Addition of volatile sulfur compounds to yeast at the early stages of fermentation reveals distinct biological and chemical pathways for aroma formation.

Volatile sulfur compounds (VSCs) greatly influence the sensory properties and quality of wine and arise via both biological and chemical mechanisms. VSCs formed can also act as precursors for further downstream VSCs, thus elucidating the pathways leading to their formation is paramount. Short-term additions of exogenous hydrogen sulfide (H2S), ethanethiol (EtSH), S-ethylthio acetate (ETA), methanethiol (MeSH) and S-methylthio acetate (MTA) were made to exponentially growing fermentations of synthetic grape medium. The VSC profiles produced from live yeast cells were compared with those from dead cells and no cells. Interestingly, this experiment allowed the identification of specific biochemical and/or chemical pathways; e.g. most of the conversion of H2S to EtSH, and the further step from EtSH to ETA, required the presence of live yeast cells, as did the conversion of MeSH to MTA. In contrast, the reaction from MTA to MeSH and ETA to EtSH was due primarily to chemical degradation. Ultimately, this research unravelled some of the complex interactions and interconversions between VSCs, pinpointing the key biochemical and chemical nodes. These pathways are highly interconnected and showcase the complexity of both the sulfur pathways in yeast and the reactive chemistry of sulfur-containing compounds.

A new analytical method to measure S-methyl-l-methionine in grape juice reveals the influence of yeast on dimethyl sulfide production during fermentation.

BACKGROUND: Dimethyl sulfide (DMS) is a small sulfur-containing impact odorant, imparting distinctive positive and / or negative characters to food and beverages. In white wine, the presence of DMS at perception threshold is considered to be a fault, contributing strong odors reminiscent of asparagus, cooked cabbage, and creamed corn. The source of DMS in wine has long been associated with S-methyl-l-methionine (SMM), a derivative of the amino acid methionine, which is thought to break down into DMS through chemical degradation, particularly during wine ageing. RESULTS: We developed and validated a new liquid chromatography-tandem mass spectrometry (LC-MS/MS) method with a stable isotope dilution assay (SIDA) to measure SMM in grape juice and wine. The application of this new method for quantitating SMM, followed by the quantitation of DMS using headspace-solid phase micro-extraction coupled with gas chromatography-mass spectrometry (HS-SPME/GC-MS), confirmed that DMS can be produced in wine via the chemical breakdown of SMM to DMS, with greater degradation observed at 28 °C than at 14 °C. Further investigation into the role of grape juice and yeast strain on DMS formation revealed that the DMS produced from three different Sauvignon blanc grape juices, either from the SMM naturally present or SMM spiked at 50 mmol L-1 , was modulated depending on each of the four strains of Saccharomyces cerevisiae wine yeast used for fermentation. CONCLUSION: This study confirms the existence of a chemical pathway to the formation of DMS and reveals a yeast-mediated mechanism towards the formation of DMS from SMM during alcoholic fermentation. © 2019 Society of Chemical Industry.

The impact of postharvest ultra-violet light irradiation on the thiol content of Sauvignon blanc grapes.

Sauvignon blanc grapes were exposed to an ultra-violet (UV) light source post-hand harvest (whole bunches) or post-machine harvest. The thiol precursors S-3-(hexan-1-ol)-l-cysteine (Cys-3MH) and S-3-(hexan-1-ol)-l-glutathione (GSH-3MH) were quantified in the juices before and after UV treatment. Results showed that irradiation of the grapes with UV light had little to no effect on the thiol precursors. Wines were fermented from the corresponding juices and 18 aroma compounds were quantified. Differences were found between UV treatments of the wines for 3-mercaptohexanol, hexan-1-ol, ethyl butanoate, ethyl hexanoate, ethyl octanoate and phenylethyl alcohol. However, these changes were not significant (p < 0.05) for both grape media trialled. Future studies involving larger sample sizes and replicate numbers should be completed in order to ascertain any changes in aroma chemistry as a result of UV light application to grapes postharvest.

