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Distillers say that as much as 70% of the flavor of Straight Bourbon or Rye comes from the maturation process in charred barrels that had not been used before the whiskey is entered. The barrel is a crucial part of this process so it is important to know what is happening in the wood that contributes to the flavor profile of the whiskey. I will give a brief description here of what I have come to understand happens in the barrel.

When a cooper makes a barrel he is creating many different levels of flavor in the wood. In order to bend the wood staves into a tight container that will hold whiskey, the cooper applies heat to the wood. This heat helps make the wood flexible so the staves can be bent and shaped. The heat is not so great that the wood catches fire, but it is hot enough to break down the lignin in the wood into vanillin, which will contribute vanilla flavors to the whiskey. The process is called “toasting the barrel” and different distillers use different levels of “toast” in making the barrels for their products.

After the barrel is shaped, then it is charred on the inside. Charring is simply allowing the inside of the barrel to catch fire creating a layer of charcoal on the wood. This charcoal is measured by different grades from 1 to 4 with level 1 char being the lightest char and level 4 being the deepest char. It should be noted that if the char is too deep it will destroy the layer of toasted wood almost completely and reduce the amount of vanilla flavors the barrel can contribute to the final whiskey. The charcoal contributes tannins to the whiskey that gives the liquid a red color and a bitter, dry wood flavor.

Behind the charred wood is what is often referred to as the “red layer” or a layer of natural sugars from the wood that have been caramelized by the charring process, Trees store their energy in the form of sugar while living and much of that sugar remains in the wood after the tree is cut down and the wood dried. These sugars will caramelize during the charring process and will contribute caramel and maybe even a little butterscotch flavors to the whiskey.

The secret to whiskey aging is the fact that you have whiskey surrounded by wood. The barrel is filled with the new make spirit and placed into a warehouse to age. About three gallons of whiskey will soak into the wood in the next several hours.  When the warehouse is hot in the summer or steam heated in the winter, the liquid expands and some of the liquid will evaporate, building pressure in the barrel forcing liquid into the wood. When the temperature drops at night or during the winter, the gasses will condense and the pressure inside the barrel will lessen and liquid will come out of the wood bringing the flavors from the red layer and the toasted wood with it. This is where barrel entry proof can make a difference in the flavor of the whiskey. Sugars dissolve better in water than alcohol so the lower the barrel entry proof, the more water to bring out the sugars in the wood so you get a sweeter flavor at a younger stage of maturity.

The “Angel’s Share” is the evaporation of the whiskey into the atmosphere during the aging process. Wood looks solid but it is porous which allows the whiskey to soak into the wood and remove the flavors from the charred oak staves. However when the pressure inside the barrel is great enough, some of the atomized liquid will escape through the pores completely and create that wonderful smelling aroma in the warehouse known as the “Angel’s Share”. Alcohol evaporates at 172 degrees and water at 212 degrees Fahrenheit.  Where temperature rises enough that both water and alcohol evaporate then water being the smaller molecule escapes from the wood at a greater rate and the proof of the liquid left behind will increase. In cooler sections of the warehouse it is mostly the alcohol that evaporates and thus more escapes through the wood, lowering the proof of the whiskey left behind.

As this process is happening, a certain amount of air is entering the barrel allowing for oxidation to take place, breaking down some flavors to create other flavors. Lincoln Henderson once said to me that you get a lot of flavor from the wood in the first six weeks of aging and you spend the next four years trying to get rid of them. He was talking about the oxidation process. This is the one process that cannot be hurried. Only time works to create the flavors and to remove that “young whiskey” flavor.

The maturation process does involve many different factors and the barrel is only one of them. It is a very large part of the process, but still only a part. To understand it completely you must also look at warehouses, location of the water house and even the location of the barrel in the warehouse. The barrel is just a piece of the puzzle but an important piece.

Photos Courtesy of Maggie Kimberl