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July 2015 P R A C T I C A L W I N E R Y & V I N E YA R D 57 W I N E M A K I N G The sensory properties of a group of oak ellagitannins include astringency and bitterness. However, these compounds are rather unstable in wine. The ellagitan- nins can be hydrolysed into monomers and ellagic acid, precipitated with pro- teins of wine. The comparison of the amount of ellagitannins and sensory-active prod- ucts of their transformation in wine with their sensory thresholds does not allow to judge them as direct impact compounds, considering the presence of grape tannins in much higher concen- trations. 8,9,14,23 In contrast, ellagitannins seem to have an indirect impact on the sensory prop- erties of wine due to their implication in redox reaction and scavenging of sulfur compounds. 13,23 More recently, A. Marchal identified and sensory-characterized other classes of sensory-active wood compounds: quercotriterpenosides and lignanes, involved respectively in perception of sweetness and bitterness. 11,12 Major com- pounds of these classes are characterized by low sensory threshold (quercotriter- penoside 1 is perceived as sweet at 0.59 mg/L, and lyoniresinol is perceived as bitter at 1.5 mg/L) and thus could be considered as direct contributors to the taste of oak-aged wines. Correlation of wood composition with sensory impact Wine is a complex mixture, and its sen- sory perception is the result of interac- tion of flavors. Thus, to estimate the role of flavor compounds, the consideration of OAVs is not enough; the correlation study is necessary to know the impact of flavor compounds on flavor attributes. The wine matrix effect can strongly affect the perception of the volatile components, depending on their chemical nature. F. Sauvageot observed either positive correlation between cis-whisky-lactone and sensory descriptors such as toasty, coconut, woody, vanilla, or negative ones with pharmaceutical, hay and clove in a study of Pinot Noir wine. 16 In another study, the authors applied the partial least squares model to sensory and chemical results and found excellent correlation between the woody-vanilla- cinnamon descriptor and cis-whisky-lac- tone and less pronounced with vanillin and eugenol. 2 The cis-whisky-lactone also contributes to intensity of sweet-candy- cocoa descriptor and vanillin to the "fruity" one. In a 2004 study, the authors showed that cis-whisky-lactone content correlates positively with coconut, berry, coffee and dark chocolate descriptors in red wine. 19 Vanillin content in white wine is not directly correlated to the vanilla descrip- tor. On the other hand it is correlated to cinnamon and smoky descriptors includ- ing guaiacol, 4-methyl-guaïacol, 4-eth- ylphenol, furfural and 5-methyl-furfural for the latter. T. Thomas-Danguin studied the sen- sory perception of woody and fruity notes and found an increase in fruity perception with a small concentration of woody compounds and a decrease when the concentration of woody compounds became too high. 1,10 In 2014, P. Stamatopoulos found that wood extractives (trans, cis-whisky-lac- tones and eugenol) enhance overripe orange-perception and typicality of sweet Bordeaux wines. 20 In our studies we pursued the afore- mentioned research with the accumula- tion of a large amount of data, allowing us to make more general conclusions. Two examples of research are presented below: In the first study we compared a range of different wines, with different types of barrels in élevage, using both sensory