Ask Arok: Your Questions, My Answers... A Question About Barrel Proof

A few days ago, I got an email from Tom asking about barrel proof.

Hi Arok......
I need a little clarification. I just received a bottle of "Booker’s” in a nice box, as a gift. The label says single barrel 127 proof. It aroused my curiosity. Then I read that Garrison Brothers have a new release "Cowboy Bourbon" @ 134 proof (not for the faint of heart)....! I thought to be "Bourbon.” whiskey had comply with certain criteria one of which is that it couldn't be barreled at higher than 125 proof....... what’s the story here? 
Thanks for your help.

Tom asks a great question. To answer it we need to dig a little into the science behind aging. While it is true that bourbon can’t be barreled at higher than 125 proof, that is only true for the liquid going into the barrel. What happens after that is up to nature. 

Let’s take a look at what happens during aging. There are three basic things going on: extraction of flavor, chemical reactions, and then the interaction with the surrounding environment (which is where Tom's question comes in). So to look at each in turn: 

Extraction of flavor: Alcohol is a solvent and like all solvents, it loves to dissolve things. In the case of bourbon that is all the caramel and vanilla flavors that burning a piece of oak allows the alcohol access to. This happens pretty quickly in the grand scheme of things. It’s why you can get something that tastes like “bourbon” at six months or less in a small barrel. It doesn’t taste exactly like the large, mainstream bourbons, but it has a lot of the same characteristics. At this point it is basically a wood extract, kind of like the vanilla extract you’d find in your kitchen cupboards. Only in this case, it’s wood flavors dissolved in alcohol not vanilla bean.

Chemical Reactions: This is a function of time. Certain things happen to that extract as time passes while it is in the presence of oxygen. Molecules break down and recombine into tasty combinations that give a well matured whiskey a lot of the tasty flavors we associate with it. How does that oxygen get into the barrel? A properly constructed barrel is very good at keeping liquid inside, but luckily isn’t so good at keeping air inside (or outside). 

Which brings us to the answer to Tom's question: interaction with the environment. In general terms, if you were to look at both the ethanol molecule and the water molecule, you would notice something. The ethanol molecule is much larger. As such, the water molecule can more easily pass through the grain of the oak being used in the barrel. This means that two things can happen: 

  1. In a hot environment, such as the upper floors of a rick house in Kentucky, water and ethanol evaporate. The water passes through the wood, but the alcohol stays behind. As such the alcohol per volume of the liquid goes up as the volume of liquid goes down due to the water escaping. This is why a barrel proof bourbon such as Bookers or Stag can be higher in proof than the liquid that originally went into the barrel. 
  2. In a cool moist environment, such as the bottom floor of a rick house with a dirt floor, just the opposite happens. It’s cool enough that there isn’t as much evaporation happening, but the air going in and out of the barrel, being moist, is bringing water into the barrel. And so the total alcohol by volume goes down as the volume of water goes up. This is why the new Wild Turkey 17 year old can be barrel proof at well below the proof that it went into the barrel.

Of course, this is just a simplified version of the science behind aging. People with degrees in more than art probably can give all the charts and more specific reasons behind these processes, but this is how it was explained to me. 

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