Indications of the prominent role of elemental sulfur in the formation of the varietal thiol 3-mercaptohexanol in Sauvignon blanc wine

Hydrogen sulfide production during early yeast fermentation correlates with volatile sulfur compound biogenesis but not thiol release

Yeasts undergo intensive metabolic changes during the early stages of fermentation. Previous reports suggest the early production of hydrogen sulfide (H2S) is associated with the release of a range of volatile sulfur compounds (VSCs), as well as the production of varietal thiol compounds 3-sulfanylhexan-1-ol (3SH) and 3-sulfanylhexyl acetate (3SHA) from six-carbon precursors, including (E)-hex-2-enal. In this study, we investigated the early H2S potential, VSCs/thiol output, and precursor metabolism of 11 commonly used laboratory and commercial Saccharomyces cerevisiae strains in chemically defined synthetic grape medium (SGM) within 12 h after inoculation. Considerable variability in early H2S potential was observed among the strains surveyed. Chemical profiling suggested that early H2S production correlates with the production of dimethyl disulfide, 2-mercaptoethanol, and diethyl sulfide, but not with 3SH or 3SHA. All strains were capable of metabolizing (E)-hex-2-enal, while the F15 strain showed significantly higher residue at 12 h. Early production of 3SH, but not 3SHA, can be detected in the presence of exogenous (E)-hex-2-enal and H2S. Therefore, the natural variability of early yeast H2S production contributes to the early output of selected VSCs, but the threshold of which is likely not high enough to contribute substantially to free varietal thiols in SGM.

Modulation of Volatile Thiol Release during Fermentation of Red Musts by Wine Yeast

During the alcoholic fermentation of grape sugars, wine yeast produces a range of secondary metabolites that play a critical role in the aroma profile of wines. One of the most impactful yeast-modified compound families, particularly in white wines, are the ‘fruity’ polyfunctional thiols, which include 3-mercaptohexan-1-ol (3-MH) and 4-mercapto-4-methylpentan-2-one (4-MMP). While the formation and stylistic contribution of these thiols have been extensively researched in white wines, little is known about the conditions leading to their formation in red wines. In this study, we explored the ability of yeast strains to modulate the release of these aroma compounds during the fermentation of two red musts. In laboratory-scale Pinot Noir fermentations, the formation of 3-MH strongly correlated with yeast β-lyase activity, particularly with the presence of certain genotypes of the flavour-releasing gene IRC7. Subsequent production of Grenache wine at the pilot scale, with detailed compositional and sensory analysis, was undertaken to confirm laboratory-scale observations. A commercial wine strain used for expressing ‘fruity’ thiols in Sauvignon Blanc was shown to produce wines that exhibited more intense red fruit aromas. These results reveal an opportunity for winemakers to shape red wine aroma and flavour by using yeasts that might typically be considered for white wine production.

Time course accumulation of polysulfides in Chardonnay and model juice fermentations

Over the last five years, polysulfides in wine have become a topic of interest. The finding that both yeast and the wine composition could contribute to the formation and evolution of these compounds has prompted several studies, further corroborated by the discovery of a link between polysulfides and H₂S release during ageing. In the present study, the accumulation of cysteinylated and glutathionylated polysulfides was followed during fermentation for the first time. Synthetic grape media and Chardonnay juice treated with CuSO₄, elemental sulfur or both were fermented, and subsequently analysed using an in-house UHPLC-HRMS method. Differences in polysulfide accumulation were observed between the two media, highlighting the role of the matrix composition. Elemental sulfur, especially when combined with CuSO₄, resulted in significantly increased accumulations of polysulfides compared to controls in both media. Polysulfides with longer S-chains generally appeared later in the fermentation, an observation that prompted further questions on these metabolites' formation.

