When wine is exposed to air it begins to oxidize due to the chemical reaction that occurs between oxygen and alcohol. Depending on the situation, oxidation is either beneficial or undesirable. A less-than- airtight cork, for example, can turn an expensive bottle of wine into an expensive bottle of vinegar. Wine that has a stale or caramelized taste and a dark golden color also shows signs of oxidation. Many barrel-aged wines, however, actually benefit from a very slow process of gentle oxidation, which enhances their flavor. See also aeration.