Just last night I hosted a small beer evaluation session for a couple of friends. One of the beers we tasted was Sam Adams Double Bock. We had two bottles, one fresh, the other cellared for one year in a cool basement.

Aside from the proteinaceous flakes that were floating in the more aged beer, and a different cap, it looked just like the younger. Its flavor was reminiscent of many of the European double bocks I encountered during my college years in central New York: the aroma was sweet, very similar to sherry, and there were also some hints of toffee and maybe even a bit of honey. The flavor wasn’t as rich as the nose portended, with a sort of empty space between the initial rush over the tongue and the finish.

In my Rolling Rock-filled ignorant youth, I rejoiced when I experienced beers like this: they actually tasted like something! I quickly started spending my limited beer budget on expensive imports, trading quantity for perceived quality (this was before the American craft brewing revolution). Flavors, especially sweet flavors, were more important to me than the buzz. I would often wonder about the thick layer of dust on some brands, but back then I believed that beer could not go bad.

Shortly after my introduction to flavorful beers, I took my first trip to Europe. Although I only spent a short time in Germany, the Bocks astounded me. They tasted so rich, so malty, and so unlike the beers of the same label bought in the States. That empty space in the middle was filled with luscious and interesting notes of toasty, caramel-like malt.

I found myself wondering why they should taste so different? Did the brewers use different recipes for various markets? It was only a few years later, when I started brewing at home, that I realized how age can effect a beer.

The fact is, beer can go bad, and does so quite predictably.

The problem for beer is that nothing that grows in it can hurt a human; while the flavor will absolutely change, usually for the worst, nobody will become ill as they would drinking aged milk. It is this durability that leads people to believe that beer can withstand all sorts of abuse.

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. (Geek fact: elements and compounds that take electrons from others are called oxidizers. Aside from oxygen, other common oxidizers include chlorine bleach and nitric acid).

Oxygen is a major component of Earth’s atmosphere, and as such it is a very common beer oxidizer. Brewers work hard to keep it out of the product, but some ingress is inevitable. Damaging oxygen can be introduced at many points in the brewing process: during mashing, runoff, boiling, whirlpooling, filtration, and packaging. Oxygen uptake during any of the first four is referred to as “hot side aeration” because the wort is hot during those process points. Cold side aeration occurs when the beer is – you guessed it – cold, as it is when filtering and packaging.

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.

Many American consumers of European beers have come to expect flavors similar to bread and toffee in their beer. While these flavors 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.

As an aside, that is the main reason one should not use bread as a palate cleanser when conducting beer evaluations. Bread is not flavorless, and some of its flavors can occur in beer. If one chomps on some bread, then tastes a beer, how can they know if the bread-like flavor originated in the beer or in the bread that is still stuck between their teeth?

The last major oxidative flavor 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 purveyance of diacetyl-free beers.

Some breweries, using more aged techniques either by necessity, choice, or ignorance, produce beers that are prone to become buttery over time. While this flavor is not offensive to all consumers, it is almost impossible for the brewer to control the amount that ends up in their customer’s glass. Rather than adding interesting complexity to the beer, the diacetyl may overwhelm the palate, embarrassing the brewery.

The easiest way to learn about oxidative flavors 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 well-made beers. You are safest getting beers from large breweries because they almost always have the best quality control. Do not buy any imported beers for this experiment; none of them are fresh by the time they get here! It is desirable to obtain beers in six-packs or cases, so that you are sure they are from the same batch.

Take some of the beers and store them in your refrigerator. Put a few in a very warm place, like the trunk of your car in summer or on your furnace in winter. Start tasting the beers on the fifth day of warm storage, comparing the flavor to the refrigerated example each time. Conduct a tasting every five days or so until you can stand it no longer.

I recommend the following beers for this test: Coors light, Coors, Sierra Nevada Pale Ale, and canned Guinness stout (the stuff for the U.S. is made in Canada now, so it is reasonably fresh). 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!