Cork taint is usually the explanation for why a bottled wine goes bad, aside from improper storage or significant age. The cork may appear moldy or wine may have permeated through the cork. Alternately the wine itself may taste moldy, or may have an unpleasant smell. The smell of the cork or wine with cork taint has numerous colorful comparisons like wet dog, or moldy newspaper.
About 1-15% of bottled wines will show evidence of cork taint. This is why restaurants that serve wine open the bottle at the table. The patron can inspect the cork, and the wine can be smelled and tasted for evidence of cork taint. Upon finding cork taint, it is perfectly acceptable to send back a wine.
While some cork taint is the result of a contaminated cork, some wine spoilage may be due to storage in oak barrels, which can also grow fungi. The fungi is harmless to drink, but does render a wine tasteless or unpleasant.
Cork taint occurs when the cork or the wine becomes contaminated with a chemical called 2,4,6-Trichloroanisole. This chemical may develop if fungi come into contact with environmental pollutants called chlorphenols. Chlorphenols are often present in pesticides and may be found in cork trees or oak trees that are later turned into barrels. They also may be used as a wood preservative. Oak barrels with this wood preservative are much more likely to exhibit cork taint.
Ironically, perhaps, cork taint may also occur when people attempt to make wine sanitary and safe to drink. Many companies used to bleach corks with chlorine, which can produce chlorphenols. Since this cause of cork taint has been well established, other bleaching methods for corks are now preferred. Most companies that manufacture corks use hydrogen peroxide, or other peroxides as a safer alternative.
Since the cork is not always involved in cork taint, wine with synthetic corks may still suffer from cork taint. Wines with screw caps, which are often pooh-poohed by wine traditionalists is actually subject to a different kind of taint. The cap can let off odors, which taints the smell and taste of the wine.
Other chemicals may also be responsible for cork taint but occur less frequently. Most of these chemical compounds produce unpleasant smells, quite different from cork taint odor. An unusual and unpalatable odor may not be “traditional” cork taint, but is still a good reason to send back a wine, especially when one is paying a high mark-up for the wine in a restaurant setting.