You've Been Storing Vermouth All Wrong
Vermouth is one of those ingredients you buy to stock your bar with and tend to only use on occasion, like when making martinis or Negronis. But apart from those classic cocktails, vermouth tends to go neglected, simply taking up space at the back of your bar. Fortunately, vermouth is really making a comeback in many drinks, and you might find yourself reaching for it more often. However, if you plan to drink vermouth cocktails more frequently, you should know how to properly store it.
Vermouth is actually a fortified wine, which means it has a higher alcohol by volume level due to alcohol added to the wine (via The Crafty Cask). Because it is a wine product rather than a distilled liquor or spirit, vermouth tends to oxidize more easily. So as soon as you open the bottle, the air that runs into it will begin to affect the vermouth inside, Bon Appetit explains. That's why vermouth, no matter which of the three types it is — sweet red, sweet white, or dry — must be properly stored to extend its shelf life and preserve its flavor.
You should always put vermouth in the fridge
When room temperature air begins to meld with unused vermouth inside its bottle, it causes the fortified wine to spoil more quickly. That means your cocktails won't be nearly as good. According to The Crafty Cask, vermouth should be used within three months of being opened. But no matter how quickly you're going through it, you definitely need to put your vermouth in the fridge.
Bon Appetit explains that vermouth will have a distinctly off flavor if it has spoiled. The publication also recommends taking a sip of the fortified wine by itself to see if it is still good before you waste other ingredients mixing it into a cocktail. If the vermouth has gone bad, its flavor should have changed to taste "flat and dull." If it is really too far gone, then you will be able to smell that it has turned too. So avoid taking chances by always placing an open bottle of vermouth in the fridge to keep it good for as long as possible. Cheers!