Genetic characterization of strains of Saccharomycescerevisiae responsible for 'refermentation' in Botrytis-affected wines

Chemical Methods for Microbiological Control of Winemaking: An Overview of Current and Future Applications

Preservation technologies for winemaking have relied mainly on the addition of sulfur dioxide (SO2), in consequence of the large spectrum of action of this compound, linked to the control of undesirable microorganisms and the prevention of oxidative phenomena. However, its potential negative effects on consumer health have addressed the interest of the international research on alternative treatments to substitute or minimize the SO2 content in grape must and wine. This review is aimed at analyzing chemical methods, both traditional and innovative, useful for the microbiological stabilization of wine. After a preliminary description of the antimicrobial and technological properties of SO2, the additive traditionally used during wine production, the effects of the addition (in must and wine) of other compounds officially permitted in winemaking, such as sorbic acid, dimethyl dicarbonate (DMDC), lysozyme and chitosan, are discussed and evaluated. Furthermore, other substances showing antimicrobial properties, for which the use for wine microbiological stabilization is not yet permitted in EU, are investigated. Even if these treatments exhibit a good efficacy, a single compound able to completely replace SO2 is not currently available, but a combination of different procedures might be useful to reduce the sulfite content in wine. Among the strategies proposed, particular interest is directed towards the use of insect-based chitosan as a reliable alternative to SO2, mainly due to its low environmental impact. The production of wines containing low sulfite levels by using pro-environmental practices can meet both the consumers’ expectations, who are even more interested in the healthy traits of foods, and wine-producers’ needs, who are interested in the use of sustainable practices to promote the profile of their brand.

Response to Sulfur Dioxide Addition by Two Commercial Saccharomyces cerevisiae Strains

Sulfur dioxide (SO2) is an antioxidant and antimicrobial agent used in winemaking. Its effects on spoilage microorganisms has been studied extensively, but its effects on commercial Saccharomyces cerevisiae strains, the dominant yeast in winemaking, require further investigation. To our knowledge, no previous studies have investigated both the potential SO2 resistance mechanisms of commercial yeasts as well as their production of aroma-active volatile compounds in response to SO2. To study this, fermentations of two commercial yeast strains were conducted in the presence (50 mg/L) and absence (0 mg/L) of SO2. Strain QA23 was more sensitive to SO2 than Strain BRL97, resulting in delayed cell growth and slower fermentation. BRL97 exhibited a more rapid decrease in free SO2, a higher initial production of hydrogen sulfide, and a higher production of acetaldehyde, suggesting that each strain may utilize different mechanisms of sulfite resistance. SO2 addition did not affect the production of aroma-active volatile compounds in QA23, but significantly altered the volatile profiles of the wines fermented by BRL97.

Functional Analysis of the FZF1 Genes of Saccharomyces uvarum

Being a sister species of Saccharomyces cerevisiae, Saccharomyces uvarum shows great potential regarding the future of the wine industry. The sulfite tolerance of most S. uvarum strains is poor, however. This is a major flaw that limits its utility in the wine industry. In S. cerevisiae, FZF1 plays a positive role in the transcription of SSU1, which encodes a sulfite efflux transport protein that is critical for sulfite tolerance. Although FZF1 has previously been shown to play a role in sulfite tolerance in S. uvarum, there is little information about its action mechanism. To assess the function of FZF1, two over-expression vectors that contained different FZF1 genes, and one FZF1 silencing vector, were constructed and introduced into a sulfite-tolerant S. uvarum strain using electroporation. In addition, an FZF1-deletion strain was constructed. Both of the FZF1-over-expressing strains showed an elevated tolerance to sulfite, and the FZF1-deletion strain showed the opposite effect. Repression of FZF1 transcription failed, however, presumably due to the lack of alleles of DCR1 and AGO. The qRT-PCR analysis was used to examine changes in transcription in the strains. Surprisingly, neither over-expressing strain promoted SSU1 transcription, although MET4 and HAL4 transcripts significantly increased in both sulfite-tolerance increased strains. We conclude that FZF1 plays a different role in the sulfite tolerance of S. uvarum compared to its role in S. cerevisiae.

