Skunked beer is a fault caused by the reaction of isomerized alpha acids from hops (isohumulone) with riboflavin in the presence of light.
The compound produced is MBT (3-Methyl-2-butene-1-thiol), which is very similar to the chemical butyl mercaptan or skunk spray. The off flavor is often referred to as light-struck beer.
MBT is not appropriate in any beer, but some beers, notably Heineken and Corona are packaged in light green or clear bottles, making the production of MBT and the off flavors inevitable. Most people now consider the off flavors as part of the style.
Make sure you bottle your homebrews in dark brown glass and don't expose the wort to sunlight or fluorescent light after the boil.
When clear bottles are exposed to sunlight, the beer becomes skunked in a matter of minutes. For education purposes only, this is a quick way of producing the fault for BJCP training. Just bottle one bottle in clear glass and then expose it to direct sunlight for several minutes.
If you find this site helpful, please link to us!