How Does Oak Affect Flavor?

What does oak aging do to a whiskey? It makes it better. The end.

Okay, okay. I’ll say more. 

Firstly, not every spirit is improved with oak aging. Vodka, gin, brandy and agave spirits like tequila and mezcal all have styles that are oak aged and styles that aren’t. A dominant philosophy in Oaxaca is that far from making mezcal taste better, oak will make it taste worse. They see it as an adulterating force, especially if they want to showcase the profile of a rare or wild-harvested agave, so most top-shelf mezcal has never touched oak. 

So, oak isn't good for its own sake. But it is great for whisky/ey, rum, cognac, tequila, and other things. The reason for this is that when you put new make spirit into a barrel and let it sit for a while, you get lots of benefits. In the spirits industry, we categorize those benefits under six processes that are more or less coextensive during barrel aging: (1) extraction, (2) evaporation, (3) oxidation, (4) concentration, (5) filtration and (6) coloration.

Extraction means that the spirit is pulling congeners (flavor/aroma compounds) out of the wood as it soaks in the barrel. Alcohol is a solvent, which means that it helps other things dissolve into the spirit. Those other things are quite tasty, as we have found. American white oak (Quercus Alba), for example, has a high concentration of vanillin in it. As you can imagine, it tastes like vanilla. When oak barrels are toasted on the inside, the naturally present sugars are caramelized and degraded, making it easier for them to be absorbed by the spirit. That's why bourbon, which is required by law to be aged for at least two years in new, charred white oak barrels, has as its dominant flavors vanilla and caramel. You have got the oak to thank for that.

Evaporation happens because the oak is porous and allows airflow through the barrel. Over time, alcohol escapes in the “Angel’s Share,” but depending on conditions outside the barrel, the water might evaporate more quickly than the alcohol. That’s why in humid environments, the Angel’s Share is extremely high and the spirit loses alcohol by volume (ABV) over a short time. But in places like Kentucky, the spirit will generally get a higher ABV as years pass. 

When the spirit leaves the barrel through evaporation, it is replaced by air. The oxygen in our air then dissolves and transforms the spirit through oxidation. But oxidation doesn’t just happen directly to the spirit. The wood also oxidizes as it is exposed to sunlight and heat. These further degrade the barrel and result in more congeners for the spirit to extract. (That’s why the barrels chosen for Blanton’s Bourbon are called honey barrels, because they are located by the windows of the warehouse and develop a light brown exterior due to oxidation.)

As water and alcohol evaporate, the net volume of the spirit decreases in the barrel, but most of the congeners don’t leave. As a result, the flavors are concentrated. When you sip a whisky/ey that has been aging for 25 years, for example, you can immediately notice that it has twice the flavor and sweetness of a much younger dram. This is true in a real sense, as the liquid in your glass has a higher percentage of flavor compounds due to the water and alcohol being gradually removed by nature. 

I mentioned before that barrels are often either toasted or charred. Actually, most often they are both toasted and charred. The toasting penetrates deep into the wood and is what caramelizes it. As caramelization degrades the molecules in the wood, the amount of available congeners for extraction is associated with whether the oak is toasted. 

But charring does something else entirely, which is the fifth process: filtration. Activated char has long been recognized for its ability to filter out impurities. When oak is exposed to high heat, the surface of the barrel’s interior turns into a layer of activated charcoal. So as the spirit ages in the barrel, it is gradually having its bitter or astringent parts filtered out. Crazy right? Now you know where “smoothness” comes from. 

There are four levels of charring. Level 1 means that the barrel is exposed to high heat for 15 seconds. Level 2 is a 30-second burn. Level 3 is burned for 45 seconds, and level 4 (referred to as alligator char because it looks like their shiny, scaly exteriors) is burned for 55 seconds. The level 4 is the highest degree of char, and the majority of barrels are not put under such high heat for so long.

The last process is coloration. If you have read my article explaining how to taste whiskey, you will know that in most countries the addition of artificial caramel coloring is permitted. But when the spirit is naturally colored, that color comes from the barrel aging process. The most important factor in how much color the spirit will extract is tannin. Yes, the same “tannin” that wine lovers talk about all of the time. It’s basically a family of acids that are present in all plants. So wine can get tannins from the grape skin or stems, but a good deal of it comes from the oak too. And that’s definitely the case with spirits too. 

Some species of oak have more tannin than others. Each species also varies in the amount of flavor compounds like eugenol, syringol, vanillin or lactone. You will also find the tightness of grain in the oak to differ, resulting in more or less extraction. All of these things contribute to what kind of barrel you might want your spirit to age in. 

The main types of oak used for barrel aging are Quercus robur (European oak) and Quercus alba (American white oak). There are others. For example, Quercus petraea is a type of European oak that is common in the production of wine, although it is used for a few spirits. But Q. robur and Q. alba are the big two. The grain is quite different between these. Q. robur is porous and has a rough grain to it, so it imparts more tannin (color) and spicy, fruity flavors. Q. alba has a smooth grain to it and makes the spirit taste sweet and creamy, with more vanilla.

Some spirit brands will try to approximate the benefits of oak aging by putting little, toasted oak staves into the spirit for a brief time. Doing so will give you extraction and coloration, but not evaporation, oxidation, concentration or filtration. Consider this when brands advertise products to you that celebrate “new technology” in “aging” a spirit. There is a reason we have been storing alcohol the same way for millennia. 

(And I mean millennia. Romans adopted oak aging from Germanic tribes after the Gallic Wars). 

There are also some spirits that fit in categories which generally improve with oak aging, but should be considered best when bottled early. If you are a fan of Scotch whisky, for example, you might think that it doesn’t reach its “peak” until after 15 to 20 years or longer. But some Scotch is peated, meaning that the barley is smoked over burning peat (decayed plant matter) for hours. If the smokiness in peated whisky is desirable to you, the whisky you might prefer will be young. Perhaps only 5 years old. This is because as the whisky ages, those flavors that come from the grains are gradually replaced by flavors imparted by the oak. Think of it as a sliding scale. If you take a peated Scotch and let it age for about 25 years, most of the peatiness will be gone. 

The last thing I’ll say is that oak is fantastic, and many of our most beloved spirits would not exist without it. I’ll encourage you to read more of my articles on those spirits, like the posts on brandy (coming soon) or bourbon. Also, if you haven’t yet read it, consider my article about how whiskey was invented. My purpose for writing these is to give you free insight into the world of booze. So, I’ve made information available to you that I had to spend quite a lot of resources and time acquiring. 

On behalf of Distillare, I hope it benefits you as a consumer of oaky ambrosia.

Art generated by AI