D espite a history stretching back two millennia, barrels remain some of the most complex and misunderstood elements used in winemaking. In order to obtain a barrel that matches your desired style, it is important to understand and evaluate the numerous options available. In this article, we will delve deeper than the classic American versus French debate and explore how other properties and factors can influence the wine, while checking the validity of popular anecdotes.
To get started, let’s review how wood flavors can affect your end product. Oak storage can impact wine in several ways, but two are particularly significant: first, it allows a small amount of oxygen to interact with the wine, and second, it contributes aroma compounds that can alter the flavor. The amount of oxygen that moves into the wine is largely dictated by the wood grain; generally, the tighter the wood grain, the more oxygen you can expect to interact with the wine inside of the barrel. This interaction can be beneficial for certain types of wine, particularly red wines and richer white wines. Oak also has a standard set of flavors that it tends to impart on wine. These flavors are affected by the oak species, location of growth (even within the same forest), seasoning, and toasting. Some of the main compounds responsible for these aromas are shown in the table below.
The main oak-derived aromas found in wines.
When discussing barrels, the most frequently used descriptor is the provenance of the oak. There is a widespread notion that American oak is more aromatic and French oak is subtler but lends more tannin. Hungarian oak, which has been increasing in popularity, is said to fall somewhere in the middle. However, those assumptions are not always true. American oak usually refers to the species Q. alba. Meanwhile, there are two French oak species used in cooperage: Quercus robur and Q. petraea. Q. robur has coarser (wider) grain and typically lower concentrations of aroma compounds than Q. petraea, making the former better suited for aging spirits and the latter more complimentary to wine. Certain French forests grow one species or another, however many have a combination of both. Therefore, when we talk about American or French oak, it is important to note that we are discussing a difference in species as well as environmental growing conditions. The distinction between the two French species is, unfortunately, not well addressed (or sometimes confused) in non-academic and academic publications alike, leading to many of the misconceptions we have today.
When people state that American oak is more “aromatic” than French, this is typically attributed to the cis-oak lactone compound that lends a woody/coconut/sweet aroma. There is also a trans- isomer, however it has a low odor impact, so most people focus on the cis- portion when discussing lactones. There have been multiple studies comparing the relative concentrations of cis-oak lactone between the oak species, yet they are they are not always in agreement. While American oak is purported to have higher levels of cis-oak lactone than French, this is only definitively proven in comparison to the Q. robur species. While there is a lot of tree-to-tree variation in Q. alba, Q. petraea shows even higher levels of variability, and that makes comparing the two species challenging. In one study, total content of oak lactones in six Q. petraea trees from Tronçais ranged between ~1-78 μg/g (dry wood), while six Q. alba from Missouri and Virginia ranged from ~24-77 μg/g (Masson et al. 1995). The table shown below illustrates just how much variation there can be.
The lactone content in wine depends partly on the type of oak in which it is aged.
What all of this means is that the probability of an American oak barrel having those lactone aromas is higher than a French one. However, it is all together possible to purchase a French oak (Q. petraea, specifically) barrel that actually has more oaky character than American! This underlines the importance of tasting through individual barrels during the winemaking progress, as this variation and unpredictability can lead to very different flavors in your wine.
While the “oak flavor” debate remains hazy, there are important differences between American and French oak that have been successfully quantified. One of them doesn’t require a science experiment to figure out: pricing. French oak barrels are typically the most expensive, followed by American, and then Hungarian are the most affordable. While some of this has to do with demand (currently, Hungarian oak doesn’t carry the same prestige as French), it is also due to physiological differences between European and American oak species. The internal structure of American oak allows it to be sawn into staves, while European oak must be split along the grain in order to be watertight. The need to split European logs translates into only 25% of it being available for stave production, while American logs can yield 50% (Chatonnet and Dubourdieu, 1998). As you might expect, this greatly increases the cost of French oak.
The physiological differences between the species also include a trait referred to as “grain”. Grain describes the width between growth rings, with “coarse” meaning larger and “fine/tight” indicating smaller. American oak typically grows faster than the French Q. petraea, which results in coarser grain. Interestingly, Q. robur, the other French species, also has coarse grain. There is a widely circulated belief that grain directly impacts the aroma intensity of wine, however this theory has been disproved. Studies have failed to show any correlation between grain size and sensory descriptors or levels of oak lactone. Rather, Sauvageot and Feuillat (1999) suggest the anecdotal reports of grain impact can actually be explained by species, e.g., when comparing coarse-grained Q. robur and tight-grained Q. petraea, which we’ve discussed as having very different concentrations of oak lactones. Although grain doesn’t influence the level of aroma compounds are imparted on the wine, it does affect how much oxygen enters. Somewhat counter-intuitively, tight-grained oak is more porous than coarse grain, and will therefore allow more oxygen ingress. This can be an important factor to take into account when choosing barrels for your desired style.
Examples of tight grain (left) and coarse/wide grain (right) in barrel staves. Wood grain in tree growth is influenced by several factors and has significant impact on the wood as a storage vessel.
