When a hangover occurs, people try to blame it on alcohol or on sulphites in wine. Looks like there’s another candidate for the penance burden: acetaldehyde or ethanal. Acetaldehyde is naturally contained in fruit (0.2-230 ppm), vegetables (0.2-400 ppm), in dairy products (0.001-76 ppm) and bakery products (4.2-9.9 ppm).  It is one of the components of tobacco smoke, and is addictive to tobacco, alcohol and some drugs. Some of its "sisters" develop flavours that make them useful in the food industry: benzaldehyde (almond smell), cinnamaldehyde (cinnamon smell), vanillin (vanilla smell), octanal (orange smell) or in cosmetics: heptanal (raw green grass smell) and citral (lime smell).
Acetaldehyde has long been used as a precursor to acetic acid (industrially produced vinegar), but it has been found to be much more economical to produce vinegar from methyl alcohol (methanol). In some alcoholic beverages it combines with glycerol and gives rise to secondary metabolites (cyclic acetals) that may smell like nuts, cinnamon or port wine.
Acetaldehyde in alcoholic beverages can be blamed for the development of cancer of the liver, oesophagus or upper gastrointestinal tract. Acetaldehyde interferes with the DNA (deoxyribonucleic acid) chain at the cellular level and blocks cellular regeneration function. DNA is the ‘instruction manual’ of cells, which controls their normal growth and function. When DNA is damaged, a cell can start to grow out of control and generate a cancerous tumour. An accumulation of acetaldehyde to a toxic level can increase the risk of cancer. 
According to a study at the frontier of knowledge, there is significant behavioural evidence implicating acetaldehyde in the mechanisms underlying the psychopharmacological effects of ethanol (ethyl alcohol). The energizing, euphoria-inducing effect at low concentrations has been inextricably linked to its involvement in alcohol dependence. The critical role of acetaldehyde in potentiating the energizing effect of ethyl alcohol confirms the original report, made in 1953 by a physician, after observing that his patients did not perceive aversive effects from small amounts of ethanol when treated with disulfiram, an aldehyde dehydrogenase (ALDH) inhibitor, suggesting that ALDH inhibition might enhance the euphoric and pleasurable effects of low doses of ethanol by increasing the availability of acetaldehyde.
Acetaldehyde has a positive effect on the development of red winesbecause it reduces their astringency. As an additive in a tannic red wine, it stabilizes the colour, prevents the formation of what we call "wine jacket", but in high concentrations leads to instability. Two critical points arise during winemaking: 1) during alcoholic fermentation excess ethanal is produced and yeast must be inoculated quickly, the use of sulphur dioxide is limited and chitosan is used for antioxidant protection during fermentation, or 2) after the end of fermentation In alcoholic wine, when it is the most appropriate time to add sulphur dioxide, it will not be added at all, if there is already free reactive ethanal in the wine, capable of chemically combining with the available compounds.
Among the additives with a preservative role, sulphites stabilizes ethanal, and glutathione can block its side reactions. Microxygenation decreases the proportion of free sulphur dioxide (SO2 free), but if the drop reaches zero, the ethanal content increases sharply and secondary oxidation occurs, which is the most dangerous for wines. Replacing sulphuric anhydride with chitosan in red wine production, especially in the post-fermentation phase, leads to a high content of polymeric pigments and a low content of tannins reactive with salivary proteins (we feel the bitterness of tannins because of the reaction they have with our own saliva!).
Relative acetaldehyde (ethanal) content of wines: 4 – 212 mg/ litre in red wines; 11 – 493 mg/ litre in white wines; 188 – 248 mg/ litre in sweet wines; 90 – 500 mg/ litre in fortified wines and 63 – 308 mg/ litre in wine distillates.
Depending on its concentration in the wine, ethanal can impart different odours: 0 – 2.7 mg/ litre = red fruit odour; 2.7 – 17 mg/ litre = dried fruit odour, dried apples; over 25 mg/ litre = pastry odour (canella, muffins), often found in wines of the variety Syrah ; 40 – 70 mg/ liter = smell of oxidized cut green apples, in wine from Syrah and over 100 mg/liter stink of rotten, oxidized apples. In a 1:1 reaction with sulphur dioxide (sulphur dioxide), the smell of oxidised cut apples is reduced by 90%. Here’s a good reason not to give up the much-hyped sulphites in the antioxidant protection of wine.
Red wine rich in anthocyanins will benefit from the positive effect of ethanol on colour, but wines with more flavanols and less anthocyanins, at concentrations above 50 mg/ litre ethanol, show a high phenolic instability (loss of colour) during maturation.
A component of wine, with a complicated name, with duplicitous activity – it can be good or bad, acetaldehyde could rid alcohol of some of its accusations – including hangovers – and could be beneficial when well controlled and used wisely.