296 M.S. P ́erez-Coello and M.C. D ́ıaz-Maroto
stabilizing reactions that affect wine colour and clarity, structural changes in the
phenolics and new aroma formation all take place during oak wood aging. None of
these reactions occurs in tanks or bottles, which are inert, do not interact with the
wine, and do not allow the controlled oxidation that takes place in oak barrels.
The release of oak wood aromas is one of the primary reasons why wines are
made to undergo aging in the wood. This makes it necessary to take into account the
different factors regulating the release of volatile components from the oak to the
wine. These factors include the type of oak employed and the geographical origin of
the oak (Mosedale and Ford 1996; Chatonnet and Dubourdieu 1998; P ́erez-Coello
et al. 1999; Fern ́andez de Sim ́on et al. 2003), the drying treatment to which the oak
has been subjected (Sefton et al. 1993; Masson et al. 2000b; Doussot et al. 2002),
the degree of toasting (Chatonnet 1999; Hale et al. 1999; Cadah ́ıa et al. 2003), and
the length of time a wine spends in the barrel, along with barrel age and usage, i.e.,
the number of times a barrel has been used (Singleton 1995; Escalona et al. 2002;
P ́erez-Prieto et al. 2002).
Several chemical reactions that happen in the barrel during aging can also affect
the final volatile composition of wines. Among these reactions is found the oxidation
due to the entry of oxygen across the pores of the wood, which modify both the wine
volatile compounds and those extracted from wood. On the other hand, microorgan-
isms, proceeding from the wine or from the barrel, especially lactic bacteria and
yeasts, can produce important volatile compounds from a sensorial point of view,
like vinylphenol and vinylguaiacol (Jarauta et al. 2005).
Finally, the adsorption of certain wine volatiles by the lees and the barrel itself
(Chatonnet et al. 1992a; Ram ́ırez Ram ́ırez et al. 2001; Jim ́enez Moreno and Anc ́ın
Azpilicueta 2007), together with the possible changes that can occur during the addi-
tional bottle aging period (P ́erez-Prieto et al. 2003), will also cause modifications in
the final aromatic composition of the wine.
The long aging process normally increases the price of the final product, since the
wine has to be left in the barrels before it can be brought to market. For this reason,
the use of oak chips or staves as an alternative for the aging process has been widely
considered in the last few years (Ducournau et al. 1999; Guti ́errez Afonso 2002;
Frangipane et al. 2007). However, since oak barrel aging involves something more
than the simple transfer of compounds from the wood to the wine, this new type of
skill will have to be optimised to obtain a quality product.
8C.2 Volatile Composition of Oak Wood and Sources of Variation
Only oak heartwood is used to manufacture barrels. The basic composition varies
little from one species of oak to another. Oak wood consists mainly of cellulose
(50%) and hemicellulose (20%), which provide the framework and matrix for the
wood. Lignin is the other large polymer that makes up 30% of dry wood. It is a three-
dimensional, reticulated polymer comprising phenylpropane with units of guaiacyl
(2-methoxyphenol) and syringyl (2,6-dimethoxyphenol) that are cross-linking by
oxidation. Lignin is mainly present in the cell walls.