The influence of nitrogen and biotin interactions on the performance of Saccharomyces in alcoholic fermentations

Physiological role of the EHL gene in sake yeast and its effects on quality of sake

The EHL1/2/3 genes were identified by whole-genome sequencing of Kyokai No. 7 (K7), which is a well-known representative Japanese sake yeast Saccharomyces cerevisiae. The genes are present in K7, but not in laboratory strain S288C. Although the genes were presumed to encode epoxide hydrolase based on homology analysis, their effect on cellular metabolism in sake yeast has not yet been clarified. We constructed ehl1/2/3 mutants harboring a stop codon in each gene using the haploid yeast strain H3 as the parental strain, which was derived from K701, and investigated the physiological role and effects of the EHL1/2/3 genes on sake quality. Metabolome analysis and vitamin requirement testing revealed that the EHL1/2/3 genes are partly responsible for the synthesis of pantothenate. For fermentation profiles, ethanol production by the ehl1/2/3 mutant was comparable with that of strain H3, but succinate production was decreased in the ehl1/2/3 mutant compared to strain H3 when cultured in yeast malt (YM) medium containing 10% glucose and during sake brewing. Ethyl hexanoate and isoamyl acetate levels in the ehl1/2/3 mutant strain were decreased compared to those of strain H3 during sake brewing. Thus, the EHL1/2/3 genes did not affect ethanol production but did affect the production of organic acids and aromatic components during sake brewing.

QTL mapping reveals novel genes and mechanisms underlying variations in H2S production during alcoholic fermentation in Saccharomyces cerevisiae

Saccharomyces cerevisiae requirement for reduced sulfur to synthesize methionine and cysteine during alcoholic fermentation, is mainly fulfilled through the sulfur assimilation pathway. Saccharomyces cerevisiae reduces sulfate into sulfur dioxide (SO2) and sulfide (H2S), whose overproduction is a major issue in winemaking, due to its negative impact on wine aroma. The amount of H2S produced is highly strain-specific and also depends on SO2 concentration, often added to grape must. Applying a bulk segregant analysis to a 96-strain-progeny derived from two strains with different abilities to produce H2S, and comparing allelic frequencies along the genome of pools of segregants producing contrasting H2S quantities, we identified two causative regions involved in H2S production in the presence of SO2. A functional genetic analysis allowed the identification of variants in four genes able to impact H2S formation, viz; ZWF1, ZRT2, SNR2, and YLR125W, and involved in functions and pathways not associated with sulfur metabolism until now. These data point out that, in wine fermentation conditions, redox status, and zinc homeostasis are linked to H2S formation while providing new insights into the regulation of H2S production, and a new vision of the interplay between the sulfur assimilation pathway and cell metabolism.

Characterizing the impact of species/strain-specific Lactiplantibacillus plantarum with community assembly and metabolic regulation in pickled Suancai

To investigate the colonization and impact of the specific Lactiplantibacillus plantarum strains, four isolated strains were applied in pickled Suancai which is a traditional pickled mustard (Brassica juncea). Results showed that strain-8 with the highest lactic acid bacteria (LAB) counts and acetic acid (p < 0.05). There were 11.42 % ∼ 32.35 % differential volatile compounds detected, although nitriles, esters, and acids were predominant. L. plantarum disturbed the microbial community, in which the microbial composition of strain-11 was most similar to the naturally fermented sample. Amino acids, carbohydrate metabolism, and metabolism of cofactors and vitamins were the main functional classes because of the similar dominant microbes (Lactiplantibacillus and Levilactobacillus). The functional units were separated based on NMDS analysis, in which bacterial chemotaxis, amino acid-related units, biotin metabolism, fatty acid biosynthesis, and citrate cycle were significantly different calculated by metagenomeSeq and Benjamin-Hochberg methods (p < 0.05). The contents of most flavor compounds were consistent with their corresponding enzymes. In particular, glucosinolates metabolites were different and significantly related to the myrosinase and metabolic preference of LAB. Therefore, this study revealed the impact mechanism of the specific L. plantarum strains and provided a perspective for developing microbial resources to improve the flavor diversity of fermented vegetables.

