Earlier in October, The Academic Wino reported on a study investigating the influence of toasting on the antioxidant capacity and phenolic composition of oak-aged wines (click here for that article). The main finding of that research was that model wines aged in non-toasted oak contained significantly higher antioxidant and total phenol levels than model wines aged in oak barrels that had been toasted. One question I posed at end of the article was whether different types of oak possessed similar antioxidant capacities, or if the levels of antioxidant varied depending upon the origin of the wood, and the species of plant used to create the oak barrels. The same authors that penned the previous article have since followed-up with a new study answering that very question I posed.
Many alcoholic beverages are aged in oak barrels in order to change and improve the sensory characteristics of the finished product. At least three mechanisms occur to change these characteristics, including aromatic complexity, changes in phenolic compounds to improve taste, and mild oxidation to change the astringency and color in the finished product. The extent to which flavors and aromas are extracted from the wood depend on the initial concentrations in the wood itself, the type of wood used, and the chemical composition of the wood.
The most traditional and frequently used wood for alcohol aging are the oak species Quercus alba (“American oak”), Quercus robur L. (“pedunculate oak”; a European species), and Quercus petraea Liebl (“sessile oak”; a European species). One up and coming species of oak for use in alcohol aging is Quercus pyrenaica Willd, which is a species grown in the Iberian Peninsula in Spain. When comparing the American oak species to the European oak species, there appears to be a greater difference in chemical composition, then if one were to compare just the European species to each other.
Aging alcoholic beverages in wood barrels not only changes their sensory characteristics, but also changes their antioxidant capacity. Some beverages (i.e. cognac, whiskey, brandies, etc) have been found to show increased antioxidant capacities when aged in wood barrels for longer periods of time. This increased antioxidant capacity is due to the increase in total phenol levels of the beverage over time. From these observations, one can hypothesize that the type and concentrations of polyphenols in differ types of wood will determine the antioxidant capacity of the finished beverage after aging.
The current study, which is a follow-up of the previous study performed by the same authors, aimed to study the antioxidant capacity available for extraction from a range of wood species commonly used in cooperage.
The species examined in this study were chestnut (Castanea sativa), cherry (Prunus avium), and oak (Q. alba, Q. petraea, Q. robur, and Q. pyrenaica). Q. petraea, Q. robur, and Q. pyrenaica were sampled from several forests in northwest Spain (oceanic with mild temperatures and high precipitation). Q. alba was sampled from the Appalachian region of the United States, near Nashville, Tennessee. All other wood samples were collected from the forest of Lugo (northwest Spain).
For each tree sampled (four per species), discs of wood were collected from a height of 1.3 meters from the base. From each of these discs, heartwood samples measuring 20x20x40mm were obtained. These heartwood samples were then dried, ground with a mechanical mill, and sieved to obtain a homogenous sample of sawdust.
Total phenol content was measured according to the Folin-Ciocalteau procedure and the resulting absorbance measured using a UV-visible spectrophotometer. Antioxidant capacity was measured using several procedures, since there is no one procedure of yet that all investigators use consistently (ABTS assay, FRAP assay, and the ORAC assay). Individual phenolic compounds were analyzed using HPLC protocols.
- The species of wood had an influence on antioxidant capacity.
o The lowest antioxidant capacity levels and total phenol levels were found in cherry wood.
o American oak (Q. alba) and two European oaks (Q. pyrenaica and Q. petraea) contained intermediate levels of antioxidants and total phenols.
o The highest antioxidant capacity levels and total phenol levels were found in chestnut and one European oak (Q. robur).
§ There were significant differences in antioxidant capacity levels of Q. robur compared to all other species except chestnut using the FRAP and ORAC assays, however, when using the ABTS assay, there were no significant differences between the three European oaks and the chestnut.
- Correlation analysis of the three assay methods indicated that there was a high correlation between them, meaning that all three assays provided comparable values when estimating the antioxidant capacity of wood.
- Correlation analysis showed that antioxidant capacity and total phenol content were highly positively correlated with one another, indicating that phenols are primarily responsible for the antioxidant capacity of the wood.
- Oak wood contained more ellagic acid than chestnut wood.
