# The Application of Wood Species in Enology: Chemical Wood Composition and Effect on Wine Quality
## 3 Impact of Wood Species on Wine Chemical Composition  
During the wine aging in wood, numerous chemical and physical changes occurred, such as transferring volatile and phenolic compounds from wood to wine to improve the wine's quality [91]. However, reactions including only wine compounds and evaporation of volatile compounds can also take place. The key wood extractives are phenolic compounds [92], lactones (_cis_- and _trans_-\(\beta\)-methyl-\(\gamma\)-octadecones), aldehydes [76], and furfuryl compounds (5-methylfurfural, furfural, and furfuryl alcohol) [90]. Moreover, wines aged in wood are also continually exposed to small quantities of atmospheric oxygen (for oak barrels, it has been estimated at 10-45 mg/L per year) through the stave pores. This natural micro-oxygenation enhanced the condensation and polymerization reactions between flavonoid compounds (tannin-tannin and tannin-anthocyanin), which positively influenced the evolution of wine phenolic composition, by the formation of new stable anthocyanin and tannin derivatives, with consequent color stabilization and loss of astringency [53; 76; 93]. On the other hand, the wine compounds can also be fixed on the wood and by wine lees [94; 95], so this factor will also impact the volatile composition of wine. Therefore, the wine aged in wood undergoes important modifications that influence the wine volatile profile (a potential more complex aroma), color stability, and clarification; however, these changes are dependent on numerous factors, such as the wood species, initial wine composition, and the aging time. The type and quantity of wood extractives are highly dependent on the aging time and on the number of compounds that are potentially extractable from the wood to the wine, which is influenced by the wood species employed [76], geographical origin, wood grain [13; 15] drying and the toasting methods [16; 96; 97] and on barrel utilization time [43; 98]. Nevertheless, the wine to be aged in wood needed to have a good initial structure and body to balance the adverse influence of oxygen. [PERSON] et al. [99] studied the concentration of the volatile compounds removed from the wood, namely the syringaldehyde, _cis_- and _trans_-whiskylactones, vanillin, furfural, 5-methylfurfural, guaicaol, eugenol, \(p\)-ethyl-phenol, and \(p\)-ethylguaicol in twelve red monovarietal wines aged in new American oak barrels. These authors conclude that the removal of these compounds was quicker during the first 4 or 9 months of aging time in wood. Moreover, the wine alcohol content influences the removal process of the compounds from the wood. Thus, [PERSON] [100] studied the removal of _cis_- and _trans_-oak lactones, from American oak (_Q. alba_) in model wines with diverse ethanol levels (0%, 10%, 20%, 40%, and 60%), showing that the maximum levels of oak lactones were achieved in the samples with 40% ethanol. The removal of volatile compounds from wood barrels throughout the wine's aging process with diverse alcohol content and pH (Cabernet Sauvignon, 12.3% \(v/v\), pH, 3.45; Merlot, 13.6% \(v/v\), pH, 3.7) was studied by [PERSON] et al. [101]. These