The higher the proof, the more volatile the whisky is inside the bottle. It'll change over time. The first time you pop-open a whisky bottle it'll be tight on flavor, and less expressive on the nose. You'll need to leave it out in the glass longer to get it to show-up with its full flavors. The closer to empty the bottle gets, the more the flavors will either flatten out or exaggerate (depending on the whisky).
That's been my experience. Science is less certain on this.
When the industry does quality control, brilliant people in lab coats have proven whisky is a shelf-stable product. Whether it’s been bottled for a day, or for a hundred years, so long as it’s been sealed properly and kept out of the sunlight you’re going to be tasting whisky. This is in sharp contrast to wine, which continues fermenting inside the bottle. But the experience of many whisky enthusiasts is subtle whisky flavor does change in the bottle.
What Happens Inside a Bottle of Whisky?
This is an under-studied part of the whisky making process. As I said at the beginning, as far as the people in lab-coats are concerned, whisky remains identical from first drop to the last. According to many people's palates, whisky changes. A bottle that's three-quarters to four-fifths empty is likely seeing these affects faster than a freshly opened bottle.
Peated whiskies lose their smoky notes. Ryes get sweeter. Scotches flatten out and then start showing more acidic notes like lemon and lime. In some cases whisky starts losing its composure, and in other cases it simply flattens out losing its complexity.
It's not a surprise that oxygen reacts with the whisky. Oxidation plays a key role in whisky maturation. Oxidation changes the flavour of whisky. In the early stages of barrel maturation, the alcohol molecules are complex. We generally don't find these complex molecules favorable. Oxidation slowly breaks down the alcohol molecule, as well as the tannins from the oak. Oxidation is one of many steps of barrel maturation.
However, oxidation is slow acting, especially within a properly sealed bottle. Don Livermore told me his company (Wiser's) has done tests on partially filled bottles for up to five years, and the conclusion is there is no change in the whisky within the bottle. Oxidation is not the likely culprit.
But something is happening.
As Drew Mayville (Master Blender) from Buffalo Trace told me, for unfiltered higher-proof whiskies the vatting process is key in the whisky-making process. "I won't rush it," he told me. Part of the change in the vat is, again, whiskies interaction with the air. Vatting brings about other benefits, such as allowing the whisky from various barrels to properly blend.
Mary Reynier, formally of Bruichladdich ownership and now the owner of Waterford Distillery, likewise told me that vatting and the length of time the whisky remains in the vat is an important part of the process. He also borrowed a term from the wine industry: Bottle Shock. Mary Reynier believes that some whiskies don't show themselves fully right off the bottling line.
When you pour a glass of whisky, you can smell the alcohol, caramel, vanillas, and smoke (if it's peated) from an arm's length away. Whisky is volatile. When you pour your whisky, those molecules dispersed into the room. Chemists and physicists call this dissipation. It's a chemical process that can't be reversed. I recall, on a podcast that briefly touched on this topic, Davin de Kergommeaux was saying, "once the flavors are gone, they're gone." That's dissipation.
Don Livermore introduced me to the concept of dissipation during our talks on whisky and how it might change in the bottle. On The Whisky Topic Episode 67, Eric of WhiskyAnalysis.com had a similar theory. He suggested it wasn't the amount of time whisky is left in a mostly-empty bottle, but rather each time that whisky is poured the air inside the bottle is displaced affecting future pours. Dissipation is less-frequently talked about because oxidation is a more popular term, but the two meanings couldn't be any different.
As these volatile molecules leave the bottle, this changes the overall flavor of the whisky. At first, the harsher volatile molecules depart smoothing out the flavor and rounding out the whisky. Next, though, we start losing enjoyable notes causing the whisky to flatten out over time.
Drink Your Heels, Enjoy Them at Their Best
A little while ago I enjoyed a treat with a friend: Willet 23 Year Old Single Barrel at cask strength. It's rumored to have come from the same distillery as the original Pappy Van Winkle (Stitzel-Weller distillery). It tastes like Pappy with a bigger blast of flavor. My buddy had been saving the bottle, but even after just a year, it lost some of that character. This was a high-proof (69%+ ABV) monster, and the likelihood of losing key flavour notes is more likely.
Whisky heels are bottles near-empty. I used to keep them around for special occasions. These days, though, I'd rather taste the whisky at its best than have the last sip be at the whiskies weakest. For whiskies bottled at or just above 40% ABV, these changes are more subtle, but for higher-proof whiskies noticeable loss of flavor is bound to happen. When your bottle gets closer to empty, finish it up and move on to the next one.