# Tannin additions decrease the concentration of malodorous volatile sulfur compounds in wine-like model solutions and wine

Marlize Zaretha Bekker, Allie Cherie Kulcsar, Alicia Loreto Jouin, Victor

Felipe Laurie

► To cite this version:

Marlize Zaretha Bekker, Allie Cherie Kulcsar, Alicia Loreto Jouin, Victor Felipe Laurie. Tannin additions decrease the concentration of malodorous volatile sulfur compounds in wine-like model solutions and wine. Food Chemistry, 2025, 471, pp.142777. 10.1016/j.foodchem.2025.142777. hal-04940967

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Submitted on 11 Feb 2025

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# Tannin additions decrease the concentration of malodorous volatile sulfur compounds in wine-like model solutions and wine

Marlize Zaretha Bekker<sup>a,b,*</sup>, Allie Cherie Kulcsar<sup>a</sup>, Alicia Loreto Jouin<sup>a,c</sup>, Victor Felipe Laurie<sup>d</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

- Tannin
- Hydrogen sulfide
- Ethanethiol
- Methanethiol
- Reductive aromas
- Wine oxidation

## ABSTRACT

Hydrogen sulfide (H₂S), methanethiol (MeSH) and ethanethiol (EtSH) are volatile sulfur compounds (VSCs) produced during winemaking and are associated with negative 'reductive' aromas in wine. Anecdotal evidence suggests that oenological tannins may be used to remediate the 'reductive' character of wines, yet little scientific evidence or explanation supporting this observation has been published. In this study, it was found that the addition of oenological tannins significantly decreased H₂S, MeSH, and EtSH in model wine by up to 92%, 90% and 86%, respectively, after two weeks of storage. Furthermore, the removal of H₂S from real wine matrices was accelerated at high pH levels. The reaction products of polyphenol oxidation and their ability to ameliorate the presence of MeSH and EtSH were studied in model systems and wines, and vescalagin/castalagin adducts of MeSH and EtSH were subsequently produced. This study provides evidence for the mechanism through which oenological tannins may diminish 'reductive' aromas in wine.

## 1. Introduction

Volatile sulfur compounds (VSCs) are a group of chemicals commonly found in wine, which are responsible for various aromas and flavours, ranging from fruity and floral to pungent and sulfurous. While some of these compounds can contribute positively to the sensory properties of wine, others may impart negative effects, such as off-flavours or 'reductive' aromas that detract from wine quality. Sulfur compounds such as hydrogen sulfide (H₂S), methanethiol (MeSH) and ethanethiol (EtSH) are often produced during winemaking and may negatively impact wine aroma, perceived quality, and consumer preference (Bekker, Wilkes, & Smith, 2018; Kreitman, Elias, Jeffery, & Sacks, 2019; Müller & Rauhut, 2018; Zhang, Bekker, Kulcsar, Espinase Nandorfy, & Clark, 2024).

To avoid the unwanted consequences of developing these compounds, winemakers may resort to certain prevention or remediation tools and strategies, some of which are selective in their abilities to prevent and/or remediate distinct types of sulfur compounds or could even further their negative impact (Bekker et al., 2020; Day et al., 2021; Ferreira, Franco-Luesma, Vela, Lopez, & Hernandez-Orte, 2018). For instance, copper sulfate has limitations on when it can be applied, the

amounts that can be used are regulated, and it has also been shown to produce detrimental effects on wine aroma, wine flavour, and shelf life (Kontoudakis et al., 2019; Pavelescu, Ehsani, & Spangl, 2021; Ugliano et al., 2011).

Anecdotal evidence suggests that the use of oenological tannins may facilitate the removal of the 'reductive' character of wines, yet little scientific evidence supporting or explaining this observation has been published.

Tannins are important in wine production as they can affect colour, mouthfeel, and stability (Pavez et al., 2022; Vignault et al., 2018). Proanthocyanidins and hydrolysable tannins are two groups of compounds naturally found in grapes or different wood sources. The former are oligomers or polymers of flavan-3-ols extracted from grape pomace, and the latter may be gallo or ellagitannins (i.e., most notably castalagin and vescalagin) recovered from diverse types of woods (Vignault et al., 2018).

During wine processing, tannins may undergo a number of transformations, including oxidation, hydrolysis, and even nucleophilic-type reactions with other wine substances such as flavanols, anthocyanins, or glutathione (Petit, Jacquet, Pouységu, Deffieux, & Quideau, 2019). During wine oxidation, the nucleophilic addition of quinones with

<footnote><sup>a</sup> The Australian Wine Research Institute, P.O. Box 197, Glen Osmond, SA 5064, Australia</footnote>

<footnote><sup>b</sup> School of Agriculture and Food Sustainability, The University of Queensland, St Lucia, QLD 4072, Australia</footnote>

<footnote><sup>c</sup> Univ. Bordeaux, Bordeaux INP, Bordeaux Sciences Agro, INRAE, OENO, UMR 1366, ISVV, Villenave d'Ornon F-33140, France</footnote>

<footnote><sup>d</sup> Departamento de Horticultura, Facultad de Ciencias Agrarias, Universidad de Talca, 2 Norte 685, Talca, Chile</footnote>

<footnote>* Corresponding author at: School of Agriculture and Food Sustainability, The University of Queensland, St Lucia, QLD 4072, Australia
*E-mail address: m.bekker@uq.edu.au (M.Z. Bekker).</footnote>
various electrophiles, including VSCs, has been suggested as a possible mechanism of wine aroma change (Laurie et al., 2012; Nikolantonaki & Waterhouse, 2012). Quinones are key reactive electrophilic oxidation intermediates in wine and are typically produced through metal-catalysed reactions involving polyphenolic compounds (Danilewicz, 2007). Metals, such as copper and iron, can act as catalysts facilitating the electron transfer process required to transform the polyphenolic compound into a quinone, which may then react with nucleophiles such as VSCs.

More recently, studies describing potential interactions between the in-vitro reactions of ellagitannins or catechins and odoriferous thiols (i. e., 3-sulfanylhexan-1-ol and 2-furanmethanethiol) have been studied (Petit et al., 2019; Petit, Jacquet, Pouységu, Deffieux, & Quideau, 2015). As such, the in-vitro reactivity of condensed tannins and thiols (including EtSH) has also been studied (Suc, Rigou, & Mouls, 2021).

Moreover, preceding evidence of these types of reactions could also be found in reports showing that polyphenols play a role in deodorising aqueous or gas matrices containing MeSH, H₂S, or *trans*-2-nonenal (Ham, Kim, & Lim, 2015; Lodhia et al., 2008; Yasuda & Arakawa, 1995). Yasuda and Arakawa (1995) reported the deodorising mechanism of (–)-epigallocatechin gallate (EGCg) against MeSH at pH 7.5, proposing the addition of a methylthio-group to the ortho-quinone generated during EGCg oxidation. However, the prior was not observed when a phosphate buffer at pH lower than 5 was employed, given that at this condition, the oxygen-induced formation of ortho-quinones would be less favoured.

