(This is a work in progress)
When aging whiskey (or other distilled spirits), what is the difference between using barrels made from American oak or from European oak?
American oak is denser, so the spirit interacts with the wood less. It is said to contribute sweetness, and flavors of vanilla, and perhaps a hint of coconut.
European Oaks are less dense. Some say that they add more spicey, savoury, or peppery flavors.
Complications: Many people believe that American oak gives darker colours. But much of a whiskey’s color can come from the amount of barrel char, the time aged, the climate that iyt was aged in, and the amount of added caramel coloring (*) Color also depends on whether one uses a virgin barrel, or a barrel that had previously been used. So unless one ran a controlled experiment, where the only variable was the type of wood, it is very hard to say how much the wood source would affect the color.
We need to be careful about what species of tree we are referring to. Only a few species are traditionally used in aging spirits. Oak could refer to any of 600 species of trees & shrubs in the genus Quercus. Because it is an old word, used long before modern scientific classification was more precise, the name “oak” also is used for some mostly unrelated plants, so one should be careful about what plant someone is referring to.
Oak/Quercus is native to the Northern Hemisphere, and includes deciduous and evergreen species. Common ones used in whiskey aging include:
Quercus alba (American white oak)
Quercus sessiliflora (French sessil oak)
Quercus robur (French robur oak)
Quercus pyrenaica (Spanish oak)
It is difficult to say exactly how each wood affects the aging of whiskey. Peter Konrad Thornburg Burger did a literature review on just the differences in volatiles (because there’s even less data on non-volatiles) in these woods.
What are volatiles? Organic molecules that evaporate readily at normal temperatures (let’s stay, temperatures above freezing, and less than a hot summer day) They are molecules that have a low molecular weight, so they vaporize easily into the air.
He studied volatiles from Quercus alba (American white oak), Quercus sessiliflora (French sessil oak), Quercus robur (French robur oak), Quercus pyrenaica (Spanish oak), and more distantly related species like Castanea sativa (which is a chestnut, not oak.)
The net result is that there is a ton of information on extractives from oaks, but the studies aren’t terribly meaningful, especially against one another, because the extraction and analysis methods are inconsistent and dramatically affect the results.
“But, there are some generalizations that are reasonable: assuming you mean oak as used in wet cooperage (that is, seasoned and toasted or charred), the biggest difference is the concentration of lactones (cis- and trans-β-methyl-γ-octalactone; generally characterized as being cocoanut/vanilla for cis-, and woody/spicy for trans-). “
“American oak has a much higher whisky lactone versus any European oak. Interestingly, the toasting process might actually reduce a lot of that difference, but there’s only been one study I could find on this.”
“The next big difference are polyphenol concentrations, with American oak having higher concentrations than the European ones. Eugenol and its derivatives are characterized as spicy, while Guaiacol is characterized as smoky.”
and here are phenolic acids in oak wood.
This infographic shows us the influence of Oak on maturation.
Aromatic extractives from different wood species used in spirit maturation
Peter K. T. Burger, MSc Brewing and Distilling, A11PJ Project Studies, The School of Engineering and Physical Sciences [2016-17]
(*) Adding caramel coloring doesn’t affect the taste of a whiskey, which is why even whiskey purists are usually ok with this. Doing so in fact is standard in the Scottish and Canadian whiskey industry, and in non-bourbon American whiskey production.