Wine In Wood And Wood In Wine: How different factors impact on the chemical composition of oak

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WINE IN WOOD AND WOOD IN WINE
How different factors impact on the chemical composition of oak
by Charl Theron
The supply of barrels and alternative oak products to wine industries is an extremely competitive industry involving a product that, as a result of its origin and nature, may impart a wide range of additional flavour and taste characteristics to wine. Suppliers use these so-called unique characteristics of their products for marketing purposes. This article discusses some of the unique characteristics.
Different factors may influence the chemical composition of oak, the most important being the oak species, Quercus alba, Quercus robur, Quercus petraea, the individual trees, wood age, grain and the geographical origin of the oak. Various researchers have also shown that the pedunculate oak, Quercus robur, contains more ellagic tannins than the sessile oak, Quercus petraea; on the other hand, the pedunculate oak contains fewer flavour components, especially the so-called whisky lactone, than the sessile oak. (The terms pedunculate oak and sessile oak are used to distinguish between the different species and refer to the way in which the leaf is set.)
The grain of the wood, or the average width of the annual rings is determined by the species and the growing conditions of the tree. Cooperages and winemakers often consider the grain to be an index of the chemical composition of the wood. It has been proven, for example, that there is a correlation between the oak grain and dry extract and ellagic tannin content of the oak, but that it correlates negatively with the volatile component content of the oak.
Origin is often used by coopers as a so-called indication of quality of the oak. The influence of geographic origin has not been correctly established from a scientific point of view, because evaluated samples have usually been mono-specific. It is therefore difficult to separate the influence of species from ecological circumstances when comparing two sites with different species ratios. Oak being characterised by a wide variation in the quality of the volatile and phenolic compounds, it is only possible to understand the influence of origin by using large sample sizes. The influence of a tree’s age and the distribution of the ellagic tannins in the tissue of the wood may also complicate conclusions and lead to the wrong interpretations.
The purpose of this research project was to improve understanding of the influence of species and the circumstances of origin, respectively. The trial site, situated in western France (48.08 degrees N and 0.17 degrees W), comprised 5 hectares with 286 oak trees and 1 beech tree, the planting density of the trees therefore being 57 trees per hectare. The 286 oak trees consisted of 158 pedunculate oaks, 118 sessile oaks and 10 trees with an intermediate morphology. The trees were on average 100 years old and grew under the same conditions. The site consisted of three ecological areas, namely a small valley, a plateau and a transitional slope. Both oak species, Quecus robur and Quercus petraea, occurred on the site.
Heartwood from the various trees was chemically analysed and the data statistically analysed. Since it has already been shown that various ellagic tannins, the so-called whisky lactones (coconut character) and eugenol (clove character), are the most important compounds of untoasted oak that contribute to the sensorial characteristics of wine, these formed the focus of the study. The results of the project may be summarised as follows:

The sessile oak, Quercus petraea, generally contains more volatile components than the pedunculate oak, Quercus robur. The difference applies especially to the whisky lactone flavours that are 12 to 20 times more. In some trees the amount did not differ between the species.
The pedunculate oak, Quercus robur, contains more tannins than the sessile oak, Quercus petraea.
No significant differences with regard to the volatile component levels of ellagic tannin content occurred among the various species in the different ecological zones. It is therefore possible to contend that different trees from different origins may have similar aromatic and tannic characteristics.
No significant correlation could be found between the oak grain and the whisky lactone content of the oak.

The project confirmed, therefore, that with regard to the chemical composition of oak, the various oak species are the most important factor. Differences in origin may be disregarded as a factor, this being largely eliminated by the natural variation of oak. The selection of oak based on origin is in fact therefore irrelevant, unless the species distribution of a specific origin is dominated by a specific species. The natural variation of the chemical composition of oak for barrels makes it hugely problematic to produce barrels with identical characteristics.
Reference
Prida, Andrei & Puech, Jean-Louis. 2008. Australian & New Zealand Wine Industry Journal 23(5): 42 - 46.
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