oak trees have provided the raw material for barrels in which to store and transport alcoholic beverages since Roman times and perhaps earlier. The beverage industry has adopted oak as its dominant choice for maturation vessels and frequently uses it for fermentation vessels as well. It is prized for its pliability, its strength, and its molecular-level permeability, as well as the (relatively) modest influence it has upon flavors and aromas.
Oak is a member of the Fagaceae family, the beeches, and has two genera, only one of which, the genus Quercus, is used in producing spirits; it includes more than 250 different species. For oak barrels, the relevant subgenus is Euquercus, for which there are six groups of species. Europe utilizes eight indigenous species of oak for alcohol beverage: the three most important are:
Quercus robur—a relatively open-grained oak, most famously found in France’s Limousin forest. Important for aging cognac and (in the guise of the “black oak” harvested in the Monlezun forest in southwestern France) Armagnac. Q. robur is also dominant in the forests of the Baltic, which formerly supplied a high percentage of the barrel staves used in Great Britain.
Quercus petraea or sessiflora—a somewhat tighter-grained, less tannic oak, most famously found in France’s Tronçais forest. Also important for cognac.
Quercus pyrenaica—also sometimes used for aging spirits; it was, however, the species that traditionally provided Portuguese producers with their oak barrels, prior to their late twentieth century adoption of French oak.
North America is reliant upon Quercus alba, or American white oak, for its barrels; it has a specific internal structure that allows its staves to be sawn and yet resist leaking. European oak staves are created by splitting along the grain; sawing will cause leaking.
European oaks are generally tight-grained due to slow growth in continental climate. US oaks from cool areas such as Ohio and Montana exhibit tight grain, while a wider grain is found in oaks from warmer areas such as Tennessee and Missouri. Lesser-utilized North American oaks include Quercus macrocarpa (in Minnesota and central Canada) and Quercus garryana (Oregon).
With oak often bent for shipbuilding in the ancient world, early northern European Celts crafted barrels and other oak containers; their cylindrical shapes offered easier and safer transportation than traditional clay amphorae. The Romans took notice and created oak barrels of their own. When cognac distillers began deliberately barrel aging their spirits, in the seventeenth century, they naturally turned to local oak, although other woods, such as chestnut, were sometimes also used. When it began aging whisky a century or so later, the United Kingdom, lacking France’s extensive oak forests, imported its barrel staves first from Memel in the Baltic and then, increasingly, from New Orleans and Quebec, switching to American white oak in the process.
Oak’s special characteristics are not merely physical; organoleptic elements leach into beverages in oak containers. Those flavors, aromas, and textures can be influenced by the oak’s geographic source and treatment; toasting and charring of barrels will have a profound impact upon those beverages. Elements that may be present in toasted or charred oak include furanes (with characteristics of almonds, grilled nuts), lactones (vanilla, nuts, cocoa, coffee, and raw wood), phenols (smoke, pepper, flowers, clove, and caramel), and aldehyde phenols (vanilla and fruit).
As oak is transformed from raw wood into barrels, the steaming, heating, firing, and/or charring that are part of the process will affect each of these compounds, with the exception of cellulose, which has little bearing on beverage character, flavor, and texture.
Because hemicellulose includes sugars, such as glucose, xylose, rhamnose, mannose, and galactose, as well as some polysaccharides, the application of heat can create Maillard-like reactions. Various chemical chains provide furanes, furfurals, aldehydes, and furfuraldehydes during the toasting, heating, and burning. As carbohydrates break down, available aromatics include almonds, butterscotch, caramel, or cherry.
As phenols slowly oxidize during a spirit’s barrel maturation, there are numerous potential flavors and aromas that can arise including fruit, vanilla, cinnamon, almond, tonka bean, coffee, smoke, pepper, flowers, clove, sawdust, caramel, brown sugar, walnuts, molasses, candy, spice, eugenol, vanilla, leather, and others.
As lignins change in the presence of heat, the aromatics offered include vanilla, eugenol (clove), and 4-methylguaiacol (or vanilla/spicy/smoky aromas).
There are endless types of tannins, themselves a subset of polyphenols. But just as tannin acts as a preservative with leather (leather was originally tanned by curing animal skin with the juice of oak galls), it slows oxygen’s degradation of fruit character in wine. In spirits, its bitter and astringent characteristics also provide a sense of weight, texture, and mouthfeel; the mouth, throat, and body have their own physiochemical receptors for bitterness.
Tannin also has the ability to bind with saliva’s proteins. leading to astringency, dryness, and even increased friction in the mouth, all of which lend to a wine or a spirit’s perceived weight.
These characteristics are influenced as well by the source wood and barrel processes:
• Species of oak—American oak generally has fewer tannins and more sweet, nutty coconut notes.
• Origin of the wood—research is ongoing, but climate will impact growth and the oak’s constituent elements.
• The seasoning of the wood—some wood is kiln dried and some is air dried for months or even years.
• The grain of the wood and other physical characteristics.
• The toasting and/or burning that is part of the coopering process.
Finally, the wood supplies all these many sensory attributes, but oxygen ingress (mostly through the bung, but also through the grain of the oak) sees the polymerization of tannins and the further development of aldehydes. These will be strongly influenced by the location of the barrels, temperature, humidity, and proximity to influences like ocean air.
Buxton, Ian, and Paul S. Hughes. The Science and Commerce of Whisky. London: Royal Society of Chemistry, 2014.
Emen, Jake. “Why and How Oak Matters in Whisky.” Whisky Advocate, Spring 2017.
Johnson, Hugh. The World of Trees. London: Mitchell Beazley, 2016.
By: Doug Frost