Identification of Hydrogen Disulfanes and Hydrogen Trisulfanes in H2S Bottle, in Flint, and in Dry Mineral White Wine

Smoky Characters in Wine: Distinctive Flavor or Taint?

The frequency of wildfires has significantly increased in recent years, posing concerns for many grapegrowers and winemakers. Exposure of grapes to smoke can result in wines with notable smoky notes, which in severe cases are described as "smoke tainted". However, smoky aromas in wine are not a priori quality defects but may be considered desirable in some styles of wines, as also widely found and appreciated in many spirits. In this perspective, we summarize recent research on sources and assessment of smoky sensory attributes in wine and provide an outlook on opportunities for managing excessive smoky characters.

Accurate measurement of sulfhydryls and TCEP-releasable sulfhydryls in the liquid phase of wine that contribute to 'reductive' aromas using LC-MS/MS

Volatile sulfur compounds (VSCs) are important aroma and flavour characters in food and beverage products. The identification and quantification of these extremely reactive and volatile compounds pose analytical challenges which demand selective and sensitive methods. In this study, a novel quantification method was developed to analyse sulfhydryls as well as the total pool of sulfhydryls which can be released after tris(2-carboxyethyl)phosphine (TCEP) addition from disulfides, polysulfides, metal-bound and other yet to be identified sources naturally present in wine. The majority of methods for VSC quantification analyse VSCs in wine headspace, whereas this method measures sulfhydryls and TCEP-releasable sulfhydryl species, which likely include free and metal-bound sulfhydryl forms, in the liquid phase of wine using UHPLC-MS/MS. Sulfhydryls were derivatised with N-(2-ferroceneethyl) maleimide (FEM), subsequently, followed by differential labelling of sulfhydryls released after TCEP addition with ferrocenecarboxylic acid-(2-maleimidoyl)ethylamide (FMEA). Analysis of commercial wines revealed the presence of hydrogen sulfide, methanethiol, ethanethiol, and 2-mercaptoethanol at aroma-active concentrations. Significant positive correlations were found between MeSH and CH3-S-R TCEP-releasable species, and significant positive correlations were found between EtSH and CH3-CH2-S-R TCEP-releasable species. This method provides important information on sulfhydryls, and may also provide insights into a wine's risk of developing 'reductive' faults post-bottling from latent sources.

Neogrisemycin, a Trisulfide-Bridged Angucycline, Produced upon Expressing the Thioangucycline Biosynthetic Gene Cluster in Streptomyces albus J1074

Neogrisemycin (1) was isolated from recombinant Streptomyces albus J1074 strain SB4061 expressing an engineered thioangucycline (TAC) biosynthetic gene cluster (BGC). The structure and absolute configuration of 1 were established by a combination of mass spectrometry, nuclear magnetic resonance, and single-crystal X-ray diffraction analyses. Like the TACs, 1 was also proposed to derive non-enzymatically from the common epoxide (8), the nascent product encoded by the tac BGC, mediated by endogenous hydrogen trisulfide.

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.

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.

Evolution and Correlation of cis-2-Methyl-4-propyl-1,3-oxathiane, Varietal Thiols, and Acetaldehyde during Fermentation of Sauvignon blanc Juice

cis-2-Methyl-4-propyl-1,3-oxathiane (cis-2-MPO) was recently identified in wine and proposed to arise from the reaction of 3-sulfanylhexan-1-ol (3-SH) and acetaldehyde. However, the evolution profile of cis-2-MPO during alcoholic fermentation (AF) and storage and its relationship with varietal thiols and acetaldehyde production were unknown. These aspects were investigated by fermenting Sauvignon blanc juice with J7 and/or VIN13 yeast strains and assessing the stability of cis-2-MPO during wine storage. Moderate to strong Pearson correlations verified similar evolution trends between acetaldehyde, 3-sulfanylhexyl acetate, and cis-2-MPO, with initial increases and a peak during the early to middle stages of AF before consecutive decreases until the end. Contrarily, 3-SH correlated moderately only at the end of AF. A consistent decrease observed for cis-2-MPO when spiked into Sauvignon blanc wine and assessed during 1-year storage revealed its general instability, but acetaldehyde addition (100 mg/L), pH 3.0, and storage at 4 °C all appeared to retain cis-2-MPO. These results have implications for wine aroma and the potential for cis-2-MPO to act as a sink (or source) for 3-SH in wine over time.

