Relationship between wine composition and temperature: Impact on Bordeaux wine typicity in the context of global warming-Review.

Weather conditions throughout the year have a greater influence than other factors (such as soil and cultivars) on grapevine development and berry composition. Temperature affects gene expression and enzymatic activity of primary and secondary metabolism which determine grape ripening and wine characteristics. In the context of the climate change, temperatures will probably rise between 0.3°C and 1.7°C over the next 20 years. They are already rising and the physiology of grapevines is already changing. These modifications exert a profound shift in primary (sugar and organic acid balance) and secondary (phenolic and aromatic compounds) berry metabolisms and the resulting composition of wine. For example, some Bordeaux wines have a tendency toward reduced freshness and a modification of their ruby color. In this context it is necessary to understand the impact of higher temperatures on grape development, harvest procedures, and wine composition in order to preserve the typicity of the wines and to adapt winemaking processes.

Nebulized water cooling of the canopy affects leaf temperature, berry composition and wine quality of Sauvignon blanc.

BACKGROUND: The present paper details a new technique based on spraying nebulized water on vine canopy to counteract the negative impact of the current wave of hot summers with temperatures above 30 °C, which usually determine negative effects on vine yield, grape composition and wine quality. RESULTS: The automatized spraying system was able to maintain air temperature at below 30 °C (the threshold temperature to start spraying) for all of August 2013, when in the canopy of uncooled vines the temperature was as high as 36 °C. The maintenance of temperature below 30 °C reduced leaf stress linked to high temperature and irradiance regimes as highlighted by the decrease of H2 O2 content and catalase activity in the leaves. A higher amount of total polyphenols and organic acids and lower sugars characterized the grapes of cooled vines. Wine from these grapes had a higher content of some volatile thiols like 3-sulfanylhexanol (3SH) and 3-sulfanylhexylacetate (3SHA), and lower content of 4-methyl-4-sulfanylpentan-2-one (4MSP). CONCLUSION: Under conditions of high temperature and irradiance regimes, water nebulization on the vine canopy can represent a valid solution to reduce and/or avoid oxidative stress and associated effects in the leaves, ensure a regular berry ripening and maintain high wine quality. The consumption of water during nebulization was acceptable, being 180 L ha-1 min-1 , which lasted an average of about 1 min to reduce the temperature below the threshold value of 30 °C. A total of 85-90 hL (from 0.8 to 0.9 mm) of water per hectare per day was required. © 2016 Society of Chemical Industry.

Vine nitrogen status and volatile thiols and their precursors from plot to transcriptome level.

BACKGROUND: Volatile thiols largely contribute to the organoleptic characteristics and typicity of Sauvignon blanc wines. Among this family of odorous compounds, 3-sulfanylhexan-1-ol (3SH) and 4-methyl-4-sulfanylpentan-2-one (4MSP) have a major impact on wine flavor. These thiols are formed during alcoholic fermentation by the yeast from odorless, non-volatile precursors found in the berries and the must. The present study investigates the effects of vine nitrogen (N) status on 3SH and 4MSP content in Sauvignon blanc wine and on the glutathionylated and cysteinylated precursors of 3SH (Glut-3SH and Cys-3SH) in the berries and the must. This is paralleled by a RNA-seq analysis of gene expression in the berries. The impact of N supply on the expression of the glutathione-S-transferase 3 and 4 (VviGST3 and VviGST4) and the γ-glutamyltranspeptidase (VviGGT), considered as key genes in their biosynthesis, was also evaluated. RESULTS: N supply (N100 treatment) increased the 3SH content in wine while no effect was noticed on 4MSP level. Furthermore, N supply increased Glut-3SH levels in grape berries at late berry ripening stages, and this effect was highly significant in must at harvest. No significant effect of N addition was noticed on Cys-3SH concentration. The transcript abundance of the glutathione-S-transferases VviGST3 and VviGST4 and the γ-glutamyltranspeptidase (VviGGT), were similar between the control and the N100 treatment. New candidate genes which might be implicated in the biosynthetic pathway of 3SH precursors were identified by whole transcriptome shotgun sequencing (RNA-seq). CONCLUSIONS: High vine N status has a positive effect on 3SH content in wine through an increase of Glut-3SH levels in grape berries and must. Candidate GSTs and glutathione-S-conjugates type transporters involved in this stimulation were identified by RNA-seq analysis.

