The effects of copper fining on the wine content in sulfur off-odors and on their evolution during accelerated anoxic storage

Enhancing wine shelf-life: Insights into factors influencing oxidation and preservation

Background

Understanding the shelf life of wine is complex and involves factors such as aroma preservation, flavour development and market acceptance. Ageing potential, crucial for flavour complexity, exposes wine to oxidation, influenced by oxygen, temperature and light, with an impact on quality. This type of oxidation is non-enzymatic, is catalyzed by metal ions and alters colour and flavour.

Scope and approach

This review examines the dynamics of wine preservation, focusing on oxidation and the impact of closure. Corks allow controlled oxygen transfer, while screw caps offer a nearly hermetic closure. Oxygen transfer rates vary, with natural corks having fluctuating rates and synthetic corks causing over-exposure. Additives such as sulphur dioxide and alternative substitute such as lysozyme and ascorbic acid are examined for their role in preventing oxidation and ensuring microbiological stability.

Key findings and conclusions

Closure choice significantly affects wine preservation. Balancing oxygen exposure, temperature, and light is vital. Effective management, including the strategic use of preservatives and additives, is crucial for maintaining quality and extending shelf life. This review underscores the delicate equilibrium necessary for preserving wine quality from production to consumption.

The distribution of beneficial mutational effects between two sister yeast species poorly explains natural outcomes of vineyard adaptation

Domesticated strains of Saccharomyces cerevisiae have adapted to resist copper and sulfite, two chemical stressors commonly used in winemaking. S. paradoxus, has not adapted to these chemicals despite being consistently present in sympatry with S. cerevisiae in vineyards. This contrast represents a case of apparent evolutionary constraints favoring greater adaptive capacity in S. cerevisiae. In this study, we used a comparative mutagenesis approach to test whether S. paradoxus is mutationally constrained with respect to acquiring greater copper and sulfite resistance. For both species, we assayed the rate, effect size, and pleiotropic costs of resistance mutations and sequenced a subset of 150 mutants isolated from our screen. We found that the distributions of mutational effects displayed by the two species were very similar and poorly explained the natural pattern. We also found that chromosome VIII aneuploidy and loss of function mutations in PMA1 confer copper resistance in both species, whereas loss of function mutations in REG1 were only a viable route to copper resistance in S. cerevisiae. We also observed a single de novo duplication of the CUP1 gene in S. paradoxus but none in S. cerevisiae. For sulfite, loss of function mutations in RTS1 and KSP1 confer resistance in both species, but mutations in RTS1 have larger average effects in S. paradoxus. Our results show that even when the distributions of mutational effects are largely similar, species can differ in the adaptive paths available to them. They also demonstrate that assays of the distribution of mutational effects may lack predictive insight concerning adaptive outcomes.

A Review on Wine Flavour Profiles Altered by Bottle Aging

The wine flavour profile directly determines the overall quality of wine and changes significantly during bottle aging. Understanding the mechanism of flavour evolution during wine bottle aging is important for controlling wine quality through cellar management. This literature review summarises the changes in volatile compounds and non-volatile compounds that occur during wine bottle aging, discusses chemical reaction mechanisms, and outlines the factors that may affect this evolution. This review aims to provide a deeper understanding of bottle aging management and to identify the current literature gaps for future research.

Construction of a water-soluble fluorescent probe for copper (II) ion detection in live cells and food products

The quality of wine can be affected by excess Cu²⁺ due to the occurrence of oxidation reactions or precipitation. Therefore, it is essential to use simple and effective testing methods to ensure the Cu²⁺ content in wine. In this work, we designed and synthesized a rhodamine polymer fluorescent probe (PEG-R). The water solubility of PEG-R was improved by the introduction of polyethylene glycol, which improved the performance and broadened its application in the food field. The PEG-R was characterized by high sensitivity, selectivity and fast response to Cu²⁺ and was able to complete the response process within 30 s, with approximately 29-fold fluorescence enhancement of the probe after exposure to Cu²⁺, the limit of detection (LOD) was 1.295 × 10^-6 M. The probe can be used for the determination of Cu²⁺ in living cells, zebrafish, white wine and food products, and it was made into practical gels and test strips.

