Whiskey aging is often referred to as a long slumber, but the process is much more active than that makes it sound. The whiskey is actually eating the barrel.
Aging occurs in cycles. On hot summer days, the whiskey expands through the char, through the red layer, and into the well-dried but otherwise untreated wood beneath. While in the wood, the whiskey dissolves various delicious compounds. In the cool of evening, the liquid contracts and brings all of the dissolved wood goodies out with it, where they begin to flavor the body of the whiskey.
The char layer reacts with and either traps or softens any bad congeners that remain in the spirit, taming them and rendering them either innocuous or tasty.
Once the aging process has begun the product is called whiskey, but like the intern who is called ‘doctor’ but can’t do anything without supervision, it isn’t much good until it has been in wood for at least two years, at which point it can be called straight whiskey.
After three years it can be sold in Europe as whiskey.
After four years, the maker no longer has to put its age on the label.
After about eight years they want to put the age on the label. Most American whiskey is aged between four and six years, but some is aged longer, up to twenty years or more.
In addition to extraction, the other important processes that take place in the barrel are oxygenation and evaporation.
After about twelve years, barrel flavors start to take over. The whiskey can easily go south at this point. In some cases it gets more interesting. Some whiskeys aged 12+ years will continue to improve for a few more, but older is not necessarily better. Older is only always … older; which means more wood notes, so less of everything else.
Aging is expensive, so old age tends to rationalize a high price, but neither is any guarantee of quality. A great whiskey must be balanced and most very-olds are not.
Sometimes older is interesting, though not necessarily great; sometimes older is awful; and sometimes, but only very rarely, older is singularly outstanding.
For a producer, continuing to age a barrel that is fully mature is a risky proposition. If you miscalculate and let it go too far, you may have to ‘blend it away,’ which is jargon for adding a few barrels of over-aged whiskey to a typically young, commodity product so everything balances out and the imperfections go unnoticed. Financially, this represents a failure, since the monetary return will be much less than originally anticipated.
Corn whiskey, a kind of proto-bourbon, is a very small part of the American straight whiskey market. Unlike all other types, it can be aged but it doesn’t have to be. To call it straight corn whiskey it must be aged for at least two years in either new un-charred or used barrels. If it is aged in new charred barrels, it is bourbon, even if the mash bill is 100 percent corn.
Only the United States regards un-aged corn whiskey as whiskey. It may not be labeled as whiskey in Canada or Europe.
Whiskey aging warehouses are big boxes full of 53 gallon barrels that are themselves full of high proof whiskey. They are a pretty great place to be. The scent of whiskey suffuses the air.
It is comforting to see how much whiskey there is, standing by, pulling itself together, getting ready for future thirsty multitudes.
Out in the countryside, the aging warehouses are on hilltops, spread far apart, copiously adorned with windows, and sheathed only in a thin skin of corrugated metal. In Frankfort and Louisville, they are more likely to be brick, with fewer windows, and capable of being heated to simulate summer conditions during winter, a process known as heat cycling. Some warehouses use electric fans to enhance air circulation. Most just rely on nature.
Brown-Forman is the only company that uses heat cycling in all warehouses at both of its Kentucky distilleries.
Different types of warehouses, different warehouse locations, and different locations within a warehouse all age whiskey differently. The barrels themselves are another variable, the age of the tree when it was harvested, the soil in which it was grown, how long the cut wood was seasoned, and so on.
Aging whiskeys are periodically tested, i.e., tasted, to assess how they are progressing. In some now more and more rare cases (because it’s so expensive to do) barrels are moved to a different location where they will age differently. This process, once ubiquitous but now rare, is called barrel rotation. Maker’s Mark is the only distillery that does it routinely and they don’t do it for every barrel, just the ones that need it.