To be or not to be required: Yeast vitaminic requirements in winemaking

Although vitamins are prime actors in yeast metabolism, the nature and the extent of their requirement in Saccharomyces cerevisiae in winemaking remains little understood. To fill this gap, the evolution of 8 water-soluble vitamins and their diverse vitamers during its alcoholic fermentation in a synthetic must medium was monitored, providing the first evidence of the consumption of vitamers by five commercial S. cerevisiae strains, and highlighting the existence of preferential vitameric sources for its nutrition. The vitamins required by the yeast, B1, B5, and B8, were then identified, and the nature of their requirement characterized, strongly asserting the required trait of B1 for fermentation, B8 for growth, and B5 for both processes. The extent of the requirement for B5, that with the most impact of the three vitamins, was then quantified in three S. cerevisiae strains, resulting in the conclusion that 750 μg.L-1 should prove sufficient to cover the yeast's requirements. This investigation offers the first insight into S. cerevisiae vitaminic requirements for winemaking.

To each their own: Delving into the vitaminic preferences of non-Saccharomyces wine yeasts

Considering the growing interest in non-Saccharomyces wine yeasts, and notably in the context of mixed fermentations with S. cerevisiae, understanding their nutritional behaviors is essential to ensure better management of these fermentations. The vitaminic consumption of three non-Saccharomyces yeasts (Starmerella bacillaris, Metschnikowia pulcherrima and Torulaspora delbrueckii) was investigated during their growth in wine-like conditions, providing initial evidence that they consume different vitamers. The vitamin consumption profiles during their growth highlighted releases of certain vitamers by the yeasts before re-assimilation, strongly suggesting the existence of synthesis pathways. Not only did the essential character of vitamin B1, in particular, appear to be a trait common to these yeasts, since all its vitamers are consumed, this investigation also provided evidence of the existence of species-dependent preferences for their vitaminic sources. These different behaviors were quite striking in certain vitamers, as was observed in nicotinamide: while it was consumed by T. delbrueckii, it was left untouched by S. bacillaris and produced by M. pulcherrima during growth. Furthermore, this offers grounds for further investigation into these yeasts' requirements, and provides the first tool for managing vitamin resources during mixed fermentations with S. cerevisiae, and for preventing nutritive deficiencies from occurring.

The bio3 mutation in sake yeast leads to changes in organic acid profiles and ester levels but not ethanol production

Biotin is an essential coenzyme that is bound to carboxylases and participates in fatty acid synthesis. The fact that sake yeast exhibit biotin prototrophy while almost all other Saccharomyces cerevisiae strains exhibit biotin auxotrophy, implies that biotin prototrophy is an important factor in sake brewing. In this study, we inserted a stop codon into the biotin biosynthetic BIO3 gene (cording for 7,8-diamino-pelargonic acid aminotransferase) of a haploid sake yeast strain using the marker-removable plasmid pAUR135 and investigated the fermentation profile of the resulting bio3 mutant. Ethanol production was not altered when the bio3 mutant was cultured in Yeast Malt (YM) medium containing 10% glucose at 15 °C and 30 °C. Interestingly, ethanol production was also not changed during the sake brewing process. On the other hand, the levels of organic acids in the bio3 mutant were altered after culturing in YM medium and during sake brewing. In addition, ethyl hexanoate and isoamyl acetate levels decreased in the bio3 mutant during sake brewing. Metabolome analysis revealed that the decreased levels of fatty acids in the bio3 mutant were attributed to the decreased levels of ethyl hexanoate. Further, the transcription level of genes related to the synthesis of ethyl hexanoate and isoamyl acetate were significantly reduced. The findings indicated that although the decrease in biotin biosynthesis did not affect ethanol production, it did affect the synthesis of components such as organic acids and aromatic compounds. Biotin biosynthesis ability is thus a key factor in sake brewing.

