All you could ever want to know about light and skunked beer. Light skunks beer. Period. And quickly too.
The hop compounds that are responsible for making beer bitter are called isomerized alpha-acids. These chemicals, along with sulfur compounds found in beer, are also culpable in beer skunking. When light hits beer, it provides the energy necessary to drive a reaction that transforms the iso-alpha-acids into 3-methyl-2-butene-1-thiol. The “thiol” part of that somewhat cumbersome name indicates that there is sulfur present. Sulfur compounds often have strong, offensive aromas. Some musteline animals, like skunks, have evolved the ability to produce this chemical, and use it for self-defense.
In a sense, the aroma of light-struck beer doesn’t just resemble skunk spray, it is skunk spray! It’s the same stuff!
This photochemical reaction is the only cause of skunked beer. Warm storage, while damaging to the flavor of beer, does not skunk it. Cycling the temperature of beer from warm to cold and back again is also not implicated. Storing beer in the dark is the simple way to prevent skunking.
Blue light, and to a lesser extent green and a bit of near ultraviolet are the most damaging to beer. Most wavelenghts of ultraviolet light are not a concern because glass blocks them quite effectively (that’s why you don’t get sunburned in your car). The color of glass is the color of the light that it transmits, so green bottles allow the green light though. Similarly, blue light passes unhindered through pretty, cobalt-blue bottles. Clear bottles transmit all of the visible light. That is the reason beer in green, blue, and clear bottles is almost always skunked. Yes, even some very expensive imports.
The photochemical reaction that skunks beer occurs very quickly; a well-hopped beer in clear glass can become noticeably offensive with just 30 seconds of exposure to sunshine. Brown glass transmits less visible light than the previously mentioned colors, and therefore offers some protection from skunking. It does allow some light through, so beer in brown bottles will skunk after a few hours of light exposure.
Since light is an essential ingredient in the skunking process, beers packaged in kegs, cans, and opaque bottles cannot be skunked. Beers with very little hops, like all of the insipid mass-market American lagers, cannot become as skunky as overtly hoppy styles like pale ales, simply because there is less iso-alpha-acid available.
