Pretty much everyone knows that bourbon spends some time aging in wooden barrels. Why wood? Why American oak? Read on.
Wood, instead of any other material, is the medium of choice (and necessity) for the storage and aging of bourbon and other alcohol because wood provides an idea place to bourbon to mellow out. When bourbon is first introduced into a barrel, it isn't the lovely shades of reddish-amber and brown you see in the bottled end product - it's clear and has a taste that you would never remember fondly. The wood of the barrel chemically interacts with spirit, and when the acids in the wood are introduced to the infant whiskey, over time the unpalatable harshness is blunted. Theoretically, and up to a point, the more time in the barrel creates a smoother drink. Yes, it is very much a science as it is an art.
Oak is used because of its grain. The very tight nature of oak wood's grain allows an ideal amount of oxygen both in and out of the barrels during aging. This ebb and flow of air through the pores of the oak is an essential part of the maturation process, and imparts taste to the bourbon. And, with the ebb and flow of air comes the absorption of the spirit into the wood itself. From the hot summers to the cold winters, bourbon expands and contracts into and out of the wooden barrel walls, sucking the acids, sugars, and colors from the charred wood to give bourbon its color and, in significant part, taste. It's manipulated nature at its best - 100% natural processes that wouldn't occur without human intervention.
The process of maturation is a topic unto itself, and will be discussed later in greater detail. In the meantime, I hope you understand that bourbon creation depends on all things natural. Indeed, a whiskey that has anything artificial added to it cannot properly be called "bourbon," nor should it ever be.