All oak barrels allow small amounts of oxygen to interact with the wine as it ages. These minute levels of oxygen can provide significant benefits during aging, especially for red wines. In fact, the presence of oxygen is required for essential chemical reactions that stabilize the wine’s color. For example, anthocyanins, color molecules that are susceptible to the color-loss process known as “bleaching,” react with tannin in the presence of oxygen to form much more stable color molecules, polymeric pigments. Oxygen can also impact the tannins by speeding up the rate at which they bind to one other. This binding and transformation process results in tannins that consumers perceive as being “smoother,” rather than more astringent.
The permeability of the wood that allows oxygen to move into the wine also allows water and ethanol to evaporate out of it. Depending on the humidity level where the barrels are stored, this can cause the percentage of alcohol in the wine to increase or decrease, sometimes dramatically. The loss of “wine” (technically just water and ethanol) through this process—colloquially referred to as the “angels’ share”—can impact more than just the alcohol percentage. The loss of water and ethanol can help to increase the concentration of flavor and aroma compounds, since they are too large to pass through the oak. As discussed above, choice of wood grain can help direct how much these changes will occur, and can therefore impact the final product.
The coopering and toasting of barrels involves a very controlled amount of fire inside the barrel.
Another significant feature that can be selected is the toast level of the barrel. The interiors of barrels are usually toasted to bring particular flavors to the wine by caramelizing the barrel’s internal surface. This process breaks down the wood’s natural carbohydrates and lignins to create aromatic compounds. Typically, the greater the toast level, the lower the concentration of lactones. On the other hand, the lower the toast level, the more the amount of tannins are present, which can sometimes present a more astringent or bitter taste, or provide structure. Each flavor compound follows its own pattern with toasting: some are increased with toast, some are decreased, and others peak somewhere in the middle and then fall off. The most common, however, is for a compound reach a peak at a medium toast, and then when it passes a certain toast level, the aroma compounds begin to break down. It should be noted that many experiments on toast level have shown conflicting results, so these trends should certainly not be considered hard rules.
Despite their ubiquitous use and very lengthy history, oak barrels remain one of the least understood aspects in winemaking. This is, in part, due to the nature of nature: barrels are made from living things and therefore exhibit a high level of variability. The inconsistency in raw wood is high to begin with, but there are so many steps that occur between tree and barrel that can cause a cascade of effects. With the difficulty of accounting for all of this variability, the scientific community has struggled to agree on how a particular barrel will affect a wine. This gap in knowledge has left a lot of space for anecdotal “evidence” to become widespread and accepted, despite it often being directly opposed to what scientific evidence we do have. To have the best chance of obtaining your desired oak style, it’s important to sort the fact from the fiction.
An oak tree grove. Oak is a wood commonly used to make wine barrels (Photo courtesy of Forest Preserves of Cook County)
Citations
Chatonnet, P. 1991. Incidences du bois de chêne sur la composition chimique et les qualités organoleptiques des vins. Applications technologiques. Mémoire de DER, Institut d’Oenologie, Université de Bordeaux II.
Chatonnet, P., and D. Dubourdie. 1998. Comparative study of the characteristics of American white oak (Quercus alba) and European oak Quercus petraea and Q. robur) for production of barrels used in barrel aging of wines. Am. J. Enol. Vitic. 49:79-85.
Feuillat, F., L. Moio, E. Guichard, M. Marinov, N. Fournier, and J.-L. Puech. 1997. Variation in the concentration of ellagitannins and cis- and trans-β-methyl-γ-octalactone extracted from oak wood (Quercus robur L., Quercus petraea Liebl.) under model wine cask conditions. Am. J. Enol. Vitic. 48:509-515.
Masson, G., E. Guichard, N. Fournier, and J.-L. Puech. 1995. Stereoisomers of β-methyl-γ-octalactone. II. Contents in the wood of French (Quercus robur and Quercus petraea) and American (Quercus alba) oaks. Am. J. Enol. Vitic. 46:424-428.
Masson, G., E. Guichard, et al. 1996. Dosage des stéréoisomèrs de la β-méthyl-γ-octalactone dans le bois des chênes européens et américains. Application aux vins élevés en fûts. In: Aline Lonvaud-Funel, Oenologie 95, 5e Symposium International d’Oenologie. Lavoisier, Londres, Paris, New-York: 451-454.
Pérez-Coello, M.S., J. Sanz, and M.D. Cabezudo. 1999. Determination of volatile compounds in hydroalcoholic extracts of French and American oak wood. Am. J. Enol. Vitic. 50:162-165.
Rodríguez-Rodríguez, P., and E. Gómez-Plaza. 2011. Effect of volume and toast level of French oak barrels (Quercus petraea L.) on Cabernet Sauvignon wine characteristics. Am. J. Enol. Vitic. 62:359-365.
Sauvageot, F., and F. Feuillat. 1999. The influence of oak wood (Quercus robur L., Q. petraea Liebl.) on the flavor of Burgundy Pinot noir. An examination of variation among individual trees. Am. J. Enol. Vitic. 50:447-455.
Towey, J.P., and A.L. Waterhouse. 1996. The extraction of volatile compounds from French and American oak barrels in Chardonnay during three successive vintages. Am. J. Enol. Vitic. 47:63-172.
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