The question: Does aging end after cognac is bottled? I have a bottle of Rémy XO that was bought around 1990. I wonder if it has improved or not.
The answer: Unequivocal no, it hasn't improved – or deteriorated …
Wine's reputation for delivering dividends with time in bottle has unfairly spilled over into the spirits world. Once sealed in glass, cognac – or any other high-alcohol spirit – is essentially embalmed. Time itself does not cause the flavour to evolve. Only in contact with wood does a spirit's character change. The fluid draws out not only colour from the cask's fibres (that's why brown spirits are brown) but also nuances that can suggest vanilla, caramel, spice, even fruit. The specific flavours depend on the type of wood as well as the temperature and humidity in the distiller's cellars.
Incidentally, after many years, a spirit will drop in strength and volume, too, as the volatile alcohol and, to a lesser degree, water evaporate through the vessel's pores. That contact with air sometimes also adds a nuance of its own, which some people compare with the nuttiness of sherry.
Wood extract is the main ingredient that distinguishes cognac from, say, clear vodka, though the former is distilled from wine and the latter from grain or potatoes, so there's a grape-based character to cognac as well.
Wine is more of a living thing, highly sensitive to oxygen and often containing minute solids from grape skins that contribute to flavour evolution.
Age designations for cognac, such as VS and XO, refer to the time spent in wood. A VS spirit is blended from distillates that have rested in cask for at least two years, while XO designates a minimum of six years.
There's essentially no difference between your excellent Rémy purchased in 1990 and one you can buy today, except perhaps that over 21 years your spirit will have evaporated ever so slightly through the porous cork, as you may notice from the fill level. No harm done, though. On the plus side, you probably got that 1990 for a song relative to today's prices because of inflation. To replace that Rémy XO today, you'd have to pay about $225.
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