Interspecific hybridisation among diverse Saccharomyces species: A combined biotechnological solution for low-temperature and nitrogen-limited wine fermentations

A consortium of different Saccharomyces species enhances the content of bioactive tryptophan-derived compounds in wine fermentations

In recent years, the presence of molecules derived from aromatic amino acids in wines has been increasingly demonstrated to have a significant influence on wine quality and stability. In addition, interactions between different yeast species have been observed to influence these final properties. In this study, a screening of 81 yeast strains from different environments was carried out to establish a consortium that would promote the improvement of indolic compound levels in wine. Two strains, Saccharomyces uvarum and Saccharomyces eubayanus, with robust fermentative capacity were selected to be combined with a Saccharomyces cerevisiae strain with a predisposition towards the production of indolic compounds. Fermentation dynamics were studied in pure cultures, co-inoculations and sequential inoculations, analysing strain interactions and end-of-fermentation characteristics. Fermentations showing significant interactions were further analyzed for the resulting indolic compounds and aroma profile, with the aim of observing potential interactions and synergies resulting from the combination of different strains in the final wine. Sequential inoculation of S. cerevisiae after S. uvarum or S. eubayanus was observed to increase indolic compound levels, particularly serotonin and 3-indoleacetic acid. This study is the first to demonstrate how the formation of microbial consortia can serve as a useful strategy to enhance compounds with interesting properties in wine, paving the way for future studies and combinations.

Differences in metabolism among Saccharomyces species and their hybrids during wine fermentation

This study aimed to investigate how parental genomes contribute to yeast hybrid metabolism using a metabolomic approach. Previous studies have explored central carbon and nitrogen metabolism in Saccharomyces species during wine fermentation, but this study analyses the metabolomes of Saccharomyces hybrids for the first time. We evaluated the oenological performance and intra- and extracellular metabolomes, and we compared the strains according to nutrient consumption and production of the main fermentative by-products. Surprisingly, no common pattern was observed for hybrid genome influence; each strain behaved differently during wine fermentation. However, this study suggests that the genome of the S. cerevisiae species may play a more relevant role in fermentative metabolism. Variations in biomass/nitrogen ratios were also noted, potentially linked to S. kudriavzevii and S. uvarum genome contributions. These results open up possibilities for further research using different "omics" approaches to comprehend better metabolic regulation in hybrid strains with genomes from different species.

Application of Strain Selection Technology in Alcoholic Beverages: A Review

The diversity of alcohol beverage microorganisms is of great significance for improving the brewing process and the quality of alcohol beverage products. During the process of making alcoholic beverages, a group of microorganisms, represented by yeast and lactic acid bacteria, conducts fermentation. These microorganisms have complex synergistic or competitive relationships, and the participation of different microorganisms has a major impact on the fermentation process and the flavor and aroma of the product. Strain selection is one of the key steps. Utilizing scientific breeding technology, the relationship between strains can be managed, the composition of the alcoholic beverage microbial community can be improved, and the quality and flavor of the alcoholic beverage products can be increased. Currently, research on the microbial diversity of alcohol beverages has received extensive attention. However, the selection technology for dominant bacteria in alcohol beverages has not yet been systematically summarized. To breed better-quality alcohol beverage strains and improve the quality and characteristics of wine, this paper introduces the microbial diversity characteristics of the world's three major brewing alcohols: beer, wine, and yellow wine, as well as the breeding technologies of related strains. The application of culture selection technology in the study of microbial diversity of brewed wine was reviewed and analyzed. The strain selection technology and alcohol beverage process should be combined to explore the potential application of a diverse array of alcohol beverage strains, thereby boosting the quality and flavor of the alcohol beverage and driving the sustainable development of the alcoholic beverage industry.

Beyond S. cerevisiae for winemaking: Fermentation-related trait diversity in the genus Saccharomyces

Saccharomyces cerevisiae is the yeast of choice for most inoculated wine fermentations worldwide. However, many other yeast species and genera display phenotypes of interest that may help address the environmental and commercial challenges the wine industry has been facing in recent years. This work aimed to provide, for the first time, a systematic phenotyping of all Saccharomyces species under winemaking conditions. For this purpose, we characterized the fermentative and metabolic properties of 92 Saccharomyces strains in synthetic grape must at two different temperatures. The fermentative potential of alternative yeasts was higher than expected, as nearly all strains were able to complete fermentation, in some cases more efficiently than commercial S. cerevisiae strains. Various species showed interesting metabolic traits, such as high glycerol, succinate and odour-active compound production, or low acetic acid production, compared to S. cerevisiae. Altogether, these results reveal that non-cerevisiae Saccharomyces yeasts are especially interesting for wine fermentation, as they may offer advantages over both S. cerevisiae and non-Saccharomyces strains. This study highlights the potential of alternative Saccharomyces species for winemaking, paving the way for further research and, potentially, for their industrial exploitation.

