Applying the term “skunk” to a beer sounds nasty and perhaps a little extreme. Beer goes bad, but why do we use the word “skunked” or “skunky” to describe this?
Skunking of beer refers to a specific process that takes place in hoppier beers that are exposed to light. Certain wavelengths of UV or blue light react with the chemical compounds in hops and break them down. Another term for skunked beer, then, is lightstruck beer.
Skunked Beer Mechanism
One result of this is that a chemical found in skunk spray is released! Now, there are many chemicals in skunk spray. It is primarily a mixture of sulfur-containing chemicals called thiols or mercaptans. Thiols can smell like rotten eggs, garlic, or other things high in sulfur. They are responsible for the foul smell of skunk spray but also garlic, rotting chicken, and bad breadth. We humans, although we may not have the best noses in the animal kingdom, can smell these chemicals in extremely small amounts. For this reason, they are added to natural gas, which would otherwise be odorless.
The specific thiol produced in lightstruck or skunked beer is 3-methyl-2-butene-thiol (prenyl mercaptan), abbreviated MBT. When beer is made, compounds are extracted from the hops, just as they are when you brew a tea. Some of these compounds are isomerized alpha acids (iso-alpha acids). One of these is called isohumulone. This compound, when exposed to the right wavelengths of UV light, undergoes protolysis to form the MBT. The process can take place at any time after iso-alpha acids are formed, which means even the wort must be protected from light.
Riboflavin, or vitamin B-2, is also strongly implicated in the process since flavins absorb visible blue light and may promote protolysis of iso-alpha acids. The process by which visible light contributes to the sun-struck reaction in beer, though, is not completely understood. It stands to reason that reducing flavins in beer may reduce the reaction, but this is easier said than done.
Paler beers, and as said, hoppier beers are more likely to be damaged in this way. This makes sense when you think about it. Light does not penetrate through a dark beer as easily as a light beer. If the light cannot get through, the iso-alpha acids or flavins cannot absorb it. The stronger malty taste of dark beers may also mask the little bit of skunkiness that occurs and since they tend to contain less hops, the lightstruck flavor is less likely.
Clear Glass and Green Bottles Lead to Light-Struck Beer
The skunking process is very robust and very quick, but everything depends on whether the beer is exposed to light. Clear glass bottles let through pretty much all the UV and blue-light rays so beers in clear glass bottles exposed to light will skunk quickly. Even the lights from the refrigerator will do. Sunlight is, of course, the worst. And, the fluorescent lighting often found in liquor stores is particularly bad. liquor Regular old-fashioned incandescent light bulbs are not as bad for beer, but even this kind of light should be limited.
Lest you think that fancy green or blue bottles are protective, think again. They may look pretty but they do no adequately protect a beer from light damage. Green bottles do screen out a good amount of the UV light, but they let the visible blue light through. Some experimentation on beer exposed to sunlight has suggested that MBT production is twice as high in clear bottles as in green bottles, and around six times as high in green bottles as in brown bottles.
The absolute best type of glass bottle for protecting a beer from light, then, is a dark brown bottle, which blocks around 98% of the UV light and most of the visible light. For total protection, though, cans or ceramic bottles, something you don’t see much, are best. Obviously, beer kegs are completely cut off from light. But keep in mind that heat matters too and certain processes of oxidation take place even when a beer is completely sealed and not exposed to oxygen. This is called auto-oxidation. Extreme heat speeds up this process, as revealed in this article about letting a cold beer get warm and then cold again. The idea that rewarming a once cold beer is the mechanism is a skunked beer myth, but keep in mind that spoiled beer results from more than just the skunking reaction.
In the article Does Beer Get Better as It Ages?, I mentioned Corona beer. Corona is infamous for being skunked. Corona uses clear glass bottles and, according to some experts, a hops variety that is vulnerable to skunking. The bad reputation the beer has among some drinkers is probably due to this. It is rumored that Corona plays up the tradition of putting a lime wedge in the beer because it helps cover up this skunky flavor. Informal experiments have revealed that Corona skunks extremely quickly. If left in the sunlight, it only takes a couple of minutes to start producing the off-flavors. Since just sitting on liquor store shelves exposed to (usually) fluorescent lights cause the lightstruck reaction, it is often suggested that the vast majority of Corona drinkers are drinking skunky beer.
