Sulfur compounds play an important role in the sensory characteristics of wine. These molecules can derive from the grape, in which the non-volatile forms are usually present as glycosylated molecules, the metabolic activities of yeast and bacteria, the chemical reactions taking place during the wine aging and storage, and the environment. The sulfur compounds include molecules positively correlated to the aromatic profile of wine, namely the volatile thiols, and are responsible for certain defects, imparting notes described as cabbage, onion, rotten egg, garlic, sulfur and rubber. Due to the low concentration of these molecules in wine, their high reactivity and the matrix complexity, the analytical methods which enable their detection and quantification represent a challenge. The solid phase microextraction (SPME) technique has been developed for sulfur compounds associated with off-flavors. The analysis of volatile thiols usually requires a derivatization followed by gas chromatography (GC)-MS or UPLC-MS methods. Besides the sulfur-containing aromas, another sulfur compound that deserves mention is the reduced glutathione (GSH) which has been widely studied due to its antioxidant properties. The analysis of GSH has been proposed using a liquid chromatography technique (HPLC or UPLC) coupled with fluorescence, MS and UV detectors.
- sulfur off-flavors
- volatile thiols
- reduced glutathione
- sample preparation
Sulfur-containing compounds strongly affect the sensory properties of wine and must. They include aromatic molecules, off-flavors and a well-known non-volatile compound with antioxidant properties such as glutathione. The sulfur-containing compounds can be derived from the grape in which the non-volatile forms are usually present, the metabolic activities of yeast and bacteria, the chemical reactions taking place during the wine aging and storage, and the environment . Their presence in wine can be the result of both enzymatic and non-enzymatic mechanisms. In the first case, the sulfur compounds represent the products of metabolic and fermentative pathways whose substrates are both amino acids and some sulfur-containing pesticides. When yeast and bacteria metabolize these thiols, the released sulfur compounds are generally considered off-flavors . Non-enzymatic processes include photochemical, thermal and other chemical reactions of sulfur compounds during winemaking and storage . The reactions involving the sulfur-containing amino acids, in particular, can bring about the light-struck taste in case the bottled white wine is exposed to light greatly affecting the sensory properties [4, 5]. The sulfur-containing off-flavors impart negative notes such as cabbage, onion, rotten egg, garlic, sulfur and rubber . Among them, hydrogen sulfide is probably the best-known sulfur compound in wine. Hydrogen sulfide is a very reactive species and can trigger reactions generating compounds such as mercaptans, dimethyl sulfide and polysulfide, which also have a negative impact on the wine aroma. The long-chain polyfunctional sulfur compounds, also known as volatile thiols, are one of the most important groups of aroma compounds in wine, which confer pleasant aromatic notes at trace levels; at high concentrations, these compounds can be objectionable yet [3, 7, 8, 9]. Volatile thiols such as 4-mercapto-4-methylpentan-2-one (4MMP), 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA) are particularly important, for example, for characterizing the typical aroma of Sauvignon Blanc wine . Moreover, in some wines, 4-mercapto-4-methylpentan-2-ol (4MMPOH) is also detected but its concentration is lower than the thiol-related aromas mentioned above .
Besides the sulfur compounds contributing to the sensory characteristics of wine, the glutathione is of particular interest due to its antioxidant properties. This tripeptide can limit the browning of white must as it reduces back the
2. Aromatic compounds containing sulfur
2.1. Sulfur off-flavors
Sulfur-containing compounds are mainly associated with off-odors due to the presence of hydrogen sulfide and mercaptans whose concentrations are typically low as well as their perception threshold . The most abundant sulfur off-flavors found in wine are represented by hydrogen sulfide, methanethiol, ethanethiol, dimethylmercaptans (dimethylsulfide, dimethyldisulfide, dimethyltrisulfide)  and other sulfur-containing compounds responsible for off-flavors in wine (Table 1). Hydrogen sulfide is the most recognized sulfur compound associated with the rotten egg aroma, it is highly volatile and has a low perception threshold (10–80 μg/L). Even if hydrogen sulfide could be easily removed by copper treatment , it is very reactive specie participating in the generation of other mercaptans, such as dimethylsulfide and polysulfide, and ethanthiol, the latter due to its combination with ethanol or acetaldehyde .
