Beer should also be kept away from bright light, especially sunlight, and especially if it has been bottled in clear or green glass: otherwise it will develop a strong sulfurous odor. A cup of beer at a picnic can go skunky in a few minutes; bottled beer in a fluorescent-lit display case may deteriorate in a few days. It turns out that light in the blue-green to ultraviolet parts of the light spectrum reacts with one of the hop acids to form an unstable free radical, which in turn reacts with sulfur compounds to form a close relative of chemicals in the skunk’s defensive arsenal. Brown glass can absorb blue-green wavelengths before they get to the beer inside, but green bottles don’t. As a result, green-bottled German and Dutch beers are often sulfurous, and many consumers now expect this! One American brewer with trademark clear bottles developed a modified hop extract that’s free of the vulnerable hop acid, and this prevents its beer from going skunky.