Beer contains hops (it also contains sulphur). Hops contain alpha acids. When hops are boiled the alpha acids get isomerized and become isohumulone. When light hits the isohumulone compounds it breaks them down and causes a reaction with the sulphur in the beer and produces some of the same stinky chemicals that skunks spray. Hence the term ‘skunked’ beer.
Brown bottles offer the best protection from those beer-harming UV rays, green bottles don’t help much and clear bottles offer no protection against light. Open a beer in a clear bottle and it will have a similar dusty, funky character to any other beer in a clear bottle; that's not some unique British hop you're smelling, that's skunked beer. All bottles let through some light and therefore all are susceptible to being light-struck. Casks, kegs and cans are safe from the light.
Beer can be light-struck almost immediately in direct sunlight; it takes a little longer in non-direct daylight and a little longer still in fluorescent light (the sort of light that brightens the supermarkets which have aisles filled with beer...), but it can happen quickly. And it’s the hops which make it go bad; Coke or wine in clear glass bottles aren’t affected the same way.
I’ve never smelt a skunk but I have smelt and tasted skunked beer and it’s bloody horrible. I now won’t buy beer in clear bottles because I hate the aroma and flavour of light-struck beer. Sometimes it’s just a little moldy and funky, like over-stewed vegetables, dust or damp cardboard, other times it’s like sewage or really bad breath. Sometimes it’s like all of the above. Never is it nice.
I don’t understand why any brewery would make beer and then put it in clear bottles when they know it will be affected by light. I’m guessing it’s a marketing choice but if that decision is done on aesthetics while knowingly spoiling decent beer then what’s the point? Some well known beers have an essential quality of being light-struck (Newcastle Brown Ale, Shepherd Neame and Becks to name just a few) so the brewers are making decent beer and then having to let the light do its damage to create the flavour. Why?
I also think beer in clear bottles looks cheap and horrible.
It is possible to use clear bottles without it getting light-struck but it involves using tetra-hop extract, a lab produced hop replacement that is pre-isomerised and doesn’t get affected by light. The trouble is that it doesn’t taste much like real hops (the beers it gets used in don’t taste much like real beer though...).
Beer in clear bottles is bad. If you want to taste it for yourself then buy two beers in clear bottles (although they will already be affected by the time you get them), leave one in the light for a week or two and the other in a dark cupboard and then try them side-by-side (for more extreme results leave it in direct sunlight but watch out because it’s horrific).
Do you buy beers in clear bottles or not?
If I’ve missed any interesting science stuff then let me know below. If you know why a brewery uses clear bottles then also please tell me. Of all the off-flavours I think this is the stupidest because it can largely be avoided simply by using a different coloured bottled. And sure, most people are used to that flavour in the beer, but that doesn't make it right, does it?