By the way, the reason the keg Heineken doesn't task skunky is probably because it isn't in a green bottle. The green bottles do not block UV light....UV light makes beer skunky. But it might also be the fact that beer in a bottle is a more concentrated experience than beer in a keg. Maybe the makers of Heineken want the beer to be somewhat skunky.
UV exposure: Hop-derived molecules, called isohumulones, are basically ripped apart. Some of these parts bind with sulfur atoms to create that “skunk†character, which is similar in character to a skunk's natural defense and is such a potent compound that parts-per-trillion can be detected and even ruin a beer. Although brown bottles aid in protecting beer from being light-struck, it hardly makes the beer invincible. Green or clear bottles provide little to no protection. And it's been said that bottled beer can become light-struck in less than one minute in bright sun, after a few hours in diffuse daylight, and in a few days under normal fluorescent lighting.