Additionally, oenological tannins have been shown to limit the outcomes of the light-struck taste phenomenon (LST) by restricting the formation of VSCs (Fracassetti, Limbo, Messina, Pellegrino, & Tirelli, 2021; Fracassetti, Limbo, Pellegrino, & Tirelli, 2019; Fracassetti, Messina, Saligari, & Tirelli, 2023) in white wines affected by photo-oxidative degradation (Diaz, Castro, Ubeda, Loyola, & Laurie, 2021; Grant-Preece, Barril, Schmidtke, Scollary, & Clark, 2017). Considering the potential of oenological tannins to bind VSC, as is supported by anecdotal evidence from winemakers, we hypothesise that their use in wine plays a significant role in decreasing the concentration of malodorous VSCs, potentially leading to improved wine quality.

Given the significance of VSCs and tannins in wine production, it is important to gain a deeper understanding of their interactions and potential impact on the chemical properties of wine. This study explores the role that tannins play in the presence and release of VSCs in model solutions and wines. Through a better understanding of these factors, winemakers may be able to optimise wine production and improve the overall quality of their products. Specifically, the formation of polyphenol oxidation reaction products and their ability to ameliorate the presence of H₂S, MeSH, and EtSH were studied.

## 2. Materials and methods

### 2.1. Reagents and chemicals

All chemicals used were of analytical reagent grade, whilst ultrapure water (18.2 Ω) was prepared onsite using a Milli-Q purification system (Millipore, North Ryde, NSW, Australia). Acetonitrile (HPLC-grade), ethanol (HPLC-grade), methanol (HPLC grade) and sodium chloride (99.5 %) were purchased from Merck (Frenchs Forest, NSW, Australia). Sodium sulfide nonahydrate, Na₂S·9H₂O (98 %), ethanethiol, EtSH (99.7 %), methanethiol, MeSH (98 %), potassium l-tartrate monobasic (99 %), l-(+)-tartaric acid (≥ 99.5 %), and iron (II) sulfate heptahydrate (FeSO₄·7H₂O) were purchased from Sigma-Aldrich (Darmstadt, Germany), whilst copper (II) sulfate pentahydrate (CuSO₄·5H₂O) was obtained from ACE chemical company (Cardem park SA, Australia). Formic acid (98 %) was purchased from Rowe Scientific Pty Ltd. (Lonsdale, SA, Australia).

The tannins tested were commercial preparations of oak ellagitannins and grape procyanidins called Quertanin® (TanninQ) and

TanninVR Grape® (TanninV), respectively (Laffort-Australia, Woodville North, SA, Australia).

### 2.2. Model wine solutions and wines

Model solutions consisted of 12.0 % ethanol in water containing 0.011 M potassium l-tartrate monobasic 99 % and 0.008 M l-(+)-tartaric acid to give a final pH of 3.60 ± 0.1. Metals, such as copper and iron, act as catalysts to transform polyphenolic compounds into quinones. Subsequently, iron and/or copper ions were added using FeSO₄·7H₂O and CuSO₄·5H₂O to reach approximate concentrations of 2.5 and 0.25 mg/L, respectively to facilitate the formation of reactive quinone species.

A finished, ready-to-bottle Shiraz wine from the 2017 vintage was sourced from McLaren Vale (SA, Australia). The basic chemical composition of the wine was as follows: pH 3.50, alcohol 16.0 % (v/v), volatile acidity (VA, as acetic acid) 0.52 g/L, free sulfur dioxide (FSO₂) 53 mg/L total sulfur dioxide (TSO₂) 100 mg/L, 0.19 mg/L copper, and 1.32 mg/L iron. Finished Sauvignon blanc wine from the 2018 vintage was sourced from the Adelaide Hills (SA, Australia), having the following chemical composition: pH 3.00, alcohol 12.0 % (v/v), VA 0.49 g/L, FSO₂ 14 mg/L, TSO₂ 112 mg/L, 0.04 mg/L copper, and 0.23 mg/L iron.

### 2.3. Experimental design

The following two independent experiments were conducted to evaluate the ability of tannins to lower the concentration of VSCs from model solutions and wines, as well as to propose a possible mechanism for such an effect.

#### 2.3.1. Effects of oenological tannins and the effects of pH on the removal of volatile sulfur compounds

Experiments were performed in (i) model wine containing 2.5 mg/L of iron (as detailed in section 2.2), (ii) red wine, (iii) and white wine, all of which were spiked with H₂S, MeSH, and EtSH (50 µg/L). Tannin treatments were prepared as follows: control samples containing no tannins (Control) and treated samples containing 100 mg/L of commercial Quertanin (“TanninQ”) or Tannin VR Grape (“TanninV”), respectively, as per manufacturer’s recommendations. All samples were prepared in triplicate in an aerobic environment. The pH levels of the wines were adjusted using sodium hydroxide or sulfuric acid to produce “low” or “high pH” (i.e., pH 3.00 and pH 4.00, respectively) in amber vials (22 mL) sealed with solid polypropylene screw caps with PTFE liners (Adelab Scientific, Thebarton, SA, Australia). Enough samples were prepared for analysis at three time points (Day 0, Week 2, and Month 12). After preparation, all samples were stored in a vinyl anaerobic chamber (Coy Laboratories, MI, USA) equipped with a CAM-12 anaerobic monitor (Coy Laboratories, MI, USA) (oxygen concentration ~ 0 mg/L), protected from light, and at room temperature (20 °C).

#### 2.3.2. Elucidating the reaction products between tannin and sulfhydryls

Freshly made model solutions at low pH (pH 3.20) containing copper (0.25 mg/L), iron (2.5 mg/L), and either MeSH or EtSH (5 mg/L each) were prepared in 40 mL vials. TanninQ (50 mg/L) was added to the model wine, and samples were sealed with solid polypropylene screw caps with PTFE liners. Samples were prepared in triplicate in an aerobic environment, stored at room temperature (20 °C), and protected from light. Control samples of the variables were also prepared containing no added sulfhydryls. Analysis was performed at three time points (Day 0, Week 2, and Month 1).

## 2.4. Chemical analyses

### 2.4.1. Volatile sulfur compound analysis

Volatile sulfur compound analysis was performed on an Agilent 8355 sulfur chemiluminescence detector coupled to an Agilent 7890B gas
chromatograph (Forest Hill, VIC, Australia. The standard preparation, sample preparation and chromatographic conditions were as described by Siebert, Solomon, Pollnitz, and Jeffery (2010), using an Agilent DB Sulfur SCD column (0.75 µm, 40 m × 0.32 mm) (Agilent, Mulgrave, VIC, Australia) with 1 m of retention gap (0.53 mm), and a flow rate of 2.7 mL/min.

