Identification of odour-active trace compounds in toasted oak wood: impact on wines and spirits aroma

The aging of wine in oak barrels is an important step in the maturation of quality wines. The oak toasting process generates several nuances that can be released into the wine. Significant research has been devoted to identifying the wood-derived volatile compounds that influence wine aromas [1, 2] (e.g., whisky-lactone, maltol, eugenol, guaiacol, vanillin). However, these compounds only partially explain oak wood aromas and their effect on wine complexity. The purpose of our research was to gain a greater understanding of toasted oak aromas. To achieve this, we used a sensory-guided approach based on the purification of organic extracts of crude oak wood using a preparative high-performance liquid chromatography (HPLC) technique and gas chromatography analysis paired with olfactometry and high-resolution mass spectrometry (GC-O-TOF MS). This approach revealed odorous zones in oak wood extracts with reminiscence of metal and pastry aromas. The odorous zone with metallic aroma was attributed to (E)-4,5-epoxy-(E)-2-decenal, while the one with pastry aroma was attributed to 2,4,6-nonatrienal isomers. The injection of standard confirmed the identification of (E)-4,5-epoxy-(E)-2-decenal. Two isomers were detected for the 2,4,6-nonatrienal compound (E,E,Z and E,E,E). We used a chemical organic synthesis method (Wittig reaction) to synthesize the isomers. An additional purification step using semi-preparative HPLC allowed us to isolate the (E,E,Z) isomer with 99% purity. These compounds were identified for the first time in wood, wine and spirits by using SPE extraction and optimized GC-MS NCI (NH3) separation and detection. Furthermore, their very low olfactory-detection threshold in a hydro-alcoholic model solution demonstrated their impact on the aroma of these beverages. Their quantitation in numerous samples of oak wood also revealed the influence of toasting intensity on their formation. Additional results on their precursors in oak and wine were also discussed