USE OF 13C CP/MAS NMR AND EPR SPECTROSCOPIC TECHNIQUES TO CHARACTERIZE MACROMOLECULAR CHANGES IN OAK WOOD(QUERCUS PETRAEA) DURING TOASTING

Abstract

For coopers, toasting process is considered a crucial step in barrel production during which oak wood (Q. petraea) develops several aromatic nuances released to the wine during its maturation. Toasting consists of applying different degrees of heat to a barrel for a specific period. As the temperature increases, thermal degradation of oak wood structure produces a huge range of chemical compounds. Many studies have identified the main key aroma volatile compounds (whisky-lactone, furfural, eugenol, guaiacol, vanillin). However, detailed information on how the chemical structure of oak wood degrades with increasing toasting level is still lacking.

In this study, we characterised the structural changes of the main components (cellulose, hemicellulose and lignin) of oak wood staves subjected to heat treatments (non-toasted to 240 °C) by cross-polarisation/magic angle spinning nuclear magnetic resonance (CP/MAS NMR, 800 MHz) and electron para-magnetic resonance (EPR, X-band) spectroscopy techniques applied to solids. Furthermore, the results will be compared to those obtained by quantitative GC-MS (EI) analysis of oak wood volatile compounds generated by the same heat treatments. The 13C CP/MAS NMR data highlighted concomitant phenomena of demethoxylation and depolymerisation of syringyl units leading to the formation of guaiacyl units and the release of monomer units. EPR results revealed the same phenomena and led us to hypothesize that the nature of the radical formed evolved during toasting from syringyl to guaiacyl form. Our results are consistent with data on others wood species and confirm that 200 °C is a threshold temperature above which degradation of the macromolecular structure leads to the formation of radicals and volatile compounds. Overall, the study offers interesting perspectives for the application of spectroscopic techniques, particularly EPR spectroscopy, to monitor radical formation during barrel aging and, consequently, to assess the oxidative stability of wine.