### 2.4.2. Liquid chromatography-mass spectrometry

Tannin-sulphydryl adducts were analysed using a Thermo Scientific Vanquish Horizon high-performance liquid chromatography (HPLC) system (Thermo Scientific, Waltham, MA USA) equipped with an autosampler, binary pump, column oven, and diode array detector. The HPLC was connected to an Orbitrap IDX Tribrid High-Resolution Fourier Transform Mass Spectrometer (HRFTMS) (Thermo Scientific, Waltham, MA, USA).

A Phenomenex Kinetex column (F5, 100 Å, 2,6) (Phenomenex Australia, Lane Cove, NSW, Australia) was used for separation with a flow rate of 0.4 mL/min. The solvents used were (A) 99.4 % water with 0.5 % methanol and 0.1 % formic acid and (B) 50.9 % methanol with 40 % acetonitrile, 2 % water and 0.1 % formic acid.

The tannin-sulphydryl adducts present in the TanninQ treated samples were measured using a gradient for solvent B: 0 min, 0 %; 20 min, 20 %; 35 min, 40 %; 40 min, 100 %; 41 min, 100 %; 41.2 min, 0 %; 41.4 min, 100 %; 41.6 min, 0 %; 41.8 min, 100 %; 42 min, 0 %; 43 min, 0 %. The mass spectrometer conditions were as follows: ion spray voltage 3300 V; sheath gas 40 psi; auxiliary gas 7 psi; negative mode; ion transfer tube 275 °C, vaporisation temperature 200 °C.

The mass range was measured between 200 and 2000 m/z. Morin (50 mg/L) was added to all the samples and calibration curves as an internal standard (Luximon-Ramma et al., 2005). For the ellagitannin method, a calibration curve of vescalin (0.25 to 20 mg/L) was injected at the beginning of each sequence. The ellagitannin-sulphydryl adducts concentration was reported in vescalin concentration equivalents.

### 2.5. Statistical analysis

The chemical datasets were assessed using either one-way ANOVA with Tukey's multiple comparisons post hoc test, or two-way ANOVA, for samples analysed at three time points, with Tukey's multiple comparisons post hoc test using GraphPad Prism 10.0.0 (GraphPad Software, San Diego, CA, USA) to determine significant differences between treatments. All treatments were prepared in triplicate (n = 3). Datasets were evaluated for outliers using Grubb's outlier test (α = 0.1). If outliers were identified, they were removed, and ANOVA analysis was performed with two replicates followed by a Sidak post hoc analysis. Statements ascribing the level of statistical evidence are as follows: not significant (ns) for *P* > 0.05; * *P* ≤ 0.05; ** *P* ≤ 0.01; *** *P* ≤ 0.001; **** *P* ≤ 0.0001.

## 3. Results and discussion

### 3.1.1. Effectiveness of oenological tannins in removing volatile sulfur compounds

The effectiveness of oenological tannin in removing H₂S, MeSH, and EtSH was first evaluated in model wine, Sauvignon blanc, and Shiraz wine at pH 3.00. The decrease in concentration of VSCs was measured immediately after their addition to the wines (T0), two weeks after addition (W2) or 12 months after VSC addition (M12), in samples that contained either no tannin (control), or either TanninV or TanninQ (Fig. 1). The greatest effect of the tannin in VSC removal was observed in the simple model wine system (Figs. 1a – 1c). As the complexity of the wine matrix increased when Sauvignon blanc and Shiraz wines were used (Figs. 1d – 1i), the effect of tannin addition on VSC removal was less

pronounced. This is most likely due to the interaction of the tannins with other wine matrix compounds, and the competition with other molecules for binding with VSCs. Notably, H₂S reacted faster with the tannin compared to MeSH and EtSH, with significant decreases of H₂S concentrations measured at T0 (Fig. 1a), however, the greatest effects on H₂S, MeSH, and EtSH concentrations were observed after two weeks of storage (Fig. 1a – c). After 12 months of storage, the majority of the VSC concentrations decreased to less than 0.5 µg/L in the model wine samples, and no significant differences in VSC concentrations between the tannin treatments and the control were measured in any of the wine samples.

When H₂S concentrations were measured immediately after the addition of tannins (T0), the model wines showed significantly less H₂S compared to the control samples without added tannin (Fig. 1a, Table S1). TanninQ-treated samples contained 29 % less H₂S than the control, and samples treated with TanninV contained 43 % less H₂S than the control (Fig. 1a). After two weeks of storage, 92 % of the H₂S remaining in the samples was removed by TanninQ, and TanninV had removed 91 % of H₂S, and no significant differences in the effectiveness of H₂S removal were observed when TanninV and TanninQ were compared (Fig. 1a, Table S1). In the Sauvignon blanc wine, only TanninQ removed significant amounts of H₂S (31 %) at T0 (Fig. 1d, Table S1); whereas, in the Shiraz wine, only TanninV removed significant amounts of H₂S (34 %), compared to the control, at T0 (Fig. 1g, Table S1).

Model wines containing TanninQ removed significant amounts of MeSH at T0, with TanninQ removing 15 % of MeSH (Fig. 1b, Table S1). After two weeks of storage, TanninQ removed significantly greater amounts of MeSH from the model wines, with 90 % of MeSH removed by TanninQ, and 70 % of MeSH removed by TanninV (Fig. 1b, Table S1). In the Sauvignon blanc wines, only TanninV removed significant amounts of MeSH (21 %) after two weeks of storage (Fig. 1e, Table S1). Instead, in the Shiraz wines, only TanninV removed significant amounts of MeSH (14 %) but only when measured at T0 (Fig. 1h, Table S1).

The effect of oenological tannin on EtSH concentrations was only apparent in the model wines and only after 2 weeks of storage. TanninQ removed significantly greater amounts of EtSH from the model wines with 86 % of EtSH removed by TanninQ, and 45 % of EtSH removed by TanninV (Fig. 1c, Table S1).

It is important to note that no copper was added to this experiment. The model wines without added copper provide a simplistic model for how the oenological tannin would act as VSC scaling agents under the right conditions. However, when interpreting the results of the Sauvignon blanc and Shiraz wine, it is important to consider the effects that the natural residual copper could have on the sulphydryls. It is known that copper readily reacts with sulphydryls (Viviers, Smith, Wilkes, Smith, & Johnson, 2014) and removes “free” sulphydryls from the wine matrix and produces “bound” copper-sulphydryl complexes (Franco-Luesma & Ferreira, 2014). The decreases in sulphydryl concentrations cannot only be attributed to the formation of tannin-VSC complexes, as there would be competition between quinones and copper to bind with VSCs.