Copper in grape and wine industry: Source, presence, impacts on production and human health, and removal methods

Heavy metals are of particular concern in grape and wine processing, especially copper. The sources of copper are diverse, including vineyard soil, copper-containing pesticides on the fruit surface, copper wine-making equipment, and exogenous addition in winemaking. Copper has potential risks to human nerves, metabolism, and others. It can inhibit yeast growth, delay fermentation, and also mediate oxidation reactions, which has a huge impact on the nutritional quality and sensory quality of fresh wine and aged wine. It is therefore crucial to detect, quantify, and remove copper from grapes and wine. However, the copper situations in the wine industries of various countries are complicated and diverse, and the existing forms of copper are quite different, which makes the research challenging. This review summarizes and analyzes the existence and influence of copper in the wine industry by analyzing the sources of, the current situation regarding, and the detection and removal methods for copper in wine. With the study, a better understanding of copper's impact on wine production will be gained, facilitating further control of copper in wine and helping the wine industry grow.

Response Surface Methodology Approach to Evaluate the Effect of Transition Metals and Oxygen on Photo-Degradation of Methionine in a Model Wine System Containing Riboflavin

A Box-Behnken experimental design was implemented in model wine (MW) to clarify the impact of copper, iron, and oxygen in the photo-degradation of riboflavin (RF) and methionine (Met) by means of response surface methodology (RSM). Analogous experiments were undertaken in MW containing caffeic acid or catechin. The results evidenced the impact of copper, iron, and oxygen in the photo-induced reaction between RF and Met. In particular, considering a number of volatile sulfur compounds (VSCs) that act as markers of light-struck taste (LST), both transition metals can favor VSC formation, which was shown for the first time for iron. Oxygen in combination can also affect the concentration of VSCs, and a lower content of VSCs was revealed in the presence of phenols, especially caffeic acid. The perception of "cabbage" sensory character indicative of LST can be related to the transition metals as well as to the different phenols, with potentially strong prevention by phenolic acids.

Wine Minerality and Funkiness: Blending the Two Tales of the Same Story

In wines, minerality is a complex concept with increasing popularity in scientific research and the wine press. The flavour conceptual space of mineral wines comprises sulphur-reduced aromas, such as flint, wet stone or chalk associated with freshness and lingering mouth perceptions. Professionals do not consider the perception of sulphur-reduced flavours as an off-flavour. Indeed, this sort of reduction is a cue for the recognition of minerality under a likely top-down mental process. However, untrained consumers perceive these aromas as unpleasant. This different qualitative assessment hampers the communication between professionals and amateurs. This review aimed to describe the perceptions of minerality by experts and novices to promote their mutual understanding. Funkiness is proposed as a descriptor of mineral wines when tasted by unexperienced consumers. The chemical basis of minerality and winemaking options were explored to understand their implications on sensory perception. Mineral flavours have two main features. The first comprises ephemeral aromas that may be described as funky, given their association with sulphur-reduced molecules. The second is linked to the fresh and vivacious lingering mouthfeel perceptions that remain after the reductive aromas vanish. Consumers recognise this dual perception by demonstrating positive emotional responses of surprise during tasting. Then, the perception of minerality is a question of cognition and not of particularly developed sensory skills. Appropriate tasting approaches encompassing emotional responses and emergent properties (e.g., harmony, depth, persistence, complexity) appear essential to understand the nature of wine minerality and to determine when it may be regarded as a surrogate for fine wine quality.

Wine Faults: State of Knowledge in Reductive Aromas, Oxidation and Atypical Aging, Prevention, and Correction Methods

The review summarizes the latest scientific findings and recommendations for the prevention of three very common wine faults of non-microbial origin. The first group, presented by the reductive aromas, is caused mainly by excessive H₂S and other volatile sulfur compounds with a negative impact on wine quality. The most efficient prevention of undesirable reductive aromas in wine lies in creating optimal conditions for yeast and controlling the chemistry of sulfur compounds, and the pros and cons of correction methods are discussed. The second is browning which is associated especially with the enzymatic and non-enzymatic reaction of polyphenols and the prevention of this fault is connected with decreasing the polyphenol content in must, lowering oxygen access during handling, the use of antioxidants, and correction stands for the use of fining agents. The third fault, atypical aging, mostly occurs in the agrotechnics of the entire green land cover in the vineyard and the associated stress from lack of nutrients and moisture. Typical fox tones, naphthalene, or wet towel off-odors, especially in white wines are possible to prevent by proper moisture and grassland cover and alternating greenery combined with harmonious nutrition, while the correction is possible only partially with an application of fresh yeast. With the current knowledge, the mistakes in wines of non-microbial origin can be reliably prevented. Prevention is essential because corrective solutions for the faults are difficult and never perfect.