Sulfane Sulfur Compounds as Source of Reappearance of Reductive Off-Odors in Wine

Reactive compounds with one or more sulfane sulfur atoms can be an important source of reductive off-odors in wine. These substances contain labile sulfur, which can participate in microbiological (enzymatic) and chemical transformations (including in the post-bottling period), releasing malodorous hydrogen sulfide (H2S) and its derivatives (MeSH, EtSH, etc.). The following sulfane sulfur compounds were considered in this review as important precursors in the wine chemistry of reductive aromas: elemental sulfur (S8), persulfides (R-S-S-H), polysulfanes (R-Sn-R(′)), polythionates (−O3S-Sn-SO3−), thiosulfate (S2O32−) and derivatives of (poly)sulfane monosulfonic acids (R-Sn-SO3H). This review discusses the formation of these compounds, their reactivity and chemical transformations in wine, including reactions of nucleophilic substitution. In particular, the reactions of thiolysis, thiosulfatolysis and sulfitolysis of sulfane sulfur compounds are described, which lead in the end to reductive aroma compounds. In this way, the review attempts to shed light on some of the mysteries in the field of sulfur chemistry in wine and the reappearance of reductive off-odors after bottling.

A single run liquid chromatography-tandem mass spectrometry method for the analysis of varietal thiols and their precursors in wine

This research details the development of a novel LC-MS/MS method involving a QuEChERS-based extraction that, for the first time, achieves the fast and concurrent analysis of both free varietal thiols (3-sulfanylhexan-1-ol, 3-sulfanylhexyl acetate, and 4-methyl-4-sulfanylpentan-2-one) and their S-glutathionylated/S-cysteinylated precursors in wine samples. Limits of detections for the free thiols were comparable to or lower than those obtained with previously reported more laborious methods. Repeatability for the free thiols ranged between 3-19% and from 0.1-7.7% for the thiol precursors. The method was subsequently applied to the analysis of a selection of 22 wines from a range of different grape varieties. The rapidity of this method combined with its sensitivity makes it suitable for both research and industry applications.

Volatile-Based Prediction of Sauvignon Blanc Quality Gradings with Static Headspace-Gas Chromatography-Ion Mobility Spectrometry (SHS-GC-IMS) and Interpretable Machine Learning Techniques

The analytical scope of static headspace-gas chromatography-ion mobility spectrometry (SHS-GC-IMS) was applied to wine aroma analysis for the first time. The method parameters were first fine-tuned to achieve optimal analytical results, before the method stability was demonstrated, in terms of repeatability and reproducibility. Succinct qualitative identification of compounds was also realized, with the identification of several volatiles that have seldom been described previously in Sauvignon Blanc wine, such as methyl acetate, ethyl formate, and amyl acetate. Using the SHS-GC-IMS data in an untargeted approach, computer modeling of large datasets was applied to link aroma chemistry via prediction models to wine sensory quality gradings. Six machine learning models were compared, and artificial neural network (ANN) returned the most promising performance with a prediction accuracy of 95.4%. Despite its inherent complexity, the ANN model offered intriguing insights on the influential volatiles that correlated well with higher and lower sensory gradings. These findings could, in the future, guide winemakers in establishing wine quality, particularly during blending operations prior to bottling.

Effects of antioxidant and elemental sulfur additions at crushing on aroma profiles of Pinot Gris, Chardonnay and Sauvignon Blanc wines

The antioxidants sulfur dioxide (50 ppm) and ascorbic acid (100 ppm) were added to grapes soon after harvest at crushing. The chemical composition and sensory profile of Sauvignon Blanc, Pinot Gris and Chardonnay wines were examined, made from grapes collected at three different sites for each variety. With good antioxidant protection of the juices, reflected in low absorbances at 420 nm, remarkable increases in the polyfunctional mercaptans, 3-mercaptohexanol (3MH) and its acetate ester (3MHA), were seen in the wines. Moreover, high levels of these compounds were produced in the Pinot Gris and Chardonnay wines, equally high as with Sauvignon Blanc. The Pinot Gris wines maintained varietal characteristics in sensory profiles, even with high levels of polyfunctional mercaptans. When elemental sulfur was included with the grapes at crushing, extra increases in polyfunctional mercaptans were observed. However, this led to the production of unwanted reductive aroma compounds in some wines.

