On the most recent episode of “The Cocktail Spirit” on the Small Screen Network, mixology expert and historian Robert Hess explains how an old vermouth will negatively affect the quality of a good cocktail.
Vermouth is a wine. And like any wine, it will oxidize over time, which will impact its flavor. Vermouth is what is known as a fortified/aromatized wine (Port and Sherry are simply fortified wines). Fortification simply means adding an alcohol to the wine, usually brandy. This originally was done to help preserve it, the higher alcohol content would make it last longer. …These botanicals also had a side-effect of giving the wine a longer shelf-life, not because it reduced oxidation, but because it would sort of mask the effects of oxidation. Even with fortification and aromatization vermouth is still a wine, and so its shelf life, once opened, is limited. Those dusty bottles of vermouth you might have on your shelf are not going to do anything good for any drink you use them in.