Thiamine and Biotin: Relevance in the Production of Volatile and Non-Volatile Compounds during Saccharomyces cerevisiae Alcoholic Fermentation in Synthetic Grape Must

Vitamins are major cofactors to numerous key metabolic pathways in enological yeasts, and both thiamine and biotin, notably, are believed to be essential to yeast fermentation and growth, respectively. In order to further assess and clarify their role in winemaking, and in the resulting wine, alcoholic fermentations of a commercial Saccharomyces cerevisiae active dried yeast were conducted in synthetic media containing various concentrations of both vitamins. Growth and fermentation kinetics were monitored and proved the essential character of biotin in yeast growth, and of thiamine in fermentation. The synthetic wine volatile compounds were quantified, and notable influences of both vitamins appeared, through a striking positive effect of thiamine on the production of higher alcohols, and of biotin on fatty acids. Beyond the evidence of this influence on fermentations and on the production of volatiles, this work proves, for the first time, the impact held by vitamins on wine yeasts' exometabolome, investigated through an untargeted metabolomic analysis. This highlighted chemical differences in the composition of synthetic wines through a notably marked influence of thiamine on 46 named S. cerevisiae metabolic pathways, and especially in amino acid-associated metabolic pathways. This provides, overall, the first evidence of the impact held by both vitamins on the wine.

Development of a New Assay for Measuring H2S Production during Alcoholic Fermentation: Application to the Evaluation of the Main Factors Impacting H2S Production by Three Saccharomycescerevisiae Wine Strains

Hydrogen sulfide (H2S) is the main volatile sulfur compound produced by Saccharomycescerevisiae during alcoholic fermentation and its overproduction leads to poor wine sensory profiles. Several factors modulate H2S production and winemakers and researchers require an easy quantitative tool to quantify their impact. In this work, we developed a new sensitive method for the evaluation of total H2S production during alcoholic fermentation using a metal trap and a fluorescent probe. With this method, we evaluated the combined impact of three major factors influencing sulfide production by wine yeast during alcoholic fermentation: assimilable nitrogen, sulfur dioxide and strain, using a full factorial experimental design. All three factors significantly impacted H2S production, with variations according to strains. This method enables large experimental designs for the better understanding of sulfide production by yeasts during fermentation.

Engineering oxygen-independent biotin biosynthesis in Saccharomyces cerevisiae

An oxygen requirement for de novo biotin synthesis in Saccharomyces cerevisiae precludes the application of biotin-prototrophic strains in anoxic processes that use biotin-free media. To overcome this issue, this study explores introduction of the oxygen-independent Escherichia coli biotin-biosynthesis pathway in S. cerevisiae. Implementation of this pathway required expression of seven E. coli genes involved in fatty-acid synthesis and three E. coli genes essential for the formation of a pimelate thioester, key precursor of biotin synthesis. A yeast strain expressing these genes readily grew in biotin-free medium, irrespective of the presence of oxygen. However, the engineered strain exhibited specific growth rates 25% lower in biotin-free media than in biotin-supplemented media. Following adaptive laboratory evolution in anoxic cultures, evolved cell lines that no longer showed this growth difference in controlled bioreactors, were characterized by genome sequencing and proteome analyses. The evolved isolates exhibited a whole-genome duplication accompanied with an alteration in the relative gene dosages of biosynthetic pathway genes. These alterations resulted in a reduced abundance of the enzymes catalyzing the first three steps of the E. coli biotin pathway. The evolved pathway configuration was reverse engineered in the diploid industrial S. cerevisiae strain Ethanol Red. The resulting strain grew at nearly the same rate in biotin-supplemented and biotin-free media non-controlled batches performed in an anaerobic chamber. This study established an unique genetic engineering strategy to enable biotin-independent anoxic growth of S. cerevisiae and demonstrated its portability in industrial strain backgrounds.

