Butterscotch, sultanas, vanilla, Christmas Cake, dates, walnuts… The wondrous tasting notes of whiskey include some of our favourite flavours on the planet. Indeed, it’s why we love the liquid so darn much. But where are all of these delicious elements coming from? We’re glad you asked. Because we’ve got an expert answer.

Let’s start off by looking at the most popular styles of whiskey on the planet: Irish whiskey, Scotch, Bourbon. Not only are these the most widely consumed, they’re also among the most fiercely protected categories of spirit in terms of production techniques. Meaning that you can’t just dump actual vanilla or butterscotch (or any flavor additives, for that matter) into the mix. 

The luscious notes of dark cherry and chocolate in that bottle of Brollach, as an awesome example, are mainly owed to the barrels in which the liquid matured. In the case of the aforementioned Irish whiskey, that means ex-Bourbon American oak casks as well as ex-Madeira French oak barrels. Slowly and surely, over two decades, a double-distilled single malt absorbed flavours from the surrounding staves. 

In fact, when it was first distilled—like all whiskey—it rolled off the still a clear liquid. If you were to have tasted it then, it would sip much like the malted barley that went into its production. You’d probably make comparisons to porridge, breakfast cereal, or maybe a toasted lager beer. Once it lays down in the wood, everything changes. From here the whiskey will pick up all of its color and up to 80% of its overall flavour. 

And that flavour will be shaped not only by the type of wood used, but by what the cask might have held before, how it was treated, where the barrel sits in the warehouse—even where the warehouse, itself, is located. Indeed, there are dozens of variables that contribute to the final result. 

In the United States, native white oak is used to build Bourbon barrels. It must be a new barrel, and it must be charred before the whiskey can enter for aging. The distant species of wood, known as Quercus Alba, is responsible for Bourbon’s telltale characteristics of vanilla, cinnamon, caramel, and coconut. In Europe, the native species is called Quercus Robur. Often sourced from Spain, it provides elements of dried fruit, orange citrus and baking spice. It’s most commonly used for storing wine, but when whiskey makers get their hands on this expensive, elegant oak, it’s quite magical indeed. 

Other significant wood varietals for whiskey aging include Quercus Mongolica—better known as Mizunara oak. It’s famous for the incense and sandalwood sorts of flavours that have helped make Japanese whisky a global sensation. Lesser established, Quercus Petraea is actually the national tree of Ireland—although it more often ends up in the cognac industry, and can impart a tartness to the liquid it holds. 

But whichever wood you start with, eventually there’s a mechanical process that’s going to occur once its filled with whiskey… And bare with us while we get a bit geeky here: complex polymers in the wood grain, known as lignins, interact with the alcohol over time to slowly convert chemicals such as tannins and acetic acid into pleasing compounds called esters. And esters are organic compounds largely responsible for the flavour and smell of everyday foods.

Still with us? 

Well we’re not done just yet! How the barrel was conditioned—or seasoned—prior to being filled will also have a significant impact on the final result. Cooperages, responsible for building the barrels, will often leave staves out in the open air for upwards of a year in order to dry out the oak and leach out some of the tannins that can result in unpleasant astringency. 

After they are built they can be filled with other sorts of liquid altogether before ever being exposed to whiskey. This is quite a common occurrence in the southwest of Spain, where massive 500-liter Sherry Butts are filled with the semi-sweet fortified wine of the region and then left to soak up those flavors—deep into the wood—for months at a time. They are then dumped and the now-seasoned barrel can be filled with whiskey distillate. It will extract the characteristics of that previous liquid: prune, raisins, sultana, walnuts. Just a brief glimpse at what you can expect from a whiskey that was aged in this way. 

All this is to say: when it comes to adding flavour to whiskey, producers have so many tools in their arsenal. Or more accurately, so many spices in the spice rack. And just like a Michelin-starred chef, a world-class whiskey-maker is eager to innovate, making available as many of the flavouring agents as possible. But because the lion’s share of that development will come together well after the distillation of the whiskey itself, the quality and the source of the wood is what’s determining most of what you’re ultimately tasting in the glass. 

You always want to be sure that corners weren’t cut in the stillhouse, of course. However, you can start with the best distillate on the planet—if it’s not stewarded properly during the maturation process, you are not going to be dumping unforgettable whiskey out of the cask. And we are only interested in unforgettable whiskey. So we are always paying careful attention to the cooperage. The next time you’re considering how your favourite whiskey acquired those incredible hints of toffee, molasses, tobacco…whatever, you might want to go hug a nearby tree. Because you’ve got good wood to thank for great whiskey.