kramerog
February 13, 2012, 6:09pm
21
hubie:
As I understand the chemistry, that is not true. The skunking comes from interactions with the isomerized alpha acids. Riboflavin (vitamin B) is one mechanism that contributes to this through a series of chemical reactions, but the iso-acids are also broken up by direct UV exposure. I believe the energetics are such that direct light exposure requires energies approaching the UV range, while the chemistry involving riboflavin is caused by light with a broader band of wavelengths out to as long as 500 nm (green light). Though you get riboflavin from yeast, it is also present in malt.
The skunking compound is part of the thiol family which has a ridiculous detection threshold, something like a few nano-grams per liter. Because of the very low detection threshold and the general unpleasant odor, thiols are put in things like natural gas to make it easy to detect.
As you’d expect, there are all sorts of factors involved that will determine how much it skunks and how easy it is to detect (hop level, beer color, pH, etc.). However, the worst light you can exposure your beer to is green (bad) to blue (worse) to UV (even worse). It is funny because I am writing this as I watch the Superbowl ads for Bud Light Platinum which appears to be packaged in blue bottles. For their sake I hope they are using those special lightstruck-resistent hop extracts for that.
Possibly people don’t detect skunking of the wort in the finished beer because fermentation scrubs out the thiols and because the yeast reduces thiols to H2S which is scrubbed out too during fermentation.