Identification and characterisation of thiolated polysulfides in must and wine using online SPE UHPLC-HRMS

3-Mercaptohexanol and 4-methyl-4-mercaptopentanone are volatile thiols with a low perception threshold and are found in relatively low concentrations in several types of wines, e.g. Sauvignon blanc. They contribute positively to the flavour of a wine when in their free form, but they can oxidise, especially in the presence of Cu²⁺ and sulfur residues originating from pesticide treatments on the grapes. This condensation reaction results in the formation of polysulfides, which during storage can cause the release of H₂S: a compound known to give rise to off-flavour in wine. The formation of these polysulfides has been proposed to originate from cysteinyl and glutathionyl S-conjugate precursors, but they have not yet been characterised. In this work, a method using ultra-high-performance liquid chromatography coupled to high-resolution mass spectrometry and online solid-phase extraction was designed and optimised for detection of these S-conjugates. The method allowed the detection of 21 suspected symmetrical and asymmetrical S-conjugates in thiol-supplemented aqueous solutions, of which 17 were also recovered in supplemented synthetic musts and wines. Moreover, the proposed method was used to investigate polysulfide formation upon addition of two different types of sulfur. Differences in formation of S-conjugates were evident in the synthetic samples: a higher relative abundance was observed upon addition of wettable sulfur compared to washed sulfur. For the commercial wines, differences in polysulfide formation were minor and merely related to the differences between the wines and not to the type of sulfur added.

Protective Effect of foliar application of sulfur on photosynthesis and antioxidative defense system of rice under the stress of Cd

This paper investigates the effect of foliar application of sulfur on photosynthesis and antioxidative defense system of rice under the stress of Cd. The initial field studies showed that foliar spray of S was effective for reducing Cd concentration in rice and increasing the grain yield. However, the physiological mechanisms remain less clear on how the foliar application of S alleviates Cd toxicity in rice. Chlorophyll fluorescence, as a measure of photosynthesis, was taken to estimate the efficiency of photosystem II (PSII) photochemistry after the foliar application of S. The increase of photosynthetic parameters, i.e. the maximum photochemical efficiency of PSII reaction center (Fv/Fm), the actual PSII photochemical efficiency (ΦPSII), the photochemical quenching coefficient (qP), indicated that the foliar treatment alleviated the toxicity of Cd to PSII. The decrease of non-photochemical quenching coefficient (NPQ) indicated the increase of photochemical reaction efficiency with more absorbed light energy for photochemical reactions. Fourier Transform Infrared (FTIR) spectra showed that the foliar treatment stimulated the syntheses of lignin, lipids, aliphatic acid, polysaccharides, carboxylate and proteins. Micrographs of transmission electron microscope (TEM) also revealed the reduced mobility of Cd in cells. Foliar application of S effectively reduced the damage of Cd stress by maintaining the integrity of cell structure and participating in metabolic activities such as protein synthesis.

Comparison of an Offline SPE-GC-MS and Online HS-SPME-GC-MS Method for the Analysis of Volatile Terpenoids in Wine

The aroma profile is an important marker for wine quality. Various classes of compounds are responsible for the aroma of wine, and one such class is terpenoids. In the context of this work, a validated gas chromatography-mass spectrometry (GC-MS) method for the quantitation of terpenoids in red and white wine using headspace solid-phase microextraction (HS-SPME) and solid-phase extraction (SPE) was established. Calibrations were performed in the respective base wine using both sample preparation methods. The linearity, precision and accuracy evaluated for the respective matrices were excellent for both sample preparations. However, the HS-SPME approach was more sensitive and more accurate. For both sample preparations, the quantification limits were lower than the odor thresholds in wine. The terpenoid concentrations (µg/L) were evaluated for 13 white wines using both sample preparation methods. Importantly, the online HS-SPME approach was more sensitive than the offline SPE method. The major terpenoids identified in the white wines evaluated were linalool (0.2-63 µg/L), geraniol (nd-66 µg/L) and α-terpineol (nd-85 µg/L).

Mitigation of Cd accumulation in rice from Cd-contaminated paddy soil by foliar dressing of S and P

