The colourless and very volatile liquid (ethanal, obsolete ethanal) is the aldehyde of ethanol. The name is often mispronounced, the correct one is "acet-aldehyde". It is a natural component of almost all plant matter. The substance is formed during alcoholic fermentation with the release of carbon dioxide as a preliminary stage of the final product alcohol. Towards the end of the fermentation process, the proportion normally decreases very sharply. Acetaldehyde is present in all wines in normally small quantities and, in low concentrations, does not have any negative effects on the taste. It reacts with anthocyanins, catechins and sulphur dioxide, which means that it is involved in the formation of pigments (dyes). In the bottle, faulty corks can cause it to react with oxygen and thus impair the aroma, especially in white wines. Acetaldehydes bind sulphur in wine or make it a sulphur-eater.
In the human body, acetaldehyde is produced as an intermediate product during the breakdown of ethanol by the so-called alcohol dehydrogenase (see under ADH). It is responsible for the hangover(headache, nausea) after excessive alcohol consumption. Studies have shown an association between the consumption of alcoholic beverages and an increased risk of cancer of the digestive tract. There is an increased formation of oxygen radicals which damage the membranes of the cells. Crotonaldehyde, which is formed from acetaldehyde, was identified as the cause. In Germany, more than 1,500 spirits were examined in 2008 and high concentrations of acetaldehyde were found in mainly brandies, port wine and sherry. The highest values were found in Chinese and Mexican spirits. The lowest were found in beer (11 ± 9 mg/l) and wine (36 ± 42 mg/l). The European food law only includes the limit value of 0.5 grams of acetaldehyde per hectolitre of pure alcohol; no limits are defined for all other product groups. Up to this value, health hazards are excluded. Nevertheless, a re-evaluation seems necessary.
Acetaldehyde in its pure form develops an anaesthetic odour and, when present in large quantities in wine, causes an unpleasant, sharp-bitter, severe tone which is easily recognisable by trained tasters even in much smaller quantities than the above limit value. It is partly responsible for hangover symptoms such as headaches, nausea and vomiting. It is also the cause for different taste tones such as firn (age firn), as well as airy taste (aldehyde tone), which is the desired result as sherry tone. In pronounced form or with the "wrong wine", like a sherry tone with normal white wine, this means a wine defect. When reacting with hydrogen sulphide, malodorous thiols (mercaptans) can be formed which cause the buckling. See also a list of similar wine faults under Alterston.