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A Technical Guide for Wine Producers
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WINE IN WOOD AND WOOD IN WINE
The flavour contribution of wood to wine
by Charl Theron, guest lecturer in Oenology, Tim Rypstra, professor in Wood Science and Jan Swart, director: Wood and Fibre Institute, Stellenbosch University.
Barrels have been used in the wine industry for various reasons for over two thousand years. Although ceramic vessels were intially used to contain wine, these were not sufficiently durable for transport purposes and the lack of modern-day steel or plastic meant that the obvious solution, due to its price, durability, limited porosity and structural strength, had to be wood. In the contemporary wine industry, however, the role of wood has evolved to being far more than a simple container.
The contemporary use of wood in vinification may influence wine composition in various ways. Compounds may be extracted from the wood, slow oxidation of the wine components may occur and the microflora in the wood, either new or used, may also play a role in the wine composition.
The nature and concentration of compounds extracted from wood are determined by factors such as the moisture content of the wood, origin, drying, toasting level and the toasting period. The best-known compounds that may be extracted are the following:
Deriving from untoasted wood: Lactones with a coconut character and nonenes with a sawdust character.
Deriving from toasted wood: Vanillin with a vanilla character, eugenol with clove, guaiacol with smoke and furfural with an almond character.
Deriving from microbial action: Furfuryl mercaptan which has a coffee character.
The concentration of some compounds varies depending on the extent and period of toasting. Vanillin concentrations, for example, are low in untoasted wood, medium in medium toasted wood and lower again in heavily toasted wood as a result of degradation; the furfural concentration in wood, on the other hand, increases with increased toasting.
Due to the porous nature of wood gradual oxidation will always occur in barrels. If long maturation occurs in barrels, oxidised products such as sherry and Madeira may therefore be expected. Since barrel maturation also goes hand in hand with time, it is obvious that chemically such wines will be more stable due to the time factor.
The porous nature of wood presents the ideal breeding ground for micro-organisms and is therefore also very hard to clean effectively. Contrary to expectations, the potential for microbiological spoilage of wines appears to be much greater in new barrels. This may be explained by the higher concentration of hydrolisable sugars and more available oxygen in new barrels.
Barrel fermentation is often employed by winemakers to ensure strong wood character, but also to allow integration of the wood and wine flavours. The alcoholic fermentation is a reductive process, however, unlike normal barrel maturation which entails slow oxidation. This may cause some wood compounds to be less prominent due to the fermentation in the barrel, because they are reduced instead of oxidised. Vanillin is a typical example. If the same wine is placed in the same barrel before or after alcoholic fermentation, the latter will have a more pronounced vanilla character than the former.
Oak is a natural product and variation in grain density, porosity and chemical composition of the wood may therefore be expected to occur. This implies that barrels are bound to differ from each other and that the problem may be solved only by using alternative wood products seeing that the manufacturing process thereof results in greater homogeneity.
Various alternative wood products are available. The basic difference between barrels and the use of alternative wood products is that in the case of the former the wood is around the wine, while in the case of the latter the wine is around the wood. Basically this is the reason why micro-oxygenation (MOX) is generally used in conjunction with alternative wood products. The contact surface of oak chips is obviously much larger than that of a wooden barrel, thereby promoting the extraction of more wood compounds. The extraction tempo of oak components from wooden chips is not always linear, however, which is why preliminary trials are recommended. The most important characteristics of oak chips compared to oak barrels are the following:
(The first two characteristics are applicable in South Africa only if the supplier in question is able to confirm this in the product specifications.)
References:
Bowyer, P.K., Bompas, C. & Murat, M-L. 2007. The aromatic science of oak and oak chips. Australian & New Zealand Grapegrower & Winemaker, April 2007: 61 - 64.
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