Academic literature on the topic '3-Sulfanylhexyl acetate'

Volatile polyfunctional thiol compounds, particularly 3-sulfanylhexan-1-ol (3SH) and 3-sulfanylhexyl acetate (3SHA), are key odorants contributing to the aroma profile of many wine styles, generally imparting tropical grapefruit and passionfruit aromas. 3SH and 3SHA are present in negligible concentrations in the grape berry, juice, and must, suggesting that they are released from non-volatile precursors present in the grape. The exploration of the nature and biogenesis of these precursors to 3SH and 3SHA has proven important for the elucidation of polyfunctional thiol biogenesis during alcoholic fermentation. The development and validation of appropriate analytical techniques for the analysis of 3SH precursors in enological matrices have been extensive, and this review explores the analysis and discovery of these precursor compounds. The development of analytical methods to analyze 3SH precursors, from the selection of the analytical instrument, sample preparation, and methods for standardization, will first be discussed, before highlighting how these techniques have been used in the elucidation of the biogenesis of 3SH and 3SHA in grape wines. Lastly, the future of thiol precursor analysis will be considered, with the development of new methods that greatly reduce the sample preparation time and enable multiple precursors, and the thiols themselves, to be quantitated using a single method.

The levels of varietal thiols and the role these compounds play in Colombard wine have not been investigated in detail. This study assessed the levels of the varietal thiols 4-methyl-4-sulfanylpentan-2-one (4MSP), 3-sulfanylhexyl acetate (3SHA) and 3-sulfanylhexan-1-ol (3SH) and their sensory effects in 24 young South African Colombard wines. Levels of 3SHA and 3SH were, in general, in line with those previously reported in South African Chenin blanc and Sauvignon blanc wines. Levels of 4MSP were, in general, found at a narrower range than those reported for Sauvignon blanc wines. Twelve of these wines were also sensorially analysed by a panel of wine industry experts. Aroma descriptors, such as guava, passion fruit, sweat and tomato leaf, which have previously been linked to 3SH and 3SHA, were also found, especially in the wines containing higher levels of these two compounds. Good correlations between 3SHA and sweat and guava were found. This study contributes to the knowledge of varietal thiol levels and their role played in 24 Colombard white wines. It could also lead to wine producers adapting their winemaking g practices to increase levels of varietal thiols if the aroma characteristics linked to these compounds are sought after in this single cultivar wine.

Polyfunctional thiols have previously been shown to be key aroma compounds in Sauvignon blanc and more recently in Chardonnay wines. Their role in other wine varieties such as those made from three popular indigenous Cypriot grape varieties has remained unexplored. As an extension of a previous project that profiled the sensory and chemical characteristics of Cypriot wines and their comparison to Australian wines, this study aimed to investigate five potent thiols in Xynisteri, Maratheftiko, Giannoudhi, Pinot gris, Chardonnay and Shiraz wines. Wines were analysed utilising Stable Isotope Dilution Assay (SIDA) with derivatisation and High-Performance Liquid Chromatography–Tandem Mass Spectrometry (HPLC-MS/MS). The varietal thiols measured were 4-methyl-4-sulfanylpentan-2-one (4MSP) that has an aroma of “boxwood” and “cat urine” at high concentration, 3-sulfanylhexan-1-ol (3SH) which has been described as having a “grapefruit/tropical fruit” aroma, and 3-sulfanylhexyl acetate (3SHA) that has also been described as having an aroma of “passionfruit”. Additionally, two other potent thiols were measured including benzyl mercaptan (BM) that has an aroma of “smoke and meat” and furfuryl thiol (FFT) that has been described as having a “roasted coffee” like aroma. The reason these thiols are known as potent thiols are due to their very low aroma detection thresholds in the low ng/L (ppt) range. Of the thiols that were measured, 3SH was the only varietal thiol detected in the red wine samples. All of the white wine samples contained 3SH, BM and 3SHA, whereas 4MSP was only detected in Pinot gris and three Xynisteri wines. The potent thiol, FFT, was detected only in the Chardonnay and four of the Xynisteri wines. Interestingly the thiols that were present in the samples were found at concentrations above their aroma detection thresholds (determined in hydroalcoholic solutions), especially 3SH which was found in an order of magnitude above its aroma detection threshold. These findings provide early knowledge of the presence of these thiols in Cypriot wines, compared with Australian wines and establish any relationships between this chemical data with previous wine sensory profile data.

