How long does Scotch whisky last after you open the bottle?
In my house? About two days. (Kidding!) Scotch is very stable.
Unlike wine, Scotch (or any other whisky) does not evolve once bottled. Maturation of distilled spirits occurs only prior to bottling, when the liquid sits in cask mingling with and drawing components from wood. It similarly does not change much after you pull the cork or twist the cap. At 40-per-cent alcohol or higher, it's embalmed, and there's precious little in that bottle that can oxidize.
I once put the question to a famous English whisky writer with a huge collection of opened bottles and he told me whisky will last for a year or so before it begins to change, if it changes much at all. And when it does change, it does so at a glacial pace. It might become faintly more caramel-like in flavour and texture or more candied – sweeter, if you will – but that's just my subjective impression. Some people claim to like whisky better after it's been open for a few months or even years.
The alteration, if you can detect any, will depend on the volume of spirit left in the bottle. If it's much less than 50 per cent, it will change more rapidly because there's more air with which to react and more space for the alcohol and water to evaporate into. We're talking barely perceptible degrees here.
You can perform a simple test to accelerate the process and see what you think: Pour a tiny dram into a drinking glass and leave it, uncovered, on the counter for a day or two. Alcohol, which is more volatile than water, will evaporate more quickly, so the spirit should get slightly less strong, and the solid extract might become faintly stickier and more syrupy as both fluids evaporate. Eventually you'll be left with a sad, brown deposit on the inside of a dry, empty glass – that's the whisky component that is neither water nor alcohol.
This said, I have whiskies at home that have been open for a few years (a crime, I know) and I'm not too worried. They still taste fine, if not precisely as they did when I first cracked the seals.
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The Flavour Principle, by Lucy Waverman and Beppi Crosariol, was named best Canadian Food & Drinks Book in the 2014 Gourmand World Cookbook Awards. Published by HarperCollins.