American oak (Quercus alba) has about 21% non-tannic phenolic content while its French (and Hungarian) counterpart (Quercus robur), contains around 14%. However, French (and Hungarian to a lesser content) has 2.5 times the extraction of total phenolics than does the American oak. In everyday English, this means that American Oak will be much more perfumed, but French and Hungarian will generally have better inherent structuring abilities. Other than these basic differences, the two different species generally react in the same way to toasting (more on this later).
The way in which the raw wood is processed has a major affect on the final flavor and aroma profile of the oak, regardless of species variation. When oak for winemaking is cut, it has to undergo a period of drying and conditioning before it can be used, and this is referred to as seasoning. This period usually lasts between 2 to 3 years and basically involves stacking the staves in the open air and letting the elements (rain and sun) work its magic on the oak. The stacks are usually stacked and re-stacked throughout this period so that the staves on the top one year are at the bottom during the next year, and so on. This is done to in order to better equalize the seasoning differences that exist between the top of the stack (more exposure to sun and air) and the bottom (more moisture and less light). All throughout the seasoning period, basically what is going on is that various fungal micro flora attack and colonize the wood. As they do so, they release a series of enzymes that are responsible for the following desired reactions: the wood extract becomes lighter in color and less astringent, the harsher and bitter elements of the wood are greatly reduced, and various positive aromatic compounds are boosted; including vanilla, clove, and especially coconut. Besides being interesting on its own, what is even more fascinating about all of this is that it turns out that the amount and ratio of these compounds that are transformed in the wood turns out to be site-specific. In fact, experiments done at the Bouchard cooperage in France with the same wood that was seasoned in two different regions and then brought together and identically coopered to the same toast level in the same facility produced two different sets of flavors and aromas! This was directly attributed to the differences in the seasoning conditions of the two woods. Therefore, in addition to species differences, it is important to keep in mind that the way in which a wood is seasoned also will affect the final qualities of the oak once it is toasted.
As for the toasting itself, it should be noted that the duration and the intensity of the heat during the coopering and toasting process has a tremendous effect on the amount of individual compounds that are produced in a barrel, even from the same woods which have received the same seasoning. However, there are in fact some basic generalities for how some of the various compounds in oak will behave when they are toasted. Understanding these can only help when trying to decide which level of toasting will be more apt to give the desired character to a particular wine:
Hemicellulose: A class of compounds comprised of several simple sugars that when toasted give caramelized products which have a sweet, toasty quality and which help to contribute to the “body” of a wine. The more intense the heat, the “darker” the caramel flavors become.
• Furfural is “sweet” and “caramel-like”,
• 5-methyl-furfural is more of a “butterscotch” type of flavor.
Lignin: is made up of two building blocks: Guaiacyl and Syringyl. Sweet vanilla increases up to a medium plus toast, but then it starts to decrease as the heat is raised towards a more heavy toast or a char. Interestingly enough, with the higher heat also comes the appearance of more smoke and spice (clove) notes.
• Vanillin is vanilla,
• Guaiacol is “smoky”,
• 4-methyl-guaiacol is “spicy” & “smoky”,
• Eugenol is “clove-like”.
Lipids: are made up of the oils, fats, and waxes found in the wood and are responsible for the oak lactones. Seasoning greatly increases the level of lipids in the wood. With toasting levels up to medium/medium plus, the level of oak lactones increases, however it breaks down and decreases after that as the heat is raised further.
• Cis-oak lactone is “woody” and “fresh oak” like,
• Trans-oak lactone is “coconut-like”
• The lower the toast, the more tannins (“structure”) and lactones (“wood-like” and “coconut”) will be present in each of the oaks.
• The higher the toast, the more spice and smoke notes will be present
• The deeper the toast, the more deep the caramel tones will be (moving into butterscotch at medium plus).
• Vanilla will increase up through a medium-plus toast and then decrease with a heavy toast and char.
• American oak will be more aromatic, but French oak will give more structure (Hungarian will give less than the French but more than the American).
• The greater the toast level, the lower the lactones (“wood” and “coconut”) for all three woods.
Medium plus is the most complex of all of the toast levels, and the most popular. Depending on the wine being made, this may or may not be a good thing!
For additional information on oak toasts and types, check out our Oak Information Paper.
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