aldehydes
From The Oxford Companion to Beer
are organic compounds produced by the removal of hydrogen from (i.e., oxidation of) alcohols. Conversely, the reduction of aldehydes leads to the production of alcohols. In a brewing context, the best known of these reducing reactions is the reduction of acetaldehyde to ethanol.
Aldehydes tend to be highly flavor active, are detectable in lower concentrations than their equivalent alcohols, and can contribute a range of characters to beer, especially to aged beer. Many aldehydes are used in the production of perfumes, giving them their distinctive fragrances. In beer, particular attention has been paid to E-(2)-nonenal, which possesses a pronounced cardboard character. However, quantitatively, the more important aldehyde in beer is often acetaldehyde, with its distinctive aroma of green apples.
Aldehydes will react with sulfur dioxide (metabisulfite) to form adducts that display far less flavor potency than the aldehydes per se. This is believed to be one of the mechanisms by which sulfur dioxide protects beer from staling.
It is also believed that aldehydes produced by the oxidation of alcohol in the body are the prime mediators of hangovers and headaches through their reactivity with diverse components of cells.
This definition is from The Oxford Companion to Beer, edited by Garrett Oliver. © Oxford University Press 2012.