A Comparative Study on Volatile Compounds and Sensory Profile of White and Red Wines Elaborated Using Bee Pollen versus Commercial Activators

Lack of nutrients in grape may cause problems for a proper alcoholic fermentation process, resulting in an altered aromatic profile of the wines. To avoid this situation, commercial winemakers often use fermentation activators, which are usually combinations of ammonium salts, inactivated yeast and thiamine. In addition, it has been shown that bee pollen addition to the grape can help to improve fermentation, resulting in better volatile compound profile of wines responsible for sensory quality. For this reason, the aim of this research work was to carry out a comparative study using bee pollen versus commercial fermentation activators in white and red winemaking. The same dose of bee pollen and commercial activators (0.25 g/L) were used in all experiments. Volatile compounds were analyzed by gas chromatography-mass spectrometry, odor activity values were determined to assess odorant impact of various volatile compound families, and finally a descriptive sensory analysis was carried out. Then, the triangular test and the ranking assay were used to identify perceptible differences as well as preference among the wines elaborated. Compared to commercial activators, bee pollen wines increased volatile compound formation, mainly higher alcohols, esters, and terpenes, enhancing fruity and floral odorant series. On the other hand, triangular test showed significant differences between wines, and the ranking assay showed a greater preference for bee pollen wines.

Vitamins in wine: Which, what for, and how much?

Vitamins are essential compounds to yeasts, and notably in winemaking contexts. Vitamins are involved in numerous yeast metabolic pathways, including those of amino acids, fatty acids, and alcohols, which suggests their notable implication in fermentation courses, as well as in the development of aromatic compounds in wines. Although they are major components in the course of those microbial processes, their significance and impact have not been extensively studied in the context of winemaking and wine products, as most of the studies focusing on the subject in the past decades have relied on relatively insensitive and imprecise analytical methods. Therefore, this review provides an extensive overview of the current knowledge regarding the impacts of vitamins on grape must fermentations, wine-related yeast metabolisms, and requirements, as well as on the profile of wine sensory characteristics. We also highlight the methodologies and techniques developed over time to perform vitamin analysis in wines, and assess the importance of precisely defining the role played by vitamins in winemaking processes, to ensure finer control of the fermentation courses and product characteristics in a highly complex matrix.

The non-Saccharomyces yeast Pichia kluyveri for the production of aromatic volatile compounds in alcoholic fermentation

Alcoholic fermentation is influenced by yeast strain, culture media, substrate concentration and fermentation conditions, which contribute to taste and aroma. Some non-Saccharomyces yeasts are recognized as volatile compound producers that enrich aromatic profile of alcoholic beverages. In this work, 21 strains of Pichia kluyveri isolated from different fermentative processes and regions were evaluated. A principal component analysis (PCA) showed statistical differences between strains mainly associated with the variety and concentration of the compounds produced. From the PCA, two strains (PK1 and PK8) with the best volatile compound production were selected to evaluate the impact of culture media (M12 medium and Agave tequilana juice), stirring speeds (100 and 250 rpm) and temperatures (20°C, 25°C and 30°C). Increased ester production was observed at 250 rpm. Greatest effect in alcohols and ester production was found with A. tequilana, identifying PK1 as higher alcohol producer, and PK8 as better ester producer. Regarding temperature, PK1 increased ester production with decreased fermentation temperature. PK8 presented maximum levels of ethyl acetate and ethyl dodecanoate at 20°C, and finally isoamyl acetate increased its production at 30°C. Therefore, P. kluyveri strains are of great interest to produce different aromatic profiles that are affected by factors including medium, agitation and temperature.

Evaluation of Saccharomyces cerevisiae Wine Yeast Competitive Fitness in Enologically Relevant Environments by Barcode Sequencing

When a wine yeast is inoculated into grape juice the potential variation in juice composition that confronts it is huge. Assessing the performance characteristics of the many commercially available wine yeasts in the many possible grape juice compositions is a daunting task. To this end we have developed a barcoded Saccharomyces cerevisiae wine yeast collection to facilitate the task of performance assessment that will contribute to a broader understanding of genotype-phenotype relations. Barcode sequencing of mixed populations is used to monitor strain abundance in different grape juices and grape juice-like environments. Choice of DNA extraction method is shown to affect strain-specific barcode count in this highly related set of S. cerevisiae strains; however, the analytical approach is shown to be robust toward strain dependent variation in DNA extraction efficiency. Of the 38 unique compositional variables assessed, resistance to copper and SO₂ are found to be dominant discriminatory factors in wine yeast performance. Finally, a comparison of competitive fitness profile with performance in single inoculum fermentations reveal strain dependent correspondence of yeast performance using these two different approaches.