Enhancement of volatile thiol release of Saccharomyces cerevisiae strains using molecular breeding.

Cysteine-conjugated volatile thiols are powerful aromatic compounds that contribute to the fruity notes of many white wines and especially Sauvignon Blanc. Genetic selection programs of wine yeast starters able to produce more volatile thiols constitute, therefore, an important goal for the wine industry. Recent investigations on yeast metabolism suggested that the ß-lyase Irc7p and the control of its gene expression by nitrogen catabolite repression constitute a rational way for yeast genetic improvement. This work demonstrates that the use of a natural ure2 mutation can be used to design wine starters with an enhanced capacity of volatile thiols production. By applying backcrosses driven by molecular markers, this allelic form was introduced in different starter backgrounds. Our investigations demonstrate that the ure2 inheritance is able to enhance the production of 4MMP (recently renamed 4MSP) and 3MH (recently renamed 3SH). For 4MMP, this effect depends of the presence of the allele IRC7LT encoding a long form of the Irc7 protein. Moreover, a correlation in between the expression level of this allelic form and 4MMP production was found within industrial starters. All together, these results emphasised the use of molecular breeding for improving quantitative traits of industrial strains without the use of genetically modifying strategies.

Impact of the Closure Oxygen Transfer Rate on Volatile Compound Composition and Oxidation Aroma Intensity of Merlot and Cabernet Sauvignon Blend: A 10 Year Study.

This study evaluated the impact of closure type on unoaked 100 %-Merlot, oak-aged 70%-Merlot/30%-Cabernet Sauvignon, and 30%-Merlot/70%-Cabernet Sauvignon during a 10 year period. Closures were microagglomerate corks, screw caps, and synthetics with the known oxygen transfer rate (OTR), ranging from 0.1 to 4.6 mg/y, including natural corks. Oxidation intensity perception, dissolved oxygen, sulfite, and 3-methyl-2,4-nonanedione (MND) were monitored on a regular basis. After 10 years of aging, additional aroma impact markers were evaluated (3-sulfanylhexan-1-ol, H2S, DMS, methional, and phenylacetaldehyde). Low OTR levels (≤0.3 mg/y) delayed the oxidation of red wines in this long-term experiment. In addition, our results led us to hypothesize that the MND concentration in young wines might be linked with their ability to produce it during bottle aging that is with their aging potential. Finally, we found that the kinetic accumulation of MND in wines was first strongly impacted by its intrinsic composition and thereafter by the OTRT0 of the stopper.

Yeast and Filamentous Fungi Microbial Communities in Organic Red Grape Juice: Effect of Vintage, Maturity Stage, SO2, and Bioprotection.

Changes are currently being made to winemaking processes to reduce chemical inputs [particularly sulfur dioxide (SO2)] and adapt to consumer demand. In this study, yeast growth and fungal diversity were investigated in merlot during the prefermentary stages of a winemaking process without addition of SO2. Different factors were considered, in a two-year study: vintage, maturity level and bioprotection by the adding yeast as an alternative to SO2. The population of the target species was monitored by quantitative-PCR, and yeast and filamentous fungi diversity was determined by 18S rDNA metabarcoding. A gradual decrease of the α-diversity during the maceration process was highlighted. Maturity level played a significant role in yeast and fungal abundance, which was lower at advanced maturity, while vintage had a strong impact on Hanseniaspora spp. population level and abundance. The presence of SO2 altered the abundance of yeast and filamentous fungi, but not their nature. The absence of sulfiting led to an unexpected reduction in diversity compared to the presence of SO2, which might result from the occupation of the niche by certain dominant species, namely Hanseniaspora spp. Inoculation of the grape juice with non-Saccharomyces yeast resulted in a decrease in the abundance of filamentous fungi generally associated with a decline in grape must quality. Lower abundance and niche occupation by bioprotection agents were observed at the overripened stage, thus suggesting that doses applied should be reconsidered at advanced maturity. Our study confirmed the bioprotective role of Metschnikowia pulcherrima and Torulaspora delbrueckii in a context of vinification without sulfites.