- Chestnut wood contained more gallic acid than oak wood.
o This pattern did not hold for Q. robur, which had similar gallic acid levels as chestnut.
- Cherry wood contained the lowest levels of ellagic acid.
- Q. robur and Q. petraea contained the highest levels of protocatechuic acid and vanillic acid.
- Chestnut contained the highest levels of 4-hydroxybenzoic acid and p-coumaric acid.
- Q. petraea contained the highest levels of sinapic acid and syringic acid.
- Ferulic and caffeic acids were present in all woods, though in very low amounts.
- Cherry wood contained high levels of protocatechuic aldehyde, and coniferaldehyde.
- Q. petraea contained high levels of syringaldehyde and sinapaldehyde.
- There were considerable amounts of scopoletin in the oak species and chestnut, but was not detected in the cherry wood samples.
- Ellagitannin content was significantly different among species.
o The most significant was the low ellagitannin content in the cherry wood samples. Small concentrations of vescalagin and castalagin were detected while roburins and grandinin were not detected.
o All oak species and chestnut contained high concentrations of ellagitannins.
- The ellagitannin monomers of grandinin, vescalagin, roburin E and castalagin were found at their highest concentrations in the oak and chestnut samples.
o Castalagin was the primary monomer found in the oak samples.
o Vescalagin was the primary monomer found in the chestnut samples.
o Q. alba contained the lowest ellagitannin content.
o Q. robur contained the highest ellagitannin content.
- Q. robur contained the highest concentration of roburin AD and grandinin, followed by Q. pyrenaica and Q. petraea.
- Chestnut contained the highest concentration of vescalagin, roburin E, and castalagin.
Correlations between antioxidant content and phenolic composition
- Strong correlations were found between antioxidant capacity and gallic, protocatechuic, ellagic, vanillic, and p-coumaric acid concentrations for low-molecular weight phenols.
o These compounds greatly contribute to the antioxidant power of the wood species.
o Higher antioxidant capacities could very well be due to the relatively high concentrations of phenols in the wood.
o Certain phenolic compounds do not play a role in the antioxidant capacity of the wood (protocatechuic aldehyde, vanillin, and siringaldehyde).
- Strong correlations were found between radical-scavenging capacity measurements and the concentrations of ellagitannins, which suggest that high-molecular weight polyphenols play an important role in the antioxidant capacity of woods.
The results of this study were straight forward and clear. To put it simply, the species of wood used in cooperage for winemaking or any other alcoholic beverage aging has a significant influence on the antioxidant capacity of that wood. These differences were due to specific polyphenol content differences between the different wood species. If a winemaker is strictly looking to maximize the antioxidant capacity of the finished wine, they may opt to select a type of wood that is higher in specific phenol concentrations and overall antioxidant capacity, such as chestnut or the European oak Q. robur.
Of course, using different types of wood for aging will also change the sensory characteristics of the wine, so a lot needs to be considered before choosing a particular species. If the overall goal is to create a wine with specific sensory characteristics, then this type of analysis may not be particularly important. However, if one is designing a wine to be “healthier”, by containing higher levels of phenols and antioxidant capacity, then studies like these are of utmost importance.
Since different grapes contain significantly different antioxidant and phenol levels, determination of which variety to use is very important. After that decision has been made, then studies like this that explore antioxidant capacities of different types of wood are very important in maximizing the total potential antioxidant capacity of the finished wine.
This study was a great next step in this line of research. Next, I’d like to see a study age model wines in different types of wood (using different varieties of grapes) while measuring the antioxidant capacity and phenol content of the finished wine and potentially correlating that to the antioxidant capacity and phenol content of the wood itself.
I’d love to hear what you all think! Please feel free to comment below!
Source: Alañón, M.E., Castro-Vázquez, L., Díaz-Maroto, M.C., Hermosn-Gutiérrez, I., Gordon, M.H., and Pérez-Coello, M.S. 2011. Antioxidant capacity and phenolic composition of different woods used in cooperage. Food Chemistry 129: 1584-1590.
I am not a health professional, nor do I pretend to be. Please consult your doctor before altering your alcohol consumption habits. Do not consume alcohol if you are under the age of 21. Do not drink and drive. Enjoy responsibly!