Hydrogen sulphide production by Saccharomyces cerevisiae UCD 522 in a synthetic grape juice medium deficient of thiamin (vitamin B1 ) and/or pyridoxine (vitamin B6 )

The impacts of thiamin and pyridoxine along with YAN on alcoholic fermentation and hydrogen sulphide production by Saccharomyces cerevisiae were studied. Using a synthetic grape juice medium, three fermentation trials were conducted; (i) 2 × 3 factorial design with thiamin (0, 0·2, or 0·5 mg l^-1 ) and YAN (60 or 250 mg l^-1 ) as variables, (ii) 2 × 3 factorial design with pyridoxine (0, 0·25, or 0·5 mg l^-1 ) and YAN (60 or 250 mg l^-1 ) as variables, and (iii) 3 × 3 factorial design with thiamin (0, 0·2 or 0·5 mg l^-1 ) and pyridoxine (0, 0·25 or 0·5 mg l^-1 ) as variables in media containing 60 mg l^-1 YAN. Although the progress of fermentations was affected by thiamin or pyridoxine, YAN had a larger impact than either vitamin. H₂ S production was significantly lower with increasing amounts of thiamin in those fermentations under low YAN (60 mg l^-1 ) while even lower amounts (<30 µg l^-1 ) were produced under high YAN (250 mg l^-1 ) with or without the vitamin. The highest amounts of H₂ S were synthesized in those fermentations without any pyridoxine (>110 µg l^-1 ), with the lowest production in media with pyridoxine and high YAN (<20 µg l^-1 ). SIGNIFICANCE AND IMPACT OF THE STUDY: Concentrations of thiamin, pyridoxine and yeast assimilable nitrogen (YAN) influenced the synthesis of hydrogen sulphide (H₂ S) by Saccharomyces cerevisiae in a synthetic grape juice medium. With a few exceptions, an increase in the concentration of either vitamin or YAN resulted in less H₂ S released. This is the first report to demonstrate that both thiamin and pyridoxine along with YAN affected H₂ S production, emphasizing the need to assess yeast nutrients to lower risks of off-odours during fermentation.

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.

Altered Fermentation Performances, Growth, and Metabolic Footprints Reveal Competition for Nutrients between Yeast Species Inoculated in Synthetic Grape Juice-Like Medium

The sequential inoculation of non-Saccharomyces yeasts and Saccharomyces cerevisiae in grape juice is becoming an increasingly popular practice to diversify wine styles and/or to obtain more complex wines with a peculiar microbial footprint. One of the main interactions is competition for nutrients, especially nitrogen sources, that directly impacts not only fermentation performance but also the production of aroma compounds. In order to better understand the interactions taking place between non-Saccharomyces yeasts and S. cerevisiae during alcoholic fermentation, sequential inoculations of three yeast species (Pichia burtonii, Kluyveromyces marxianus, Zygoascus meyerae) with S. cerevisiae were performed individually in a synthetic medium. Different species-dependent interactions were evidenced. Indeed, the three sequential inoculations resulted in three different behaviors in terms of growth. P. burtonii and Z. meyerae declined after the inoculation of S. cerevisiae which promptly outcompeted the other two species. However, while the presence of P. burtonii did not impact the fermentation kinetics of S. cerevisiae, that of Z. meyerae rendered the overall kinetics very slow and with no clear exponential phase. K. marxianus and S. cerevisiae both declined and became undetectable before fermentation completion. The results also demonstrated that yeasts differed in their preference for nitrogen sources. Unlike Z. meyerae and P. burtonii, K. marxianus appeared to be a competitor for S. cerevisiae (as evidenced by the uptake of ammonium and amino acids), thereby explaining the resulting stuck fermentation. Nevertheless, the results suggested that competition for other nutrients (probably vitamins) occurred during the sequential inoculation of Z. meyerae with S. cerevisiae. The metabolic footprint of the non-Saccharomyces yeasts determined after 48 h of fermentation remained until the end of fermentation and combined with that of S. cerevisiae. For instance, fermentations performed with K. marxianus were characterized by the formation of phenylethanol and phenylethyl acetate, while those performed with P. burtonii or Z. meyerae displayed higher production of isoamyl alcohol and ethyl esters. When considering sequential inoculation of yeasts, the nutritional requirements of the yeasts used should be carefully considered and adjusted accordingly. Finally, our chemical data suggests that the organoleptic properties of the wine are altered in a species specific manner.

