300 M.S. P ́erez-Coello and M.C. D ́ıaz-Maroto
toasting brings about an increase in the volatile phenols (vanillin, syringaldehyde,
guaiacol, eugenol). On the other hand, temperatures higher than 250–300◦C reduce
the content of these compounds as a result of losses due to carbonization (P ́erez-
Coello et al. 1997; Chatonnet 1998,1999).
Toasting can raises the levels of the oak lactones by releasing them from their
precursor, first isolated by Otsuka et al. (1980), recently joined by one additional
precursor, 6′-O-gallate derivative of -D-glucopyranosyloxy-3-methyloctanoic acid
(Masson et al. 2000a).
Heating the wood also results in the formation of carbohydrate (cellulose and
hemicellulose) breakdown products, e.g., the furanic derivatives furfural, 5-methyl-
furfural, furfuryl alcohol, etc. (Chatonnet 1998).
Heating also produces a series of other substances as a result of Maillard reac-
tions. These include cyclotene, maltol and its derivatives (hydroxymaltol, dihydro-
maltol), 2,3-dihydro-5-hydroxy-2-methyl 4(H) pyranone (DHM) and its 5-hydroxy
derivative (DDMP) and furaneol. The sensory impact of these components can play
an important role in developing the “toasted”, “burnt”, or “caramel” aromas of some
barrel-aged wines (Cutzach et al. 1997,1999).
Barrel toasting is of fundamental importance in the leaching of volatile compo-
nents into the wine and in many cases may mask the attributes intrinsic to the species
or geographical region of origin of the oak (Francis et al. 1992).
As a consequence, the variability inherent to oak wood is augmented by further
variability at the barrel cooperage, resulting in very high levels of variation even
within the same batch of manufactured barrels. On examining the furfural, oak
lactone, syringol, and guaiacol contents of wines aged in American and French
oak wood barrels manufactured by four different cooperages, Towey and Water-
house (1996) recorded levels of variation ranging between 15% and 40%, depending
on the substance considered.
8C.3 Sensory Relevance of the Volatile Components of Oak Wood
Not all of the volatile components present in oak wood play a decisive role in
determining the aroma of aged wines. While concentrations in the oak wood will
be a conditioning factor, final aroma isthe result of each component’s olfac-
tory detection threshold and the synergistic effects between it and the other wine
components.
In a study of Pinot Noir wines aged in different types of oak wood, the wines
that earned the highest scores for the attributes “woody”, “toasted”, “vanilla” and
“coconut” contained thehighest quantities of thecisoak lactone isomer (Sauvageot
and Feuillat 1999). This component exerts the greatest sensory effect in wines aged
in oak wood, and thecisisomer is present in quantities greater than the detection
threshold even in species low in lactones, likeQ. robur(D ́ıaz-Maroto et al. 2004).