What's Really Happening Inside a Whiskey Barrel?

When visitors come on tours at our distillery, less than half know that whiskey is brown because of the barrel it aged in. Actually, any distilled spirit—whether it’s vodka, gin, whiskey, brandy, or rum— comes off of the still clear. When spirits are aged in barrels, though, they pick up color from the barrel. But, barrels don't just contribute color that barrels to the wine and spirits that age in them. Barrels are integral to the flavor of spirits and wine in several different ways.

Whether you’re one of the people who knew why whiskey is brown or one of those who didn’t, I’m now here to tell you about as much as you could possibly want to know about the beautiful marriage of alcohol and wood, without getting intensively into organic chemistry.

Very close to 100% of the barrels used for aging alcohol are oak. This was originally a happy accident that happened way, way in the past. At least 2,000 years ago, people were making oak barrels, which came into popularity as a vessel for transporting wine during the Roman Empire. Oak was, and is, a remarkable wood in that it’s both water tight and slightly porous. Because it was water tight, wine didn’t leak out, and because it was porous it allowed tiny amounts of oxygen to travel through it. People discovered this aeration improved the wine that was in the barrel, making it softer and smoother. Similarly, as spirits began to appear in the 1400’s and 1500’s, they were stored in barrels out of necessity—glass bottles didn’t really appear on the scene until the 1600’s—and drinkers found spirits that had been shipped in oak barrels tasted better than, well, the moonshine that comes straight from the still.

So, what’s happening in the barrel?

Some of the structure of oak comes from a type of compound called lignins. These are an important component in the cell walls of all wood cells, so they’re not at all unique to oak. However, what’s special about the lignins in oak is they break down into flavor molecules that absorb into the alcohol inside the barrel. One of the most notable of these compounds is vanillin, which (no surprise) gives a vanilla flavor and smell. In fact, the amount of vanillin that comes from American oak is so high, oak lignin is sometimes used to make imitation vanilla extract. Other flavor compounds that come from oak lignins—like eugenol, furfural, and lactones—give wine or spirits fruity, spicy, nutty, or buttery flavors. These flavors come out most strongly if the aging barrel is new, that is, it hasn’t already been used for aging another batch of wine of spirits. If the barrel has been previously held something else—like sherry, or port, or bourbon—it will impart a little of those flavors into the barrel’s next occupant.

Now, as I previously mentioned, oak is slightly porous. Because of this, it allows for a process called micro-oxygenation. Basically, small amounts of oxygen travel in and out of the barrel. The tiny, gradual amounts of oxygen mean that the wine or spirit doesn’t oxidize, but it does catalyzes a variety of chemical reactions that can only happen when there is oxygen present. Molecules swap atoms and functional groups, and make new flavor compounds, particularly a group of compounds called esters, that taste better and more complex (for example, fruity, creamy, floral) than the molecules that were originally present. In particular, these are the hallmarks of a fine Scotch or cognac.

A final thing to think about is some wines and spirits are aged in American oak, while some are aged in French or European oak. European oak is denser, and it has more tannins. Yes, oak has its own tannins, separate from the tannins that come from the grape skins in wine making. These oak tannins can give some astringency, but also create active sites for reactions that create more delicate, complex scents and flavors. The denser structure of the European oak also means the micro-oxygenation process is slower. Yes, aging takes longer, but, again, in the end this slow-process can generate some of the incredible balance and complexity people consider integral to great wine and spirits.

Opposite of European oak, American oak is bolder and brasher (go figure!). It gives more intense and distinct flavors of vanilla, butterscotch, and even coconut. This is the signature flavor of most bourbon, as well as some of the butch American styles of Cabernet Sauvignon or Chardonnay.

See: The barrel’s a beautiful marriage of alcohol and wood. It’s something everyone, whether or not you knew beforehand why whiskey is brown, can agree on.

Fiveandspice, a.k.a. Emily Vikre, is a writer, self-described "food policy wonk," and co-founder of Vikre Distillery. She lives in Duluth, Minnesota. You can read more of her writing here.

Do you have any questions about barrel-aging alcohol? Let us know in the comments below!