Nitrogen and carbon source balance determines longevity, independently of fermentative or respiratory metabolism in the yeast Saccharomyces cerevisiae

Dietary regimens have proven to delay aging and age-associated diseases in several eukaryotic model organisms but the input of nutritional balance to longevity regulation is still poorly understood. Here, we present data on the role of single carbon and nitrogen sources and their interplay in yeast longevity. Data demonstrate that ammonium, a rich nitrogen source, decreases chronological life span (CLS) of the prototrophic Saccharomyces cerevisiae strain PYCC 4072 in a concentration-dependent manner and, accordingly, that CLS can be extended through ammonium restriction, even in conditions of initial glucose abundance. We further show that CLS extension depends on initial ammonium and glucose concentrations in the growth medium, as long as other nutrients are not limiting. Glutamine, another rich nitrogen source, induced CLS shortening similarly to ammonium, but this effect was not observed with the poor nitrogen source urea. Ammonium decreased yeast CLS independently of the metabolic process activated during aging, either respiration or fermentation, and induced replication stress inhibiting a proper cell cycle arrest in G0/G1 phase. The present results shade new light on the nutritional equilibrium as a key factor on cell longevity and may contribute for the definition of interventions to promote life span and healthy aging.

Metagenomic sequencing reveals the relationship between microbiota composition and quality of Chinese Rice Wine

Chinese Rice Wine (CRW) is a common alcoholic beverage in China. To investigate the influence of microbial composition on the quality of CRW, high throughput sequencing was performed for 110 wine samples on bacterial 16S rRNA gene and fungal Internal Transcribed Spacer II (ITS2). Bioinformatic analyses demonstrated that the quality of yeast starter and final wine correlated with microbial taxonomic composition, which was exemplified by our finding that wine spoilage resulted from a high proportion of genus Lactobacillus. Subsequently, based on Lactobacillus abundance of an early stage, a model was constructed to predict final wine quality. In addition, three batches of 20 representative wine samples selected from a pool of 110 samples were further analyzed in metagenomics. The results revealed that wine spoilage was due to rapid growth of Lactobacillus brevis at the early stage of fermentation. Gene functional analysis indicated the importance of some pathways such as synthesis of biotin, malolactic fermentation and production of short-chain fatty acid. These results led to a conclusion that metabolisms of microbes influence the wine quality. Thus, nurturing of beneficial microbes and inhibition of undesired ones are both important for the mechanized brewery.

Ecological and Genetic Barriers Differentiate Natural Populations of Saccharomyces cerevisiae

How populations that inhabit the same geographical area become genetically differentiated is not clear. To investigate this, we characterized phenotypic and genetic differences between two populations of Saccharomyces cerevisiae that in some cases inhabit the same environment but show relatively little gene flow. We profiled stress sensitivity in a group of vineyard isolates and a group of oak-soil strains and found several niche-related phenotypes that distinguish the populations. We performed bulk-segregant mapping on two of the distinguishing traits: The vineyard-specific ability to grow in grape juice and oak-specific tolerance to the cell wall damaging drug Congo red. To implicate causal genes, we also performed a chemical genomic screen in the lab-strain deletion collection and identified many important genes that fell under quantitative trait loci peaks. One gene important for growth in grape juice and identified by both the mapping and the screen was SSU1, a sulfite-nitrite pump implicated in wine fermentations. The beneficial allele is generated by a known translocation that we reasoned may also serve as a genetic barrier. We found that the translocation is prevalent in vineyard strains, but absent in oak strains, and presents a postzygotic barrier to spore viability. Furthermore, the translocation was associated with a fitness cost to the rapid growth rate seen in oak-soil strains. Our results reveal the translocation as a dual-function locus that enforces ecological differentiation while producing a genetic barrier to gene flow in these sympatric populations.