|Compound||Olfactory description||Perception threshold (μg/L)||Range in wine (μg/L)|
|Hydrogen sulfide||Rotten eggs, reduced taste||0.001–150||0–370|
|Methanethiol||Cooked cabbage, reduced taste||0.3||0–16|
|Ethanthiol||Onion, rubber, putrefaction||1.1||0–50|
|Dimethyl sulfide||Cabbage, asparagus, corn, molasses||10–160||0–910|
|Carbon disulfide||Cabbage, rubber||>38||0–18|
|Dimethyl trisulfide||Cabbage, onions, cooked vegetable||0.1||0–111|
|Dimethyl disulfide||Cooked cabbage, asparagus, onions||20–45||0–160|
|Diethyl disulfide||Garlic, onion, burnt rubber||4.3–40||0–160|
|2-Mercaptoethanol||Barnyard-like, poultry, farmyard||130||0–400|
|Methylthioacetate||Sulfurous, rotten vegetables||300||0–115|
|2-(Methylthio)-1-ethanol||Cauliflower, French bean||250||0–139|
|2-(Methylthio)-1-propanol||Cauliflower, cooked cabbage||1200||0–5655|
|2-(Methylthio)-1-butanol||Onion, garlic, earthy||100||0–180|
The sulfur-containing compounds are associated with the occurrence of light-struck taste, a defect affecting white wine bottled in clear bottles and exposed to light. The photochemical oxidations may affect different wine components including phenols, acids and alcohols [31, 32]. The formation of the light-struck taste is related to the presence of riboflavin, a vitamin highly sensitive to light and methionine. The pathway proposed by Maujean and Seguin  involves the photoxidative degradation of methionine to give methional and reduced riboflavin. Methional is unstable when exposed to light and decomposes to acrolein and methanethiol (Figure 1). Two molecules of the latter eventually yield dimethyl disulfide (DMDS). The light-struck taste is described as cooked cabbage mainly due to the formation of methanethiol and DMDS though hydrogen sulfide may have a role [33, 34, 35]. Light exposure can also lead to the formation of other undesirable compounds, such as furfural  which has been positively correlated to the “cooked vegetable” aroma of white wines stored under oxidative conditions [37, 38]. Not all wines are susceptible to developing this defect. The concentration of riboflavin plays the major role [5, 39] and when it is lower than 80–100 μg/L, the risk of the light-struck taste decreases. The riboflavin content in the grape hardly exceeds a few tens of micrograms per liter and the metabolic activities of
2.2. Varietal thiols
The varietal thiols including 3-mercaptohexan-1-ol (3MH), 3-mercaptohexylacetate (3MHA) and 4-mercapto-4-methylpentan-2-one (4MMP) are sulfur-containing aromas associated with the typical flavor of Sauvignon Blanc wines [11, 43, 44]. Moreover, in some wine, 4-mercapto-4-methylpentan-2-ol (4MMPOH) was also detected, but its level was lower than the volatile thiols mentioned above  (Table 2). They are characterized by some fruity aromas, like cassis , grapefruit , passion fruit  and guava . The impact of sulfur compounds on wine aroma has been updated in literature and the varietal character is affected by several of these molecules if their concentration is close to the perception threshold [22, 49, 50]. 3MH and 3MHA have olfactory perception thresholds of 60 and 4 ng/L, respectively and they are responsible for passion fruit-like and grapefruit-like olfactory notes . These compounds were also found in red wines . The 4MMP has an olfactory perception threshold of 0.8 ng/L, and its aroma is described as box tree-like, black currant-like, or even cat urine-like when occurring at high concentration . The 4MMPOH has a perception threshold of 55 ng/L in aqueous alcoholic solution and it is reminiscent of citrus zest and grapefruit . 3MH is the most abundant in wine, in concentrations generally higher than its perception threshold, while 4MMPOH in wine is generally lower than its perception threshold. The level of 4MMP is dependent on the grape cultivar and it changes between different samples in the same cultivar . The concentrations of 3MH and 3MHA in Sauvignon Blanc wines from different countries have been reported to be in the range 688–18,681 and 10–2507 ng/L, respectively [53, 54, 55]. Besides Sauvignon Blanc wines, the volatile thiols were detected in white wine made from different
The content of volatile thiols decreases during wine aging, according to the oxidative conditions. Glutathione, sulfur dioxide and anthocyanin content exert a protective effect. In contrast, increased contact with oxygen, particularly in the presence of catechin derivatives, promotes their degradation [52, 60, 61]. Three different mechanisms proposed lead to their decay with a different degradation kinetics. The volatile thiols can be easily oxidized when oxygen and iron are present, forming disulfides [62, 63]. Since the volatile thiols act as electrophiles, they can react with phenolic compounds . Moreover, the presence of oxygen and catalyst metals, namely iron and copper, lead the oxidation of phenols into the corresponding
2.2.1. Varietal thiol precursors
