In pure liquid form, acetaldehyde has a particularly penetrating and unpleasant aroma. Above a certain level it can make the wine smell ‘flat’, vapid, and oxidized. At slightly higher concentrations, it contributes to the distinctive and characteristic smell of fino sherry and other flor wines. Acetaldehyde binds with sulfur dioxide. It also adds to anthocyanin pigments, catechins, and proanthocyanidins (condensed tannins) and it is thus involved in the formation of pigmented tannins and other derived pigments in wines.