Large group of volatile organic sulphur compounds (also alkanethiols, thioalcohols and, as an obsolete trivial term, mercaptans). They belong to the organic acids with a somewhat higher degree of acidity than, for example, alcohols and phenols. Thiols also undergo the same reactions as alcohols, but instead of an oxygen atom, a sulphur atom is exchanged. They are derivatives of hydrogen sulphide. They are formed during decomposition and rotting processes of organic material, for example during or after the fermentation of grape must from a chemical reaction with ethanol (drinking alcohol) or acetaldehyde. This process is favoured, among other things, by yeast sediment storage, which is common especially with white wines.
Thiols are found as flavouring substances mainly in milk, cheese, onions and garlic. They have an intense, leek- and herb-like to putrid odour that is often disgusting (several thioalcohols are also found in the glandular secretions of skunks). This is why they are added to industrial natural gas as an odour warning. The type ethanthiol (ethyl mercaptan) causes the dreaded wine defect boekser (sulphur boeker). Certain thiols, however, are responsible for the positive cassis, lime and grapefruit aromas in wine that are typical of bouquet varieties. And benzylthiol (toluolthiol, benzyl mercaptan) is the cause of the equally positive flint aroma, which is counted among the mineral tones. See also under wine faults.