The majority of stale beer flavors are formed by chemical reactions classified as "oxidation." As its name suggests, oxygen can be involved, although other chemicals can act as oxidizers.
Oxidative reactions are always occurring in beer, regardless of how it is stored. As with most chemical reactions, heat and motion accelerate oxidation. Storing beer cold at all times will help preserve it. That is why all imported beers taste oxidized to some degree: the heat and motion experienced during shipping are brutal!
Oxidation effects beer flavor in a few ways. The most unpalatable occurs mostly in light-colored beers: the formation of trans-2-nonenal. This compound, which is classified as an aldehyde, tastes like paper or lipstick (lipstick and paper actually taste quite similar!). It has a very low flavor threshold; as little as 0.1 parts per billion can be discerned in beer!
The aroma of many light-colored beers will also be compromised by oxidation. The malt character that is originally present in a fresh example may become honey-like due to the formation of 2,3-pentanedione. While this may not be unpleasant, it may not be what the brewer intended.
Dark beers tend to be affected differently. As they age, rich malt aromas are replaced by sweet, sherry-like tones. Many people find this aroma enticing, although it is much different from the original malty character of the fresh beer. Most concerning, the malt flavor of the beer disappears, leaving an emptiness in the palate that can be quite disappointing.
These sherry characteristics are the result of the oxidation of malty-tasting chemicals called melanoidins. Their oxidation products have a wide range of flavors, one of which is the almond-like benzaldehyde. Together, the different compounds are responsible for the flavor of sherry.
A degree of sherry-like flavor adds complexity to certain strong beer styles, like barley wine and dark Belgian ales. It is usually not considered appropriate in lower alcohol beers, and too much oxidation will even render strong, dark beers monotonous.
Some beers develop an unpleasant metallic flavor as they age. This seems to be independent of beer color. This flavor is often most prominent when the head of the beer is sampled, and if it is not too strong, it seems to fade as the beer breathes. This is most likely due to the tongue becoming accustomed to the flavor and ceasing to respond to it.
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