Identification of key odorants in complex mixtures occurring in nature

Covering: up to 2019Soon after the birth of gas chromatography, mass spectrometry and olfactometry were used as detectors, which allowed impressive development to be achieved in the area of odorant determinations. Since the mid-80s, structured methods of gas chromatography-olfactometry have appeared, allowing the determination of which odor constituents play a key role in materials. Progressively, numerous strategies have been proposed for sample preparation from raw materials, the representativeness evaluation of extracts, the identification of odor constituents, their quantification, and subsequently, the recombination of the key odorants to mimic the initial odor. However, the multiplicity of options at each stage of the analysis leads to a confusing landscape in this field, and thus, the present review aims at critically presenting the available options. For each step, the most frequently used alternatives are described, together with their strengths and weaknesses based on theoretical and experimental justifications according to the literature. These techniques are exemplified by many applications in the literature on aromas, fragrances and essential oils, with the initial focus on wine odorants, followed by a short overview on the molecular diversity of key odorants, which illustrates most of the facets and complexities of odor studies, including the issues raised by odorant interactions such as synergies.

Minerality in Wine: Towards the Reality behind the Myths

Tasting minerality in wine is highly fashionable, but it is unclear what this involves. The present review outlines published work concerning how minerality in wine is perceived and conceptualised by wine professionals and consumers. Studies investigating physico-chemical sources of perceived minerality in wine are reviewed also. Unusually, for a wine sensory descriptor, the term frequently is taken to imply a genesis: the sensation is the taste of minerals in the wine that were transported through the vine from the vineyard rocks and soils. Recent studies exploring tasters’ definitions of minerality in wine support this notion. However, there are reasons why this cannot be. First, minerals in wine are nutrient elements that are related distantly only to vineyard geological minerals. Second, mineral nutrients in wine normally have minuscule concentrations and generally lack flavour. Results of reviewed studies overall demonstrate marked variability in both wine professionals’ and wine consumers’ definitions and sensory-based judgments of minerality in wine, although there is some consensus in terms of the other wine attributes that associate with the term mineral. The main wine composition predictors of perceived minerality involve a complex combination of organic compounds dependent on grape ripeness and/or derived from wine fermentations and redox status.

Converting organosulfur compounds to inorganic polysulfides against resistant bacterial infections

The use of natural substance to ward off microbial infections has a long history. However, the large-scale production of natural extracts often reduces antibacterial potency, thus limiting practical applications. Here we present a strategy for converting natural organosulfur compounds into nano-iron sulfides that exhibit enhanced antibacterial activity. We show that compared to garlic-derived organosulfur compounds nano-iron sulfides exhibit an over 500-fold increase in antibacterial efficacy to kill several pathogenic and drug-resistant bacteria. Furthermore, our analysis reveals that hydrogen polysulfanes released from nano-iron sulfides possess potent bactericidal activity and the release of polysulfanes can be accelerated by the enzyme-like activity of nano-iron sulfides. Finally, we demonstrate that topical applications of nano-iron sulfides can effectively disrupt pathogenic biofilms on human teeth and accelerate infected-wound healing. Together, our approach to convert organosulfur compounds into inorganic polysulfides potentially provides an antibacterial alternative to combat bacterial infections.

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.

Revisiting the evaluation strategy of varietal thiol biogenesis

The varietal thiols 3-mercaptohexan-1-ol (3MH), 3-mercaptohexyl acetate (3MHA), and 4-mercapto-4-methylpentan-2-one (4MMP) are key aroma compounds in wine due to the tropical notes they impart. They are released by yeast during alcoholic fermentation from different precursors. However, a large part of 3MH origin remains unknown. In this study, we focused on dipeptide forms arising from glutathione S-conjugates to 3MH and 4MMP. Using labelled tracers, we showed in spiked must the release of varietal thiols from 4 different compounds. We highlighted the interconversion between different forms of precursors under defined enological conditions. Cysteinyl-glycine S-conjugates are partially degraded into cysteine S-conjugates, contrary to γ-glutamyl-cysteine S-conjugates. Glutathione S-conjugate to 3MH can be partially degraded to γ-glutamyl-cysteine S-conjugate to 3MH. For the first time, all these labeled forms of precursors were found to release 3MH or 4MMP between 0.17 and 1% molar conversion yield. Two different yeasts were compared without any significant difference.

Copper(II)-Mediated Hydrogen Sulfide and Thiol Oxidation to Disulfides and Organic Polysulfanes and Their Reductive Cleavage in Wine: Mechanistic Elucidation and Potential Applications

Fermentation-derived volatile sulfur compounds (VSCs) are undesirable in wine and are often remediated in a process known as copper fining. In the present study, the addition of Cu(II) to model and real wine systems containing hydrogen sulfide (H₂S) and thiols provided evidence for the generation of disulfides (disulfanes) and organic polysulfanes. Cu(II) fining of a white wine spiked with glutathione, H₂S, and methanethiol (MeSH) resulted in the generation of MeSH-glutathione disulfide and trisulfane. In the present study, the mechanisms underlying the interaction of H₂S and thiols with Cu(II) is discussed, and a prospective diagnostic test for releasing volatile sulfur compounds from their nonvolatile forms in wine is investigated. This test utilized a combination of reducing agents, metal chelators, and low-oxygen conditions to promote the release of H₂S and MeSH, at levels above their reported sensory thresholds, from red and white wines that were otherwise free of sulfidic off-odors at the time of addition.