Wine Chemistry and Biochemistry

(Steven Felgate) #1

8C Volatile Compounds and Wine Aging 299


In terms of its volatile composition, Spanish oak is comparable to French oak of


the same species. There are also two native species,Q. pyrenaicaandQ. faginea,


which have amounts of the oak lactones, vanillin, and eugenol that are statistically


comparable to those in French oak, though wines aged inQ. fagineabarrels have


received particularly poor ratings by taste panels, earning low scores for the charac-


teristic oak wood sensory attributes (Fern ́andez de Sim ́on et al. 2003, 2006; Cadah ́ıa


et al. 2003).


The ratio between thecisandtransisomers of oak lactones has been used as


a means of differentiating oaks of different origins. The oak lactone isomer ratio


is higher (between 5 and 9) in American oak. The ratio differs somewhat for


French oak, between 1 and 5, depending onthe geographic region of origin (Vos-


ges, Centre, and Allier) (Waterhouse and Towey 1994; P ́erez-Coello et al. 1999,


2000a), withQ. roburhaving particularly low levels of both isomers (Feuillat et al.


1997).


For barrel manufacturing, in its turn, the wood has to undergo a series of pre-


liminary processing steps. First of all, the wood for the staves used to make the


barrels has to be dried to decrease the moisture content of the newly cut wood.


Traditionally, drying took place outdoors in the open air for at least three years, but


now to save time artificial kiln dryingis commonplace, or a combination of the two


may be employed.


The quantity of volatile compounds in newly harvested wood is quite low. These


components form as a result of oxidation taking place during drying. Traditional


drying through outdoor seasoning increases levels of aromatic aldehydes in the
wood, especially when seasoning is carried out in countries with a hot climate, like


Australia (Chatonnet et al. 1994; Spillman et al. 2004a). Some researchers have


reported lower levels of eugenol and oak lactones and a variable effect on vanillin


concentrations depending on the temperature used in kiln drying (Sefton et al. 1993;


Masson et al. 2000).


Toasting the inner surface of the staves is a common practice employed in barrel


making. Toasting helps the wood to bend when the barrels are being made and also


offers other advantages, e.g., pyrolysis of the lignin helps increase levels of phenolic


aldehydes and other components with desirable sensory effects (Maga 1984). French


coopers perhaps use toasting most of all to lower the tannin levels that could increase


wine astringency.


The level of toasting is decided on empirically by coopers based on the degree of


darkening of the wood and toasting time. Chatonnet and Boidron (1989) measured


the temperature on the toasted surfaces of wood undergoing different levels of toast-


ing: light toasting for 5min reached a temperature of 180◦C, whereas 230◦Cwere


reached for heavy toasting (17.5min).


Heating the wood during toasting altersthe volatile composition of the wood, and


for this reason solid-phase microextraction (SPME) followed by gas chromatog-


raphy has been put forward as a quick and simple method of analysing volatile


compounds and providing information about the volatile composition of the wood


that can be correlated with the toasting level (Chatonnet et al. 1994; D ́ıaz-Maroto
et al. 2004). The heat breakdown of the lignin that takes place during medium

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