How long does vieux keep after opening?

Vieux does not spoil like fresh goods: drinks with an alcohol content of 10 percent by volume or more are exempt from the best-before date under Regulation (EU) 1169/2011.

That exemption is in Article 24 and Annex X of the labelling regulation. Vieux is well above it: in the now-lapsed 2009 Regulation on the Naming of Spirits and Weak Spirits, vieux had a minimum of 35 percent by volume. At that strength, bacteria and yeasts get no foothold. What does change is the flavour.

In an opened bottle, ethanol and volatile aroma compounds evaporate, and what remains reacts with the oxygen above the drink.

The Oxford Companion to Spirits and Cocktails summarises the effect briefly: colours become darker, aromas flatten and flavours change. In a drink like vieux, in which aroma and flavour compounds play a large role, that is noticed sooner than in a neutral drink. We are talking months, not days. A bottle that is empty within a few weeks hardly changes.

The less drink there is in the bottle, the more oxygen above it and the faster aroma and flavour shift.

A bottle that is still three-quarters full has a quarter air above the liquid. With a bottle that is still a quarter full, it is three-quarters, so three times as much. That explains why it is precisely the last bit at the bottom of the bottle that changes fastest. Pouring it into a smaller, well-sealing bottle reduces that space and slows both evaporation and oxidation.

Store the bottle upright, dark and cool, with the closure tightly shut and not next to a heat source.

Upright keeps the drink away from the closure, which helps with both a screw cap and cork. Dark and cool slow down the reactions in the bottle; a windowsill or the spot above the stove is the worst place in the house. Put the cap back on straight after pouring, because most evaporation happens in a bottle left open on the counter.

An unopened bottle hardly changes, because maturation stops as soon as the drink is bottled and little oxygen can get in.

Spirits mature on wood, not in glass. As long as the closure is intact and the bottle stands upright, dark and not too warm, the contents stay the same for years. Over a long series of years some moisture can evaporate past a cork; that can be seen from the level in the bottle. For that reason a best-before date is missing on the label.