304 M.S. P ́erez-Coello and M.C. D ́ıaz-Maroto
Once the oak components have been extracted, they undergo a series of micro-
biological alterations. These include the reduction of furanic and phenolic alde-
hydes to their respective alcohols and subsequent formation of their ethyl esters
(Boidron et al. 1988; Spillman et al. 1998b). Furfural is especially sensitive to
such changes, forming furfuryl alcohol and the corresponding ethyl ester. Thus, the
presence of furfuryl alcohol indicates that microbial activity has taken place in the
barrels for at least part of the wine’s aging period (Spillmanet al. 1998a). Vanillin
and 5-methylfurfural undergo alterationsof this type to a lesser extent (Spillman
et al. 1998), which limits the sensory impact of these compounds, since the alcohols
have higher olfactory detection thresholds than the corresponding aldehydes.
This effect is particularly important for wines fermented in the barrel, a practice
that is widely used to add a touch of oak to white wines while avoiding the oxida-
tion effects commonly encountered during prolonged aging (Aleixandre et al. 2003;
Herjavec et al. 2007).
Eugenol and guaiacol seem to be more stable than phenolic aldehydes, and to
date no alterations during wine aging in oak barrels have been reported; hence the
concentration of these compounds depends solely on the extent to which they are
extracted (P ́erez-Prieto et al. 2003; G ́omez-Plaza et al. 2004; Garde-Cerd ́an and
Anc ́ın-Azpilicueta 2006). Their concentration in wine increases over the initial
months of aging and the levels off and remains virtually constant after 12 months in
the barrel (Garde-Cerd ́an et al. 2002).
Similarly,cisandtransoak lactones undergo little alteration during aging, and
their concentration profile in wine is similar to that of eugenol (Spillman et al. 1998),
though it has been reported the possible release of oak lactones by their precursor
during the aging period (Wilkinson et al. 2004).
Vinylphenol and vinylguaiacol are present at low concentrations in oak wood, but
the amounts found in wine are chiefly a product of yeast metabolism, including the
yeasts taking part in alcoholic fermentation, which are able to produce these com-
pounds by decarboxylizing the coumaric acid and ferulic acid in the grapes. Vinyl-
guaiacol concentrations in young wines canreach the olfactory detection threshold
of 380μg/L in red wines and 440μg/L in white wines (Boidron et al. 1988), afford-
ing a spicy aroma. Both these components are highly reactive at wine pH, giving rise
to ethanol adducts and stable wine pigments in red wines (Hakansson et al. 2003),
and hence concentrations tend to be lower in red wines aged for longer periods of
time (Spillman et al. 2004a).
The formation of 4-ethylphenol and 4-ethylguaiacol contaminating microorgan-
isms represents a more serious problem. These two compounds are present at trace
quantities in oak wood but can reach values approaching their olfactory detection
thresholds (605μg/L for the former and 110μg/L for the latter in red wines) in wines
aged for lengthy periods, giving rise to unpleasant “horse stable” and “medicinal”
aromas (Chatonnet et al. 1992b).
Wineries commonly reuse their barrels. The useful lifetime of a barrel, however,
is between five and six years, not just because of the depletion of volatile com-
ponents that can leach into the wine but also because of the growth of unwanted
microorganisms such as Brettanomyces/Dekkera. This yeast can synthesize