### 3.1.2. Effects of pH on the removal of volatile sulfur compounds using oenological tannins

The effects of pH on the ability of commercial tannin products to scalp H₂S, MeSH, and EtSH were evaluated. It is known that pH affects the formation of quinones and subsequent adduct formation (Friedman & Jurgens, 2000), as well as the rate of wine oxidation and the accumulation of Strecker aldehydes (Marrufo-Curtido, Ferreira, & Escudero, 2022). It is also known that pH affects the binding of sulphydryls to metal ions (Kontoudakis et al., 2017). Interestingly, the effects of pH on VSC removal were less pronounced in model systems (Fig. 2a – 2c) compared to real wine (Fig. 2d – 2i), and H₂S was removed more readily than MeSH and EtSH (Fig. 2a, 2d, g vs 2b, 2c, 2e, 2f, 2h, 2j).

The pH level had a significant effect on the concentrations of H₂S in
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<figure><img src="image_12.png" /><figcaption>**Fig. 1.** The concentration of hydrogen sulfide ($H_2S$), methanethiol (MeSH), and ethanethiol (EtSH) measured in model wine (MW, a - c), Sauvignon blanc (SB, d - f), and Shiraz (SZ, g - i) at day 0 (T0) and after two weeks (W2) and 12 months (M12) of storage.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 2.** The concentration of hydrogen sulfide ($H_2S$), methanethiol (MeSH), and ethanethiol (EtSH) measured in model wine (MW, a – c), Sauvignon blanc (SB, d – f), and Shiraz (SZ, g – i) at day 0 (T0) and after two weeks (W2) and 12 months (M12) of storage. The pH levels of the wines were adjusted to pH 3.00 (low pH) or pH 4.00 (high pH) and the effects of pH on the ability of TanninV and TanninQ to remove $H_2S$, MeSH, and EtSH were evaluated.</figcaption></figure>

Sauvignon blanc and Shiraz wines (Fig. 2d, g), but not so in the model wine (Fig. 2a). All wines, with or without tannin addition, contained significantly lower $H_2S$ concentrations in the samples with higher pH compared to wines at lower pH (Sauvignon blanc, Fig. 2d; Shiraz, Fig. 2g, Table S2). These results suggest that certain compounds that are naturally present in wine facilitated the binding and removal of $H_2S$, irrespective of the addition of tannins (Fig. 2d, g). This is confirmed by the results of the model wine, with or without added tannins, in which tannin addition provided no additional benefits in $H_2S$ removal at either low or high pH (Fig. 2a). Interestingly, all model wines treated with either TanninQ or TanninV showed lower $H_2S$ concentration compared to the control (Fig. 2a, Table S2), suggesting that these phenolics are involved in this effect (Table S3, Fig. S1). In samples containing phenolics, such as white and red wines, the losses of $H_2S$ observed could potentially be related to binding reactions between phenol-derived electrophilic quinones, which would be favoured at high pH, provided that oxygen has triggered the oxidation of phenolics.

Previous results (Bekker, Mierczynska-Vasilev, Smith, & Wilkes, 2016) have shown that copper treatments may increase the $H_2S$ concentrations in Chardonnay and Shiraz wines compared to samples without added copper. In Chardonnay, they observed that lowering the wine's pH (3 vs. 3.46) helped decrease the concentration of $H_2S$ in the copper-treated samples but not so in the samples without added copper.

In contrast, they showed that in Shiraz wine at lower pH, less $H_2S$ was immediately measured in samples with or without added copper (Bekker et al., 2016). Shiraz wine contains a greater concentration of tannin compared to Chardonnay; therefore, it is possible to hypothesise that the tannin compounds may have played a role in modulating the scaling of $H_2S$ and that the ability of these tannin complexes to bind sulfhydryls were affected by pH.

Additionally, the results of this study show that the type of tannin added to the wine did not significantly affect the amount of $H_2S$ removed from either Sauvignon blanc or Shiraz at pH 3 or 4 when compared to their respective controls or when comparing TanninQ to TanninV (Table S2, Fig. 2d, g).

In the case of MeSH and EtSH, no significant differences were observed at T0. However, after two weeks of storage, TanninQ removed significantly greater amounts of MeSH when at high pH, both in model wine and Sauvignon blanc (Fig. 2b, e, Table S2). This increased removal of MeSH was also seen in the control Sauvignon blanc without tannin addition (Fig. 2e) after two weeks of storage. In the model wine samples at T0, TanninV also removed significantly greater amounts of MeSH at high pH compared to the same samples at low pH (Fig. 2b, Table S2). After two weeks of storage, all model wine samples treated with tannin contained significantly lower concentrations of MeSH and EtSH (Fig. 2b, c) compared to the control wines. The fact that the effects of tannin on
MeSH and EtSH removal were most apparent after two weeks of storage, compared to H₂S, may be a reflection of their reaction rate with the tannin products.

In these experiments, iron ions ($Fe^{2+}$) were added to the model solutions and wines before TanninQ and TanninV supplementation, owing to the fact that this metal is known to promote the formation of oxidation reaction products, such as quinones, which readily and swiftly bind to nucleophiles such as sulfhydryls. Even though the combination of copper and iron leads to the rapid formation of quinones from polyphenolic compounds (Danilewicz, 2007), copper ($Cu^{2+}$) was specifically not added in order to prevent the formation of copper-sulfhydryl complexes, which would have convoluted the results of this experiment (Kreitman, Danilewicz, Jeffery, & Elias, 2016b, 2017). $Fe^{2+}$ also forms complexes with sulfhydryls, however, the reaction kinetics are much slower than for $Cu^{2+}$ (Ferreira, Bueno, Franco-Luesma, Cullere, & Fernandez-Zurbano, 2014). Also, the degree of complexation between $Fe^{2+}$ and MeSH and EtSH is significantly lower compared to the degree of complexation between $Cu^{2+}$ and these thiols (Franco-Luesma & Ferreira, 2014). As mentioned in section 3.1.1, it is important to consider the effects that the natural residual copper present in the real wine could have on the loss sulfhydryls. In the model wines, the effect of pH on VSC removal can be ascribed to the formation of tannin-VSC complexes, as no copper was present in these samples.

Nikolantonaki et al. (Nikolantonaki et al., 2012) explored the reaction rate of a range of sulfhydryls, namely 4-methyl-4-sulfanylpentan-2-one (4MSP), 3-sulfanylhexan-1-ol (3SH), 2-furanmethanethiol (2FMT); H₂S; glutathione (GSH), as well as sulfur dioxide ($SO₂$) and ascorbic acid (AA), with 4-methyl-1,2-benzoquinone. In their study, they found that sulfhydryls had increasing reaction rates ($K$) as steric inhibition declined ($K_{4MSP}$ = 0.0060 s⁻¹, $K_{3SH}$ = 0.0578 s⁻¹, and $K_{2FMT}$ = 0.0837 s⁻¹), however, these sulfhydryls had lower $K$ values than $SO₂$, GSH, and H₂S ($K_{Nu}$ = 0.3343–0.4188 s⁻¹). The results from Nikolantonaki et al. (2012), as well as the results observed in this study, suggest that the reaction rate between H₂S, MeSH, and EtSH may also be impacted by the molecular structure and R-groups attached to the sulfur nucleophile. When small hydrogen atoms are attached to the sulfur, there would be little steric repulsion between the incoming nucleophile and the electrophilic quinone compound, which would thereby increase the ease and the reaction rate of nucleophilic substitution. However, if the hydrogen atoms are replaced with an R-group such as CH₃- or CH₃-CH₂- as is the case for MeSH and EtSH, there would be increased steric repulsion between the more sterically hindered nucleophile and the electrophile, which will lead to slower reaction rates.

