Acetaldehyde

acetaldehyde, the most common member of the group of chemical compounds known as aldehydes, a natural constituent of nearly all plant material, including grapes. Acetaldehyde is the next to last substance involved in the fermentation pathway (and is therefore a minor constituent of all fermented products). Post-fermentation traces of acetaldehyde remain in all wines.

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.