Philippe Darriet

Impact of Oxygen Dissolved at Bottling and Transmitted through Closures on the Composition and Sensory Properties of a Sauvignon Blanc Wine during Bottle Storage

This work outlines the results from an investigation to determine the effect of the oxygen dissolved at bottling and the specific oxygen barrier properties of commercially available closures on the composition, color and sensory properties of a Bordeaux Sauvignon Blanc wine during two years of storage. The importance of oxygen for wine development after bottling was also assessed using an airtight bottle ampule. Wines were assessed for the antioxidants (SO(2) and ascorbic acid), varietal thiols (4-mercapto-4-methylpentan-2-one, 3-mercaptohexan-1-ol), hydrogen sulfide and sotolon content, and color throughout 24 months of storage. In addition, the aroma and palate properties of wines were also assessed. The combination of oxygen dissolved at bottling and the oxygen transferred through closures has a significant effect on Sauvignon Blanc development after bottling. Wines highly exposed to oxygen at bottling and those sealed with a synthetic, Nomacorc classic closure, highly permeable to oxygen, were relatively oxidized in aroma, brown in color, and low in antioxidants and volatile compounds compared to wines sealed with other closures. Conversely, wines sealed under more airtight conditions, bottle ampule and screw cap Saran-tin, have the slowest rate of browning, and displayed the greatest contents of antioxidants and varietal thiols, but also high levels of H(2)S, which were responsible for the reduced dominating character found in these wines, while wines sealed with cork stoppers and screw cap Saranex presented negligible reduced and oxidized characters.

Reactivity of volatile thiols with polyphenols in a wine-model medium: Impact of oxygen, iron, and sulfur dioxide

As volatile thiols are nucleophiles, they are capable of additional reactions with electrophiles. In enology, this concerns reactions between volatile or non-volatile thiols and oxidized phenolic compounds. Initial studies concerning the reactivity of volatile thiols with polyphenols showed that (+)-catechin played a detrimental role in the level of 3-sulfanylhexan-1-ol (3SH), in the absence of sulfur dioxide. Our experiment revealed that (-)-epicatechin was more reactive with volatile thiols than (+)-catechin. Furthermore, Fe (III) was shown to play a crucial role in catalyzing polyphenol oxidation reactions, by affecting the direct reaction of phenolic compounds with oxygen. It was noted that, even if the volatile thiols studied were members of the same chemical family, they exhibited a different behavior pattern under oxidation conditions. 2-furanmethanethiol (2FMT) was more reactive than 3-sulfanylhexan-1-ol with both (+)-catechin and (-)-epicatechin. In contrast, 4-methyl-4-sulfanylpentan-2-one (4MSP) was less reactive with these phenolics. Additionally, the vital role of sulfur dioxide in protecting 3-sulfanylhexan-1-ol, 2-furanmethanethiol, and 4-methyl-4-sulfanylpentan-2-one was demonstrated in the model medium.

Identification of volatile compounds responsible for prune aroma in prematurely aged red wines.

The premature aging of red Vitis vinifera L. wines is mainly associated with the formation of an intense off-flavor reminiscent of prunes. The compounds responsible for this deterioration in red wine flavor have not previously been identified. Sensory descriptive analysis associated with a gas chromatography-olfactometry (GC-O) technique was first performed to find characteristic odoriferous zones of 15 aged red wines with or without a marked prune aroma. Afterward, high-pressure liquid chromatography, gas chromatography, and multidimensional gas chromatography coupled with mass spectrometry (MDGC-MS) were used to identify the odorants reminiscent of prunes in prematurely aged red wines and in the dried fruit. Three compounds were detected with a strong odor of prunes: gamma-nonalactone, beta-damascenone, and 3-methyl-2,4-nonanedione. The perception threshold of the latter beta-diketone in a model hydroalcoholic solution is 16 ng/L. Identified for the first time in aged red wines, this very powerful volatile compound was also suggested to produce the characteristic prune aroma of prematurely aged red wines. The presence of 3-methyl-2,4-nonanedione was also detected in prunes for the first time.