The varietal thiols occur in grape berry as non-volatile cysteinyl- and glutathionyl-conjugated precursors [29, 68]. 3MH bound as cysteinyl-conjugate (Cys-3MH), glutathionyl-conjugate (GSH-3MH) and also cysteinylglycin-conjugate (CysGly-3MH) have been reported [70, 71]. 4MMP occurs in the grape and must as cysteine conjugate (Cys-4MMP) and glutathione conjugate (GSH-4MMP) [72, 73]. Moreover, 3MH can be derived from either (E)-2-hexen-1-ol or (E)-2-hexenal [74, 75]. Through a lipoxygenase/lyase sequence active in the presence of oxygen, (E)-2-hexenal can be obtained from linolenic acid and converted to GSH-3MH by coupling with glutathione [75, 76, 77]. (E)-2-hexenal may act as a precursor when hydrogen sulfide is released in the early part of the fermentation [73, 74, 78]. Under aerobic conditions,
The harvest has a significant effect on the content of thiol precursors in juice. Capone and Jeffery  showed that machine harvest and transportation for 12 h of the grapes led to higher levels of thiol precursors in comparison to hand harvest grapes immediately processed. The authors suggested the minimal berry damage results in less formation of the precursors. More than half of the total cysteine conjugates is located in the grape skin; as a consequence, an increased skin contact time augments Cys-3MH in grape musts, while it has little effect on Cys-4MMP and Cys-4MMPOH concentrations [91, 92, 93]. Stronger pressing conditions allow a major extraction of thiol precursors in comparison to free run juice. Roland and co-authors  demonstrated an easier extraction of Cys-3MH in comparison to GSH-3MH due to their distribution in different parts of the skin. The produced wines contained 3MH at higher concentrations when the juice was collected at the end of the pressing cycle. On the contrary, Patel et al.  found lower amounts of 3MH and 3MHA in wine produced from juice obtained from stronger pressing conditions in which the thiol precursors were present in higher amounts. This can be due to the higher extraction of oxidizable phenols inducing the rapid decrease of glutathione and the oxidation of varietal thiols. The pressing performed on an industrial scale caused a strong decrease of thiol precursors. The factors affecting the GSH-portion degradation of the precursors are not completely clear and mechanisms, other than oxidation or proteolysis, could induce the loss of the thiol precursors .
3. The reduced glutathione
The reduced glutathione (GSH) is a tripeptide constituted by l-cysteine, γ-glutamic acid and glycine exerting antioxidant and detoxifying activities in the cell [96, 97]. This compound exerts several activities in must and wine. The antioxidant property of GSH is well known: it can reduce the
GSH is able to limit the loss of the flavoring volatile thiols acting as a competitor for the reduction of the quinones . In fact, GSH concentration is about a thousand times higher than that of volatile thiols which are protected against oxidation. Lavigne and Dubourdieu  reported that when the GSH concentration ranged from 6 to 10 mg/L it slowed down the decrease of volatile thiols. Additionally, other aromatic compounds, such as isoamyl acetate (3-methyl-1-butyl acetate), ethyl hexanoate and linalool (3,7-dimethylocta-1,6-dien-3-ol), are better protected during bottle storage . GSH can limit the formation of sotolon (3-hydroxy-4,5-dimethyl-2(5H)-furanone), a compound responsible for the atypical aging character of white wine . It confers aroma descriptors such as dried fig and rancid and its perception threshold is 7 μg/L . Besides sotolon, 2-aminoacetophenone (1-(2-aminophenyl)-ethanone) is also responsible for the atypical aging and it has a lower perception threshold than sotolon, corresponding to 1 μg/L. Both sotolon and 2-aminoacetophenone concentrations increase due to the exposition of wine to oxygen during bottling . GSH can have a protective action on the wine aroma during the oxidative aging. GSH reduces the formation of both these off-flavors during storage. Moreover, GSH can have a positive effect on white wine color which appears to be more stable during aging [103, 107]. The first source of GSH is the grape in which it can exceed 200 mg/L of grape juice according to grape cultivar, environmental conditions and viticultural practices  and the amounts of readily assimilable nitrogen in the soil . Lower levels were found in must up to 100 mg/L  and it can be affected by exposure to oxygen, tyrosinase activity and pre-fermentative grape skin maceration [93, 109]. The loss of GSH in must production can negatively affect the formation of precursors of the varietal thiol compounds  as well as the residual content of GSH during wine aging. Glutathione was reported to be consumed by
4. Analytical methods
The analytical methods developed for the analysis of sulfur compounds in wine need to overcome the low concentrations and the high reactivity of these molecules as well as the complexity of the matrix. Sulfur compounds in wine are frequently divided into “light” (boiling point < 90°C) and “heavy” (boiling point > 90°C) compounds  indicating difficulty in using a relevant common sampling/enrichment technique.
