How long does whisky keep after opening?

Whisky does not spoil in the sense of becoming unsafe, because at 40 percent alcohol or more no bacterium or mould survives.

That is also why spirits do not need to carry a best-before date. What does happen is loss of flavour. Oxygen in the bottle reacts slowly with the aroma compounds, and volatile substances escape past the stopper. The whisky does not go bad, it becomes flatter: less aroma, a shorter finish.

An opened bottle noticeably declines as soon as there is more air than whisky in it, and that tipping point lies around halfway.

Work it out for a bottle of 70 centilitres. Right after opening, there is hardly any air above the drink. Half full, there are 35 centilitres of air to 35 centilitres of whisky. At a quarter full, that is about 52 centilitres of air to 18 centilitres of whisky, so almost three times as much air as drink. Every time you open the bottle, that air is refreshed.

Pour the whisky into a smaller glass bottle with a tight-fitting cap as soon as about a third remains.

Fill that small bottle to just below the neck, so that little air remains. Use glass and not plastic, and put a label on it with name and date. An alternative is a spray can of inert gas, which displaces the air above the drink.

Store an unopened bottle upright in a cool, dry and dark place, outside the fridge and out of direct sunlight.

Standing upright matters with spirits, because alcohol of 40 percent and higher attacks a natural cork if it is permanently in contact with it. Sunlight makes the label fade and warms the bottle, so that the pressure in the bottle fluctuates. For distilled drinks above 25 percent the Voedingscentrum gives the same advice: cool, dry, dark and not in the fridge.

Whisky does not mature any further in glass, because the age on the label is the time in cask and that stops at bottling.

The Scotch Whisky Regulations 2009 prescribe that an age statement is expressed in years and refers to the youngest whisky in the bottle. A bottle that has stood in the cupboard for twenty years has therefore not become twenty years older. What does happen in those twenty years depends on how much air is in the bottle and how warm the cupboard gets.