When a bottle is uncorked, its contents are exposed to air causing them to oxidize. if you leave the open bottle out long enough, your once tasty beverage could end up smelling like sweaty socks or wet cardboard. Bacteria naturally present in grapes can turn either the sugars in the grape juice or the alcohol in wine into acetic acid giving it a vinegar taste (and eventually producing a wine vinegar). However, these bacteria’s need oxygen to grow. Wines fermentation process is usually oxygen-free because of the large amount of carbon-dioxide produced when yeast turn sugars into alcohol. Uncorking the bottle introduces oxygen to feed the bacteria and the flavor begins to change immediately. Not all changes are undesirable; Expensive red wines in particular are said to improve in the decanter and in the glass over a short period. Some white wines oxidize in the bottle over years, producing a rich rather sour quality. Still oxygen is the enemy of most wines and there are several devices on the market to put a halt to the process. One way is the vacuum pump that pumps air out of the bottle. A nitrogen sealant is another preservation method. Releasing nitrogen into the bottle displaces oxygen theoretically keeping it from tainting the flavor.