The majority of stale beer flavours are formed by chemical reactions classified as “oxidation.” As its name suggests, oxygen can be involved, although other chemicals can act as oxidizers. (Geek fact: elements and compounds that take electrons from others are called oxidizers). Aside from oxygen, another common oxidizer is chlorine bleach.
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! So next time you see pallets of beer stacked out the back of the bottle shop in the sun, steer clear of that stale beer!
Oxidation affects beer flavour in a few ways. The most unpalatable occurs mostly in light-coloured beers: the formation of trans-2-nonenal. This compound, which is classified as an aldehyde, tastes like paper or lipstick. It has a very low flavour threshold; as little as 0.1 parts per billion can be discerned in beer!
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 flavours, one of which is the almond-like benzaldehyde. Together, the different compounds are responsible for the flavour of sherry.
A degree of sherry-like flavour 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 flavour as they age. This seems to be independent of beer colour. This flavour 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 flavour and ceasing to respond to it.
Many consumers of European beers have come to expect flavours similar to bread and toffee in their beer. While these flavours are not cause for the drinker to spit out their beer, those who have had fresh examples of the same brand will recognize the defect. Pasteurization, which heats beer to high temperatures to ensure microbial stability, and shipping beer through summer heat, will speed the formation of bread-like oxidation products.
The last major oxidative flavour is diacetyl. Regular readers of this column will recall that it tastes like butter, and is formed by the oxidation of alpha acetolactate, a normal by-product of yeast metabolism. Modern breweries have the equipment and skilled employees necessary to assure the brewing of diacetyl-free beers.
The easiest way to learn about stale beer & oxidative flavours is to experience them. If you don’t mind ruining some beer, it’s easy to do! Here’s how:
Go out and buy some fresh examples of beer you like from the same batch. Do not buy imported beers for this experiment; none of them are fresh by the time they get here! Take some of the beers and store them in your refrigerator. Put a few in a very warm place, like the boot of your car in summer- not in direct sunlight (unless you are testing for light stuck flavours). Sample one of the beers after a week of warm storage, comparing the flavour to the refrigerated example each time. Conduct a tasting every week until you can stand it no longer. Be sure to cool all of the warm samples to the same temperature as the cold-stored samples prior to tasting.
Do not be worried about invoking the wrath of the brewing gods by purposely damaging beer; they will understand that you have good reasons for your actions!