Over one thousand volatile compounds have been identified in wine and these arise from different chemical classes, varietal thiols are of one group which is particularly important given their substantial sensory impact on certain wine varieties. As such, furthering the understanding of thiol biogenesis through the analysis of known precursors and exploration of new ones, and reactivity of thiols based on factors related to viticulture, winemaking, and wine storage, is crucial to the manipulation of varietal thiols in wine and thus overall wine sensory profile. Additionally, exploring of other sulfur-containing volatile compounds in wine is one of the aims of the project, which could contribute to the appreciation of wine aroma complexity as a result of this important class of molecules. A review of the literature (Chapter 1) summarises the production and manipulation of varietal thiols in wine, including 3-sulfanylhexan-1-ol (3-SH), 3-sulfanylhexyl acetate (3-SHA), 4-methyl-4-sulfanylpentan-2-one (4-MSP), and 4-methyl-4-sulfanylpentan-2-ol (4-MSPOH), and considers the impact on non-volatile L-glutathione and L-cysteine conjugated precursors in grapes from viticultural and winemaking practices. Specialised analytical methods for studying such compounds are necessary given their chemical reactivity. The review also identified research gaps, namely knowledge regarding potential new precursors and reactivity of varietal thiols, given that the concentrations of varietal thiols in wine could not be adequately accounted for by the utilisation of known precursors. Impacts of some winemaking practices on the concentrations of thiol precursors in grape materials and varietal thiols in the resultant wines were proposed. The review highlighted the possibility of sulfur-containing volatile compounds awaiting discovery in wine, including grapefruit mercaptan and blackcurrant mercaptan, which are key contributors to the aroma of grapefruit and blackcurrant, respectively. Chapter 2 (prepared in manuscript format) reports a preliminary study aiming to identify new precursors to 4-MSP and 3-SH. 4-MSP can be released from its L-glutathionylated or L-cysteinylated conjugate (i.e., GSH-4-MSP and Cys-4-MSP) with mesityl oxide (MO) assumed to be their precursor but not been identified in grapes. It was hypothesised in Chapter 1 that one route could involve MO produced by soil bacteria near grapevine roots, with subsequent uptake and transformation of MO into 4-MSP precursors. To verify this, a feeding experiment was conducted by applying deuterium labelled MO to grape leaves and bunches of potted grapevines. Analysis of the grape tissues by HPLC-MS/MS showed the presence of deuterium labelled GSH-4-MSP or Cys-4-MSP, indicating that MO is the precursor of GSH-4-MSP and Cys-4-MSP in grapevines. However, further experiments should explore the existence of MO in vineyard soil as well as soil bacteria strains that could potentially yield MO. Additionally, this chapter studied the presence of a potential 3-SH precursor, namely the Nmalonylcysteine conjugated 3-SH (MalCys-3-SH), in Sauvignon blanc juice extracts. The extracts were screened after optimising MS parameters using an authentic MalCys-3-SH standard based on a published HPLC-MS/MS method for thiol precursors. Although MalCys- 3-SH was not identified at this stage, it cannot be concluded with the small sample set that this or other precursors are not present in juice. A greater number of grape samples should be analysed in the future, jointly with MS experiments that look for the loss of specific fragments associated with L-glutathione, for example. The research publication presented in Chapter 3 explores the reactivity of varietal thiols, particularly 3-SH. The recent identification of cis-2-methyl-4-propyl-1,3-oxathiane (cis-2-MPO) in wine was speculated to be the product of 3-SH reacting with acetaldehyde. In a continuation of this work, the evolution profile of cis-2-MPO during alcoholic fermentation was studied, revealing moderate to strong Pearson correlations with 3-SHA and acetaldehyde. Yeast strains significantly affected cis-2-MPO production during fermentation, but resulted in similar concentrations in the resultant wines. The instability of cis-2-MPO was illustrated by the continuous decline in a commercial Sauvignon blanc wine spiked with cis-2-MPO which was stored for one-year under various conditions (pH, temperature, presence of acetaldehyde or SO2). In this case, cis-2-MPO appeared to be preserved by lower pH, acetaldehyde addition, and 4 °C storage temperature. The research publication in Chapter 4 was based on the hypothesis that enantiomers of cis-2-MPO were produced from the corresponding 3-SH enantiomers upon their reaction with acetaldehyde. To