An important consideration for this experiment was the role that metal ions played in facilitating the binding and removal of sulfhydryls. Metal ions may act as catalysts of phenolic oxidation reactions and produce quinones from their phenol functional groups (Danilewicz, 2011). Metal ions also facilitate the formation of disulfides, and even polysulfides, from sulfhydryls (Kreitman, Danilewicz, Jeffery, & Elias, 2016a). Therefore, it was important to evaluate the formation of dimethyl disulfide (DMDS) and diethyl disulfide (DEDS) as well as the removal of MeSH and EtSH to understand whether the loss of MeSH and EtSH were related to the complexation of tannin oxidative reaction products with the sulfhydryls, or whether the loss of MeSH and EtSH were merely related to the formation of disulfides.

To better understand this relationship the mass balance between the molar % sulfhydryls remaining in solution, the molar % of disulfides measured in solution, and the molar % of sulfhydryls lost (presumably through complexation with tannin compounds) were evaluated in model wine. The simplicity of the model solution allows for more accurate conclusions to be drawn regarding the type of disulfide or polysulfide products that can be produced. In model systems, only symmetrical disulfides can be produced; however, in real wines, both symmetrical and asymmetrical disulfides may be formed, some of which may be large molecular mass compounds (e.g., diglutathionyl disulfide or dicysteinyl polysulfides) which cannot be quantified using the GC-SCD method; and

therefore, accurate mass balance cannot be calculated for these samples.

At Day 0, the largest fractions of MeSH and EtSH present in the model wine samples were free MeSH and EtSH (Fig. 3a & 3d). However, after two weeks, the significant effects of tannin on the removal of MeSH and EtSH can be clearly observed, with significantly greater molar % MeSH and EtSH removed from samples containing tannin products (TanninV and TanninQ,) compared to samples containing only model wine with no added tannin (Fig. 3b, Fig. 3e). The molar percentage of disulfides produced was lower than expected. TanninQ removed a larger fraction of MeSH and EtSH compared to TanninV (Fig. 3b, e). After 12 months of storage, no differences were observed when comparing the ability of TanninV and TanninQ to remove MeSH and EtSH (Fig. 3c, f). These results demonstrate that the removal of sulfhydryl is not merely related to the complexation with metal ions ($Fe^{2+}$ in this case), but that their removal is significantly enhanced with tannins, possibly through their reaction with polyphenolic oxidation products, such as quinones.

Research conducted elsewhere (Vignault et al., 2018) has shown that tannins have a distinctive capacity to consume oxygen based on their chemical nature (i.e., ellagitannins ≥ procyanidins ≥ gallotannins). Oxygen uptake could be linked with transformations of the tannins studied and an increased thiol stripping capacity that may assist in explaining the differences in the effectiveness of TanninQ and TanninV in facilitating VSC removal (Fig. 3).

This experiment demonstrated the effect of pH on the removal of sulfhydryls from wine matrices. In the majority of cases, higher pH promoted the removal of H₂S and MeSH from the samples tested at T0 for H₂S (Fig. 2d, g) and Week 2 for MeSH (Fig. 2b, e). However, due to the complex interactions between the added tannin products, added $Fe^{2+}$ ions, and the contribution of the wine matrix compounds naturally present in the Sauvignon blanc and Shiraz wines, further evidence, presented below, was required to conclusively elucidate the role of oxidative reaction products produced from tannin compounds in scaling sulfhydryls from wine.

### 3.1.3. Elucidating the reaction products between tanning and sulfhydryls

Based on the results of experiment 2, the possible mechanism by which tannin products remove sulfhydryls, was tested by adding TanninQ (50 mg/L and 100 mg/L each) to a model wine containing copper (0.3 mg/L) and iron (2.5 mg/L) as well as MeSH and EtSH (5 mg/L each). The amount of sulfhydryls added was at much greater concentrations than these compounds are normally found in wine in order to ensure that large enough quantities of tannin-sulfhydryl compounds were produced that could be detected by the Orbitrap HRFTMS. In this experiment, the formation of tannin-sulfhydryl adducts, and the scaling of MeSH and EtSH, were monitored simultaneously by measuring the non-volatile compounds using HPLC-HRFTMS, and the volatile compounds using GC-SCD.

Firstly, the samples containing sulfhydryls were evaluated for a loss in MeSH or EtSH concentration as a result of TanninQ addition, which would indicate the removal of sulfhydryls through the formation of adducts with tannins. In this experiment, MeSH and EtSH concentrations decreased as a result of treatment with TanninQ (Fig. 4), through the subsequent formation of vescalagin/castalagin adducts with MeSH and EtSH, as plotted in Fig. 5. Although, it was also noted that a certain percentage of disulfides were produced as catalysed by copper (Fig. 4).

When measured immediately after tannin addition, MeSH and EtSH were either present as free sulfhydryls or converted into a small percentage of their corresponding disulfide species (DMDS or DEDS) (Fig. 4a, d). After two weeks, there was no residual MeSH remaining in the model wine, and the produced DMDS did not account for the total % molar concentration of MeSH added to the samples, which indicated that a substantial portion of MeSH was lost (Fig. 4b). For the samples containing EtSH, after two weeks of TanninQ treatment, a loss of 25 - 30% of EtSH (% mol/L) was observed. After one month of storage, the % loss of MeSH and EtSH seemed to plateau or be reduced (Fig. 4c, f). It is possible that the binding between sulfhydryls and ellagitannin
<figure><img src="image_14.png" /><figcaption>**Fig. 3.** Molar percentages (%) of methanethiol (MeSH) remaining in the samples (blue), dimethyl disulfide (DMDS) produced (purple), and MeSH lost (pink) through binding with tannin compounds in model wine measured at 24 h (Day 0), two weeks (week 2) and 12 months (month 12) after tannin addition (a – c). Molar percentages (%) of ethanethiol (EtSH) remaining in the samples (blue), dimethyl disulfide (DEDS) produced (purple), and EtSH lost (pink) through binding with tannin compounds in model wine as measured at 24 h (Day 0), two weeks (Week 2) and 12 months (Month 12) after tannin addition (d – f). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 4.** Effect of tannin addition on the decrease of methanethiol (MeSH) (a – c) and ethanethiol (EtSH) (d – f) measured immediately after TanninQ addition (Day 0), two weeks after tannin addition (Week 2), and one month after tannin addition (Month 1). The residual MeSH and EtSH concentrations are shown in blue, the subsequent formation of disulfides (dimethyldisulfide, DMDS, and diethyldisulfide, DEDS) is shown in purple, and the molar % MeSH and EtSH loss is shown in pink. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

(TanninQ) proceeds at a faster rate and that the produced adducts are not stable. This may explain why after 2 weeks, some of the VSC scalped by tannin products were released from their tannin-sulfhydryl adducts and transformed to DMDS and DEDS by residual metals that were present in the samples.