Cadmium (Cd)-contaminated paddy soil has become a global agricultural safety issue. The application of foliage dressing with mineral elements to alleviate Cd toxicity in rice might offer a cost-effective and practical strategy for safe food production. In this study, a pot experiment was conducted to optimize foliar composition and dosage. Field experiments in two consecutive rice seasons were performed to investigate the effectiveness and mechanisms of foliage dressing. Foliar spray of S, P, and a mixture of both were effective to reduce the Cd concentration in rice grain. The maximum decrease by leaf-grain translocation was achieved at 84%, and the maximum decrease of bio-concentration was 69% in the stem. The reduction of Cd concentration in rice decreased the direct damage to the photosynthetic system, and then increased the rice growth. Foliage dressing relieved the oxidative stress of Cd to rice by decreasing the MDA content, and increasing antioxidant enzyme activities. Foliar spray with S likely reduced Cd accumulation in rice by minimizing the production of reactive oxygen species, improving the activities of enzymatic and non-enzymatic antioxidant defense systems, and manipulating glutathione synthesis. The detoxification of foliar spray with P was originated from the decrease of Cd translocation and maintaining photosynthetic machinery. These results indicated that foliage dressing with S and P has great potential for the remediation of vast agricultural fields.

A Visible Colorimetric Fluorescent Probe for Hydrogen Sulfide Detection in Wine

A new efficient and practical fluorescent probe 6-(benzo[d]thiazol-2-yl)naphthalen-2-yl-thiophene-2-carboxylate (probe 1) was synthesized to detect hydrogen sulfide (H₂S). The addition of H₂S caused the solution of probe 1 to change from colorless to yellow, and the solution of probe 1 changes to different colors with respect to different concentrations of H₂S. Importantly, probe 1 could help detect H₂S efficiently by a distinct color response as a visible detection agent. Probe 1 reacted with various concentrations of H₂S (0-200 μM), and the detection limit for H₂S was 0.10 μM. Particularly, probe 1 can be applied as a sensor to detect H₂S accurately in wine samples.

Recent Developments on the Origin and Nature of Reductive Sulfurous Off-Odours in Wine

Reductive sulfurous off-odors are still one of the main reasons for rejecting wines by consumers. In 2008 at the International Wine Challenge in London, approximately 6% of the more than 10,000 wines presented were described as faulty. Twenty-eight percent were described as faulty because they presented “reduced characters” similar to those presented by “cork taint” and in nearly the same portion. Reductive off-odors are caused by low volatile sulfurous compounds. Their origin may be traced back to the metabolism of the microorganisms (yeasts and lactic acid bacteria) involved in the fermentation steps during wine making, often followed by chemical conversions. The main source of volatile sulfur compounds (VSCs) are precursors from the sulfate assimilation pathway (SAP, sometimes named as the “sulfate reduction pathway” SRP), used by yeast to assimilate sulfur from the environment and incorporate it into the essential sulfur-containing amino acids methionine and cysteine. Reductive off-odors became of increasing interest within the last few years, and the method to remove them by treatment with copper (II) salts (sulfate or citrate) is more and more questioned: The effectiveness is doubted, and after prolonged bottle storage, they reappear quite often. Numerous reports within the last few years and an ongoing flood of publications dealing with this matter reflect the importance of this problem. In a recent detailed review, almost all relevant aspects were discussed on a scientific data basis, and a “decision tree” was formulated to support winemakers handling this problem. Since we are dealing with a very complicated matter with a multitude of black spots still remaining, these advices can only be realized using specific equipment and special chemicals, not necessarily found in small wineries. The main problem in dealing with sulfurous compounds arises from the high variability of their reactivities. Sulfur is a metalloid with a large valence span across eight electron transformations from S (−II) up to S (+VI). This allows it to participate in an array of oxidation, reduction and disproportionation reactions, both abiotic and linked to microbial metabolism. In addition, sulfur is the element with the most allotropes and a high tendency to form chains and rings, with different stabilities of defined species and a high interconvertibility among each other. We suppose, there is simply a lack of knowledge of what is transferred during filling into bottles after fermentation and fining procedures. The treatment with copper (II) salts to remove sulfurous off-odors before filling rather increases instead of solving the problem. This paper picks up the abundant knowledge from recent literature and tries to add some aspects and observations, based on the assumption that the formation of polythionates, hitherto not taken into consideration, may explain some of the mystery of the re-appearance of reductive off-odors.

