How long does vermouth keep after opening?

Vermouth consists of at least 75 percent wine, and wine reacts with oxygen as soon as the bottle is open.

Spirits start at 15 percent alcohol by volume and in practice are often around 40; because of that high content, the contents change much more slowly after opening. At 14.5 to 22 percent by volume, vermouth is closer to fortified wine than to spirits. A bottle standing next to the gin on the shelf therefore does not last just as long.

Put the bottle in the fridge closed, upright and out of the light.

Cold slows the reaction with oxygen and holds the herbal aromas longer. The emptier the bottle, the more air it contains and the faster it goes; decanting into a smaller bottle therefore helps. A pourer or a cap that does not close properly works against it. At room temperature the flavour fades noticeably faster.

The spicy, bitter side flattens out and an apple-like or nutty aroma takes its place.

That smell belongs to acetaldehyde, the substance that also characterises oxidised wine; the Australian Wine Research Institute gives a threshold of 100 to 125 milligrams per litre. White vermouth becomes darker in colour and red vermouth duller. In a cocktail you taste the difference immediately.

Usually not, because drinks with 10 percent by volume alcohol or more are exempt from the best-before date.

At 14.5 to 22 percent alcohol by volume, vermouth is well above that limit. What you will find on the bottle is a lot number. Alcohol-free vermouth is different: it falls under the ordinary rules and does carry a best-before date. Such a date applies to the unopened packaging.

Count on a few weeks in the fridge for an opened bottle of vermouth, and longer the fuller the bottle stays.

There is no legal time limit; what matters is how much air gets in and how cold the bottle is kept. If you use vermouth for the occasional cocktail each month, a small bottle is more practical than a litre bottle. If in doubt, smell it first: a loss of spiciness is the first sign.