‘Acetaldehydes’ are 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). Acetaldehyde is naturally present in wine and all wines still contain acetaldehyde after fermentation. This chemical compound is naturally produced by yeasts (some produce more than others) during alcoholic fermentation as well as by the oxidation process that can take place during the aging of wine. At this stage, the air in contact with the wine will oxidize the ethanol (alcohol) into ethanal, but in a lower dose than that produced by the yeasts. A high temperature and especially a high pH are two factors that accelerate the formation of ethanal. Ethanal is also the main molecule that naturally combines with SO2.

Acetaldehyde is a colourless, mobile, highly volatile liquid with a pleasant fruity odor, perceptible to the sense of smell at levels of the order of 0.05 ppm. This smell becomes pungent and suffocating in high concentration. It is miscible with water and most organic solvents. In pure liquid form, acetaldehyde has a particularly penetrating and unpleasant aroma. At the low concentrations normally present in wines, and mixed with wine’s many other odorants, it is not unpleasant but above a certain level can make a wine smell ‘flat’ and vapid. It helps give “Fino Sherry” wines and other flor wines their distinctive and recognizable aroma when present in slightly higher proportions.

Acetaldehyde binds with sulfur dioxide. It contributes to the synthesis of colored tannins and other derived colors in wines by combining with anthocyanin pigments, catechins, and proanthocyanidins (condensed tannins).

Because it is the first compound formed when oxygen reacts with the ethanol in wine, winemakers are careful to minimize delicate white wines’ exposure to oxygen. However, it has to be noted that this exposure to air is not so critical with heavier red wines because acetaldehyde reacts with tannins and anthocyanins which allows to better protect the wine.

White wine bottling requires extra caution since the delicate fragrances are most susceptible to being damaged when air is introduced. When a bottle of white wine is only partially emptied, the freshness of its aromas is rapidly lost and replaced by a vapid oxidized smell that is due to, among other reactions, the conversion of ethanol to acetaldehyde.

Oxidation is typically indicated by the presence of noticeable acetaldehyde and a browning of the color.

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