verify this, a validated stable isotope dilution assay (SIDA) using gas chromatography coupled to mass spectrometry (GC-MS) with a chiral GC column stationary phase was developed and utilised to analyse wine samples. Chemical formation of cis-2-MPO from the co-spiking of 3-SH and acetaldehyde standards in a commercial wine was verified and used to confirm enantiomer elution order. cis-2-MPO was revealed to consist of (2R,4S)-2-MPO and (2S,4R)-2-MPO in wine, which had strong Pearson correlations with (3S)-3-SH and (3R)- 3-SH, respectively, upon analysing the thiol enantiomers. Additionally, one enantiomer of cis- 2,4,4,6-tetramethyl-1,3-oxathiane (cis-TMO), derived from the reaction of 4-MSPOH and acetaldehyde, was identified and quantified in a few white wine samples (≤ 28 ng/L). Although presenting below the odour detection threshold (14.9 μg/L) determined in this work, its presence demonstrated a pathway for the production of 1,3-oxathianes from acetaldehyde and varietal thiols bearing a 1,3-sulfanylalkanol substitution in their structures. Chapter 5 (prepared in manuscript format) reports the preliminary method development to resolve 4-MSPOH enantiomers and identify the proposed sulfur-containing volatile compounds in wine. Bearing a chiral centre, 4-MSPOH can conceivably consist of two enantiomers. However, the identification of a single cis-TMO enantiomer (Chapter 3) implied that only a single 4-MSPOH enantiomer might be present in wine. To examine this, an assessment of two chiral stationary phases (CSPs), column temperature, eluent composition, and mobile phase flow rate were performed to separate 4-MSPOH enantiomers by adapting a method involving thiol derivatisation and HPLC-MS/MS analysis with a CSP. Base line separation of 4-MSPOH enantiomers was not achieved with the columns on hand, but the method was adequate to verify the hypothesis, and different wine samples were screened. 4- MSPOH was not identified at this stage and further method optimisation using alternative CSPs along with analysing additional wines with a more sensitive instrument may provide more conclusive results. Chapter 5 also studied the potential presence of grapefruit mercaptan (GFM) and blackcurrant mercaptan (BCM) using a published HPLC-MS/MS method after thiol derivatisation. Calibration of BCM was undertaken and a selection of wines was analysed, but BCM was not detected. Further work with a broader set of wines is required to provide evidence about the presence and concentration of BCM. In contrast, GFM appears to be unstable under light or protic conditions and the degradation product was investigated, although additional study is required to verify its identity. Research manuscripts in Chapter 6 and Chapter 7 highlight the effect of a novel grape crushing technique, known as accentuated cut edges (ACE), on the release of thiol precursors and varietal thiols in Shiraz and Sauvignon blanc during winemaking. Other practices, including water dilution and skin contact time for Shiraz, and yeast strain and lactic acid bacteria for Sauvignon blanc, were also evaluated. Concentrations of thiol precursors in Shiraz grape must and varietal thiols in Shiraz wine were not significantly affected by ACE (Chapter 6, research publication). Nonetheless, as best as it can be ascertained, this was the first time that thiol precursors were identified in Shiraz grape must. Sauvignon blanc and Pinot noir wines made with ACE or conventional crushing on a commercial scale were also analysed, giving preliminary insight into the potential of ACE for increasing concentrations of varietal thiols in Sauvignon blanc, but causing a decrease of 3-SH in Pinot noir. The impact of ACE on varietal thiols may depend on grape variety was highlighted, which was pursued further with fermentation trials involving ACE and Sauvignon blanc (Chapter 7, research publication). Varietal thiols and their precursors in the Sauvignon blanc trials were markedly increased by ACE. However, the potential for browning of white wine was raised given the higher amounts of total phenolics and hydroxycinnamates in ACE treatments. The impact of crushing method, yeast strain, and malolactic fermentation on other volatile compounds was evaluated for the Sauvignon blanc fermentations. Interactions with other winemaking parameters were observed and discussed in Chapter 6 and Chapter 7. The overall sensory quality of the Shiraz and Sauvignon blanc wines was assessed with a rate-all-that-apply (RATA) methodology, revealing that the grape crushing method and other winemaking practices could modify the wine sensory profiles, with ACE playing a relatively minor role, as outlined in the respective chapters.
Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2022