Just like the issue of whisky with vs. without ice, this question is one that is continuously popping up among whisky lovers. Much has been written about that topic, but still there is much uncertainty and confusion about it. Let me try to shed some light on this.
The Old Commonplace: It doesn’t!
One of the things any aspiring whisky afficionado learns first, is that whisky does not change after it has been bottled, under the condition that it has been properly stored. A 10yo bottled in 1970 will still be a 10yo today. Unlike wine, which does change after bottling quite a bit, the high alcohol concentration forbids anything to happen inside the liquid. So they say.
Or does It?
But nevertheless many have reported that they had found differences in taste between to bottles of the same whisky opened within a period of many years. Can this be explained only by psychology?
And then there is also the strange phenomenon that many ancient bottles containing whisky bottled at a young age seem to get much better marks than their contemporary cousins. Is this just because “they don’t make it like that anymore”? So much research has been done around whisky making that it is hard to believe that just using Golden Promise barley instead of the modern varieties and heating the stills with a direct fire could powderize all progress in whisky making in the last fifty years.
So It Does Then?
Meanwhile, more and more whisky lovers recognize the fact that their favourite tipple does indeed change over time, even in a sealed bottle. The high alcohol content may work against it, but if you look at it with a bit of common sense, it should be clear that something just must happen in the bottle after the cork has been sealed.
And What Exactly Does Happen?
The key in understanding whisky bottle ageing lies in understanding the change that red wine undergoes during bottle ageing. The two key elements here are air and tannins.
1. Just underneath the cork, there is always a bit of air in the bottle that can react with the wine. And furthermore, no cork can seal a bottle tightly enough to disallow any exchange of air with the outside. The exchange may be only minimal but it is there.
The interaction of wine with air is commonly called oxidation. This is a bit of a misconception because wine is not a homogenuous material like iron that can rust. It is a very complex mixture of many chemicals, some of which actually do oxidise and some of which are inert to the influence of oxygen.
2. The tannins contained in the wine – resulting from the stems and pips of the grapes as well as from the cask wood – slowly react with other substances to form new aromatic compounds.
In bottled whisky, exactly the same things happen. The process is much slower though because the alcohol forms a kind of coating around the reactive molecules that first has to be overcome by the reactants. And then, whisky has less tannins than red wine because the spirit itself does not contain any. All tannins present in whisky have been leeched out of the cask wood. This might also explain why especially old bourbon whiskey seem to beneftit drastically from bottle ageing. I remember one of the Malt Maniacs e-pistles where there is an amazing report about a tasting of some very old American whiskeys. Due to their maturation in fresh oak casks there should be more tannins present in those than in your average scotch.
So in my opionion it would be foolish to believe that all of the hundreds of chemical substances that are present in whisky would become totally inert right after bottling.
Time for an Experiment
The only way to really pin down changes in a bottle of whisky over time can be a long-term experiment. 20 bottles of the same whisky batch have to be stored together in a controlled environment. Every year, a bottle is opened and analysed by gas chromatography. Any volunteers?
Changing vs. Ageing
There are people who admit changes taking place in bottled whisky, but they refuse to call it ageing or maturation because these terms were legally restricted to the period the whisky rests casked in the warehouse. Let’s see what the 1990 Scotch Whisky Order has to say about that (Section 3 c):
“Scotch whisky” [...] means whisky which has been matured in an excise warehouse in Scotland in oak casks of a capacity not exceeding 700 litres, the period of that maturation being not less than 3 years.
This is the only mention of maturation in the order, the 1988 Scotch Whisky Act say the same in a slightly diferent wording. And the term “ageing” is not mentioned in either of the documents, so I think it is fair to indeed use the term “bottle ageing” for the things that happen in a whisky bottle.