Transcriptional response of Saccharomyces cerevisiae to low temperature during wine fermentation

Lifestyle, Lineage, and Geographical Origin Influence Temperature-Dependent Phenotypic Variation across Yeast Strains during Wine Fermentation

Saccharomyces cerevisiae yeasts are a diverse group of single-celled eukaryotes with tremendous phenotypic variation in fermentation efficiency, particularly at different temperatures. Yeast can be categorized into subsets based on lifestyle (Clinical, Fermentation, Laboratory, and Wild), genetic lineage (Malaysian, Mosaic, North American, Sake, West African, and Wine), and geographical origin (Africa, Americas, Asia, Europe, and Oceania) to start to understand their ecology; however, little is known regarding the extent to which these groupings drive S. cerevisiae fermentative ability in grape juice at different fermentation temperatures. To investigate the response of yeast within the different subsets, we quantified fermentation performance in grape juice by measuring the lag time, maximal fermentation rate (V_max), and fermentation finishing efficiency of 34 genetically diverse S. cerevisiae strains in grape juice at five environmentally and industrially relevant temperatures (10, 15, 20, 25, and 30 °C). Extensive multivariate analysis was applied to determine the effects of lifestyle, lineage, geographical origin, strain, and temperature on yeast fermentation phenotypes. We show that fermentation capability is inherent to S. cerevisiae and that all factors are important in shaping strain fermentative ability, with temperature having the greatest impact, and geographical origin playing a lesser role than lifestyle or genetic lineage.

Volatile Compound Screening Using HS-SPME-GC/MS on Saccharomyces eubayanus Strains under Low-Temperature Pilsner Wort Fermentation

The recent isolation of the yeast Saccharomyces eubayanus has opened new avenues in the brewing industry. Recent studies characterized the production of volatile compounds in a handful set of isolates, utilizing a limited set of internal standards, representing insufficient evidence into the ability of the species to produce new and diverse aromas in beer. Using Headspace solid-phase microextraction followed by gas chromatography-mass spectrometry (HS-SPME-GC-MS), we characterized for the first time the production of volatile compounds in 10 wild strains under fermentative brewing conditions and compared them to a commercial lager yeast. S. eubayanus produces a higher number of volatile compounds compared to lager yeast, including acetate and ethyl esters, together with higher alcohols and phenols. Many of the compounds identified in S. eubayanus are related to fruit and floral flavors, which were absent in the commercial lager yeast ferment. Interestingly, we found a significant strain × temperature interaction, in terms of the profiles of volatile compounds, where some strains produced significantly greater levels of esters and higher alcohols. In contrast, other isolates preferentially yielded phenols, depending on the fermentation temperature. This work demonstrates the profound fermentation product differences between different S. eubayanus strains, highlighting the enormous potential of this yeast to produce new styles of lager beers.

The role of yeast ARO8, ARO9 and ARO10 genes in the biosynthesis of 3-(methylthio)-1-propanol from L-methionine during fermentation in synthetic grape medium

3-(methylthio)-1-propanol (methionol), produced by yeast as an end-product of L-methionine (L-Met) catabolism, imparts off-odours reminiscent of cauliflower and potato to wine. Saccharomyces cerevisiae ARO genes, including transaminases Aro8p and Aro9p, and decarboxylase Aro10p, catalyse two key steps forming methionol via the Ehrlich pathway. We compared methionol concentrations in wines fermented by single Δaro8, Δaro9 and Δaro10 deletants in lab strain BY4743 versus wine strain Zymaflore F15, and F15 double- and triple-aro deletants versus single-aro deletants, using headspace-solid phase microextraction coupled with gas chromatography-mass spectrometry.Deletion of two or more aro genes increased growth lag phase, with the greatest delay exhibited by F15 Δaro8 Δaro9. The single Δaro8 deletion decreased methionol by 44% in BY4743 and 92% in F15, while the Δaro9 deletion increased methionol by 46% in F15 but not BY4743. Single deletion of Δaro10 had no effect on methionol.Unexpectedly, F15 Δaro8 Δaro9 and F15 Δaro8 Δaro9 Δaro10 produced more methionol than F15 Δaro8. In the absence of Aro8p and Aro9p, other transaminases may compensate or an alternative pathway may convert methanethiol to methionol. Our results confirm that Ehrlich pathway genes differ greatly between lab and wine yeast strains, impacting downstream products such as methionol.

Influence of nitrogen status in wine alcoholic fermentation

Nitrogen is an essential nutrient for yeast during alcoholic fermentation. Nitrogen is involved in the biosynthesis of protein, amino acids, nucleotides, and other metabolites, including volatile compounds. However, recent studies have called several mechanisms that regulate its role in biosynthesis into question. An initial focus on S. cerevisiae has highlighted that the concept of "preferred" versus "non-preferred" nitrogen sources is extremely variable and strain-dependent. Then, the direct involvement of amino acids consumed in the formation of proteins and volatile compounds has recently been reevaluated. Indeed, studies have highlighted the key role of lipids in nitrogen regulation in S. cerevisiae and their involvement in the mechanism of cell death. New winemaking strategies using non-Saccharomyces yeast strains in co- or sequential fermentation improve nitrogen management. Indeed, recent studies show that non-Saccharomyces yeasts have significant and specific needs for nitrogen. Moreover, sluggish fermentation can occur when they are associated with S. cerevisiae, necessitating nitrogen addition. In this context, we will present the consequences of nitrogen addition, discussing the sources, time of addition, transcriptome changes, and effect on volatile compound composition.