Suppression of reductive characters in white wine by Cu fractions: Efficiency and duration of protection during bottle aging

Cu in wine can suppress sulfidic-odours, but the active forms and duration of protection are uncertain. Additions of 0, 0.3 or 0.6 mg/L Cu(II) were made to Chardonnay and Pinot Grigio at bottling. Throughout a 12- or 14-month storage period, Cu fractions were determined by colorimetry, and sulfhydryl compounds by gas chromatography with sulfur chemiluminescence detection. After Cu(II) addition, the dominant Cu fractions were associated with Cu(II)-organic acids (fraction I) and Cu(I)-thiol complexes (fraction II), and over 8-months their concentrations gradually fell below 0.015 mg/L. During this time, a fraction of Cu, predominantly attributed to sulfide-bound Cu, increased in concentration. Suppression of free hydrogen sulfide was assured when the combined Cu fractions I and II concentrations were above 0.015 mg/L, while free methanethiol suppression required Cu fraction I concentration above 0.035 mg/L. Decay rates for Cu fractions demonstrated that the duration that Cu can actively suppress sulfidic odours is wine-dependent.

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 Highly Selective and Sensitive Colorimetric Chemosensor for the Detection of Hydrogen Sulfide: Real-time Applications in Multiple Platforms

Calorimetric chemosensors are found to be advantageous sensing systems due to their simplicity and favorable responsive properties. Although some colorimetric probes have been reported to detect hydrogen sulfide (H₂ S), the creation of rapid, highly selective, and sensitive probes for the detection of H₂ S remains a challenging target. In this work, we established dinitrosulphonamide decorated phenanthridine, 2,4-dinitro-N-(4-(7,8,13,14-tetrahydrodibenzo[a, i]phenanthridin-5-yl)phenyl)benzenesulfonamide (PHSH), for the calorimetric detection of H₂ S. H₂ S triggered thiolysis of PHSH resulted in a marked absorption enhancement alongside a visual color change from colorless to dark yellow. The result indicated that the chemosensor showed high sensitivity and selectivity with a fast response of less than 10 s with a detection limit as low as 6.5 nM. The chemosensor reaction mechanism with H₂ S was studied by UV-vis, ¹ H NMR, mass and HPLC analysis. In addition, the chemosensor has been used for the determination of H₂ S in many real-time samples.

The effects of Saccharomyces cerevisiae strains carrying alcoholic fermentation on the fermentative and varietal aroma profiles of young and aged Tempranillo wines

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10 Saccharomyces cerevisiae strains fermented must with phenolics and aroma precursors.

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Isobutanal, isopropyl isoamyl acetates, and ethyl propanoate lost by evaporation.

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Yeast strain affects levels of 45 out of 60 aroma compounds mostly after aging.

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Linalool and geraniol fermentative aroma compounds.

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Strong modulation of varietal aroma. Strains can limit levels of guaiacol or TDN.

Ten different Saccharomyces cerevisiae strains fermented semi-synthetic musts containing a Polyphenolic and Aroma Precursor Fraction (PAF) extracted from Tempranillo grapes. Aroma compounds were studied by Gas Chromatography (GC), GC-Olfactometry and GC-Mass Spectrometry (MS), during fermentation by trapping volatilized aroma, immediately after fermentation and after accelerated aging. Volatiles lost by evaporation during fermentation are mostly fermentative compounds and not grape-related odorants. Isobutanal and some esters are mostly lost during fermentation. In many cases the impact of yeast strain is evident only after aging. Strains could be classified into 3 major clusters with marked differences in fermentative and varietal profiles. Linalool and geraniol were found to have fermentative origin. S. cerevisiae yeast strains can effectively modulate varietal aroma, likely through specific enzymatic activities acting on grape phenolic acids and norisoprenoid aroma precursors and may be specifically used to mitigate some aging-related off odours, such as massoia lactone, guaiacol or TDN.

