Fermentation produces a wild variety of chemicals. Sure, you get a lot of the 2-carbon alcohol called ethanol, but you also get 1-carbon methanol, 3-carbon propanol, 4-carbon alcohols like isobutanol, 5-carbon isoamyl alcohol, etc. The higher alcohols are also known as “fusel alcohols” or “fusel oils”. And there is a zoo of other fermentation byproducts: esters, aldehydes, diacetyl, and others.

Distillation concentrates the ethanol (yay!) but also concentrates many of these congeners. Some of these congeners taste okay, like fruity esters, or are nice and interesting in small amounts, and others are just plain nasty.

Distillers know that the early part of the distillation is heavy on some nasty stuff, the middle part tastes much less bad, and then the last part is super nasty again. They refer to these regions of the distillation as the Heads, Hearts, and Tails. It’s wrong to say that all of the early nasty stuff comes out early and all of the late nasty stuff comes out late; in reality all components are present in all parts of the distillation, but they are weighted toward the front, middle, or back. So distillers commonly select some middle portion of the distillation for consumption, where the “heads” components have diminished and the nasty “tails” haven’t yet become really strong. Some people like to drink that middle cut, especially the middle cut of a second distillation, fresh and young. But young spirits, even those that result from very conservative selection of the “hearts”, can be rough and unpleasant.

At some point in history it was discovered that if spirits were left in the vessels used for storage and transportation for a longer time, they tasted better. This is the simple answer to the Why Question. We age spirits because time smooths and softens the raw spirit. When oak casks were used to store spirits, they matured and also gained pleasant complexities from the oak itself.