominance of wine Saccharomyces cerevisiae strains over S. kudriavzevii in industrial fermentation competitions is related to an acceleration of nutrient uptake and utilization

Grape must is a sugar-rich habitat for a complex microbiota which is replaced by Saccharomyces cerevisiae strains during the first fermentation stages. Interest on yeast competitive interactions has recently been propelled due to the use of alternative yeasts in the wine industry to respond to new market demands. The main issue resides in the persistence of these yeasts due to the specific competitive activity of S. cerevisiae. To gather deeper knowledge of the molecular mechanisms involved, we performed a comparative transcriptomic analysis during fermentation carried out by a wine S. cerevisiae strain and a strain representative of the cryophilic S. kudriavzevii, which exhibits high genetic and physiological similarities to S. cerevisiae, but also differences of biotechnological interest. In this study, we report that transcriptomic response to the presence of a competitor is stronger in S. cerevisiae than in S. kudriavzevii. Our results demonstrate that a wine S. cerevisiae industrial strain accelerates nutrient uptake and utilization to outcompete the co-inoculated yeast, and that this process requires cell-to-cell contact to occur. Finally, we propose that this competitive phenotype evolved recently, during the adaptation of S. cerevisiae to man-manipulated fermentative environments, since a non-wine S. cerevisiae strain, isolated from a North American oak, showed a remarkable low response to competition.

Improving the Cryotolerance of Wine Yeast by Interspecific Hybridization in the Genus Saccharomyces

Fermentations carried out at low temperatures (10–15°C) enhance the production and retention of flavor volatiles, but also increase the chances of slowing or arresting the process. Notwithstanding, as Saccharomyces cerevisiae is the main species responsible for alcoholic fermentation, other species of the genus Saccharomyces, such as cryophilic species Saccharomyces eubayanus, Saccharomyces kudriavzevii and Saccharomyces uvarum, are better adapted to low-temperature fermentations during winemaking. In this work, a Saccharomyces cerevisiae × S. uvarum hybrid was constructed to improve the enological features of a wine S. cerevisiae strain at low temperature. Fermentations of white grape musts were performed, and the phenotypic differences between parental and hybrid strains under different temperature conditions were examined. This work demonstrates that hybridization constitutes an effective approach to obtain yeast strains with desirable physiological features, like low-temperature fermentation capacity, which genetically depend on the expression of numerous genes (polygenic character). As this interspecific hybridization approach is not considered a GMO, the genetically improved strains can be quickly transferred to the wine industry.

Characterization of Saccharomyces uvarum (Beijerinck, 1898) and related hybrids: assessment of molecular markers that predict the parent and hybrid genomes and a proposal to name yeast hybrids

The use of the nuclear DNA reassociation technique has led taxonomists to consider Saccharomyces uvarum a synonym of S. bayanus. The latter, however, is not a species but a hybrid harbouring S. eubayanus (Seu) and S. uvarum (Su) subgenomes with a minor DNA contribution from S. cerevisiae (Sc). To recognize genetically pure lines of S. uvarum and putative interspecies hybrids among so-called S. bayanus strains present in public culture collections, we propose the use of four markers that were defined from the S. bayanus CBS 380T composite genome, namely SeuNTS2 (rDNA), ScMAL31, MTY1 and SuMEL1. Saccharomyces carlsbergensis CBS 1513 was found to be similar to S. bayanus except that it carries the SeuMEL1 allele. Different marker combinations revealed that among 33 strains examined only a few were similar to CBS 380T, but many pure S. uvarum lines and putative Su/Seu-related hybrids occurred. Our results demonstrated that these hybrids were erroneously considered authentic S. bayanus and therefore the varietal state 'Saccharomyces bayanus var. uvarum comb. nov. Naumov' is not valid. Our markers constitute a tool to get insights into the genomic makeup of Saccharomyces interspecies hybrids. We also make a proposal to name those hybrids that may also be applicable to other fungal hybrids.