Hybridization and spore dissection of native wine yeasts for improvement of ethanol resistance and osmotolerance

Global warming has a significant impact on different viticultural parameters, including grape maturation. An increment of photosynthetic activity generates a rapid accumulation of sugars in the berry, followed by a dehydration process which leads to a higher concentration of soluble solids. This effect is exacerbated by current viticultural practices which favor the harvest of very mature grapes to obtain wines with sweet tannins. Considering the initial hyperosmotic stress conditions and the high ethanol concentration of the produced wine, fermentation of grape musts with high sugar content could be problematic for yeast starters. In the present study, we were able to obtain by classical hybridization and spore dissection methods one hybrid and one monosporic wine yeast strain with a combined ethanol and osmotolerant phenotype. The improved yeasts were tested in vinification trials with high sugar concentration and displayed excellent fermentation performance. Importantly, the obtained wines also showed good organoleptic properties during sensory analysis. Based on our results, we believed our improved hybrid and monosporic strains can be considered good alternatives to be used as yeast starters for fermentations with high sugar content.

Generation of intra- and interspecific Saccharomyces hybrids with improved oenological and aromatic properties

Non‐wine yeasts could enhance the aroma and organoleptic profile of wines. However, compared to wine strains, they have specific intolerances to winemaking conditions. To solve this problem, we generated intra‐ and interspecific hybrids using a non‐GMO technique (rare‐mating) in which non‐wine strains of S. uvarum, S. kudriavzevii and S. cerevisiae species were crossed with a wine S. cerevisiae yeast. The hybrid that inherited the wine yeast mitochondrial showed better fermentation capacities, whereas hybrids carrying the non‐wine strain mitotype reduced ethanol levels and increased glycerol, 2,3‐butanediol and organic acid production. Moreover, all the hybrids produced several fruity and floral aromas compared to the wine yeast: β‐phenylethyl acetate, isobutyl acetate, γ‐octalactone, ethyl cinnamate in both varietal wines. Sc × Sk crosses produced three‐ to sixfold higher polyfunctional mercaptans, 4‐mercapto‐4‐methylpentan‐2‐one (4MMP) and 3‐mercaptohexanol (3MH). We proposed that the exceptional 3MH release observed in an S. cerevisiae × S. kudriavzevii hybrid was due to the cleavage of the non‐volatile glutathione precursor (Glt‐3MH) to detoxify the cell from the presence of methylglyoxal, a compound related to the high glycerol yield reached by this hybrid. In conclusion, hybrid generation allows us to obtain aromatically improved yeasts concerning their wine parent. In addition, they reduced ethanol and increased organic acids yields, which counteracts climate change effect on grapes.

Truth in wine yeast

Evolutionary history and early association with anthropogenic environments have made Saccharomyces cerevisiae the quintessential wine yeast. This species typically dominates any spontaneous wine fermentation and, until recently, virtually all commercially available wine starters belonged to this species. The Crabtree effect, and the ability to grow under fully anaerobic conditions, contribute decisively to their dominance in this environment. But not all strains of Saccharomyces cerevisiae are equally suitable as starter cultures. In this article, we review the physiological and genetic characteristics of S. cerevisiae wine strains, as well as the biotic and abiotic factors that have shaped them through evolution. Limited genetic diversity of this group of yeasts could be a constraint to solving the new challenges of oenology. However, research in this field has for many years been providing tools to increase this diversity, from genetic engineering and classical genetic tools to the inclusion of other yeast species in the catalogues of wine yeasts. On occasion, these less conventional species may contribute to the generation of interspecific hybrids with S. cerevisiae. Thus, our knowledge about wine strains of S. cerevisiae and other wine yeasts is constantly expanding. Over the last decades, wine yeast research has been a pillar for the modernisation of oenology, and we can be confident that yeast biotechnology will keep contributing to solving any challenges, such as climate change, that we may face in the future.

