## 1 Introduction  
The use of wood fragments (chips) for the ageing of wines, distillates and vinegas is a widespread practice within the European Union and complies with specific regulations to defend the Protected Geographical Indication (PGI) from possible fraud. Their use, when authorized, is intended to reduce production time and costs [1]. Different spirits, such as grappa, rum, Armagnac, cognac, brandy, whisky and tequila, are refined in barrels or, if allowed by regulations, using wood chips in their ageing processes [2]. Wood may remain in the distillate from a few days to a few weeks at a dosage of 0.5-2.5 g per L. This process results in a fast change in aromatic complexity and colour intensity [3] due to the extraction of wood volatile compounds from wood, also named xylovolatiles [4], and other extractables, such as polyphenols [5]. Most xylovolatiles arise from the depolymerisation of lignin, cellulose and hemicellulose during both the seasoning and toasting of the wood and include phenolic aldehydes(vanillin and syringaldehyde) and hydroxycinnamic aldehydes (coniferaldehyde and sinapaldehyde), as well as free phenolic acids such as vanilla, gallic, ellagic, ferulic and syringic acids. Moreover, other compounds can be found in oak-aged distillates. Some of them may derive from the Maillard reaction, which occurs during the wood-toasting phase and leads to the formation of coloured compounds (melanoidins) and several aromas with a high olfactory impact, such as pyrazines and furan derivatives [1; 6; 7]. Among factors that affect the transfer of xylvolatiles to the distillate the most, the geographic and botanical origin of the wood, the wood grain coarseness, the drying/seasoning methods, the infusion duration, the shape and size of fragments and the surface/volume ratio should be considered [1], but the wood-toasting phase is crucial to the final quality of the distillate. In fact, toasting profoundly changes the structure, chemical composition and physical properties of the wood and is strictly dependent on the applied heat intensity. Wood chips for wine ageing are exclusively obtained from pedunculate oak (_Quercus robur_), sessile oak (_Quercus petraea_) or white oak (_Quercus alba_). For the refinement of spirits, other types of wood are also available, including chestnut (_Castanea sativa_), cherry (_Prunus avium_), mulberry (_Morus alba_), fir (_Abies alba_) and walnut (_Juglans regia_) [2; 7]. The shape and dimensions are the same as those employed in winemaking: fine grains (2 mm), wood chips (2-7 mm), wooden cubes (1 or 2 cm) and small staves. Sliced wood and peeled wood or untoasted poplar chips used once during fermentation to clarify wines and improve their body could be an alternative to conventional chips or barrels for distillate ageing [1; 8]. Sliced wood is obtained by cutting the trunk using a very sharp blade of length at least equal to that of the workpiece (trunk or square portion), with a series of cuts that follow one another from its lateral surface along planes parallel to the fixing base. Wooden sheets with a thickness of a few tenths of a millimetre are obtained (the most common thicknesses are between 0.4 and 0.6 mm). Peeled wood is obtained from the trunk fixed to spindles that is rotated on its axis against a blade, obtaining a continuous sheet of wood, generally between 1 and 3 mm thick and as wide as it is along the trunk; this wooden strip is then cut transversely to obtain single sheets (possibly discarding the defective portions). A circular section with limited tapering, the regularity of the stem and the absence of ribs is required, and it need to be free of major defects (wounds, knots, etc.) depending on the use, i.e., decorative, structural or intended for packaging [9]. Some vibrational spectroscopy techniques, such as mid- and near-infrared spectroscopy (MIR and NIR), can be used to discriminate wines aged for variable periods in woods obtained from different essences (oak, acacia, chestnut) or wines aged in different ways (barriques, chips, steel) [10].