Bottle Aging and Storage of Wines: A Review

Wine is perhaps the most ancient and popular alcoholic beverage worldwide. Winemaking practices involve careful vineyard management alongside controlled alcoholic fermentation and potential aging of the wine in barrels. Afterwards, the wine is placed in bottles and stored or distributed in retail. Yet, it is considered that wine achieves its optimum properties after a certain storage time in the bottle. The main outcome of bottle storage is a decrease of astringency and bitterness, improvement of aroma and a lighter and more stable color. This is due to a series of complex chemical changes of its components revolving around the minimized and controlled passage of oxygen into the bottle. For this matter, antioxidants like sulfur oxide are added to avoid excessive oxidation and consequent degradation of the wine. In the same sense, bottles must be closed with appropriate stoppers and stored in adequate, stable conditions, as the wine may develop unappealing color, aromas and flavors otherwise. In this review, features of bottle aging, relevance of stoppers, involved chemical reactions and storage conditions affecting wine quality will be addressed.

Liquid Chromatography-Mass Spectrometry-Based Metabolomics for Understanding the Compositional Changes Induced by Oxidative or Anoxic Storage of Red Wines

The aim of this work was to study the physicochemical changes of eight red wines stored under conditions differing in O₂ exposure and temperature and time under anoxia. The methods used to analyze the wines included the measurement of volatile sulfur compounds, color, tannin (T) polymerization, and liquid chromatography-mass spectrometry untargeted metabolomic fingerprint. After 3 months, the color of the oxidized samples evolved 4-5 times more intensively than in wines stored under anoxia. The major metabolomic differences between oxidative and anoxic conditions were linked to reactions of acetaldehyde (favored in oxidative) and SO₂ (favored in anoxia). In the presence of oxygen, the C-4 carbocation of flavanols delivered ethyl-linked tannin-anthocyanin (T-A) and tannin-tannin (T-T) adducts, pyranoanthocyanins, and sulfonated indoles, while under reduction, the C-4 carbocation delivered direct linked T-A adducts, rearranged T-T adducts, and sulfonated tannins. Some of these last reactions could be related to the accumulation of reduced species, eventually ending with reductive off-odors.

Determination of Hydrogen Sulfide in Wines Based on Chemical-Derivatization-Triggered Aggregation-Induced Emission by High-Performance Liquid Chromatography with Fluorescence Detection

A chemical-derivatization-triggered aggregation-induced emission (AIE) method for the highly selective determination of hydrogen sulfide (H₂S) in wine matrices by high-performance liquid chromatography with fluorescence detection (HPLC-FLD) was developed. The detection strategy was developed based on the chemical derivatization of H₂S using a low-cost AIE-active fluorescence derivatization reagent, N-(3-iodine-2-oxopropyl)pyrene methamine (NIPM), to trigger specific AIE at 475 nm, which was red-shifted sharply to the maximum emission wavelength as compared with NIPM monomers of 375 nm, effectively quenching the interference from other thiol-containing compounds. With the aid of specific AIE and the effective separation of HPLC, the proposed method showed high selectivity and sensitivity toward H₂S. The limits of detection (LODs) at the sub-nM level of 0.25 nmol/L in the wine-beer sample and 0.30 nmol/L in red wine sample were obtained. To certify its applicability, this proposed strategy was successfully applied for the determination of H₂S in wine matrices.

Development of a new strategy for studying the aroma potential of winemaking grapes through the accelerated hydrolysis of phenolic and aromatic fractions (PAFs)

Current methods for assessing grape aroma potential are based on the fast hydrolysis of precursor fractions but provide hydrolyzates of poor aromatic quality. A new strategy based on the accelerated hydrolysis of reconstituted phenolic and aromatic fractions (PAFs) extracted from grapes is herein developed. PAFs are obtained by solid phase extraction on 10 g-C18 sorbents of partially dealcoholized "mistelle", obtained from grapes treated with ethanol. Under optimal conditions, PAFs contain all aroma precursors but the most polar ones, such as those of DMS, more than 85% of the total phenolics and just traces of metal cations and of amino acids. PAFs reconstituted in model wine, aged in strict anoxia 7 weeks at 45 °C or 24 h at 75 °C, develop strong aromas. At least 30 different odorants including lipid derivatives, volatile phenols, vanillins, norisoprenoids, terpenes, bencenoids and 3-mercaptohexanol were identified by GC-Olfactometry and GC-MS. Methodological aspects of the extraction, hydrolysis and analysis are optimized and discussed.

A novel coumarin-based fluorescent probe for sensitive detection of copper(II) in wine

A novel coumarin-based fluorescent probe (probe 1) for the detection of copper(II) was developed. The fluorescence intensity of probe 1 showed a linear relationship with the concentration of copper(II) in the range 0-16 μM (0-1.02 mg/L) and the limit of detection was 62 nM (3.94 µg/L). The luminescence of probe 1 at the maximum allowable amounts of copper(II) in wine and water could be observed with the naked eye under a 365-nm ultraviolet lamp. Moreover, probe 1 was successfully used for the qualitative and quantitative detection of copper(II) in wine.