Impact of Nitrogen Addition on Wine Fermentation by S. cerevisiae Strains with Different Nitrogen Requirements

In modern oenology, supplementation of nitrogen sources is an important strategy to prevent sluggish or stuck fermentation. The present study thoroughly determined the effect of nitrogen addition timing and nitrogen source type on fermentation kinetics and aroma production, carried out by yeast strains with low and high nitrogen requirements. The results revealed that yeast strains with different nitrogen requirements have divergent reactions to nitrogen addition. Nitrogen addition clearly shortened the fermentation duration, especially for the high-nitrogen-demanding yeast strain. Nitrogen addition at 1/3 fermentation was the most effective in terms of fermentation activity, nitrogen assimilation, and production of acetate esters. Interestingly enough, yeast cells preferentially took up amino acids related to fermentative aroma synthesis with the addition at 2/3 fermentation. The addition of nitrogen sources also largely affected the production of important metabolites. Generally speaking, acetic acid, glycerol, and succinic acid reduced with the supplementation of nitrogen sources. The results revealed significant application importance for the winemaking industry.

Phenotypic and genomic differences among S. cerevisiae strains in nitrogen requirements during wine fermentations

Nitrogen requirements by S. cerevisiae during wine fermentation are highly strain-dependent. Different approaches were applied to explore the nitrogen requirements of 28 wine yeast strains. Based on the growth and fermentation behaviour displayed at different nitrogen concentrations, high and low nitrogen-demanding strains were selected and further verified by competition fermentation. Biomass production with increasing nitrogen concentrations in the exponential fermentation phase was analysed by chemostat cultures. Low nitrogen-demanding (LND) strains produced a larger amount of biomass in nitrogen-limited synthetic grape musts, whereas high nitrogen-demanding (HND) strains achieved a bigger biomass yield when the YAN concentration was above 100 mg/L. Constant rate fermentation was carried out with both strains to determine the amount of nitrogen required to maintain the highest fermentation rate. Large differences appeared in the analysis of the genomes of low and high-nitrogen demanding strains showed for heterozygosity and the amino acid substitutions between orthologous proteins, with nitrogen recycling system genes showing the widest amino acid divergences. The CRISPR/Cas9-mediated genome modification method was used to validate the involvement of GCN1 in the yeast strain nitrogen needs. However, the allele swapping of gene GCN1 from low nitrogen-demanding strains to high nitrogen-demanding strains did not significantly influence the fermentation rate.

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.

Isotopic Tracers Unveil Distinct Fates for Nitrogen Sources during Wine Fermentation with Two Non-Saccharomyces Strains

Non-Saccharomyces yeast strains have become increasingly prevalent in the food industry, particularly in winemaking, because of their properties of interest both in biological control and in complexifying flavour profiles in end-products. However, unleashing the full potential of these species would require solid knowledge of their physiology and metabolism, which is, however, very limited to date. In this study, a quantitative analysis using ¹⁵N-labelled NH₄Cl, arginine, and glutamine, and ¹³C-labelled leucine and valine revealed the specificities of the nitrogen metabolism pattern of two non-Saccharomyces species, Torulaspora delbrueckii and Metschnikowia pulcherrima. In T. delbrueckii, consumed nitrogen sources were mainly directed towards the de novo synthesis of proteinogenic amino acids, at the expense of volatile compounds production. This redistribution pattern was in line with the high biomass-producer phenotype of this species. Conversely, in M. pulcherrima, which displayed weaker growth capacities, a larger proportion of consumed amino acids was catabolised for the production of higher alcohols through the Ehrlich pathway. Overall, this comprehensive overview of nitrogen redistribution in T. delbrueckii and M. pulcherrima provides valuable information for a better management of co- or sequential fermentation combining these species with Saccharomyces cerevisiae.

Inheritance of winemaking stress factors tolerance in Saccharomyces uvarum/S. eubayanus × S. cerevisiae artificial hybrids

Stress has been defined as any environmental factor that impairs the growth of a living organism. High concentrations of ethanol, sugars and SO₂ as well as temperature variations occurring during winemaking processes are some recognized stress factors that yeasts must overcome in order to avoid stuck or sluggish fermentations. At least two of these factors -sugar and ethanol concentrations- are strongly influenced by the global warming, which become them a worry for the future years in the winemaking industry. One of the most interesting strategies to face this complex situation is the generation of hybrids possessing, in a single yeast strain, a broader range of stress factors tolerance than their parents. In the present study, we evaluated four artificial hybrids generated with S. cerevisiae, S. uvarum and S. eubayanus using a non-GMO-generating method, in their tolerance to a set of winemaking stress factors. Their capacity to overcome specific artificial winemaking situations associated with global warming was also analyzed. All four hybrids were able to grow in a wider temperature range (8-37 °C) than their parents. Hybrids showed intermediate tolerance to higher ethanol, sugar and sulphite concentrations than their parents. Additionally, the hybrids showed an excellent fermentative behaviour in musts containing high fructose concentrations at low temperature as well as under a condition mimicking a stuck fermentation.