## 2 Volatile Wood Composition
### Ageing in Barrels with Traditional and Alternative Woods  
The compiled bibliography studies the ageing of wines from 4 months (short ageing) to 21 months (long ageing) in alternative barrels and compares them with traditional barrels. The most common wine ageing was with medium-long wood contact time (between 9 and 21 months) (Table 5). It is expected that, as the ageing time increases, the variety and the content of volatile compounds in the wine increase. However, this will also depend on the factors mentioned above (degree of toasting, toasting temperature, type of wine, etc.). [PERSON] et al., 2009 [74] studied the volatile composition of red wines aged in barrels of different alternative woods (acacia, chestnut, cherry and mulberry) and _Q. petraea_ for 9 months and observed significant differences in some of the compounds. In the case of volatile phenols, it was found that the wines aged in acacia barrels had a higher content of 4-EG (3250 ug/L), which contributes phenolic, smoky and leathery notes, than the rest of the wines aged in the other barrels. Wines aged in acacia and chestnut barrels were characterised by a higher eugenol content (21 ug/L and 26 ug/L, respectively), which contributes clove, honey, spicy and cinnamon aromas, compared to wines aged in _Q. petraea_ barrels (18 ug/L). These results are consistent with the analysis of toasted wood, where it was observed that chestnut and acacia wood had higher contents of this compound than _Q. petraea_. However, wines aged in _Q. petraea_ barrels were characterised by a higher content of certain compounds, such as furfural and 5-MF. In contrast, the use of chestnut barrels contributed more vanilla to the wines than _Q. petraea_ barrels. In addition, they observed that wines aged in mulberry barrels showed a significant decrease in fruity notes such as ethyl esters and other compounds, and yielded compounds considered aroma defects such as ethylphenol to the wine. In this study, they concluded that the wines studied could be differentiated quantitatively and qualitatively [74]. Thus, wines aged in _Q. petraea_ barrels carried aromas of caramel and toasted almonds and wines aged in acacia and chestnut barrels gave more woody and spicy aromas, such as clove, smoke and vanilla. Therefore, it can be said that the aromas mostly found in the wines were the same as in the wood. The number of literature references found was more abundant for 12-month ageing [29; 75; 76]. [PERSON] et al., 2010 [76] evaluated the aromatic composition of white wines aged in acacia barrels from two different geographical areas (Medjimurjey and Istria) against wines aged in _Q. petraea_ barrels. The results showed a significant increase after 12 months in the content of furfural, 5-MF, guaiacol, eugenol and _trans_ isoeugenol in all Malvasia white wines aged in acacia and oak barrels. After 12 months of ageing, clear differences were observed between the wines aged with these two types of wood. Thus, it was found that wines aged in acacia wood barrels were characterised by significantly higher guaiacol amounts than wines aged in _Q. petraea_ barrels. In addition, other volatile phenols such as o-cresol, p-cresol and m-cresol, not found in wines aged in _Q. petraea_ barrels, were also found in wines aged in acacia wood. On the other hand, the wines aged in _Q. petraea_ barrels had more lactones. From the sensory analysis they were able to extract that the wines aged in acacia barrels were the best scoring wines. In general, wines aged in _Q. pyrenica_ barrels were characterised by high levels of eugenol, guaiacol and other volatile phenols [29]. Regarding compounds such as _cis_ lactone or maltol, the behaviour of _Q. pyrenica_ wood was very similar to that of _Q. alba_. As for phenolic aldehydes and ketones, the levels of _Q. pyrenica_ wines were intermediate between those aged in _Q. petraea_ and _Q. alba_, with few significant differences.