Impact of Closure OTR on the Volatile Compound Composition and Oxidation Aroma Intensity of Sauvignon Blanc Wines during and after 10 Years of Bottle Storage.

The oxygen transfer rate (OTR) of closures is a well-known parameter impacting the quality of Sauvignon blanc wines (SBw) within the first years of storage, but little research has been published on its long-term effects. The chemical changes in oxidation odor intensity in three SBw sealed with natural cork and other closures that had different known OTRs, ranging from <0.1 to 4.6 mg/year, were monitored over a 10 year period. During aging, free SO2 and 3-sulfanylhexanol loss, concomitant with increases in dissolved O2, OD420, and sotolon, were correlated with closure OTR levels. After 10 years of aging, sensory analysis was conducted, supported by additional chemical analysis of aroma impact markers, including methional, phenylacetaldehyde, 2-furanmethanthiol, 4-methyl-4-sulfanylpentan-2-one, ethyl-2-sulfanylacetate, and hydrogen sulfide, as well as total SO2 and dissolved CO2. These analyses revealed that selected SBw were protected from oxidation over a 10 year aging period, provided that the closure OTR did not exceed 0.3 mg/year.

Aromatic Potential of Bordeaux Grape Cultivars: Identification and Assays on 4-Oxononanoic Acid, a γ-Nonalactone Precursor.

γ-Nonalactone has been demonstrated to be a chemical marker of dried/cooked fruit nuances detected in must and wine, but little is known about its formation pathways. Therefore, on the basis of the literature, we hypothesized 4-oxononanoic acid as a potential precursor. Using dichloromethane extraction followed by gas chromatography coupled to negative chemical ionization mass spectrometry, this keto acid was identified and quantified in Merlot and Cabernet Sauvignon musts. Its concentration ranged from traces to 60 µg/L. The biotransformation of 4-oxononanoic acid into γ-nonalactone by Saccharomyces cerevisiae during alcoholic fermentation was demonstrated using labeled d6-4-oxononanoic acid. Additional experiments shed light on the 4-oxononanoic acid role as a γ-nonalactone precursor and revealed that this biotransformation was (R)-enantioselective. Sensory and distribution studies of the enantiomers revealed that the detection threshold of R and S forms were 66 and 35 µg/L and the average ratio of R/S in grape and wine was 94:6 and 65:35.

Analysis of the diastereoisomers of the cysteinylated aroma precursor of 3-sulfanylhexanol in Vitis vinifera grape must by gas chromatography coupled with ion trap tandem mass spectrometry.

The diastereoisomeric distribution of S-3-(hexan-1-ol)cysteine (P-3SH), the cysteinylated precursor of 3-sulfanylhexan-1-ol (3SH) in Vitis vinifera grape juice, was determined by a new method. This procedure is based on the purification of P-3SH in a small volume of must (500 microL) by affinity chromatography, followed by the separation of chiral molecules in derivative forms by gas chromatography coupled with ion trap tandem mass spectrometry (GC-MS/MS). The diastereoisomers were easily separated using heptafluorobutyric anhydride and heptafluorobutanol (HFBA and HFOH) as derivatization reagents. Method validation was conducted using samples of grape juice, synthetic must, fermenting must, and wine that were fortified with P-3SH at concentrations of 0.6 and 2.5 microM. The relative standard deviation (RSD) and limit of detection (LOD) of the GC-MS/MS method were 4.6% and 1.5 nM, respectively. P-3SH assays in Bordeaux white grape juice affected by Botrytis cinerea showed an unusually increased proportion of the RS form of the precursor (approximately RR:RS=30:70) as compared to a diastereoisomer ratio (in the vicinity of 50:50) in healthy grape juice.

Surprising structural lability of a cysteine-S-conjugate precursor of 4-methyl-4-sulfanylpentan-2-one, a varietal aroma in wine of Vitis vinifera L. cv. Sauvignon blanc.

