A novel LC-HRMS method reveals cysteinyl and glutathionyl polysulfides in wine

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.

Untargeted polysulfide omics analysis of alternations in polysulfide production during the germination of broccoli sprouts

Higher consumption of broccoli (Brassica oleracea var. italica) is associated with a reduced risk of cardiometabolic diseases, neurological disorders, diabetes, and cancer. Broccoli is rich in various phytochemicals, including glucosinolates, and isothiocyanates. Moreover, it has recently reported the endogenous production of polysulfides, such as cysteine hydropersulfide (CysS2H) and glutathione hydropersulfide (GS2H), in mammals including humans, and that these bioactive substances function as potent antioxidants and important regulators of redox signaling in vivo. However, few studies have focused on the endogenous polysulfide content of broccoli and the impact of germination on the polysulfide content and composition in broccoli. In this study, we investigated the alternations in polysulfide biosynthesis in broccoli during germination by performing untargeted polysulfide omics analysis and quantitative targeted polysulfide metabolomics through liquid chromatography-electrospray ionization-tandem mass spectrometry. We also performed 2,2-diphenyl-1-picrylhydrazyl radical-scavenging assay to determine the antioxidant properties of the polysulfides. The results revealed that the total polysulfide content of broccoli sprouts significantly increased during germination and growth; CysS2H and cysteine hydrotrisulfide were the predominant organic polysulfide metabolites. Furthermore, we determined that novel sulforaphane (SFN) derivatives conjugated with CysS2H and GS2H were endogenously produced in the broccoli sprouts, and the novel SFN conjugated with CysS2H exhibited a greater radical scavenging capacity than SFN and cysteine. These results suggest that the abundance of polysulfides in broccoli sprouts contribute to their health-promoting properties. Our findings have important biological implications for the development of novel pharmacological targets for the health-promoting effects of broccoli sprouts in humans.

A method for the quantitative and reversible trapping of sulfidic gases from headspaces and its application to the study of wine reductive off-odors

Some relevant food systems release tiny amounts of sulfidic gases, whose measurement is difficult because of their inherent instability. The present paper demonstrates that Cu(I) solutions trap quantitatively and stabilize sulfidic gases. Once trapped, the gases remain stable for weeks at 4 °C and at least 8 days at 75 °C. Trapped gases can be quantitatively released with tris(2-carboxyethyl) phosphine (TCEP) and brine dilution and then determined by GC. Trapping solutions, placed in 20-mL opened vials housed in 100 mL hermetically-sealed flasks containing wine in anoxia, have been used to monitor the release of sulfidic gases by wines, revealing that at 50 °C, up to 400 μg/L of H₂S and 58 μg/L of MeSH can be released in 68 days, and 3-5 times more at 75 °C in 28 days. The possibility to differentiate between released and accumulated amounts provides key clues to understanding the fate of sulfidic gases in wine and other food systems.

Monitoring hydrogen sulfide de-novo formation from polysulfides present in wine using ion chromatography and ultra high-pressure liquid chromatography combined with fraction collection and high-resolution mass spectrometry

The study of polysulfides has been a recent topic of interest for wine research due to the possibility of these compounds to release hydrogen sulfide (H₂S) during storage. However, studying these compounds has been challenging for several reasons. Polysulfides are low in concentration in natural samples, they are chemically unstable and pure standards of the single compounds (RS_nR with n > 2) are not commercially available. In the present study, a method was developed in order to collect a single polysulfide and study its degradation and the consequent formation of H₂S. For this approach, ultra-high pressure liquid chromatography was used with an integrated fraction collector and subsequently coupled to high resolution mass spectrometry. After fractionation, the degradation of the di-cysteinyl pentasulfide (CS₅C) was induced by exposure to 30 °C and the H₂S formation was measured in parallel using ion-exchange chromatography. This method showed the evolutions of different polysulfides and the H₂S release originating from the target compound, an observation that to the best of our knowledge has never been made before. The method in the present study demonstrated promising applications for polysulfide studies and brought us a step closer to the understanding of the chemistry of polysulfides in wine.

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.

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.