New Trends in the Uses of Yeasts in Oenology

The most important factor in winemaking is the quality of the final product and the new trends in oenology are dictated by wine consumers and producers. Traditionally the red wine is the most consumed and more popular; however, in the last times, the wine companies try to attract other groups of populations, especially young people and women that prefer sweet, whites or rosé wines, very fruity and with low alcohol content. Besides the new trends in consumer preferences, there are also increased concerns on the effects of alcohol consumption on health and the effects of global climate change on grape ripening and wine composition producing wines with high alcohol content. Although S. cerevisiae is the most frequent species in wines, and the subject of most studies, S. uvarum and hybrids between Saccharomyces species such as S. cerevisiae×S. kudriavzevii and S. cerevisiae×S. uvarum are also involved in wine fermentations and can be preponderant in certain wine regions. New yeast starters of non-cerevisiae strains (S. uvarum) or hybrids (S. cerevisiae×S. uvarum and S. cerevisiae×S. kudriavzevii) can contribute to solve some problems of the wineries. They exhibit good fermentative capabilities at low temperatures, producing wines with lower alcohol and higher glycerol amounts, while fulfilling the requirements of the commercial yeasts, such as a good fermentative performance and aromatic profiles that are of great interest for the wine industry. In this review, we will analyze different applications of nonconventional yeasts to solve the current winemaking demands.

The Use of Mixed Populations of Saccharomyces cerevisiae and S. kudriavzevii to Reduce Ethanol Content in Wine: Limited Aeration, Inoculum Proportions, and Sequential Inoculation

Saccharomyces cerevisiae is the most widespread microorganism responsible for wine alcoholic fermentation. Nevertheless, the wine industry is currently facing new challenges, some of them associate with climate change, which have a negative effect on ethanol content and wine quality. Numerous and varied strategies have been carried out to overcome these concerns. From a biotechnological point of view, the use of alternative non-Saccharomyces yeasts, yielding lower ethanol concentrations and sometimes giving rise to new and interesting aroma, is one of the trendiest approaches. However, S. cerevisiae usually outcompetes other Saccharomyces species due to its better adaptation to the fermentative environment. For this reason, we studied for the first time the use of a Saccharomyces kudriavzevii strain, CR85, for co-inoculations at increasing proportions and sequential inoculations, as well as the effect of aeration, to improve its fermentation performance in order to obtain wines with an ethanol yield reduction. An enhanced competitive performance of S. kudriavzevii CR85 was observed when it represented 90% of the cells present in the inoculum. Furthermore, airflow supply of 20 VVH to the fermentation synergistically improved CR85 endurance and, interestingly, a significant ethanol concentration reduction was achieved.

Genome Diversity and Evolution in the Budding Yeasts (Saccharomycotina)

Considerable progress in our understanding of yeast genomes and their evolution has been made over the last decade with the sequencing, analysis, and comparisons of numerous species, strains, or isolates of diverse origins. The role played by yeasts in natural environments as well as in artificial manufactures, combined with the importance of some species as model experimental systems sustained this effort. At the same time, their enormous evolutionary diversity (there are yeast species in every subphylum of Dikarya) sparked curiosity but necessitated further efforts to obtain appropriate reference genomes. Today, yeast genomes have been very informative about basic mechanisms of evolution, speciation, hybridization, domestication, as well as about the molecular machineries underlying them. They are also irreplaceable to investigate in detail the complex relationship between genotypes and phenotypes with both theoretical and practical implications. This review examines these questions at two distinct levels offered by the broad evolutionary range of yeasts: inside the best-studied Saccharomyces species complex, and across the entire and diversified subphylum of Saccharomycotina. While obviously revealing evolutionary histories at different scales, data converge to a remarkably coherent picture in which one can estimate the relative importance of intrinsic genome dynamics, including gene birth and loss, vs. horizontal genetic accidents in the making of populations. The facility with which novel yeast genomes can now be studied, combined with the already numerous available reference genomes, offer privileged perspectives to further examine these fundamental biological questions using yeasts both as eukaryotic models and as fungi of practical importance.

Genetic Polymorphism in Wine Yeasts: Mechanisms and Methods for Its Detection

The processes of yeast selection for using as wine fermentation starters have revealed a great phenotypic diversity both at interspecific and intraspecific level, which is explained by a corresponding genetic variation among different yeast isolates. Thus, the mechanisms involved in promoting these genetic changes are the main engine generating yeast biodiversity. Currently, an important task to understand biodiversity, population structure and evolutionary history of wine yeasts is the study of the molecular mechanisms involved in yeast adaptation to wine fermentation, and on remodeling the genomic features of wine yeast, unconsciously selected since the advent of winemaking. Moreover, the availability of rapid and simple molecular techniques that show genetic polymorphisms at species and strain levels have enabled the study of yeast diversity during wine fermentation. This review will summarize the mechanisms involved in generating genetic polymorphisms in yeasts, the molecular methods used to unveil genetic variation, and the utility of these polymorphisms to differentiate strains, populations, and species in order to infer the evolutionary history and the adaptive evolution of wine yeasts, and to identify their influence on their biotechnological and sensorial properties.