Yeah, skunking is a thing.
@bruce_the_loon gave good resources to understand why.
The worst source of skunking is sunshine. If you want to test this, take a lighter beer, an IPA is a good choice, and put it in direct sunlight for 30 minutes. Keep it on ice if you want to test it right away. Pour into a glass and enjoy....if you can.
Or, you could use a rinsed-out and sanitized Diet Coke plastic bottle. Fill off a keg, cap, and put in the sun. Or, if you bottle rather than keg, use it as a bottle (it actually works). Keep in the dark for the normal time you let the beer condition, then put in the sun for half an hour, chill, and enjoy
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I attended an off-flavor workshop a couple years back where they took a beer, put in the sunshine for the afternoon, then served it to us. Wow, very skunky.
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BTW, skunkiness is actually a feature in a few beers. Corona is famous for this. In fact, the people putting on the off-flavor workshop claimed that the whole "A Corona gets its lime" thing was in response to skunkiness. Their bottles are clear and of course, almost impossible to avoid skunking unless you keep them in the dark forever. Anyway, they knew of the skunkiness and discovered that the lime helped mitigate the sensation of skunkiness. So now the necessity has become its virtue.
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Fluourescent light bulbs tend to put off UV light, which is what causes skunkiness. So does the sun, of course. Incandescent bulbs, not so much.
I used to ferment in my basement; I had incandescent bulbs for light, never noticed a skunkiness (and I'm very sensitive to that odor). Still, the best practice is to keep the wort/beer out of the light period. Fluourescent light is not good.
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I have a sight glass on the bottom of my stainless conical. Of course, for the most part, the beer isn't exposed to light, but that sight glass is exposed to light. I've never noticed any skunkiness from beer produced in that fermenter, but the sun does shine through the window, and the beer in the sight glass is exposed. Even though I haven't seen a problem, I'll probably cut a piece of thin cardboard to wrap around it.