MBT is the abbreviation for 3-methyl-2-butene-1-thiol, a compound formed by the reaction of isomerized alpha acids, specifically isohumulone, with riboflavin in the presence of certain wavelengths of light. MBT is very similar in chemical composition and odor to the spray of a skunk.
 MBT in Beer
MBT is not generally considered appropriate in any beer style. However, some brands of lager, notably Heineken, are packaged in such a way that the creation of some amounts of MBT (and therefore skunk flavor and aroma) is almost inevitable. Many beer drinkers, especially in the United States, have come to associate this character with the beer itself and consider it part of the beer's character.
 Causes of MBT
MBT is created when wort or beer containing isomerized alpha acids, which are extracted from hops by boiling and contribute bitterness to beer, is exposed to certain wavelengths of ultraviolet and blue visible light. In the presence of light, one iso-alpha acid, isohumulone, reacts with the riboflavin present in wort or beer to form MBT.
 Preventing MBT
MBT can be prevented by avoiding exposing wort or beer to the relevant wavelengths of light after the completion of the boil. Sunlight skunks beer faster than other kinds of light, and fluorescent light is also a problem; incandescent light contains less of the relevant wavelengths of light. Beer stored in clear, blue, or green carboys or bottles should never be exposed to sunlight or fluorescent light, and exposure to incandescent light should be limited. Beer should be bottled in brown bottles wherever possible to minimize skunking, but even brown bottles should not be left in sunlight for long periods of time. In clear or green glass, skunking can happen in a matter of minutes.
Some commercial breweries that bottle their beer in green or clear bottles avoid skunking by using special hop extracts which do not contain isohumulone, thus eliminating the problem of skunking altogether. This is not practical or desirable for most homebrewers.
 Creating MBT
To create an MBT character in beer for tasting or judging testing or calibration, leave a bottle of Heineken or another lager bottled in a green bottle in bright sunlight for fifteen minutes.