Characterization of Penicillium species isolated from grape berries by their internal transcribed spacer (ITS1) sequences and by gas chromatography-mass spectrometry analysis of geosmin production

Geosmin (trans-1,10-dimethyl-trans-9-decalol), an earthy-musty compound, has been identified in wines and in grape juice, in which its presence is highly detrimental to the aromatic quality. Geosmin has a biological origin, and the analysis of rotten grape microflora has been done on two grape varieties (Semillon, Cabernet Sauvignon) from six parcels of the Bordeaux region over 3 years (1999, 2000, 2001). Forty-three Penicillium-related species have been analyzed by gas chromatography–mass spectrometry (GC–MS) for their geosmin production. GC–MS analysis has demonstrated that the earthy odor was always correlated with the presence of geosmin. Phenotypic characterization of Penicillium spp. being ambiguous, a molecular characterization by rDNA internal transcribed spacer (ITS1) sequencing was performed for all strains. The results evidenced that all strains producing geosmin belonged to only one species, P. expansum, and that the other strains, not producing geosmin, belonged to three species: P. purpurogenum, P. thomii, and Talaromyces wortmanii.

Changes in the Sotolon Content of Dry White Wines during Barrel and Bottle Aging

GC-MS in electron ionization mode (EI) was used as a simple, sensitive method for assaying sotolon [4,5-dimethyl-3-hydroxy-2(5) H-furanone] in various dry white wines. The impact of barrel-aging conditions, that is, whether yeast lees were present or not, on the formation of sotolon in dry white wines was then studied. The sotolon content was highest in dry white wines aged in new barrels without lees, often exceeding the perception threshold (8 microg/L). These results demonstrated that yeast lees were capable of minimizing the formation of sotolon in dry white wines during aging. The sotolon and oxygen contents of several bottle of the same white wine were also compared 7 years after bottling. At the range of dissolved oxygen concentrations generally measured, between 5 and 100 microg/L, the sotolon content remained below its perception threshold in wine. The perception threshold was exceeded only in wines with oxygen concentrations above 500 microg/L. The presence of dissolved oxygen in the wine samples analyzed also resulted in a decrease in their free sulfur dioxide content.

Genetic Analysis of the Biosynthesis of 2-Methoxy-3-Isobutylpyrazine, a Major Grape-Derived Aroma Compound Impacting Wine Quality

Genetic and candidate gene approaches are combined to identify an O-methyltransferase with a major role in methoxypyrazine biosynthesis in grapevine. Methoxypyrazines (MPs) are strongly odorant volatile molecules with vegetable-like fragrances that are widespread in plants. Some grapevine (Vitis vinifera) varieties accumulate significant amounts of MPs, including 2-methoxy-3-isobutylpyrazine (IBMP), which is the major MP in grape berries. MPs are of particular importance in white Sauvignon Blanc wines. The typicality of these wines relies on a fine balance between the pea pod, capsicum character of MPs and the passion fruit/grapefruit character due to volatile thiols. Although MPs play a crucial role in Sauvignon varietal aromas, excessive concentrations of these powerful odorants alter wine quality and reduce consumer acceptance, particularly in red wines. The last step of IBMP biosynthesis has been proposed to involve the methoxylation of the nonvolatile precursor 2-hydroxy-3-isobutylpyrazine to give rise to the highly volatile IBMP. In this work, we have used a quantitative trait loci approach to investigate the genetic bases of IBMP biosynthesis. This has led to the identification of two previously uncharacterized S-adenosyl-methionine-dependent O-methyltransferase genes, termed VvOMT3 and VvOMT4. Functional characterization of these two O-methyltransferases showed that the VvOMT3 protein was highly specific and efficient for 2-hydroxy-3-isobutylpyrazine methylation. Based on its differential expression in high- and low-MP-producing grapevine varieties, we propose that VvOMT3 is a key gene for IBMP biosynthesis in grapevine.

Identification of a sotolon pathway in dry white wines.

