Exposure to extreme heat, severe temperature fluctuations, as well as damage or deterioration of the cork, can lead to oxidation of the wine. Such a wine will be thin and lose aroma and flavor. To check for oxidation, raise your glass of wine to a white background. The color should be free from browning, a sure sign of oxidation. Some experts predict that as much as 25% of wine sold in the U.S. has been damaged due to exposure to extreme heat.
One sign of possible wine damage is a raised cork. Such a condition is caused when the wine has gotten too warm, and the contents of the bottle expand, pushing upward on the cork. This allows oxygen to enter the bottle and may cause spoilage of the wine (oxidation).
When the cork is removed from a bottle of wine, it should be inspected to ensure that the bottom is moist with wine, but that this moisture has not run the length of the cork to the top. Such a condition indicates that the seal has been lost, and the wine would be subject to oxidation. (It is not necessary to smell the cork, as there is no useful purpose in doing so.)
Another sign of a wine problem is that the fill line has dropped. The typical wine level in the bottle is into the neck. A lower fill level indicates wine leakage or evaporation, which often results in oxidation of the wine. Very old wines may have a slight decrease in the fill level due to their age. Wines that are less than four years old with greater than one half inch of air space (ullage) between the cork and the liquid level of the bottle generally indicates a problem. Normally, about one eighth inch of fill space is created during the bottling process.