Skunked (or "Light Struck") Beer

Over the weekend, I was trying to explain to a friend (hint: she's Croatian) that brown bottles are better to package beer in than clear or green bottles, and that cans/kegs are really the best packaging. She wouldn't have any of it, so I figured I'd post a detailed explanation here.

When beer has been exposed to light for a period of time, the light interacts with hop-derived molecules called isohumulones. Isohumulones are the bittering compounds in beer that are created when the female flower of the hop plant is added to the boil during brewing. Light breaks down these isohumulones into a molecule called 3-methylbut-2-ene-1-thiol (according to the ever reliable Wikipedia). This new molecule is almost identical to the chemical a skunk produces, hence the term "skunked". Basically, Beer + Light = Bad. So, how does bottle color play into all this?

Colored glass actually absorbs light to some extent, and different color glass absorbs different parts of the light spectrum. The graphs that follow are from a study I found online entitled "Light Absorption by Various Beer Bottle Glass" by Dr. Bradley Sturgeon at Monmouth College in Monmouth, IL. The study measured the intensity of light at different wavelengths after being filtered through clear, green, blue, and brown glass. First off, the graph below shows the intensity of light from a standard 40W tungsten light bulb. Notice the light intensity when filtered through brown glass as opposed to green, blue, or clear.


Think about a retail store now--the lighting in most retail stores are not standard light bulbs, but rather fluorescent bulbs. The next graph measures the light intensity of fluorescent bulbs. Notice some of the large spikes in intensity and how the brown bottles filter those spikes compared with the other colors.


Lastly, we've all drank a beer in the sun on a hot summer day. This last graph, measuring the light intensity in natural sunlight, is especially telling since one of the worst parts of the spectrum for beer is UV light (less than 400 nm). Notice how the brown bottles almost completely filter out these harmful UV rays.


To make a long story short, brown bottles are the best choice for bottled beer. So why do breweries continue packaging their beer in green and clear bottles (e.g. Heineken and Corona)? The simple answer is marketing--it looks nice. Ever wonder why Corona is always served with a lime? It's because 9 times out of 10, that beer has been light struck already and the lime rounds the flavor out a bit. One interesting thing to note is that Miller uses a pre-isomerized hop extract in their beers instead of using real hops. The bittering compounds in this hop extract do not react with light, and thus Miller can bottle beer in clear bottles (like MGD) without worrying about skunking. Truth be told, the best packaging for beers is kegs or cans because the beer can't be light struck. Unfortunately, canning is expensive and only the major breweries can afford to do it--there are a few microbreweries that have started using cans, which is great (if you pour the beer into a glass there is no "metallic" taste--the cans are lined inside; it's when you put your mouth on the can that you get some metallic flavor). So now you know. And knowing if half the battle. GI Joe!!!