Optimization of ethanol, citric acid, and α-amylase production from date wastes by strains of Saccharomyces cerevisiae, Aspergillus niger, and Candida guilliermondii

The present study deals with submerged ethanol, citric acid, and α-amylase fermentation by Saccharomyces cerevisiae SDB, Aspergillus niger ANSS-B5, and Candida guilliermondii CGL-A10, using date wastes as the basal fermentation medium. The physical and chemical parameters influencing the production of these metabolites were optimized. As for the ethanol production, the optimum yield obtained was 136.00 ± 0.66 g/l under optimum conditions of an incubation period of 72 h, inoculum content of 4% (w/v), sugars concentration of 180.0 g/l, and ammonium phosphate concentration of 1.0 g/l. Concerning citric acid production, the cumulative effect of temperature (30°C), sugars concentration of 150.0 g/l, methanol concentration of 3.0%, initial pH of 3.5, ammonium nitrate concentration of 2.5 g/l, and potassium phosphate concentration of 2.5 g/l during the fermentation process of date wastes syrup did increase the citric acid production to 98.42 ± 1.41 g/l. For the production of α-amylase, the obtained result shows that the presence of starch strongly induces the production of α-amylase with a maximum at 5.0 g/l. Among the various nitrogen sources tested, urea at 5.0 g/l gave the maximum biomass and α-amylase estimated at 5.76 ± 0.56 g/l and 2,304.19 ± 31.08 μmol/l/min, respectively after 72 h incubation at 30°C, with an initial pH of 6.0 and potassium phosphate concentration of 6.0 g/l.

Impacts of variations in elemental nutrient concentration of Chardonnay musts on Saccharomyces cerevisiae fermentation kinetics and wine composition

Chardonnay, being the predominant white wine-grape cultivar in the Australian wine sector, is subject to widely varying winemaking processes with the aim of producing a variety of wine styles. Therefore, juice composition might not always be ideal for optimal fermentation outcomes. Our aim was to better understand the composition of Chardonnay juice and how compositional parameters impact on fermentation outcomes. This was achieved through a survey of 96 commercially prepared Chardonnay juices during the 2009 vintage. Common juice variables were estimated using near infrared spectroscopy, and elemental composition was determined using radial view inductively coupled plasma optical emission spectrometry. The influence of elemental composition on fermentation outcomes was assessed by fermentation of a defined medium formulated to reflect the composition and range of concentrations as determined by the juice survey. Yeast (Saccharomyces cerevisiae) strain effects were also assessed. Key parameters influencing fermentation outcomes were verified by laboratory scale fermentation of Chardonnay juice. This exploration of Chardonnay juice identified interactions between juice pH and potassium concentration as key factors impacting on fermentation performance and wine quality. Outcomes differed depending on yeast strain.

Microbial alcohol-conferred hemolysis is a late response to alcohol stress

We have reported previously that growth on alcohol vapors confers hemolytic properties on certain yeast species and strains ['microbial alcohol-conferred hemolysis' (MACH)]. In a recent study, we analyzed the genetic basis of MACH in Saccharomyces cerevisiae using the EUROSCARF mutant collection. The data suggested that intact mitochondrial and respiratory chain functions are critical for the observed alcohol-mediated hemolysis. We proposed that the uncontrolled cellular uptake of alcohol results in yeast 'hyper-respiration', leading to elaboration of hemolytic molecules such as hydrogen peroxide and lytic lipids. In the current study, we have further analyzed the molecular mechanisms involved in the MACH phenomenon in S. cerevisiae, using DNA microarrays. The patterns of regulation were confirmed by quantitative reverse transcriptase PCR. The results presented here lend further support to this hypothesis, based on upregulation of the genes responsible for coping with vast amounts of hydrogen peroxide produced as a byproduct of excessive oxidation of alcohol. These results, taken together, show that alcohol-mediated hemolysis in yeast appears to be related to the overproduction of hemolytic byproducts, particularly hydrogen peroxide, which accumulates during long-term exposure of S. cerevisiae to both ethanol and n-butanol.