4-Methyl-4-sulfanylpentan-2-one (1; 4MSP) provides a characteristic aroma compound of wines made from Vitis vinifera L. cv. Sauvignon blanc. 4MSP has a strong box-tree odor with a very low perception threshold and is derived from the cysteinylated precursor S-(1,1-dimethyl-3-oxobutyl)cysteine (4; P-4MSP). P-4MSP is transformed into 4MSP during alcoholic fermentation and is an excellent marker of varietal aroma potential. An improved synthesis of P-4MSP as well as of its deuterium-labeled analogue [D(6)]-P-4MSP is described. Several analytical methods (NMR, IR, LSI-MS, GC/MS, ESI-MS(n)) were combined to elucidate spontaneous reversible structural changes of P-4MSP at different pH values. At low pH, P-4MSP has a linear keto form. The keto-enol tautomerism was observed at neutral pH. At pH 8, the formation of N-substituted intramolecular hemiaminal was characterized by ESI-MS and ESI-MS(n) experiments. The hemiaminal loses H(2)O at high pH to produce a cycloimine, which is easily opened by acid hydrolysis. The keto-enol tautomerism explained the incorporation of only six D-atoms during the preparation of the P-4MSP deuterated standard even if [D(10)]mesityl oxide was used. Derivatization conditions for GC/MS analysis strongly affected the ratio of the monosilylated intramolecular cyclic form and the disilylated linear form of P-4MSP. The structural changes of P-4MSP may have a considerable impact on the development of methods of measuring varietal aroma potential.

1,8-Cineole in French Red Wines: Evidence for a Contribution Related to Its Various Origins.

The aromatic descriptor "green", reflecting grape unripeness in French red wines, is frequently associated with the levels of 3-alkyl-2-methoxypyrazines, particularly 3-isobutyl-2-methoxypyrazine (IBMP), which has bell pepper nuances. Nevertheless, not all green aromatic expressions in red wines correlate with 3-alkyl-2-methoxypyrazine concentrations. This study considered sensory and chemical approaches using Cabernet Sauvignon wines obtained from grapes harvested at one-month intervals during the 2014 and 2015 vintages to investigate other volatile odoriferous compounds. Semipreparative HPLC fractionation of wine extracts revealed a fraction with specific green aromas in the early harvest wines. Its sensory impact was confirmed by omission and reconstitution tests. Then, multidimensional gas chromatography coupled with olfactometry and mass spectrometry (MDGC-O-MS/TOF) was used for molecular characterization of the aroma compounds associated with the green aromas. Surprisingly, eucalyptol (1,8-cineole), with menthol odor was highlighted and assayed at concentrations sometimes above its olfactory detection threshold in Cabernet Sauvignon and Fer Servadou wines. Sensory tests confirmed its impact at several concentrations detected in French red wines (up to 2.61 ± 0.03 µg/L) on the menthol nuance and overall green perception, particularly via an additive effect with IBMP. Quantitation of 1,8-cineole in Cabernet Sauvignon and Merlot grapes during berry development in 2015 revealed its varietal origin with abundant concentrations in unripe berries and decrease during grape maturation. Moreover, the implication of an invasive plant (Artemisia verlotiorum) growing in certain vineyards was shown to be responsible for increased 1,8-cineole concentrations in some wines.

Enhanced 3-Sulfanylhexan-1-ol Production in Sequential Mixed Fermentation with Torulaspora delbrueckii/Saccharomyces cerevisiae Reveals a Situation of Synergistic Interaction between Two Industrial Strains.

