In his book, Proof: The Science of Booze, author Adam Rogers summarizes the effect of barrel-aging on whiskey:
Barrels full of booze are exciting places, chemically speaking. Of the structural components of wood, cellulose and hemicellulose are giant chains of repeating glucose molecules, and the heat of coopering breaks those into sugars—glucose, hexose, and pentose. But the third major component, lignin, is different. It’s a massive molecule, too, but with nonrepeating subunits. About half of them are vanillin (vanilla flavored), and the rest is barbecue-flavored guaiacyl, clove-flavored eugenol, and syringaldehyde. At high heat, the spicy aromatic aldehydes in the lignin undergo Maillard reactions and yield the same flavors as browned meat. When it’s hot outside, pores in the wood open up and the liquid moves inside, slurping up the tannins and other molecules that come from lignin decomposition. And the ethanol makes all those chemicals react with each other. The aldehydes mix with the acids and form fruity, tart esters.
All that activity is why it’s wise to add a couple of ice cubes. It kind of cools everything down.