A HS-SPME-GC-MS analysis of IR heated wood: Impact of the water content on the depth profile of oak wood aromas extractability

Controlled and reproducible IR heat treatments were applied to oak wood surfaces in order to establish a depth-profiled picture of the extractability of volatile compounds, with particular emphasis on the impact of the initial water content. Headspace-solid phase microextraction-gas chromatography coupled to mass spectrometry (HS-SPME-GC-MS) has been used to compare the concentrations of six aroma compounds (vanillin, furfural, eugenol; guaiacol and cis- and trans-whisky lactones) in hydroalcoholic extracts of series of slices representative of the first 8 mm of the wood facing the IR source. Results have shown that although water is supposed to have a delaying effect with respect to the thermal degradation of wood macromolecules, it can favor heat transfers and thus promote higher-than-expected transient local temperatures in a soaked wood. Yet, distinct behaviors could be observed between thermally-generated compounds (vanillin and guaiacol), where adsorbed water seemed to prevent the thermal degradation of the parent macromolecule, and thermally-degraded compounds such as eugenol where the presence of water would balance the compound degradation through a more efficient extraction process of this biogenesis molecule. Furfural exhibited a more complex behavior since its production as a result of hemicellulose degradation was thermally-favored in the presence of adsorbed water. Finally, whatever the applied heat flux and regardless of the initial water content, temperatures experienced by the wood deeper than 4 mm, were lower than 160 °C, which meant that beyond that depth, the initial wood composition was unaffected.