The aim of this work was to study the volatile thiol productions of two industrial strains of Torulaspora delbrueckii and Saccharomyces cerevisiae during alcoholic fermentation (AF) of Sauvignon Blanc must. In order to evaluate the influence of the inoculation procedure, sequential and simultaneous mixed cultures were carried out and compared to pure cultures of T. delbrueckii and S. cerevisiae. The results confirmed the inability of T. delbrueckii to release 4-methyl-4-sulfanylpentan-2-one (4MSP) and its low capacity to produce 3-sulfanylhexyl acetate (3SHA), as already reported in previous studies. A synergistic interaction was observed between the two species, resulting in higher levels of 3SH (3-sulfanylhexan-1-ol) and its acetate when S. cerevisiae was inoculated 24 h after T. delbrueckii, compared to the pure cultures. To elucidate the nature of the interactions between these two species, the yeast population kinetics were examined and monitored, as well as the production of 3SH, its acetate and their related non-odorous precursors: Glut-3SH (glutathionylated conjugate precursor) and Cys-3SH (cysteinylated conjugate precursor). For the first time, it was suggested that, unlike S. cerevisiae, which is able to metabolize the two precursor forms, T. delbrueckii was only able to metabolize the glutathionylated precursor. Consequently, the presence of T. delbrueckii during mixed fermentation led to an increase in Glut-3SH degradation and Cys-3SH production. This overproduction was dependent on the T. delbrueckii biomass. In sequential culture, thus favoring T. delbrueckii development, the higher availability of Cys-3SH throughout AF resulted in more abundant 3SH and 3SHA production by S. cerevisiae.

Identification of S-3-(hexanal)-glutathione and its bisulfite adduct in grape juice from Vitis vinifera L. cv. Sauvignon blanc as new potential precursors of 3SH.

Two main precursors (S-3-(hexan-1-ol)-l-cysteine and S-3-(hexan-1-ol)-l-glutathione) of 3-sulfanylhexanol (3SH, formerly named 3-mercaptohexanol) have been identified so far in grape juice but a correlation between precursor concentrations in grape juices and 3SH concentrations in wines is not always observed. This suggests that there may be other compounds associated with the aromatic potential. In this work, S-3-(hexanal)-glutathione (Glut-3SH-Al) and its bisulfite (Glut-3SH-SO3) adduct were identified in Sauvignon blanc grape juice by liquid chromatography coupled to Fourier transform mass spectrometry experiments. A partial purification of the compounds was carried out by Medium Pressure Liquid Chromatography (MPLC) on the reverse phase using 5L of grape juice. The addition of synthetized Glut-3SH-Al and Glut-3SH-SO3 in the synthetic medium induced a significant release of 3SH after fermentation. Moreover, we demonstrate that Glut-3SH-Al and its bisulfite adduct are present in grape juice and could be considered as new direct 3SH precursors with molar conversion yields close to 0.4%.

Comparison of electron and chemical ionization modes for the quantification of thiols and oxidative compounds in white wines by gas chromatography-tandem mass spectrometry.

A rapid, sensitive method for assaying volatile impact compounds in white wine was developed using gas chromatography-tandem mass spectrometry (GC-MS/MS) technology, with a triple quadrupole analyzer operating in chemical ionization and electron impact mode. This GC-MS/MS method made it possible to assay volatile thiols (3SH: 3-sulfanylhexanol, formerly 3MH; 3SHA: 3-sulfanylhexyl acetate, formerly 3MHA; 4MSP: 4-methyl-4-sulfanylpentan-2-one, formerly 4MMP; BM: benzenemethanethiol; E2SA: ethyl 2-sulfanylacetate; and 2FM: 2-furanmethanethiol) and odoriferous oxidation markers (Sotolon: 4,5-dimethyl-3-hydroxy-2(5)H-furanone, methional, and phenylacetaldehyde) simultaneously in dry white wines, comparing electron impact (EI) and chemical ionization (CI) modes. More molecular ions were produced by CI than protonated molecules, despite the greater fragmentation caused by EI. So, even using the best reactant gas giving the highest signal for thiols, EI was the best ionization mode, with the lowest detection limits. For all compounds of interest, the limits of quantification (LOQ) obtained were well below their detection thresholds (ranging from 0.5 to 8.5ng/L for volatile thiols and 65-260ng/L for oxidation markers). Recovery rates ranged from 86% to 111%, reproducibility (in terms of relative standard deviation; RSD) was below 18% in all cases, with correlation coefficients above 0.991 for all analytes. The method was successfully applied to the analysis of compounds of interest in Sauvignon Blanc wines from a single estate and ten different vintages.