Sotolon (3-hydroxy-4,5-dimethyl-2(5H)-furanone) is a chiral furanone, an aroma compound known to be responsible for premature-aging flavor in dry white wines. Sotolon generally results from mild oxygenation during bottle aging, and until now, its formation pathways had not been elucidated. The ability of the main precursors described in the literature under very different experimental conditions to produce sotolon was tested. In model wine solution maintained at 40 degrees C for 6 months, sotolon was produced by the oxidative degradation of ascorbic acid. By use of GC-MS, 2-ketobutyric acid, produced by the oxidative degradation of the ascorbic acid in the model wine solution, was identified as a potent precursor of sotolon in this pathway. Ascorbic acid is an exogenous compound, added before bottling, but 2-ketobutyric acid was found even in white wines that had not been supplemented. Consequently, this sotolon formation pathway is also valid in white wines with no added ascorbic acid. In addition, we showed that Saccharomyces cerevisiae strains were capable of producing variable concentrations of this ketone during alcoholic fermentation. In model wine solution, certain yeast strains released large quantities of 2-ketobutyric acid, similar to those found in oxidized dry white wines. In view of these results, the role of yeast strains in this premature-aging phenomenon of dry white wines is discussed. Finally, these investigations revealed that one chemical mechanism responsible for the low concentrations of sotolon found in prematurely aged white wines made from various grape varieties was an aldol condensation between 2-ketobutyric acid and acetaldehyde.

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.

Identification of Adducts between an Odoriferous Volatile Thiol and Oxidized Grape Phenolic Compounds: Kinetic Study of Adduct Formation under Chemical and Enzymatic Oxidation Conditions

HPLC-MS and (1)H, (13)C, and 2D NMR analyses were used to identify new addition products between 3-sulfanylhexan-1-ol (3SH) and o-quinones derived from (+)-catechin, (-)-epicatechin, and caftaric acid. The kinetics of formation of these adducts were monitored in a wine model solution and in a must-like medium by HPLC-UV-MS with the aim of understanding the chemical mechanism involved in reactions between volatile thiols and o-quinones. One o-quinone-caftaric acid/3SH adduct, three o-quinone-(+)-catechin/3SH adducts, and three o-quinone-(-)-epicatechin/3SH adducts were characterized. Caftaric acid was oxidized faster than (-)-epicatechin and (+)-catechin when these phenolic compounds were incubated in a one-component mixture with polyphenoloxidase (PPO) in the presence of 3SH. Consequently, o-quinone-caftaric acid formed adducts with 3SH more rapidly than o-quinone-(+)-catechin and o-quinone-(-)-epicatechin in the absence of other nucleophilic species. Furthermore, o-quinone-(-)-epicatechin reacted faster than o-quinone-(+)-catechin with 3SH. Sulfur dioxide decreased the yield of adduct formation to a significant extent. Under chemical oxidation conditions, the rates and yields of adduct formation were lower than those observed in the presence of PPO, and o-quinone-caftaric acid was slightly less reactive with 3SH, compared to oxidized flavan-3-ols. The identification of o-quinone-caftaric acid/3SH and o-quinone-(+)-catechin/3SH adducts in a must matrix suggests that the proposed reaction mechanism is responsible for 3SH loss in dry wines during their vinification and aging process.

Distribution and Organoleptic Impact of Sotolon Enantiomers in Dry White Wines

The enantiomers of sotolon, a flavor compound typical of oxidized white wines, were separated by preparative HPLC to determine their perception thresholds and distribution in wines. The enantiomeric ratios of chiral sotolon were evaluated in several dry white wines using gas chromatography and a chiral column (beta-cyclodextrin) connected to a 2 m precolumn (BP20). The perception threshold of (S)-sotolon (0.8 microg/L) in model wine solution was 100 times lower than that of the (R) form (89 microg/L), indicating that (S)-sotolon contributes to the characteristic aroma of prematurely aged dry white wines. Both enantiomers are detected in white wines. Analysis of commercial dry white wines from various vintages and origins revealed three types of distribution patterns: the racemic form, an excess of R, and an excess of S. The proportions found in these wines may be partially explained by the slow racemization kinetics (20 months) of optically active sotolon.