Exploring and unravelling the complex toasted oak wood (Q. sp.) volatilome using GCxGC-TOFMS technique

Abstract

For coopers, toasting process is considered as a crucial step in barrel production where oak wood develops several specific aromatic nuances released to the wine during its maturation. Toasting is applying varying degrees of heat to a barrel over a specific amount of time. Today it is well known that as the temperature increases, thermal degradation of oak wood structure produces a huge range of chemical compounds. Indeed, many works were conducted to identify key aroma volatile compounds (e.g., whisky-lactone, furfural, maltol, eugenol, guaiacol, vanillin) using the traditional gas chromatography coupled with olfactometry and mass spectrometry (GC-O-MS).

Inspired by recent untargeted approaches in food “omics”, this work aims at expanding our knowledge on oak wood volatile composition by bi-dimensional comprehensive gas chromatography-time of flight mass spectrometry (GCxGC-TOFMS, BT4D, Leco).

In a first experiment, five toasting levels were selected and applied to Q. sessilis oak wood samples (control, 160 °C, 180 °C, 200 °C and 220 °C, 30 min, n=3). Organic extracts were prepared (dichloromethane, 50 g/L) and analysed by GCxGC-TOFMS on conventional column combination nonpolar/midpolar (DB-5ms/Rxi-17Sil). The separation was followed by a non-targeted approach for data processing. The resulting mass spectra (TIC) were de convoluted (ChromaTOF software) and compared to spectra from a database for tentative peak identification. It was necessary to restrict the number of processed peaks by applying some “filters” such as signal to noise (S/N > 50), linear retention index (LRI ± 30), mass spectra similarity (> 750) and repeatability level. Supervised multivariate and univariate statistical approaches were used to identify potential markers of toasting intensity. Thanks to R script, reproducible peaks number was reduced from about 15000 to 568. By comparing observed retention indices with those found in the literature, 77 of the identifications have been confirmed and associated with an increase in toasting intensity. Some of them were sensory active and well known in oak wood, such as guaiacol, creosol and isoeugenol. Others were identified for the first time in toasted oak wood such as 2-methylbenzofurane (burnt) and 2-hydroxy-2-cyclopenten-1-one (caramel).Additional results were also discussed on the capability of GCxGC-TOFMS to identify oak wood botanic origins (Q. robur, Q. alba