The use of oak in wine making, be it with oak barrels or oak chips, has a significant effect on the flavor, aroma, texture, and color of the finished wine. Traditionally, three different species of oak have been used for wine barrel fermentation and/or aging: Quercus alba L. (American oak), Quercus petraea (Matt.) Liebl., and Quercus robur L (both French oak). As a result of overcropping of the oak trees, or the harvesting off more oak trees than can be regrown for future use, in addition to the quest for more variety, new sources of oak for wine making are being explored. Oak from Eastern European countries such as Ukraine, Russia, Romania and Hungry are starting to be used more in wine making, including similar species as used in France (Q. petraea and Q. robur) and another species used less frequently: Q. pyrenaica, which frequently hails from Spain.
The use of oak barrels in wine making significantly influences the volatile composition of the finished wine, which in turn affects the flavor, aroma, and taste of the wine. Studies have shown that there are quantitative differences in the volatile composition of wine made from American versus French oak barrels. Furthermore, studies have shown that the Spanish oak, Q. pyrenaica, also shows significant differences in volatile composition of the wood, including higher levels of eugenol, guaiacol, and other volatile phenols and furanic aldehydes. Looking at phenolic aldehydes and ketones, Spanish oak appears to retain levels between that of French and American oak.
The amounts of these volatile compounds that are extractable from the oak wood are extremely important in determining the overall aromatic profile of the finished wine. As a result of this, understanding the chemical composition of Spanish oak (Q. pyrenaica) is extremely important. Studies from other oak species have shown that there is strong variability in the volatile composition of oak wood within the same species, tree, forest, stand, etc, due to various environmental and geographical factors.
The study presented today aimed to add to the literature of oak wood volatile composition by examining Spanish oak, Q. pyrenaica, and to evaluate the effect geographical location, site, and silvicultural parameters had on them.
The sample set included 107 samples of Q. pyrenaica that were collected from several stands in three geographical locations in the northwestern Iberian peninsula (from the provinces of Ourense, Lugo, and Pontevedra).
From each tree, disks of wood were collected at a height of 1.3m above the ground.
From each of these disks, test tubes of heartwood were taken (20mm x 20mm x 40mm).
Heartwood samples were dried, then ground with a mechanical mill and sieved (<1mm) in order to obtain sawdust of a homogenous size.
Volatile compounds were isolated and then analyzed using gas chromatography and mass spectrometry.
- The qualitative profile of volatile compounds obtained from Spanish oak was very similar to what has been reported for other oak species.
- There was high variability on several levels, including the levels of individual trees, forests, and geographical locations.
- Q. pyrenaica samples were high in cis- and trans-β-methyl-γ-octalactone (cis- was the dominant form).
- Samples were high in phenolic aldehydes, including vanillin, syringaldehyde, coniferaldehyde and sinapaldehyde.
- Eugenol and isoeugenol were the major phenolic compounds found in all Spanish oak samples.
- Phenolic compounds also found in all Spanish oak samples were α-terpinol, γ-cadinene, δ-cadinene, 3-oxo-α-ionol, and vomifoliol (fruity and floral aromas).
- The most common semi-volatile present in the samples was β-sitosterol (antioxidant properties).
- The most influential silvicultural parameter on volatile composition of oak samples was altitude.
o The volatiles that were most affected were α-terpineol, eugenol, and vitamin E.
o All correlations were negative: the higher the altitude, the lower the concentration of volatile compounds.
- Organic matter and average annual temperature had some influence on volatile composition, specifically α-terpineol, vanillin, oxo-α-ionol, vitamin E, and α-amyrin.
- Distance from tree center, average annual precipitation, and number of trees per hectare did not influence the volatile composition of wood.
- Volatile composition of oak in different soil types was similar.
o Significant differences were found for the volatile aldehydes syringaldehyde, sinapaldehyde, and coniferaldehyde; and for tritperpenic compounds such as β-amyrin and derivatives and β-sitosterol (all with antioxidant properties).
o For those compounds with important sensory influence, significant differences were found with α-terpineol, and 3-oxo-α-ionol (floral notes), and trans-β-methyl-γ-octalactone (coconut/woody notes).
- The greatest difference for all the above compounds were found in loamy soils.
o Samples grown in loamy soils had the lowest levels of all of the above volatile compounds.
- Samples from Lugo and Pontevedra were very similar.
- Samples from Ourense had lower levels of many volatile compounds compared to the other two locations.
o Significant differences were found for guaiacol and vinyl guaiacol (smoky odor), syringol, eugenol (clove aroma), α-terpineol (floral odor), and coniferaldehyde.
- According to linear discriminate analysis, different geographical locations could determine/distinguish different chemical composition of wood samples of the same species.
The results of this study showed that some silvicultural parameters, such as altitude, organic matter and average annual temperature influence the volatile composition of Spanish oak, Q. pyrenaica. Conversely, other silvicultural parameters, such as distance from tree center, average annual precipitation, and number of trees per hectare do not influence volatile composition.
Even though it was shown that Q. pyrenaica grown in loamy soils show some significant differences in regards to volatile composition compared to other soil types, linear discriminate analysis showed that soil type is a poor determinant in volatile composition of wood in Spanish oak. On the other hand, as with French and American oak, geographical location does have a significant influence on oak volatile composition, and is a good factor for volatile composition classification.
The results of this study should arm wine makers with information they need in order to make a decision on whether or not Spanish oak is right for the style of wine they wish to create, and which forest/province they should harvest the trees from. Ultimately, however, I think the next step in this line of research would be to create a wine using barrels from Spanish oak from different geographical locations, and compare the oak volatile composition results with the finished wine volatile content as well as a sensory analysis.
What do you all think of this topic? How many of you use Spanish oak barrels or Spanish oak chips in your wine making practice? Please feel free to leave your comments below (no html tags, please).
Source: Alañón, M.E., Pérez-Coello, M.S., Díaz-Maroto, I.J., Martín-Alvarez, P.J., Vila-Lameiro, P., and Díaz-Maroto, M.C. 2011. Influence of geographical location, site and silvicultural parameters, on volatile composition of Quercus pyrenaica Willd. wood used in wine aging. Forest Ecology and Management 262: 124-130.
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