Thiols are a large group of sulfur-containing organic compounds that contain an SH group (a sulfur atom bound to a hydrogen atom). Volatile thiols often emit a very unpleasant smell, for example aromas of sulfur, garlic, rotten eggs, struck match, cabbage or rubber. They arise in wines that are produced (too) reductive, or in old white wines. However, other thiols may also contribute to fruit aromas in the wine. For example, the thiols 3-mercaptohexanol (3MH), 3-mercaptohexyl acetate (3MHA) and 4-methyl-4-mercaptopentan-2-one (4MMP) give aromas of boxwood, guava, tropical fruit, passion fruit and grape fruit, and are especially important contributors to the varietal aroma of Sauvignon blanc.