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A Technical Guide for Wine Producers
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WINE IN WOOD AND WOOD IN WINE
The flavour components of oak chips
Wood has long been much more than merely the substance from which oak barrels are made to serve as a durable container of wine. Winemakers around the world realise that oak imparts an additional dimension to flavour, taste and colour stability. The advent of various oak alternatives further expanded the oak profile of wine. This was proven by the local creation of “Coffee Pinotage”. Initially only one brand had this association, but the commercial success thereof, led to the creation of many others. Wine folk will obviously differ in their opinion of this notion. Wine purists consider it a violation of the cultivar character, while creative winemakers and marketers see it as innovative. Whatever the case, cooperages are able to produce the flavour desired by the winemaker.
The most important compounds that may be extracted from oak to influence the characteristics of wine are lactones, furfurals, vanillin, guaiacols, ethylphenols, eugenol and iso-eugenol.
Untreated oak contains mainly lactones, eugenol and iso-eugenol. During the toasting of the oak its lactone quality decreases, while the furfural and vanillin qualities increase and at even higher toasting temperatures the furfural and vanillin are gradually displaced by guaiacol. During the toasting of oak, the eugenol and iso-eugenol qualities in the oak increase. It goes without saying that the toasting temperature and duration of the process have a decisive influence on the flavour profile of the particular oak product.
Furfurals impart an attractive aroma to wine which may be described as sweet, butter caramel, light caramel or light almond. The furfurals are formed during the toasting process as degradation products of the carbohydrates that are present in oak. Oak chips in used barrels produce a higher furfural content than chips used in stainless steel tanks for wine contact. The furfural content of the wine increases in both instances after periods ranging from 6 to 9 months. In all instances the use of new barrels produces wines with the highest furfural content.
Guaiacol compounds impart smoke, tar and spice flavours to wines. These are formed as a result of the degradation of the oak lignins at high toasting temperatures. In order to ensure that these flavour components have a substantial influence on the wine, the toasting temperature should be above 230°C.
Lactones in various forms are the most important flavour compounds extracted from oak. A fresh oak and coconut flavour is typical, although the flavour may also be herbal. More lactones are extracted from oak chips or cubes than from powder. The extraction of lactones from oak is a rapid process, able to produce the highest quality within 3 months. The extraction of lactones from oak chips is also quicker than from new barrels. In the latter case the staves first have to be penetrated by the wine before extraction may take place.
Ethyl phenols described as leathery or sweaty are not naturally present in oak, but are usually the result of Brettanomyces spoilage. In wine that has undergone oak treatment, it occurs as a result of contaminated oak products. Organisms of this nature may occur up to 8 mm inside barrel staves. The highest levels of such flavour compounds usually occur after 6 to 9 months oak contact.
Vanillin, which is associated with vanilla beans, is also present in untreated oak. The quality thereof usually increases with medium toasting levels, but decreases with high toasting levels. Vanillin flavours in wine may also derive from an origin other than wine. They are usually extracted from oak chips within 3 months at most, while the extraction from new barrels takes longer.
Eugenol compounds produce spicy and clove flavours in wines.
In general, flavour compounds are extracted more readily from oak chips than new barrels, with used barrels falling somewhere in between. The general observation is that while oak chips are a cost-effective way of adding oak character to wines, the general quality of wines matured in new barrels is better (Guerra, 2009).
Reference
Guerra, Bibiana. 2009. Are Chips a Good, Cheap Option? Wine Business Monthly, 16 April 2009: 40 - 43.
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