The same samples that showed decreased MeSH and EtSH

concentration after TanninQ were then analysed and the tannin-VSC adducts characterised using HPLC coupled to electrospray ionisation high-resolution mass spectrometry. Structural hypotheses on the reaction products were first formulated according to the structure of the most common oak ellagitannins and the structure of the sulfhydryl nucleophiles, as well as knowledge concerning ortho-quinone reactivity
<figure><img src="image_16.png" /><figcaption>**Fig. 5.** The formation of vescalagin/castalagin adducts with methanethiol (MeSH) and ethanethiol (EtSH) in model wine containing either 50 mg/L or 100 mg/L of TanninQ and treated with 5 mg/L of MeSH or EtSH. The formation of (a) (Vescalagin/Castalagin)-S-CH₃, and (b) (Vescalagin/Castalagin)-S-CH₂-CH₃ was monitored at three time points (Day 0, Week 2, and Month 1).</figcaption></figure>

towards sulfhydryls (Cilliers & Singleton, 1990; Laurie et al., 2012; Nikolantonaki et al., 2012; Nikolantonaki & Waterhouse, 2012; Quideau, Feldman, & Appel, 2002). The putative tannin-VSC adducts evaluated in this study are shown in Table S4. The molecular ions [M – H]⁻ corresponding to the putative reaction products of expected ellagitannins and MeSH and EtSH were screened on the full ion chromatograms over the mass range 200–2000. Only products that incorporated vescalagin/castalagin (referred to in this manner due to the inability of the analytical methods to discriminate between the two isomeric forms of this tannin compound), gallic acid, and ellagic acid in their structure were considered (Table S4). The screening mainly included structures

consisting of a single tannin compound containing one or two sulfhydryl moieties (Table S4).

The reaction between tannins with malodorous sulfhydryls to produce tannin-VSC products under wine-like conditions was demonstrated through accurate mass spectrometry. In this study, vescalagin/castalagin adducts with MeSH and EtSH were measured (Scheme 1, Table 1).

Vescalagin/castalagin adducts with MeSH and EtSH were measured in model wine treated with 50 mg/L and 100 mg/L of TanninQ (Fig. 5). For the tannin-VSC adducts characterised in this study, greater concentrations of adducts were measured in the samples treated with 100 mg/L of tannin products compared to samples treated with 50 mg/L

<figure><img src="image_17.png" /><figcaption>**Scheme 1.** Structural hypothesis of reaction products vescalagin/castalagin with methanethiol (MeSH) and ethanethiol (EtSH).</figcaption></figure>
<caption>**Table 1**</caption>

Mass spectrometric analytical results of major reaction products of vescalinin/castalinin with methanethiol (MeSH) and ethanethiol (EtSH).

<table><thead><tr><th rowspan="2">Hypothetical Structures</th><th colspan="2">High Resolution Mass Spectrometry</th></tr><tr><th>Calculated</th><th>Measured</th></tr></thead><tbody><tr><td>[(Vescalinin/Castalinin + -SCH<sub>3</sub> - H) - H]<sup>-</sup>, C<sub>42</sub>H<sub>28</sub>O<sub>26</sub>S</td><td>979.0521</td><td>979.0516</td></tr><tr><td>[(Vescalinin/Castalinin + -SCH<sub>2</sub>CH<sub>3</sub> - H) - H]<sup>-</sup>, C<sub>43</sub>H<sub>30</sub>O<sub>26</sub>S</td><td>993.0621</td><td>993.0673</td></tr></tbody></table>

tannin products. The concentrations of (Vescalinin/Castalinin)-S-CH₃, and (Vescalinin/Castalinin)-S-CH₂-CH₃ also increased between Day 0 and Week 2 (Fig. 5a – 5b).

Moreover, the extracted ion chromatograms of vescalinin/castalinin-S-CH₃ and vescalinin/castalinin-S-CH₂-CH₃ (Fig. S2) suggest the existence of different isomers of these adducts, which highlights the diverse potential binding sites of VSCs on these types of tannins.

This experiment provided evidence for the mechanisms in which ellagic tannins may bind and remove unwanted sulfhydryls from wine and decrease the impact that these highly volatile compounds have on wine aroma.

## 4. Conclusion

These experiments showed that the oenological tannins removed VSCs and that H₂S was removed at the most rapid rate. Overall, both TanninQ and TanninV were effective in removing up to 90 % of H₂S in simple model wine systems after 2 weeks of storage. In real wines, the effects of the tannin were less pronounced, likely due to competitive reactions with sulfhydryls with other wine matrix compounds. In model wines, TanninQ was more effective than Tannin V in removing MeSH and EtSH, and the effect on their concentrations was only measurable after two weeks of storage, whilst in real wine.

A larger proportion of H₂S was removed when wine was at pH 3.00 compared to pH 4.00, whereas larger proportions of MeSH and EtSH were removed when wine samples were at a higher pH (pH 4.00) compared to pH 3.00. This study confirmed the important role that pH plays in the amelioration action of the oenological tannins.

Importantly, this study provided evidence for the decrease of VSCs and the subsequent increase in tannin-VSC adducts, providing evidence towards remediation strategies, as well as the possible mechanism driving the removal of VSCs when using commercial ellagic tannin products to improve ‘freshness’ of ‘reductive’ wines.

### CRediT authorship contribution statement

**Marlize Zaretha Bekker:** Writing – review & editing, Writing – original draft, Supervision, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Allie Cherie Kulcsar:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation. **Alicia Loreto Jouin:** Writing – original draft, Investigation, Formal analysis. **Victor Felipe Laurie:** Writing – review & editing, Writing – original draft, Validation, Supervision, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors thank Dr. Keren Bindon (AWRI) for guidance and helpful discussions regarding tannin chemistry and Dr. Luca Nicolotti for expert assistance with mass spectrometry. V.F. Laurie thanks the 2018 Australia Awards-Endeavour Research Fellowship (Department of Education and Training, Commonwealth of Australia). This work was supported by Wine Australia, with levies from Australia’s grapegrowers and winemakers and matching funds from the Australian Government. The AWRI is a member of the Wine Innovation Cluster in Adelaide, SA.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.foodchem.2025.142777.

