Large group of volatile organic sulphur compounds (also alkanthiols, thioalcohols and, as an obsolete trivial term, mercaptans). They belong to the group of organic acids with slightly higher acidity than alcohols and phenols, for example. Thiols also undergo the same reactions as alcohol, but instead of an oxygen atom a sulphur atom is exchanged. They are descendants of hydrogen sulphide. They are formed during decomposition and rotting processes of organic material, for example during or after fermentation of grape must from a chemical reaction with ethanol (potable alcohol) or acetaldehyde. This process is favoured, among other things, by the storage of yeast sediment, which is common especially for white wines.
Thiols are found as aromatic substances, especially in milk, cheese, onions and garlic. They have an intense, leeky and herbaceous to putrid odour, which is often nauseating (several thioalcohols are also found in the glandular secretion of the skunk). Therefore they are added to industrial natural gas as an odour warning. The species ethanthiol (ethyl mercaptan) causes the dreaded wine defect Böckser (sulphur Böckser). However, certain thiols are responsible for the positive cassis, lime and grapefruit aromas in the wine, which are typical of bouquet varieties. And benzyl thiol (toluene thiol, benzyl mercaptan) is the cause for the equally positive flint aroma, which is counted among the mineral tones. See also under wine faults.