What Exactly Happens to your Whiskey as it Ages?
You can't have whiskey without barrels. In fact, the way barrels are used in the distilling process is often a determining factor of whether a product can be legally marketed as "whiskey," depending on where you're selling. Until relatively recently, most whiskey lovers didn't understand exactly what the barrel-aging process does to whiskey. But today, distilleries everywhere can use their knowledge to create very specific types of whiskey, and even monitor the changes going on inside individual barrels!
Before a barrel can be used to create a proper whiskey, the inside is lightly charred. This helps the wood release aromatic compounds called phenols that contribute to the whiskey's flavor. The charring process also breaks down some of the wood's cellular structure into sugary byproducts that give the whiskey additional notes of flavor, like caramel and vanilla.
As the whiskey sits in the barrel, it's subjected to years of seasonal changes in temperature and humidity. These subtle environmental fluctuations cause the whiskey to expand and contract, forcing the liquid in and out of the wood. The specific location of where the barrels are stored while they age may seem trivial, but it's actually a critical part of what, legally, makes a whiskey "Tennessee Whiskey." One of the legal requirements of Tennessee Whiskey is, naturally, that the product must be aged in Tennessee.
US laws require almost all whiskeys to be aged in new barrels only, with corn whiskey being an exception. However, many other countries (e.g. Ireland, Scotland, Canada, etc.) allow barrels to be reused for aging whiskey. But as I'm sure you've probably figured out, reusing the barrel doesn't make another batch of the previous whiskey. Due to the number of factors that contribute to the aging process, it's nearly impossible to accurately predict how whiskey aged in a used barrel will taste.
To solve this problem, Don Livermore, Hiram-Walker Distillery's Master Blender, built an infrared sensor that can be placed inside a barrel to measure the levels of different chemicals within. By analyzing the results of the scanner, you can tell exactly how the barrel will change the whiskey as it ages. Livermore built this scanner as part of his dissertation project for his PhD in Brewing and Distilling. Unfortunately, it's not practical for most distillers to use because it's so large that the barrel must be disassembled before the scanner can fit inside.
Livermore has said that he hopes to partner with an engineering company to create a better design. If that happens, maybe we could enjoy a whole new age of whiskey, Tennessee, or otherwise.