Group of plant substances that occur in many fruits. They are also present in varying concentrations in many grape varieties. The content depends very much on environmental conditions; the highest concentrations are found in warm growing regions with intensive solar radiation. The so-called C13-norisoprenoids are formed from chemical processes such as the enzymatic oxidation of carotenoids of grape berries. Here they are initially present in glycosylated (sugar-bound) and odourless form and only become odour-active through acid-catalytic splitting off of the sugar residue and other reactions. However, they are also formed later during fermentation and during bottle ageing or ageing of the wine. Due to low odour and perception thresholds, they make a decisive contribution to the varietal aroma of the wine. Norisoprenoid derivatives are damascenone (β-damascenone) and the TDN (trimethyl dihydronaphtaline) which is responsible for the petroleum clay. See also under flavouring substances.