Effect of Cluster Thinning Vitis vinifera cv. Pinot Noir on Wine Volatile and Phenolic Composition.

The effects of two cluster thinning regimes (low and moderate) with Vitis vinifera cv. Pinot noir, in vineyards located in Central Otago, New Zealand, on wine composition, were studied across three consecutive seasons. There were strong correlations between the extent of cluster thinning and pH and bunch weights, and the concentrations of the C13-norisoprenoids, monoterpenes, fatty acids, cinnamic esters, ß-phenylethyl alcohol, and all polyphenols. The sensory terms herbaceous and acidic consistently received the highest ranking in the control wines without cluster thinning. The thinning treatments produced wines with higher marks in the descriptors fruity, spice, sweet, and body. Cluster thinning also had a measurable effect on timing of harvest, in addition to effects on chemical composition and wine sensory, which demonstrate benefits to wine quality that can be expected with cluster thinning of Pinot noir.

Facile gas chromatography-tandem mass spectrometry stable isotope dilution method for the quantification of sesquiterpenes in grape.

Sesquiterpenes are a widespread class of compounds of increasing interest found in grapes and wines, amongst many other natural sources. Due to a lack of standards and the complexity of the mass spectra fragmentation, accurate quantification of these low concentration compounds had not previously been accomplished. The current paper presents a new method for the concurrent quantification of several sesquiterpenes. The multivariate method optimisation is presented. Synthesised isotopic standards were utilised in conjunction with solid phase microextraction (SPME) and gas chromatography-tandem mass spectrometry (GC-MS/MS) to perform a standard isotope dilution assay (SIDA). The method was successfully applied to several grape must samples of four different cultivar. To the best of our knowledge this was the first time some of these sesquiterpenes were quantified in grape.

Pre-fermentation fining effects on the aroma chemistry of Marlborough Sauvignon blanc press fractions.

In the wine industry, fining agents are commonly used with many choices now commercially available. Here the influence of pre-fermentation fining on wine aroma chemistry has been explored. Free run and press fraction Sauvignon blanc juices from two vineyards were fined using gelatin, activated carbon, polyvinylpolypyrrolidone (PVPP) and a combination agent which included bentonite, PVPP and isinglass. Over thirty aroma compounds were quantified in the experimental wines. Results showed that activated carbon fining led to a significant (p<0.05) concentration decrease of hexan-1-ol and linalool in the experimental wines when compared to a control, consistent across all vineyard and fraction combinations. Other aroma compounds were also influenced by fining agent, even if vineyards and press fractions played a crucial role. This study confirmed that fining agents used pre-fermentation can influence wine aroma profiles and therefore needs specific tailoring addressing style and origin of grape.

Hydrogen sulfide production during yeast fermentation causes the accumulation of ethanethiol, S-ethyl thioacetate and diethyl disulfide.

Hydrogen sulfide (H2S) is produced by yeast during winemaking and possesses off-flavors reminiscent of rotten eggs. The production of H2S during fermentation has also been associated in the finished wine with the rise of additional volatile sulfur compounds (VSCs) with strong aromas of cooked onions and vegetables. To characterize these more complex VSCs produced from H2S, we performed fermentations in synthetic grape juice. H2S production was manipulated experimentally by feeding increasing concentrations of sulfate to mutant strains that are unable to incorporate H2S efficiently as part of the sulfur assimilation pathway. In finished wines from these mutants, three VSCs - ethanethiol, S-ethyl thioacetate and diethyl disulfide - increased proportionally to H2S. (34)S-labeled sulfate fed to the MET17-deleted strain was incorporated into same three VSCs, demonstrating that they are formed directly from H2S.

Evolution of Volatile Sulfur Compounds during Wine Fermentation.