4.1. Sulfur off-flavors
The most common technique employed for the analysis of sulfur compounds associated with off-flavors in wine is static headspace analysis by means of solid phase micro extraction (SPME) combined with gas chromatography (GC) coupled with different detectors (Table 3). Methods in dynamic headspace by using Purge and Trap equipment was also described as an alternative to the static headspace. In static headspace technique, the analytes reach the equilibrium state between the liquid and gas phases and then are adsorbed in a fiber. In dynamic space, the analytes in the gas phase into the headspace and the atmosphere around the sample is constantly swept away by a flow of carrier gas, taking volatile analytes with it. Through this technique, the equilibrium state is not reached and, thus, more of the volatile dispersed in the matrix will pass into the headspace whose size results in increased sampling phase. As a consequence, the trapping stage of the analysis offers good sensitivity [43, 115]. The proper combination of sorbent and temperature may permit collection and concentration of specific analytes while venting others. Despite this, the instrumentation requires more complexity and it is more expensive than other sampling techniques, such as the static headspace. Moreover, many sources of error in Purge and Trap instruments have been reviewed . As for the dynamic headspace, the choices of the proper fiber as well as sample temperature and the presence of salt increasing the ionic strength can improve extraction yield in static headspace. In particular, the use of carboxen-polydimethylsiloxane fiber (CAR-PDMS) [117, 118] or carboxen-polydimethylsiloxane-divinylbenzene (CAR-PDMS-DVB) [22, 119] has been proposed for the analysis of fermentative sulfur compounds. The use of the latter fiber has been shown to produce good results in terms of repeatability and reproducibility . Moreover, the best modified ionic strength has been obtained by using magnesium sulfate. Due to the different boiling temperature of the fermentative sulfur compounds, a good compromise needs to be set to allow for the adsorption of these compounds through the SPME technique. As reported by Nguyen and co-authors , the incubation of the sample was carried out at 45°C for 5 min and the extraction at 45°C for 30 min under agitation. The GC equipment was coupled with MS detector and the compounds of interest were detected in single ion monitoring (SIM) mode and quantified by means of different internal standards properly chosen. The analytical methods described above are suitable for the analysis of the volatiles characterized by low boiling point (lower than 90°C) that is not the case for 3MH, 3MHA and 4MMP.
|Dynamic headspace||Purge and trap—gas chromatography||Increased sampling phase; good sensitivity||Many sources of errors|
|Static headspace||Solid phase micro extraction (SPME)—gas chromatography||Cheaper and simpler instrumentation||Compromise between sensitivity and extraction/temperature conditions needs to be properly set|
|Carboxen-polydimethylsiloxane (CAR-PDMS)a||Suggested for sulfur compounds with low boiling point||Less versatile than CAR-PDMS-DVB fiber|
|Carboxen-polydimethylsiloxane-divinylbenzene (CAR-PDMS-DVB)b||Best extraction yield for the analytical conditions applied||Suitable analytical method setting for increasing the extraction yield (sample temperature, ionic strength, extraction time)|
4.2. Varietal thiols
Among headspace sampling techniques, only purge and trap has been used to analyze 3MH and 3MHA in wines, reaching detection limits closer to their perception threshold . Despite this, the other heavy volatile sulfur aromas were not identified at perception threshold, thus leaving derivatization procedures as the most promising technique for the extraction and analysis of volatile thiols. Due to the low concentration of these compounds, in the order of magnitude of ng/L, the sample preparation has provided a liquid/liquid extraction followed by the evaporation of the organic solvent. As the varietal thiols are present in wine in low concentration as well as being highly reactive, the deuterated analogues are commonly used as internal standards compensating for the eventual loss taking place during the sample preparation .