Persistence of Two Non-Saccharomyces Yeasts (Hanseniaspora and Starmerella) in the Cellar

Different genera and/or species of yeasts present on grape berries, in musts and wines are widely described. Nevertheless, the community of non-Saccharomyces yeasts present in the cellar is still given little attention. Thus it is not known if the cellar is a real ecological niche for these yeasts or if it is merely a transient habitat for populations brought in by grape berries during the winemaking period. This study focused on three species of non-Saccharomyces yeasts commonly encountered during vinification: Starmerella bacillaris (synonymy with Candida zemplinina), Hanseniaspora guilliermondii and Hanseniaspora uvarum. More than 1200 isolates were identified at the strain level by FT-IR spectroscopy (207 different FTIR strain pattern). Only a small proportion of non-Saccharomyces yeasts present in musts came directly from grape berries for the three species studied. Some strains were found in the must in two consecutive years and some of them were also found in the cellar environment before the arrival of the harvest of second vintage. This study demonstrates for the first time the persistence of non-Saccharomyces yeast strains from year to year in the cellar. Sulfur dioxide can affect yeast populations in the must and therefore their persistence in the cellar environment.

Different mechanisms of resistance modulate sulfite tolerance in wine yeasts

From a technological point of view, yeast resistance to sulfite is of great interest and represents an important technological character for winemaking. Several mechanisms are involved, and strain-dependent strategies to obtain SO2 resistance can deeply influence wine quality, although this choice is less relevant in determining the technological performance of the strain during fermentation. In this study, to better understand the strain-specific mechanisms of resistance, 11 Saccharomyces cerevisiae strains, whose genomes have been previously sequenced, were selected. Their attitude towards sulfites, in terms of resistance and production, was evaluated, and RNA-sequencing of four selected strains was performed during fermentation process in synthetic grape must in the presence of SO2. Results demonstrated that at molecular level, the physical effect of SO2 triggered multiple stress responses in the cell and high tolerance to general enological stressing condition increased SO2 resistance. Adaptation mechanism due to high basal gene expression level rather than specific gene induction in the presence of sulfite seemed to be responsible in modulating strain resistance. This mechanism involved higher basal gene expression level of specific cell wall proteins, enzymes for lipid biosynthesis, and enzymes directly involved in SO2 assimilation pathway and efflux.

Potassium metabisulphite as a potential biocide against Dekkera bruxellensis in fuel ethanol fermentations

Dekkera bruxellensis is an important contaminant yeast of fuel ethanol fermentations in Brazil, whose system applies cell repitching between the fermentative cycles. This work evaluated the addition of potassium metabisulphite (PMB) on yeast growth and fermentative yields in pure and co-cultures of Saccharomyces cerevisiae and D. bruxellensis in two situations: addition to the acidic solution in which the cells are treated between the fermentative cycles or to the fermentation medium. In the range of 200-400 mg l(-1) , PMB was effective to control the growth of D. bruxellensis depending on the culture medium and strain. When added to the acidic solution (250 mg l(-1) ), a significant effect was observed in mixed cultures, because the inactivation of SO2 by S. cerevisiae most likely protected D. bruxellensis from being damaged by PMB. The physiological response of S. cerevisiae to the presence of PMB may explain the significant decrease in alcohol production. When added to the fermentation medium, PMB resulted in the control but not the death of D. bruxellensis, with less intensive effect on the fermentative efficiency. In co-culture with the addition of PMB, the fermentative efficiency was significantly lower than in the absence of PMB.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study is the first to evaluate the action of potassium metabisulphite to control the growth of Dekkera bruxellensis in the fermentation process for fuel alcohol production. As near as possible of industrial conditions, the study simulates the addition of that substance in different points in the fermentation process, verifying in which situation the effects over the starter yeast and alcohol yield are minimal and over D. bruxellensis are maximal. Co-culture fermentations were carried out in cell-recycled batch system. The feasibility of using this substance for this specific fermentation is discussed in light of the possible biological and chemical interactions.

Surviving in the presence of sulphur dioxide: strategies developed by wine yeasts

Sulphur dioxide has been used as a common preservative in wine since at least the nineteenth century. Its use has even become essential to the making of quality wines because of its antioxidant, antioxidasic and antiseptic properties. The chemistry of SO₂ in wine is fairly complex due to its dissociation into different species and its binding to other compounds produced by yeasts and bacteria during fermentation. The only antiseptic species is the minute part remaining as molecular SO₂. The latter concentration is both dependent on pH and concentration of free bisulphite. However, certain yeast species have developed cellular and molecular mechanisms as a response to SO₂ exposure. Some of these mechanisms are fairly complex and have only been investigated recently, at least for the molecular mechanisms. They include sulphite reduction, sulphite oxidation, acetaldehyde production, sulphite efflux and the entry into viable but not culturable state, as the ultimate response. In this review, the chemistry of SO₂ in wine is explained together with the impact of SO₂ on yeast cells. The different defence mechanisms are described and discussed, mostly based on current knowledge available for Saccharomyces cerevisiae.

