I'm sure I'm going to catch some flack for using the word "penetration" but as I understand it, a catchy title helps to capture the audience. In today's blog post I wanted to talk a bit about whisky barrel requirements by law and a bit about "The Red Layer." As borne out in the standards of identify shown below, bourbon must by law be stored in charred new oak containers.
§ 5.22 The standards of identity.
(b) Class 2; whisky
(1)(i) "Bourbon whisky, "rye whisky", "wheat whisky", "malt whisky", or "rye malt whisky" is whisky produced at not exceeding 160 proof from a fermented mash of not less than 51 percent corn, rye, wheat, malted barley, or malted rye grain, respectively, and stored at not more than 125 proof in charred new oak containers; and also includes mixtures of such whiskies of the same type.
So not only is it a legal requirement here in the states to use new containers for the storage of whiskey but the regulations further require that these new containers are charred and made of oak. Interestingly, there is a fair bit of nuance in that small phrasing that can lead to a wide variety of subtle nuances associated with how your bourbon tastes and smells.
Whiskey Barrel Break Down
When it comes to oak species used for whisky barrels (or "whiskey" as I prefer), white oak (Quercus alba) is the go-to for most distilleries. Yes, that was latin...I'm a recovering biology major/botanist so bear with me, it's like turrets, I just can't help it (this is where my wife would shake her head rather haphazardly...I worry about arthritis in her later years with so much head shaking...anyways, I digress). White oak makes sense to use as it is a fairly common oak species found extensively in bourbon country with a distribution that runs from Minnesota to Texas and hits every state east of there. Nothing in the standards of identity says that thou shalt use any specific species of oak however and so you will find a number of other oaks used at times in the making of bourbon with a worldwide speciation of nearly 500. (insert foreshadowing for future post here)....
To the left you can see the typical cross section of a bourbon barrel stave on the back side of being charred. In the first innermost layer, labeled as "1" is a layer of charcoal that has formed from the exposure to fire which is only a few millimeters thick. Despite it's small size, charcoal has enormous surface area --- 1 gram of charcoal has approximately 2,200 square feet of surface area!!!! Not only does this layer impart the beautiful brown color associated with the whiskey we love, it latches onto undesirable aromatic compounds like sulfur keeping them from making it into our dram.
The layer noted as "2" is the "red layer." As the barrel stave is heated not only does it char the inside of the barrel but the heat absorbs into the midsection of the wooden stave where due to exposure to said heat, the sugars are carmalized. As the whiskey barrel is aged, this layer also sees some penetration of alcohol due to temperature changes pushing the alcohol in the whiskey into the wood, this too adds to the color and flavor of the whiskey. The number "3" layer is simply the outer part of the wood and is incredibly boring and won't be discussed further than that.
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