Inheritance of brewing-relevant phenotypes in constructed Saccharomyces cerevisiae × Saccharomyces eubayanus hybrids

BACKGROUND

Interspecific hybridization has proven to be a potentially valuable technique for generating de novo lager yeast strains that possess diverse and improved traits compared to their parent strains. To further enhance the value of hybridization for strain development, it would be desirable to combine phenotypic traits from more than two parent strains, as well as remove unwanted traits from hybrids. One such trait, that has limited the industrial use of de novo lager yeast hybrids, is their inherent tendency to produce phenolic off-flavours; an undesirable trait inherited from the Saccharomyces eubayanus parent. Trait removal and the addition of traits from a third strain could be achieved through sporulation and meiotic recombination or further mating. However, interspecies hybrids tend to be sterile, which impedes this opportunity.

RESULTS

Here we generated a set of five hybrids from three different parent strains, two of which contained DNA from all three parent strains. These hybrids were constructed with fertile allotetraploid intermediates, which were capable of efficient sporulation. We used these eight brewing strains to examine two brewing-relevant phenotypes: stress tolerance and phenolic off-flavour formation. Lipidomics and multivariate analysis revealed links between several lipid species and the ability to ferment in low temperatures and high ethanol concentrations. Unsaturated fatty acids, such as oleic acid, and ergosterol were shown to positively influence growth at high ethanol concentrations. The ability to produce phenolic off-flavours was also successfully removed from one of the hybrids, Hybrid T2, through meiotic segregation. The potential application of these strains in industrial fermentations was demonstrated in wort fermentations, which revealed that the meiotic segregant Hybrid T2 not only didn't produce any phenolic off-flavours, but also reached the highest ethanol concentration and consumed the most maltotriose.

CONCLUSIONS

Our study demonstrates the possibility of constructing complex yeast hybrids that possess traits that are relevant to industrial lager beer fermentation and that are derived from several parent strains. Yeast lipid composition was also shown to have a central role in determining ethanol and cold tolerance in brewing strains.

Saccharomyces cerevisiae FLO1 Gene Demonstrates Genetic Linkage to Increased Fermentation Rate at Low Temperatures

Low fermentation temperatures are of importance to food and beverage industries working with Saccharomyces cerevisiae Therefore, the identification of genes demonstrating a positive impact on fermentation kinetics is of significant interest. A set of 121 mapped F1 progeny, derived from a cross between haploid strains BY4716 (a derivative of the laboratory yeast S288C) and wine yeast RM11-1a, were fermented in New Zealand Sauvignon Blanc grape juice at 12.5°. Analyses of five key fermentation kinetic parameters among the F1 progeny identified a quantitative trait locus (QTL) on chromosome I with a significant degree of linkage to maximal fermentation rate (Vmax) at low temperature. Independent deletions of two candidate genes within the region, FLO1 and SWH1, were constructed in the parental strains (with S288C representing BY4716). Fermentation of wild-type and deletion strains at 12.5 and 25° confirmed that the genetic linkage to Vmax corresponds to the S288C version of the FLO1 allele, as the absence of this allele reduced Vmax by ∼50% at 12.5°, but not at 25°. Reciprocal hemizygosity analysis (RHA) between S288C and RM11-1a FLO1 alleles did not confirm the prediction that the S288C version of FLO1 was promoting more rapid fermentation in the opposing strain background, suggesting that the positive effect on Vmax derived from S288C FLO1 may only provide an advantage in haploids, or is dependent on strain-specific cis or trans effects. This research adds to the growing body of evidence demonstrating the role of FLO1 in providing stress tolerance to S. cerevisiae during fermentation.

Evolution of Volatile Sulfur Compounds during Wine Fermentation

Volatile sulfur compounds (VSCs) play a significant role in the aroma of foods and beverages. With very low sensory thresholds and strong unpleasant aromas, most VSCs are considered to have a negative impact on wine quality. In this study, headspace solid phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME/GC-MS) was used to analyze the time course of the biosynthesis of 12 VSCs formed during wine fermentation. Two different strains of Saccharomyces cerevisiae, the laboratory strain BY4743 and a commercial strain, F15, were assessed using two media: synthetic grape media and Sauvignon Blanc juice. Seven VSCs were detected above background, with three rising above their sensory thresholds. The data revealed remarkable differences in the timing and evolution of production during fermentation, with a transient spike in methanethiol production early during anaerobic growth. Heavier VSCs such as benzothiazole and S-ethyl thioacetate were produced at a steady rate throughout grape juice fermentation, whereas others, such as diethyl sulfide, appear toward the very end of the winemaking process. The results also demonstrate significant differences between yeast strains and fermentation media.