Final answer:
Off-odors in wine are primarily caused by sulfur compounds, such as hydrogen sulfide, which can result from winemaking processes or spoilage. Alcohol and tannins contribute to flavor but not off-odors, while acidity in the form of acetic acid can cause vinegary scents when wine oxidizes.
Step-by-step explanation:
The primary cause of off-odors in wine is due to the presence of sulfur compounds. Hydrogen sulfide (H₂S), which gives a rotten egg smell, and other sulfur-containing compounds like mercaptans and thiols can produce strong, objectionable odors. These compounds may be present as a result of the winemaking process or due to spoilage.
While alcohol and tannins contribute to the aroma and taste of wine, they are not typically responsible for off-odors. However, acidity, or more specifically, acetic acid, can contribute to an off-odor when wine becomes oxidized, leading to an unpleasant vinegary scent. The oxidation of ethanol, especially in an opened bottle of wine, can produce acetic acid, giving wine a sour taste and vinegary smell.
Other off-odors can be a result of esterification, which creates esters with fruity smells when wine is either well-made or spoiled. Preservatives such as sulfur dioxide are used in winemaking to prevent oxidation and microbial growth, but in some individuals, it may cause asthmatic reactions.