Involvement of Dimethyl Sulfide and Several Polyfunctional Thiols in the Aromatic Expression of the Aging Bouquet of Red Bordeaux Wines.

The development of an aromatic bouquet during fine wine aging depends on complex transformations occurring in a reductive atmosphere, favorable to the formation and preservation of sulfur odorants, such as dimethyl sulfide (DMS) and polyfunctional thiols. The aim of this study was to address their role in the occurrence, evolution, and perceived sensory nuances of the aging bouquet of red Bordeaux wines. These compounds were quantified in 24 wines and scored by a professional wine panel for the degree to which they reflected the aging bouquet olfactory concept. Partial least square (PLS) analysis, combining sensory and quantitative chemical data, predicted that DMS, 2-furanmethanethiol, and 3-sulfanylhexanol concentrations correlated with the typicality score, discriminating highly-typical wines from less-typical ones. Several vintages from three vineyards were then subjected to sensory and chemical analysis to determine how aging bouquet typicality and the intensity of five key aromatic notes (undergrowth, truffle, fresh fruit, toasted, and empyreumatic) evolved during bottle storage in relation to these three sulfur odorants. PCA analysis emphasized their combined impact on aging bouquet typicality and their contribution to undergrowth, truffle, and empyreumatic attributes.

Vine Nitrogen Status Does Not Have a Direct Impact on 2-Methoxy-3-isobutylpyrazine in Grape Berries and Wines.

Methoxypyrazines (MP) constitute a large family of compounds that contribute to the vegetative varietal aroma of many grapevine varieties and wines. The berry content in 2-methoxy-3-isobutylpyrazine (IBMP), a major MP reminiscent of green-pepper aroma, can be influenced by environmental factors or cultural practices such as water status or mineral nutrition. To date, no study has investigated a possible direct effect of nitrogen (N) on IBMP synthesis without possible interference from water status and vigor variations. In this study, only vine nitrogen status was significantly different among treatments. Water status was controlled during the season, and vine vigor was similar among treatments. IBMP level was maximal at bunch closure and decreased during the season. There was no significant effect of nitrogen nutrition on this metabolite. Moreover, the expression profiles of VvOMT3 and VvOMT4, key genes in the IBMP biosynthetic pathway, were similar between treatments. This result indicates that when an effect of N on IBMP was found in previous studies, it was likely mediated through the modification of bunch-zone microclimate, induced by the higher vigor of high N-status vines.

3-Sulfanylhexanol precursor biogenesis in grapevine cells: the stimulating effect of Botrytis cinerea.

Volatile thiols, compounds that contribute strongly to the varietal aroma, are present in much higher concentrations in sweet wines than in dry wines. This positive effect, due to the presence of Botrytis cinerea on the berries, in fact results from a strong enrichment of cysteine S-conjugate precursors in botrytized berries. In the present study, a convenient model was investigated to reproduce and therefore study this phenomenon. A Vitis vinifera cell culture was used as a simple model, and we focused on S-3-(hexan-1-ol)-l-cysteine (P-3SH), the cysteinylated precursor of 3-sulfanylhexanol. We demonstrated that grapevine cells were able to produce P-3SH and that the presence of B. cinerea considerably increased the precursor level (up to 1000-fold). This positive result was determined to be due to metabolites secreted by the fungus. These molecules were temperature sensitive, unstable over time, and their production was activated in the presence of grapevine cells. Moreover, part of the pathway leading to P-3SH was deciphered: it was directly derived from the cleavage of S-3-(hexan-1-ol)-l-glutathione, which itself was generated after a conjugation of glutathione on (E)-2-hexenal.

The grape must non-Saccharomyces microbial community: impact on volatile thiol release.

