Bourbon whiskey—and rye, wheat, malt, and Tennessee whiskey—must be aged in new, charred oak barrels. Within that requirement, however, there are myriad variations that can affect the color, aroma, flavor, and mouthfeel of the final whiskey. Working within those barrel specifications allows distillers to craft a distinctive product, with decisions about char and toast level having major ramifications on the final whiskey.
Wood terroir is one variable. While more or less all barrels are made of white oak, the wood itself can vary by region. In colder northern climates, for example, trees grow more slowly and have a tighter grain. The flavors they offer are more concentrated, but also harder to reach and absorb.
After the wood is harvested it must be dried, or seasoned, since more than half its weight is water. Kilning—drying the wood in a heated chamber—is the fastest and easiest way, but not the best. The wood will be dry enough to make barrels, but that’s about it. With natural seasoning, rough-cut staves and headpieces are stacked up outdoors and left alone for six to 24 months (occasionally longer). They are rained and snowed on, baked in the sun, frozen, thawed, and invaded by microbes. This process reduces tannin levels and breaks down various compounds, which are then available for incorporation into the whiskey as it matures.
Natural seasoning allows the wood to begin decomposition, which is good. Fungi send out roots (hyphae) that penetrate into the wood structure and release hydrogen peroxide, a natural bleaching and oxidizing agent that helps break the wood down chemically, softening tannins and caramelizing hemicellulose (a complex carbohydrate found in plant cells), among other salutary effects. Kilning does none of that.
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