Identification and Distribution of New Impact Aldehydes in Toasted Oak Wood (Quercus petraea).

The purpose of this study was to identify aroma compounds associated with the toasting intensity of oak wood (Quercus petraea). Crude organic extracts from oak wood samples (toasted at different temperature-time couples) were analyzed by a sensory-guided approach using GC-O-TOFMS, followed by purification with semipreparative HPLC (reverse phase). This approach revealed two specific odorous zones (OZs) reminiscent of metal and puff pastry. The first OZ was identified as trans-4,5-epoxy-(E)-2-decenal (1) by coinjection of the commercial product, whereas identification of (2E,4E,6Z)-nonatrienal (2) associated with puff pastry OZ was validated by a multistep chemical synthesis approach (Wittig reaction) followed by semipreparative HPLC purification (chiral phase). Their detection thresholds in model wine solution were 60 ng/L (1) and 16 ng/L (2). Their distribution in toasted oak wood samples [GC-NCI-MS (NH3) analysis] ranged from some ng/g to 210 ng/g for (1) and 85 ng/g for (2). Finally, additional sensory experiments demonstrated the impact of newly identified aldehydes in toasted oak wood.

Contribution of Grapes and Oak Wood Barrels to Pyrrole Contents in Chardonnay Wines: The Influence of Several Cooperage Parameters.

The influence of some enological parameters on pyrrole concentrations in Chardonnay wines was studied. First, a quantitative method to assay five pyrroles was optimized and applied to determine their content in wines produced in different containers. All pyrroles were observed in wines aged in a stainless-steel tank, which indicated that they have a varietal or fermentative origin. However, their concentrations were significantly higher in wines made in new barrels than in older barrels or in a stainless-steel tank, so oak wood may largely contribute during the winemaking process. A quantitative method to assay pyrroles in oak wood extract was also developed to study the influence of several cooperage parameters such as different types of traditional toasting, as well as the temperature and the time of toasting. Significant differences were observed on pyrrole concentrations in oak wood extracts according to these different cooperage parameters. These findings bring new perspectives to the monitoring of winemaking and the aging of Chardonnay wines.

Identification of Dialkylpyrazines Off-Flavors in Oak Wood.

Volatile extractive compounds from high-quality oak wood (Quercus sp.) are responsible for important pleasant olfactory notes, such as coconut, wood, vanilla, caramel, and spice. Recently, a new off-flavor reminiscent of rancid butter has been detected in oak wood. Using gas chromatography-olfactometry (GC-O) coupled to several detection modes, such as nitrogen-phosphorus detection (GC-O-NPD) or mass spectrometry (GC-O-MS) and multidimensional GC-O coupled to time-of-flight mass spectrometry, six compounds containing nitrogen atoms were identified. The volatiles were suggested to belong to 2,5-disubstituted pyrazines family, which was confirmed by comparison with synthetic reference compounds. For this purpose, symmetric and dissymmetric 2,5-dialkylpyrazines were prepared from methyl esters of corresponding aliphatic amino acids (Val, Leu, and Ile) by a three-step, one-pot reaction under mild reducing conditions. Organoleptic descriptors and odor detection thresholds were also determined, whereas a bacterial origin explaining these off-flavors was hypothesized.

New Approach for Differentiating Sessile and Pedunculate Oak: Development of a LC-HRMS Method To Quantitate Triterpenoids in Wood.

Oak aging is a crucial step in winemaking during which the organoleptic properties of wine are modified. Various parameters affect the chemical composition of oak wood including botanical origin, which has been previously shown to be a determinant factor. This study focused on the development of a LC-HRMS method to assay four recently discovered taste-active triterpenes (three sweet and one bitter). The method was applied to evaluate the effect of oak species (Quercus petraea and Quercus robur) on the concentration of these molecules in wood. The results showed that sessile oak was richer in sweet triterpenes and poorer in the bitter one than pedunculate oak, with high interindividual variations within species. Furthermore, a triterpenoid index was calculated to reveal the triterpenoid composition of oak wood. This index appears to be a promising tool for the unambiguous discrimination of oak species and could offer new insights into oak wood selection by coopers and the monitoring of oak aging by winemakers.

Role of glycoconjugates of 3-methyl-4-hydroxyoctanoic acid in the evolution of oak lactone in wine during oak maturation.

Oak lactone is a natural component of oak wood, but it also exists in glycoconjugate precursor forms. This study concerned the role of glycoconjugates of 3-methyl-4-hydroxyoctanoic acid, specifically a galloylglucoside, glucoside, and rutinoside, in the evolution of oak lactone during cooperage and maturation. The glycoconjugate profiles of 10 French oak samples were obtained by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) using stable isotope dilution analysis. The galloylglucoside was found to be the predominant glycoconjugate precursor and ranged in concentration from 110 to 354 µg/g. Maturation trials indicated the galloylglucoside undergoes acid-catalyzed hydrolysis after extraction into wine; after 12 months of maturation, the glucoside was the most abundant precursor, present at between 2- and 11-fold higher concentrations than those observed for powdered oak. Thermal degradation of glycoconjugates was observed only when oak samples were heated at 200 °C for 30 min, demonstrating their thermal stability.

Dry vs soaked wood: modulating the volatile extractible fraction of oak wood by heat treatments.

The aim of this study was to analyze the impact of the water content of wood on the concentrations of volatile compounds which can be extracted after heat treatments. Head Space-Solid Phase Micro Extraction Gas Chromatography coupled to Mass Spectrometry (HS-SPME GC-MS) has been used to compare the concentrations of six aroma compounds (vanillin, furfural, eugenol, guaïacol and cis- and trans-whisky lactones) in hydroalcoholic extracts of heated oak wood samples either previously soaked in hot water or not. Except for eugenol, concentrations of extracted aromas appeared to be lower in soaked woods than in dry woods for temperatures up to 200 °C. If a delaying effect of water could explain such overall lower extracted concentrations from soaked woods, a PCA analysis revealed that for the longer duration (25 min of heat treatment), the adsorbed water could promote a higher impact of furfural, eugenol and both whisky lactones on the composition of hydroalcoholic extracts, suggesting that alternative mechanisms of thermal modifications of the wood macromolecular network could exist at high temperatures in presence of adsorbed water.

Influence of geographical origin and botanical species on the content of extractives in American, French, and East European oak woods.

The chemical composition of East European (Republic of Moldova, Ukraine, and Romania) oaks was investigated profoundly for the first time in the present study and compared with American and French counterparts. Taking into account the high natural variability of oak extractives contents, the wide-ranging sampling was performed for all oak origins: 276 French oaks, 102 East European oaks of both species (Quercus robur L. and Quercus petraea Liebl.), and 56 American oaks (Quercus alba). These oaks were compared with great attention paid to the extractives, which are most important for sensorial impact in wine or spirit maturation, such as ellagitannins and principal odorant substances (aromatic aldehydes, lactones and phenols). The substances in question were studied by application of HPLC and GC-MS techniques. The pattern of all studied extractive contents allowed adequate separation of oak samples according to their geographical origin or botanical species. The highest separation rate was for American and French oaks, whereas East European samples could be partially misclassified in two sets mentioned above. The most important variables for species discrimination were whiskey lactone related variables and ellagitannins, whereas the most important features for distinguishing the origin were eugenol, 2-phenylethanol, vanillin, and syringaldehyde. These substances allowed the distinction of French and East European woods of the same species. With regard to chemical composition, East European wood held the intermediary place between American and French oaks according to their ellagitannin and whiskey lactone levels; nevertheless, it was characterized by specific high values of eugenol, aromatic aldehydes, and 2-phenylethanol.