Volatile sulfur compounds (VSCs) play a significant role in the aroma of foods and beverages. With very low sensory thresholds and strong unpleasant aromas, most VSCs are considered to have a negative impact on wine quality. In this study, headspace solid phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME/GC-MS) was used to analyze the time course of the biosynthesis of 12 VSCs formed during wine fermentation. Two different strains of Saccharomyces cerevisiae, the laboratory strain BY4743 and a commercial strain, F15, were assessed using two media: synthetic grape media and Sauvignon Blanc juice. Seven VSCs were detected above background, with three rising above their sensory thresholds. The data revealed remarkable differences in the timing and evolution of production during fermentation, with a transient spike in methanethiol production early during anaerobic growth. Heavier VSCs such as benzothiazole and S-ethyl thioacetate were produced at a steady rate throughout grape juice fermentation, whereas others, such as diethyl sulfide, appear toward the very end of the winemaking process. The results also demonstrate significant differences between yeast strains and fermentation media.

Ethyl propiolate derivatisation for the analysis of varietal thiols in wine.

Varietal thiols [3-mercaptohexan-1-ol (3MH), 3-mercaptohexyl acetate (3MHA) and 4-mercapto-4-methylpentan-2-one (4MMP)] have been extensively studied in the recent literature. Nonetheless the hardest obstacle for research focussing on this class of compounds is the lack of quick, user-friendly and sensitive analytical methods. The current paper presents the use of ethyl propriolate (ETP) as a novel derivatising agent to quantify varietal thiols and the first time quantification of the thiol-ETP adducts via gas chromatography-mass spectrometry. Method optimisation including choice of the best SPE cartridge, derivatisation pH and adducts stability is presented. Validation of the method via stable isotope dilution was carried out. Detection limits in both model wine (4MMP 7.2ng/L, 3MHA 40.0ng/L and 3MH 91.2ng/L) and white wine (4MMP 24.5ng/L, 3MHA 120.9ng/L and 3MH 194.6ng/L) for the novel ETP-based method were lower than those obtained with the p-HMB method. Finally, 14 New Zealand Sauvignon blanc were analysed with both the new method and the organo-mercury based procure: good correlations were obtained for 3MH and 3MHA. Detection limits obtained with the new methods, its rapidity and reproducibility make this protocol perfectly suitable for oenological purposes.

Influence of grape-harvesting steps on varietal thiol aromas in Sauvignon blanc wines.

The intense tropical fruit aroma of Sauvignon blanc wines has been associated with the varietal thiols 3-mercaptohexanol (3MH), derived from odorless precursors in the grape, and 3-mercaptohexyl acetate (3MHA), arising from 3MH during fermentation. Grapes and juice were sourced from five locations in Marlborough, New Zealand, taking hand-picked grapes and samples at four stages during the mechanical harvesting process and pressing, which were then fermented in replicated 750 mL bottles. With each set of juices, the highest concentrations of Cys-3MH and Glut-3MH were found in the juices pressed to 1 bar, but these juices produced wines with lower 3MH and 3MHA concentrations. With three of the juices, there was an increase in varietal thiol content for wines made from juices that had been machine harvested compared to the hand-picked samples, which matched earlier findings of lower 3MH and 3MHA levels in wines made from hand-picked grapes. Juices that were more oxidized, and which showed a higher absorbance at 420 nm, were found to produce wines with lower 3MH and 3MHA concentrations.

Influence of juice pressing conditions on polyphenols, antioxidants, and varietal aroma of Sauvignon blanc microferments.

The impact of juice press fractions upon the content of varietal thiols in Sauvignon blanc has been examined for wines fermented at the laboratory scale (750 mL). Wines made from pressed juices (taken at 0.25 and 1.0 bar) contained less than half the concentration of 3-mercaptohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA), important contributors to the tropical and passion fruit character of Sauvignon blanc wines, compared to wines made from free run juices. The pressed juices and wines exhibited lower acidity values, more rapid decline in glutathione content, and more advanced polyphenol oxidation. Supplementation of the juices with glutathione (at 67 mg/L) prior to fermentation led to lower varietal thiol concentrations in the finished wines, typically by several percent, whereas treatment with polyvinylpolypyrrolidone (PVPP) made no impact on wine parameters. Pasteurization of pressed juices increased 3MHA content in the finished wines, but also led to a decline in 3MH concentrations.