Acetaldehyde is an organic compound found in almost all plant materials and the most common of the aromatic chemical compounds called aldehydes. See aldehydes.

Acetaldehyde is closely associated with ethyl alcohol (ethanol) through reduction (addition of electrons) and oxidation (removal of electrons) reactions. Acetaldehyde is produced in the early stages of fermentation and is reduced to ethanol in the latter stages of fermentation. Acetaldehyde is the immediate product of the metabolism of alcohol in the human body.

The body oxidizes ethanol back to acetaldehyde as the first step in its processing of alcohol. Beer containing excessive levels of acetaldehyde is characterized by the aroma and taste of green apples. People vary in their sensitivity to acetaldehyde, but beyond its presence as a background note, it is generally considered an off-flavor in beer.

If the yeast is not sufficiently active, either because it is not healthy or because the fermentation temperature is too low, too much acetaldehyde may remain in the beer. Bacterial infections can also interfere with yeast fermentation, leaving elevated levels of acetaldehyde in beer.

Acetaldehyde is further metabolized in the body by acetaldehyde dehydrogenases to acetate. In some people, acetaldehyde dehydrogenase enzymes are inefficient in metabolizing acetaldehyde. This can cause the well-known “flush” response observed when some people drink alcoholic beverages. This effect is genetically determined and is most prevalent in persons of Asian descent. It involves a reddening of the skin associated with a dilation of its capillaries.