Identification of MET10-932 and characterization as an allele reducing hydrogen sulfide formation in wine strains of Saccharomyces cerevisiae

A vineyard isolate of the yeast Saccharomyces cerevisiae, UCD932, was identified as a strain producing little or no detectable hydrogen sulfide during wine fermentation. Genetic analysis revealed that this trait segregated as a single genetic determinant. The gene also conferred a white colony phenotype on BiGGY agar (bismuth-glucose-glycine-yeast agar), which is thought to indicate low basal levels of sulfite reductase activity. However, this isolate does not display a requirement for S-containing amino acids, indicating that the sulfate reduction pathway is fully operational. Genetic crosses against known mutations conferring white colony color on BiGGY agar identified the gene leading to reduced H(2)S formation as an allele of MET10 (MET10-932), which encodes a catalytic subunit of sulfite reductase. Sequence analysis of MET10-932 revealed several corresponding amino acid differences in relation to laboratory strain S288C. Allele differences for other genes of the sulfate reduction pathway were also detected in UCD932. The MET10 allele of UCD932 was found to be unique in comparison to the sequences of several other vineyard isolates with differing levels of production of H(2)S. Replacing the MET10 allele of high-H(2)S-producing strains with MET10-932 prevented H(2)S formation by those strains. A single mutative change, corresponding to T662K, in MET10-932 resulted in a loss of H(2)S production. The role of site 662 in sulfide reduction was further analyzed by changing the encoded amino acid at this position. A change back to threonine or to the conservative serine fully restored the H(2)S formation conferred by this allele. In addition to T662K, arginine, tryptophan, and glutamic acid substitutions similarly reduced sulfide formation.

Differential expression of thiamine biosynthetic genes in yeast strains with high and low production of hydrogen sulfide during wine fermentation

AIMS

Release of hydrogen sulfide by fermenting yeast is a potential problem in wine production, because of its strong organoleptic impact. To identify the genetic determinants of sulfide production, we compared the transcriptomes of two wine yeast strains with similar oenological properties, but with very different levels of sulfide production, UDC522 (high sulfide producer) and P29 (low producer).

METHODS AND RESULTS

Oenological microfermentations were sampled at the peak production of sulfide. Transcription profiles of the two strains were analysed by three methods, a cDNA-based array, an oligonucleotide-based array and qRT-PCR analysis of selected transcripts. Less than 10% of yeast genes showed significant differences between the two strains. High sulfide production correlated with a general overexpression of thiamine biosynthesis genes, whereas genes linked to the catabolism of sulfur-containing compounds (like amino acids) showed no significant expression differences between both strains.

CONCLUSIONS

Our data suggest a relationship between the thiamine biosynthetic pathway and sulfide production during wine fermentation.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides a first hint which indicates that for some yeast strains, biosynthesis of thiamine (and perhaps of other sulfur-containing compounds) may be more relevant than the general nitrogen metabolism in explaining sulfide production by some yeast strains during vinification, defining new targets for genetic improvement of wine yeast strains.

The production of hydrogen sulphide and other aroma compounds by wine strains of Saccharomyces cerevisiae in synthetic media with different nitrogen concentrations

The aim of this study was to evaluate the combined effect of initial nitrogen content on the production of hydrogen sulphide and other volatile compounds during alcoholic fermentation. For that propose, three commercial wine strains of Saccharomyces cerevisiae were used to ferment synthetic grape juice media under different nitrogen concentrations. H(2)S was measured throughout fermentations and other aroma compounds were analyzed at the end of the experiments. The trigger levels at which an inverse relationship between the initial nitrogen present in media and total H(2)S production varied among the three strains tested. For UCD522 and PYCC4072, the highest H(2)S levels were produced in media with 267 mg N l(-1) of initial nitrogen, whereas the lowest levels were detected with nitrogen limitation/starvation conditions (66 mg N l(-1)). Moreover, 21 other aroma compounds belonging to different chemical classes were identified and quantified by solid phase micro-extraction (SPME) coupled to gas chromatography-mass spectrometry (GC-MS). The initial nitrogen concentration more than yeast strain had a decisive effect on the final aroma composition, suggesting that modulation of nutrients emerges as a useful tool for producing desired flavour and odour compounds.