If you have a little wine left in the bottle at the end of the evening, you don't have to pour it out. With just a little care, the wine will retain a reasonable level of quality for about three days in the refrigerator.
Once you open a bottle of wine, air gets into the bottle and the oxygen in it starts to react with the wine. Initially, this action, called oxidation, improves the flavor of the wine, particularly of red wines. Consequently, wine experts like Ed McCarthy and Mary Ewing-Mulligan, authors of Wine for Dummies, recommend that you let your red wine "breathe" by opening the bottle an hour or so before you plan to drink it. In the case of very young and very old red wines, you should pour the wine into a decanter or carafe for extra oxygen exposure.
The exposure to air that improves the taste of wine, however, eventually will cause it to spoil. This is partly because over time, oxidation will degrade the taste of the wine, but also because airborne bacteria can get into the wine once you open the bottle.
Storing Leftover Wine
If you only drink a glass or two of wine at a sitting, you are going to have some left in the bottle. Cellar Notes, an online resource for wine lovers, recommends that you recork opened bottles of wine and store them in the refrigerator. This will slow oxidation and inhibit the growth of any microorganisms that may have gotten into the wine.
Recorking can be as simple as putting the original cork back into the bottle, but you can also get fancier by using specially designed stoppers or systems that fill the neck of the bottle with an inert gas. McCarthy and Ewing-Mulligan suggest that one inexpensive way to protect leftover wine is to pour it into a smaller bottle and cork that before you put it into the refrigerator. This will reduce the amount of oxygen that the wine is exposed to.
How Long Does It Last?
Cellar Notes advises that even stored in the refrigerator, leftover wine will continue to degrade; after three days, you will be able to taste a distinct decline in its quality. At that point, you should probably either use it or throw it away.