The impact of postharvest ultra-violet light irradiation on the thiol content of Sauvignon blanc grapes

Sauvignon blanc grapes were exposed to an ultra-violet (UV) light source post-hand harvest (whole bunches) or post-machine harvest. The thiol precursors S-3-(hexan-1-ol)-l-cysteine (Cys-3MH) and S-3-(hexan-1-ol)-l-glutathione (GSH-3MH) were quantified in the juices before and after UV treatment. Results showed that irradiation of the grapes with UV light had little to no effect on the thiol precursors. Wines were fermented from the corresponding juices and 18 aroma compounds were quantified. Differences were found between UV treatments of the wines for 3-mercaptohexanol, hexan-1-ol, ethyl butanoate, ethyl hexanoate, ethyl octanoate and phenylethyl alcohol. However, these changes were not significant (p < 0.05) for both grape media trialled. Future studies involving larger sample sizes and replicate numbers should be completed in order to ascertain any changes in aroma chemistry as a result of UV light application to grapes postharvest.

Iminiumsalz-Strukturen bei der durch Pyridoxalphosphat (Vitamin B6) katalysierten Bildung von Aromastoffen und Fehlaromen im Wein

Enzymes that use pyridoxal phosphate (PLP, Vitamin B6) as cofactor constitute a ubiquitous class of biocatalysts. A variety of PLP-dependant enzymes mainly involved in biochemical pathways concerning amino acid metabolism are found in all forms of life. These enzymes also play an important role in wine production, as well in grape growing as in enological processes. The formation of pleasant aroma compounds often runs with participation of pyridoxal-dependant enzymes. But these are also brought into context with the formation of off-odors, especially from sulfur compounds (i.e. sulfur containing amino acids cysteine, methionine). The versatility of PLP-dependant bioreactions arises from its ability to covalently bind the substrate and then to function as an electrophilic catalyst, thereby stabilizing different types of carbanionic reaction intermediates, containing iminium salt structures. This article summarizes the influence of PLP on sensorically important aroma compounds in wine growing and wine processing.

Role of Elemental Sulfur in Forming Latent Precursors of H2S in Wine

The level of hydrogen sulfide (H₂S) can increase during abiotic storage of wines, and potential latent sources of H₂S are still under investigation. We demonstrate that elemental sulfur (S⁰) residues on grapes not only can produce H₂S during fermentation but also can form precursors capable of generating additional H₂S after bottle storage for 3 months. H₂S could be released from S⁰-derived precursors by addition of a reducing agent (TCEP), but not by addition of strong brine to induce release of H₂S from metal sulfide complexes. The size of the TCEP-releasable pool varied among yeast strains. Using the TCEP assay, multiple polar S⁰-derived precursors were detected following normal-phase preparative chromatography. Using reversed-phase liquid chromatography and high-resolution mass spectrometry, we detected an increase in the levels of diglutathione trisulfane (GSSSG) and glutathione disulfide (GSSG) in S⁰-fermented red wine and an increase in the levels of glutathione S-sulfonate (GSSO₃^-) and tetrathionate (S₄O₆^2-) in S⁰-fermented white wine as compared to controls. GSSSG, but not S₄O₆^2-, was shown to evolve H₂S in the presence of TCEP. Pathways for the formation of GSSSG, GSSG, GSSO₃^-, and S₄O₆^2- from S⁰ are proposed.

Yeast genes involved in regulating cysteine uptake affect production of hydrogen sulfide from cysteine during fermentation

An early burst of hydrogen sulfide (H2S) produced by Saccharomyces cerevisiae during fermentation could increase varietal thiols and therefore enhance desirable tropical aromas in varieties such as Sauvignon Blanc. Here we attempted to identify genes affecting H2S formation from cysteine by screening yeast deletion libraries via a colony colour assay on media resembling grape juice. Both Δlst4 and Δlst7 formed lighter coloured colonies and produced significantly less H2S than the wild type on high concentrations of cysteine, likely because they are unable to take up cysteine efficiently. We then examined the nine known cysteine permeases and found that deletion of AGP1, GNP1 and MUP1 led to reduced production of H2S from cysteine. We further showed that deleting genes involved in the SPS-sensing pathway such as STP1 and DAL81 also reduced H2S from cysteine. Together, this study indirectly confirms that Agp1p, Gnp1p and Mup1p are the major cysteine permeases and that they are regulated by the SPS-sensing and target of rapamycin pathways under the grape juice-like, cysteine-supplemented, fermentation conditions. The findings highlight that cysteine transportation could be a limiting factor for yeast to generate H2S from cysteine, and therefore selecting wine yeasts without defects in cysteine uptake could maximise thiol production potential.