Preserving Wine in an Open Bottle
Once any wine bottle is opened, the wine starts undergoing a process of combining with oxygen that will change the taste of the wine. Initially, wine’s taste may be improved by exposure to the air when oxygen in it combines with the flavoring compounds called phenols.
This improvement may only last from 2-6 hours. After that, wine taste slowly degrades as it becomes oxidized. The warmer the temperature at which wine is kept, the faster this oxidation occurs.
At 24 hours of room temperature, many open bottles of wine will begin to taste oxidized like a brown apple, and at 48 hours, almost all open, room temperature wines will have that nutty, sherry-like aroma of oxidation if something else is not done to retard oxygenation.
This is important because it means that bottles of wines opened for by-the-glass pours, are more likely to be spoiled (oxidized), especially if they have been open more than one day and not cooled in between when they are still open.
Thus guests are more likely to send back glasses of wine poured from previously open bottles rather than a bottle of wine just opened at their table.