### Data availability

Data will be made available on request.

### References

Bekker, M. Z., Espinase Nandorfy, D., Kulcsar, A. C., Faucon, A., Bindon, K., & Smith, P. A. (2020). Comparison of remediation strategies for decreasing ‘reductive’ characters in shiraz wines. *Australian Journal of Grape and Wine Research*, 27(1), 52–65. https://doi.org/10.1111/ajg.12459

Bekker, M. Z., Mierczynska-Vasilev, A., Smith, P. A., & Wilkes, E. N. (2016). The effects of pH and copper on the formation of volatile sulfur compounds in chardonnay and shiraz wines post-bottling. *Food Chemistry*, 207, 148–156. https://doi.org/10.1016/j.foodchem.2016.03.060

Bekker, M. Z., Wilkes, E. N., & Smith, P. A. (2018). Evaluation of putative precursors of key “reductive” compounds in wines post-bottling. *Food Chemistry*, 245, 676–686. https://doi.org/10.1016/j.foodchem.2017.10.123

Cilliers, J. J. L., & Singleton, V. L. (1990). Caffeic acid autoxidation and the effects of thiols. *Journal of Agricultural and Food Chemistry*, 38, 1789–1796.

Danilewicz, J. C. (2007). Interaction of sulfur dioxide, polyphenols, and oxygen in a wine-model system: Central role of iron and copper. *American Journal of Enology and Viticulture*, 58(1), 53–60.

Danilewicz, J. C. (2011). Mechanism of autoxidation of polyphenols and participation of sulfite in wine: Key role of iron. *American Journal of Enology and Viticulture*, 62(3), 319–328. https://doi.org/10.5344/ajev.2011.10105

Day, M. P., Espinase Nandorfy, D., Bekker, M. Z., Bindon, K. A., Solomon, M., Smith, P. A., & Schmidt, S. A. (2021). Aeration of vitis vinifera shiraz fermentation and its effect on wine chemical composition and sensory attributes. *Australian Journal of Grape and Wine Research*, 27(3), 360–377. https://doi.org/10.1111/ajgw.12490

Diaz, I., Castro, R. I., Ubeda, C., Loyola, R., & Laurie, V. F. (2021). Combined effects of sulfur dioxide, glutathione and light exposure on the conservation of bottled sauvignon blanc. *Food Chemistry*, 356, Article 129689. https://doi.org/10.1016/j.foodchem.2021.129689

Ferreira, V., Bueno, M., Franco-Luesma, E., Cullere, L., & Fernandez-Zurbano, P. (2014). Key changes in wine aroma active compounds during bottle storage of Spanish red wines under different oxygen levels. *Journal of Agricultural and Food Chemistry*, 62 (41), 10015–10027. https://doi.org/10.1021/jf503089u

Ferreira, V., Franco-Luesma, E., Vela, E., Lopez, R., & Hernandez-Orte, P. (2018). Elusive chemistry of hydrogen sulfide and mercaptans in wine. *Journal of Agricultural and Food Chemistry*, 66(10), 2237–2246. https://doi.org/10.1021/acs.jafc.7b02427

Fracassetti, D., Limbo, S., Messina, N., Pellegrino, L., & Tirelli, A. (2021). Light-struck taste in white wine: Protective role of glutathione, sulfur dioxide and hydrolysable tannins. *Molecules*, 26(17). https://doi.org/10.3390/molecules26175297

Fracassetti, D., Limbo, S., Pellegrino, L., & Tirelli, A. (2019). Light-induced reactions of methionine and riboflavin in model wine: Effects of hydrolysable tannins and sulfur dioxide. *Food Chemistry*, 298, Article 124952. https://doi.org/10.1016/j.foodchem.2019.124952

Fracassetti, D., Messina, N., Saligari, A., & Tirelli, A. (2023). Evaluation of oenological tannins for preventing the light-struck taste. *Food Chemistry*, 404(Pt A), Article 134563. https://doi.org/10.1016/j.foodchem.2022.134563

Franco-Luesma, E., & Ferreira, V. (2014). Quantitative analysis of free and bonded forms of volatile sulfur compounds in wine. Basic methodologies and evidences showing the existence of reversible cation-complexed forms. *Journal of Chromatography A*, 1359, 8–15. https://doi.org/10.1016/j.chroma.2014.07.011

Friedman, M., & Jurgens, H. S. (2000). Effect of pH on the stability of plant phenolic compounds. *Journal of Agricultural and Food Chemistry*, 48, 2101–2110.

Grant-Preece, P., Barril, C., Schmidtke, L. M., Scollary, G. R., & Clark, A. C. (2017). Light-induced changes in bottled white wine and underlying photochemical mechanisms. *Critical Reviews in Food Science and Nutrition*, 57(4), 743–754. https://doi.org/10.1080/10408398.2014.919246
Ham, J.-S., Kim, H.-Y., & Lim, S.-T. (2015). Antioxidant and deodorizing activities of phenolic components in chestnut inner shell extracts. *Industrial Crops and Products*, 73, 99–105. https://doi.org/10.1016/j.indcrop.2015.04.017

Kontoudakis, N., Schmidtke, L. M., Bekker, M. Z., Smith, M., Smith, P. A., Scollary, G. R. ... Clark, A. C. (2019). Analytical strategies for the measurement of different forms of cu and Fe in wine: Comparison between approaches in relation to wine composition. *Food Chemistry*, **274**, 89–99. https://doi.org/10.1016/j.foodchem.2018.08.084

Kontoudakis, N., Smith, M., Guo, A., Smith, P. A., Scollary, G. R., Wilkes, E. N., & Clark, A. C. (2017). The impact of wine components on fractionation of cu and Fe in model wine systems: Macromolecules, phenolic and sulfur compounds. *Food Research International*, **98**, 95–102. https://doi.org/10.1016/j.foodres.2016.11.017

Kreitman, G. Y., Danilewicz, J. C., Jeffery, D. W., & Elias, R. J. (2016a). Reaction mechanisms of metals with hydrogen sulfide and thiols in model wine. Part 1: Copper-catalyzed oxidation. *Journal of Agricultural and Food Chemistry*, **64**(20), 4095–4104. https://doi.org/10.1021/acs.jafc.6b00641

Kreitman, G. Y., Danilewicz, J. C., Jeffery, D. W., & Elias, R. J. (2016b). Reaction mechanisms of metals with hydrogen sulfide and thiols in model wine. Part 2: Iron- and copper-catalyzed oxidation. *Journal of Agricultural and Food Chemistry*, **64**(20), 4105–4113. https://doi.org/10.1021/acs.jafc.6b00642

