When leftover, uncorked champagne has been left out unrefrigerated overnight, bacteria begin to multiply and the bubbly begins to lose some of its bubble and buzz, but it is still safe to drink. The flavor and fizz begin to diminish after just a few hours, but it is still drinkable for several days.
Bacteria in Your Bubbly
While many types of bacteria – salmonella or e-coli, for example – can cause serious illness, the bacteria that helps produce wine is harmless. Beneficial bacteria such as lactic acid bacteria help ferment the wine, while fermenting fungi, such as acetobacter bacteria, increase the acidity that can eventually turn wine to vinegar.
Exposure to oxygen is the biggest culprit in wine spoilage. Wines with lower pH -- such as white wines, including champagne -- are more resistant to spoilage because of their higher acidity levels. However, once exposed to air, all wines begin to oxidize, which changes the drink's flavor and microbial concentrations. In a matter of days acetobacter bacteria begin to multiply, changing the sweet champagne to acidic vinegar. Warmer temperatures speed spoilage, so champagne left on the counter will turn sour more quickly than if it were stored in the refrigerator. Wines left out for longer periods may begin to develop a fungus layer called "mother of vinegar."
Forget the Fizz
The carbonation in champagne is due to the carbon dioxide produced during the second fermentation. Under pressure the carbon dioxide remains intact. However, once the cork is popped the pressure is released and the CO2 escapes rapidly. After just a few hours the champagne will go flat and lose its characteristic fizz.
Like a Virgin
Alcohol evaporates quickly. When left unopened and exposed to air, the alcohol in champagne quickly dissipates. The glass that left you giggly last night will hardly elicit a snicker the next day.
To safely store leftover champagne, discard the cork that came with the bottle and re-seal with a locking cork that keeps the CO2 from escaping. Pump devices can either remove the oxygen from the bottle or add a layer of inert gas that can prevent the CO2 from escaping. Refrigerate for up to three days.