One of the characteristics certain rieslings have is what is commonly called “petrol”—a potent, distinctive aroma that some wine drinkers love, and others hate. Petrol is caused by trimethyldihydronaphthalene—TDN for short. Several research studies have found that TDN is up to six times more likely to occur in riesling than in other varieties. One of the leading factors responsible for the molecule’s formation is too much sun exposure on riesling grapes as they grow. As a result, top riesling growers are always careful to allow leaves to slightly shade riesling clusters.