Several studies have reported the beneficial influence of non-Saccharomyces yeasts and their potential applications in the wine industry, mainly in mixed-culture fermentation with S. cerevisiae. The potential impact of 15 non-Saccharomyces strains from 7 species on 4-methyl-4-sulfanylpentan-2-one (4MSP) and 3-sulfanylhexan-1-ol (3SH) release in model medium and Sauvignon Blanc must was evaluated after partial fermentation. Whereas the impact of non-Saccharomyces on 4MSP release in both media was low, some M. pulcherrima, T. delbrueckii and K. thermotolerans strains had a high capacity to release 3SH, despite their minimal fermentation activity. As previously demonstrated for Saccharomyces yeast, this contribution is strain dependant. Taking into account their dynamic and quantitative presence during the whole process, the real impact of non-Saccharomyces yeast on 4MSP and 3SH release was evaluated using a recreated community simulating the yeast ecosystem. Our results revealed a positive impact on 3SH release in Sauvignon Blanc wines by promoting non-Saccharomyces yeast activity and delaying the growth of S. cerevisiae. Some non-Saccharomyces yeast strains are capable of making a positive contribution to volatile thiol release in wines, essentially during the pre-fermentation stage in winemaking, when this microbiological sub-population is dominant.

Elucidation of the 1,3-sulfanylalcohol oxidation mechanism: an unusual identification of the disulfide of 3-sulfanylhexanol in sauternes botrytized wines.

A four-step purification method was developed to isolate a citrus odorant detected by gas chromatography-olfactometry (GC-O), which was apparently specific to Sauternes botrytized wines. A fragmentation pattern of the odorant was obtained by multidimensional gas chromatography-mass spectrometry-olfactometry (MDGC-MS-O). The exact mass measurement was used to determine its elemental formula as C(6)H(12)OS. On the basis of these data, the unusual structure of 3-propyl-1,2-oxathiolane was synthesized and characterized for the first time. This confirmed its identification. Its occurrence in Sauternes wine extracts was demonstrated to result from the thermal oxidative degradation of 3-sulfanylhexanol disulfide (3,3'-disulfanediyldihexan-1-ol) in the GC injector. This disulfide was synthesized and then firmly identified for the first time in Sauternes wine. Although the presence of 3-sulfanylhexanol oxidation products had previously been reported in natural extracts (but not wine), the full oxidation pathway from 3-sulfanylhexanol to 3-propyl-γ-sultine via 3,3'-disulfanediyldihexan-1-ol was clearly established for the first time. Because the disulfide has mainly been detected in Sauternes botrytized wines, this finding suggested a singular reactivity of 3-sulfanylhexanol in botrytized wines, thus opening up a wide range of new opportunities in wine chemistry.

Aromatic potential of botrytized white wine grapes: identification and quantification of new cysteine-S-conjugate flavor precursors.

Sweet wines made from botrytized grapes contain much higher concentrations of volatile thiols, especially 3-sulfanylhexan-1-ol (3SH), than dry white wines. Three new specific volatile thiols (3-sulfanylpentan-1-ol (3SP), 3-sulfanylheptan-1-ol (3SHp), and 2-methyl-3-sulfanylbutan-1-ol (2M3SB) were recently identified in Sauternes wines. Like most volatile thiols, these compounds were almost totally absent from must, mainly being formed during alcoholic fermentation. In this work, we describe the identification and quantification of three new cysteine-S-conjugate precursors in must made from Botrytis-infected grapes. S-3-(pentan-1-ol)-L-cysteine (P-3SP), S-3-(heptan-1-ol)-L-cysteine (P-3SHp), and S-3-(2-methylbutan-1-ol)-L-cysteine (P-2M3SB) were identified by direct GC-MS analysis of their derivative forms obtained by silylation of an enriched fraction, isolated from must by affinity chromatography. Concentrations were considerably higher when Botrytis cinerea had developed on the grapes. In botrytized must, the mean levels of P-3SP, P-3SHp, and P-2M3SB were in the vicinity of 700, 50, and 500 nM, respectively, whereas concentrations in healthy must ranged from 0 to 50 nM. This indicated that these three new sulfanyl alcohols, responsible for the characteristic aroma of botrytized wines, were formed by the yeast metabolism during alcoholic fermentation from the corresponding non-volatile cysteine-S-conjugate precursors. Moreover, these results highlighted the predominant role of botrytization in developing grape aroma potential.