Among the derivatizing compounds, the use of
Other analytical approaches employ pentafluorobenzyl bromide as a derivatizing agent, which transforms thiols into their corresponding pentafluorobenzyl derivatives [125, 126, 127, 128]. The derivatizing reaction is usually carried out in a purified extract (i.e. water) , organic solvent , in-cartridge , or in-fiber . Another very promising derivatizing agent in the gas chromatography analysis of thiols is ethyl propiolate, which is able to derivatize thiols directly in wine matrix and is a suitable derivatizing reagent for the electron impact mass spectrometry detection system .
Moreover, Piano and co-authors  proposed an analytical method in which the varietal thiols are detected by liquid chromatography (UPLC) coupled with MS/MS. The sample preparation required several steps in order to protect the thiol aromas from oxidation and the liquid/liquid extraction in order to achieve the concentration of the analytes. The
4.2.1. Varietal thiol precursors
The analytical methods proposed for the determination of volatile thiol precursors include both indirect and direct methods . In the first case, the precursors are transformed in volatile compounds and determined by both GC and HPLC coupled with MS detector. In the second case, the precursors are quantified after their proper purification.
In the first method developed, the Cys-4MMP was detected by GC coupled with flame photometric detectors (FDP) by synthetizing it and 4MMP was then detected . Tominaga and co-authors  quantified the cysteinylated precursors by GC-MS after their derivatization. The addition of labeled thiol precursors allowed the quantification of those contained in the must by GC-FDP and GC-MS . The GC coupled either with atomic emission detection (AED)  or detection-capture mass spectrometry (DCMS) detector  were also proposed employing propyl thioacetate as internal standard and derivatizing the released volatile thiols with ethyl chloroformate, respectively. The precursors of 3MH were quantified using the labeled isotopic standard d10-3MH by GC-MS.
Both GC-MS and liquid chromatography techniques were used for the direct determination of thiol precursors. In the case of GC-MS, their derivatization is necessary and different derivatizing agents were proposed [133, 134, 135]. In the case of liquid chromatography, both HPLC-MS and HPLC-MS/MS were applied. The use of these types of equipment did not require a derivatization of thiol precursors, but a solid phase extraction (SPE) was applied achieving the sample purification. The quantification was carried out by means of the patterns of labeled compounds [73, 87, 136, 137, 138, 139] as well as without them [59, 72]. The quantification by liquid secondary ionization mass spectrometry (LSIMS) was also reported .
4.3. The reduced glutathione
Several analytical methods have been proposed for GSH quantification in the grape, must and wine using different analytical techniques . GSH was quantified after a treatment with glutathione reductase enzyme in white wine . In grapevine tissues, the GSH was derivatized with 5,5′-dithiobis-2-nitrobenzoic acid (DTNB) after the enzymatic treatment and the detection was performed spectrophotometrically at 412 nm . The determination of total GSH could be performed after the enzymatic treatment which allowed the breakdown of disulfide bridges. The GSH could be derivatized pre-column with
Several sulfur compounds occur in wine with a strong influence on its aromatic profile. On one hand, some of these molecules impart negative notes and their presence in wine should be counteracted and limited. The formation mechanisms are not completely clear and further researches are needed also to better understand the treatments and/or the winemaking practices potentially decreasing the appearance of these undesired compounds. On the other hand, the varietal thiols are responsible for positive characteristics of wine aroma up to a certain extent. Their protection is essential for the maintenance of the aromatic profile throughout both the winemaking and storage of wine by means of antioxidants of which reduce glutathione can represent a good natural candidate. Further studies will be necessary to investigate the fate of their precursors during winemaking on an industrial scale as well as to increase the aromatic potential of the produced wine. In the last decades, improvements of the analytical methods have been carried out in terms of sensitivity and identification of new odorants has been achieved. The goal for the researches has being the set up more sensitive and less-time consuming methods with a reduced impact on the environment in order to minimize defects and optimize the aromatic profile during winemaking.