Development and characterization of hybrids from native wine yeasts

For commercial purposes, the winemaking industry is constantly searching for new yeast strains. Historically, this has been achieved by collecting wild strains and selecting the best for industrial use through an enological evaluation. Furthermore, the increasing consumer demands have forced the industry to incorporate new strategies such as genetic engineering to obtain improved strains. In response to the lack of public acceptance of this methodology, alternative strategies based on breeding have gained acceptance in recent years. Through the use of conjugation of individual spores without the support of genetic engineering methods we generated intraspecific hybrids from wild strains with outstanding enological characteristics and interdelta fingerprinting was used to confirm the hybrid condition. A detailed enological characterization of the hybrids in synthetic and natural must indicates that physiological parameters such as sporulation, residual sugar, ethanol yield and total nitrogen uptake are within the levels determined for the parental strains, however, other parameters such as growth rate, lag phase and ethanol production show statistical differences with some parental or commercial strains. These findings allow us to propose these hybrids as new wine-making strains.

Ecological success of a group of Saccharomyces cerevisiae/Saccharomyces kudriavzevii hybrids in the northern european wine-making environment

The hybrid nature of lager-brewing yeast strains has been known for 25 years; however, yeast hybrids have only recently been described in cider and wine fermentations. In this study, we characterized the hybrid genomes and the relatedness of the Eg8 industrial yeast strain and of 24 Saccharomyces cerevisiae/Saccharomyces kudriavzevii hybrid yeast strains used for wine making in France (Alsace), Germany, Hungary, and the United States. An array-based comparative genome hybridization (aCGH) profile of the Eg8 genome revealed a typical chimeric profile. Measurement of hybrids DNA content per cell by flow cytometry revealed multiple ploidy levels (2n, 3n, or 4n), and restriction fragment length polymorphism analysis of 22 genes indicated variable amounts of S. kudriavzevii genetic content in three representative strains. We developed microsatellite markers for S. kudriavzevii and used them to analyze the diversity of a population isolated from oaks in Ardèche (France). This analysis revealed new insights into the diversity of this species. We then analyzed the diversity of the wine hybrids for 12 S. cerevisiae and 7 S. kudriavzevii microsatellite loci and found that these strains are the products of multiple hybridization events between several S. cerevisiae wine yeast isolates and various S. kudriavzevii strains. The Eg8 lineage appeared remarkable, since it harbors strains found over a wide geographic area, and the interstrain divergence measured with a (δμ)(2) genetic distance indicates an ancient origin. These findings reflect the specific adaptations made by S. cerevisiae/S. kudriavzevii cryophilic hybrids to winery environments in cool climates.

A sulphite-inducible form of the sulphite efflux gene SSU1 in a Saccharomyces cerevisiae wine yeast

Sulphite is widely used as a preservative in foods and beverages for its antimicrobial and antioxidant activities, particularly in winemaking where SO2 is frequently added. Thus, sulphite resistance mechanisms have been extensively studied in the fermenting yeast Saccharomyces cerevisiae. Sulphite detoxification, involving a plasma membrane protein encoded by the SSU1 gene, is the most efficient resistance mechanism in S. cerevisiae. In this study, we characterized the unusual expression pattern of SSU1 in the wine strain 71B. We provide, for the first time, evidence of SSU1 induction by sulphite. The study of SSU1 expression during fermentation and in different growth conditions showed that sulphite is the main regulator of SSU1 expression, explaining its specific pattern. Combining analyses of gene expression and growth behaviour in response to sulphite, we found that 71B displayed unique behavioural patterns in response to sulphite pre-adaptation that may be explained by changes in SSU1 expression. Examination of the genomic organization of the SSU1 locus and sequencing of the region revealed three different alleles in 71B, two of which corresponded to translocated VIII–XVI forms. The lack of differences between promoter regions suggests that this inducible SSU1 expression pattern is due to modification of regulatory/signalling pathways.