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.

Liberation of Hydrogen Sulfide from Dicysteinyl Polysulfanes in Model Wine

Diorganopolysulfanes can be generated when hydrogen sulfide (H₂S) and thiols are oxidized in the presence of Cu(II) under conditions usually aimed at removing H₂S from wine. This work sought to understand if polysulfanes could act as latent sources of H₂S during postbottling storage. The stability of the polysulfanes formed in situ in model wine containing cysteine, H₂S, and transition metals was dependent both on the number of sulfur linking atoms (S_n) and on the presence of a reducing agent, such as sulfur dioxide or ascorbic acid. A polysulfane containing three linking sulfur atoms was the most stable, with 84% of the relative initial amount remaining in solution after six months, compared to polysulfanes containing four or more linking sulfur atoms that decomposed rapidly, with 26% remaining after six months. Importantly, sulfur dioxide was associated with the rapid degradation of polysulfanes and subsequent liberation of H₂S. Three cysteine- S-sulfonates were also tentatively identified, which gives insight into the possible release mechanisms involved with H₂S reappearance.

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.

Elusive Chemistry of Hydrogen Sulfide and Mercaptans in Wine

This paper summarizes, discusses, and complements recent findings about the fate of H₂S and methanethiol (MeSH) during wine storage. Analytical assays to determine free volatile sulfur compounds (VSCs) and brine-releasable (BR-) VSCs in combination with accelerated reductive (AR) aging and micro-oxygenation (MOX) assays allow characterizing the different categories of species able to produce H₂S and MeSH and the processes of interconversion. Each wine seems to contain a specific total amount of H₂S and MeSH distributed into free, metal-complexed, and oxidized forms (di and polysulfides) interconnected through reversible redox equilibria whose external expression is wine redox potential. Oxidation transforms all mercaptans likely into nonvolatile disulfides and hydrodisulfides. In anoxia, these molecules are spontaneously and quantitatively reduced back. The concomitant accumulation of major wine thiols would provoke complex dissociation and the release of free H₂S and MeSH. Additionally, total amounts can increase due to the metal-catalyzed desulfhydration of cysteine and methionine.

A Reaction-Based Novel Fluorescent Probe for Detection of Hydrogen Sulfide and Its Application in Wine

A new reaction-based fluorescent probe 6-cyanonaphthalen-2-yl-2,4- dinitrobenzenesulfonate (probe 1) was designed and synthesized for detection of hydrogen sulfide (H₂ S). The addition of H₂ S to a solution of probe 1 resulted in a marked fluorescence increased accompanied by a visual color change from colorless to yellow. Importantly, this distinct color response indicates that probe 1 could be used as a visual tool for detection of H₂ S. H₂ S can be detected quantitatively in the concentration range 0 to 25 μM and the detection limit was 30 nM. Moreover, probe 1 was successfully used as a sensor to determine H₂ S levels in red wine and beer.

PRACTICAL APPLICATION

Fluorescent probe 1 could be employed as a visible sensor for H₂ S. Probe 1 could be used to detect H₂ S quantitatively in food simple.

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.

Micro-oxygenation does not eliminate hydrogen sulfide and mercaptans from wine; it simply shifts redox and complex-related equilibria to reversible oxidized species and complexed forms

This work seeks to assess the effects of micro-oxygenation (MOX) on the present and potential levels of Volatile Sulfur Compounds (VSCs) of wine. With such purpose, three red wines with a tendency to develop sulfury off-odors were subjected to three different MOX conditions (4.4-20mg/L delivered at 0.05 or 0.2mg/L/day). Samples were further subjected to Accelerated Reductive aging (AR) and analyzed for free and Brine Releasable (BR) VSCs and redox potential. Although MOX induced strong decreases in the levels of all free VSCs, hardly affected the ability of the wine to release back hydrogen sulfide and other mercaptans during AR-aging. During aging BR-levels of MOX samples became in most cases similar or higher than non-oxygenated controls. BR-levels and the fractions free/BR follow characteristic sigmoid plots when represented versus redox potential suggesting that all changes are the result of reversible equilibria between free, metal-complexed and oxidized forms of VSCs.