Skunking, of course, is not the only off-flavor and aroma that can occur as beer stales but like many others, such as burnt or sour flavors and aromas, it is possible that a small amount could be considered desirable. Sour, in fact, is a taste sought in some beer styles. People who drink lots of Corona, or Heineken, another notoriously skunky beer, may actually have gotten used to the flavor and associate it with the beer’s character.
By the way, since the term flavor refers to both taste and aroma taken together, to say that the flavor is off can also imply that the aroma is off.
It is probably unfair to single out Corona, and it may be a case of confirmation-bias due to the many bad rumors attached to the beer. The same kinds of things can be said of many pale beers in clear glass or green bottles. Given this obvious problem, why do so many major breweries continue to bottle beer in clear bottles (and less often green bottles)? Consumer preference! Being able to see that pretty amber liquid through the clear-glass can be a signal of quality to some consumers. They may also associate it with fewer calories. As well, the pretty colors of green or blue bottles can be a lure. Brown bottles are not as attractive and they ‘hide’ the contents. The question is, then, what do they do about the skunking problem?
Preventing Skunked Beer
Clear glass itself can be modified so that it blocks most of the light from reaching the beer. Miller Brewing Company uses a special glass for this purpose. Beer companies also try to encase the beer in paper or use high-sided six-pack cartons.
No matter how careful a company is in shipping their beer, or how careful liquor stores are in their lighting choice, pale beers bottled in clear bottles will be skunked. The only way to prevent this, without actually using brown bottles, is to change the chemistry of the beer itself. Preventing flavins, as above, might help, but this is not practical. Many large breweries, instead, have started using specially formulated hops extracts with light-stabilized iso-alpha acids. One company specializing in these products is Kalsec, located in Kalamazoo, Michigan. Kalsec makes many different photo-stable hop compounds, including custom-designed for large breweries. At least six of the largest breweries in the word buy these hops, and some microbreweries do as well. Some beer enthusiasts say that they affect the taste of beer negatively, and have bitter-flavor profiles that are different from natural hops. There are other examples of advanced hop products that can be used to enhance bitterness while helping to prevent the light-struck flavor.
Experiment for Yourself
If you want to taste skunky beer for yourself so you can tell the difference, it is easy, but keep in mind that you must taste a fresh and a skunked version of the same beer. Buy a six-pack of Corona or Heineken. Put one or two bottles in the direct sun for about 15 minutes. Keep the rest in a dark place. You should be able to clearly tell the difference between the light-exposed beer and the beer that you held back. Some experts suggest that you need to be very careful when you buy the beer, protecting it from light and picking a case off the back of the shelf, etc. but the reaction is robust enough that the difference in the beer you put in the sunlight and the beer you did not should be quite obvious, regardless if all the beer has had some light exposure. It still is a good idea to be careful with the beer so as to eliminate as much light as possible so you can have a clear experiment.
What About Skunked Wine?
It is often assumed that since beer is the only alcoholic beverage containing hops, it is the only one subject to this type of light-induced damage. Wine, mead, etc., it is said, are not vulnerable to light. This is not true. If you noticed that wine cellars are kept dimly-lit or dark, you will already be shaking your head at this assumption. Wine can become sunstruck as well, due to similar reactions, even though wines do not contain any hop compounds. White wine, such as Sauvignon blanc is particularly vulnerable to this reaction and since so many wine-makers insist on bottling white wine in clear bottles, it is wise to protect such wines from all light, as much as possible. The same is true of Champagne.
Riboflavin induced photooxidation, in fact, can take place in wine, forming methional, which breaks down to dimethyl disulfide, resulting in burnt and other oxidized odors. This same process can occur in milk. The skunky flavor may be unique to beer, but the process itself is not.
Methional, as well, is responsible for the problem in alcohol-free beers often described as a “worty” flavor. Methional and methanethiol can also be formed in sunstruck beer, but they do not seem to contribute much to the skunky flavor.