Kreitman, G. Y., Danilewicz, J. C., Jeffery, D. W., & Elias, R. J. (2017). Copper(II)-mediated hydrogen sulfide and thiol oxidation to disulfides and organic polysulfanes and their reductive cleavage in wine: Mechanistic elucidation and potential applications. *Journal of Agricultural and Food Chemistry*, **65**(12), 2564–2571. https://doi.org/10.1021/acs.jafc.6b05418

Kreitman, G. Y., Elias, R. J., Jeffery, D. W., & Sacks, G. L. (2019). Loss and formation of malodorous volatile sulfhydryl compounds during wine storage. *Critical Reviews in Food Science and Nutrition*, **59**(11), 1728–1752. https://doi.org/10.1080/10408398.2018.1427043

Laurie, V. F., Zúñiga, M. C., Carrasco-Sánchez, V., Santos, L. S., Canete, Á., Olea-Azar, C., ... Agosin, E. (2012). Reactivity of 3-sulfanyl-1-hexanol and catechol-containing phenolics in vitro. *Food Chemistry*, **131**(4), 1510–1516. https://doi.org/10.1016/j.foodchem.2011.09.138

Lodhia, P., Yaegaki, K., Khakbaznejad, A., Iami, T., Sato, T., Tanaka, T., Murata, T., & Komada, T. (2008). Effect of green tea on volatile sulfur compounds in mouth air. *Journal of Nutritional Science and Vitaminology*, **54**, 89–94.

Luximon-Ramma, A., Bahorun, T., Crozier, A., Zbarsky, V., Dalta, K. P., Dexter, D. T., & Aruoma, O. I. (2005). Characterization of the antioxidant functions of flavonoids and proanthocyanidins in Mauritian black teas. *Food Research International*, **38**, 357–367. https://doi.org/10.1016/j.foodres.2004.10.005

Marrufo-Curtido, A., Ferreira, V., & Escudero, A. (2022). Factors that affect the accumulation of strecker aldehydes in standardized wines: The importance of pH in oxidation. *Molecules*, **27**(10). https://doi.org/10.3390/molecules27103056

Müller, N., & Rauhut, D. (2018). Recent developments on the origin and nature of reductive sulfurous off-odours in wine. *Fermentation*, **4**(3). https://doi.org/10.3390/fermentation4030062

Nikolantonaki, M., Jourdes, M., Shinoda, K., Teissedre, P. L., Quideau, S., & Darriet, P. (2012). Identification of adducts between an odoriferous volatile thiol and oxidized grape phenolic compounds: Kinetic study of adduct formation under chemical and enzymatic oxidation conditions. *Journal of Agricultural and Food Chemistry*, **60**(10), 2647–2656. https://doi.org/10.1021/jf204295s

Nikolantonaki, M., & Waterhouse, A. L. (2012). A method to quantify quinone reaction rates with wine relevant nucleophiles: A key to the understanding of oxidative loss of varietal thiols. *Journal of Agricultural and Food Chemistry*, **60**(34), 8484–8491. https://doi.org/10.1021/jf302017j

Pavelescu, D., Ehsani, M., & Spangl, B. (2021). The influence of yeasts and copper sulphate addition on aroma of Viennese Rheinriesling wines. *OENO one*, **55**(2), 239–250. https://doi.org/10.20870/oeno-one.2021.55.2.3565

Pavez, C., González-Muñoz, B., O'Brien, J. A., Laurie, V. F., Osorio, F., Núñez, E., ... Brossard, N. (2022). Red wine astringency: Correlations between chemical and sensory features. *LWT – Food Science and Technology*, **154**. https://doi.org/10.1016/j.lwt.2021.112656

Petit, E., Jacquet, R., Pouységu, L., Deffieux, D., & Quideau, S. (2015). About the impact of oak ellagitannins on wine odoriferous thiols under acidic and oxidation conditions. *Tetrahedron*, **71**(20), 2991–2998. https://doi.org/10.1016/j.tet.2015.02.036

Petit, E., Jacquet, R., Pouységu, L., Deffieux, D., & Quideau, S. (2019). Reactivity of wine polyphenols under oxidation conditions: Hemisynthesis of adducts between grape catechins or oak ellagitannins and odoriferous thiols. *Tetrahedron*, **75**(5), 551–560. https://doi.org/10.1016/j.tet.2018.11.071

Quideau, S., Feldman, K. S., & Appel, H. M. (2002). Chemistry of gallotannin-derived o-quinones: Reactivity toward nucleophiles. *The Journal of Organic Chemistry*, **60**(16), 4982–4983. https://doi.org/10.1021/jo00121a012

Siebert, T. E., Solomon, M. R., Pollnitz, A. P., & Jeffery, D. W. (2010). Selective determination of volatile sulfur compounds in wine by gas chromatography with sulfur chemiluminescence detection. *Journal of Agricultural and Food Chemistry*, **58**(17), 9454–9462. https://doi.org/10.1021/jf102008r

Suc, L., Rigou, P., & Mouls, L. (2021). Detection and identification of oxidation markers of the reaction of grape tannins with volatile thiols commonly found in wine. *Journal of Agricultural and Food Chemistry*, **69**(10), 3199–3208. https://doi.org/10.1021/acs.jafc.0c07163

Ugliano, M., Kwiatkowski, M., Vidal, S., Capone, D., Siebert, T., Dieval, J. B., ... Waters, E. J. (2011). Evolution of 3-mercaptohexanol, hydrogen sulfide, and methyl mercaptan during bottle storage of sauvignon blanc wines. Effect of glutathione, copper, oxygen exposure, and closure-derived oxygen. *Journal of Agricultural and Food Chemistry*, **59**(6), 2564–2572. https://doi.org/10.1021/jf1043585

Vignault, A., Gonzalez-Centeno, M. R., Pascual, O., Gombau, J., Jourdes, M., Moine, V., ... Teissedre, P. L. (2018). Chemical characterization, antioxidant properties and oxygen consumption rate of 36 commercial oenological tannins in a model wine solution. *Food Chemistry*, **268**, 210–219. https://doi.org/10.1016/j.foodchem.2018.06.031

Viviers, M., Smith, M., Wilkes, E., Smith, P., & Johnson, D. (2014). The role of trace metals in wine 'reduction'. *Wine & Viticulture Journal*, **29**(1), 38–40.

Yasuda, H., & Arakawa, T. (1995). Deodorizing mechanism of (-)-epigallocatechin gallate against methyl mercaptan. *Bioscience, Biotechnology, and Biochemistry*, **59**(7), 1232–1236. https://doi.org/10.1271/bbb.59.1232

Zhang, X., Bekker, M. Z., Kulcsar, A., Espinase Nandorfy, D., & Clark, A. C. (2024). Comparison of techniques for the quantitation of reductive aroma compounds in white wine: Links to sensory analysis and cu fractions. *Journal of Agricultural and Food Chemistry*, **72**(19), 11051–11061. https://doi.org/10.1021/acs.jafc.4c01419