Beer Science: Chemistry of the "Skunk"

This science installment deals with that all-too-familiar smell and taste some beers unfortunately acquire: the dreaded "skunk" beer.  I'll admit that until recently, I wasn't really sure why a beer became skunky.  This report will help clear up some of the mysteries and chemistry behind the "skunky" beer.

What Causes the Skunk to Happen?
Now, let me first start by saying that I have definitely tasted my share of skunky beers.  I grew up in Western New York drinking both kinds of beer: Molson and Labatt.  Growing up, we had no idea what caused the skunky beer.  We did notice, however, that the green Molson bottles tended to turn skunky more than the brown Labatt bottles.  That was an interesting and (as it turns out) important observation.  Green bottles do produce more skunky beer than brown bottles.  And there is scientific evidence to support that statement.

If you ask a random beer drinker the question "Why is your beer skunky?", you will probably get a variety of answers.  Probably top of the list are "The beer is old", or "The beer got too warm".  But did you know these are not the correct reasons?

The real reason why a bottle of beer gets skunky is because the beer was exposed to light.

That's right: the beer was "light-touched".  Beer bottles that have been exposed to open and direct UV light can skunk very quickly.  This effect is obviously more pronounced by direct sunlight, but can also happen under other types of fluorescent lighting.  And green bottles tend to allow more of the "correct" wavelengths of light responsible for skunky chemistry into a bottle than brown bottles do.  However, for marketing reasons, these green bottles tend to persist despite the fact they can produce a higher percentage of skunky beers under the same lightning conditions as brown bottles.  If you have tasted a Heineken, you know this flavor , because Heineken contains low levels of skunky chemistry normally and even intentionally.

What is the Chemistry of the Skunk?
Ok, so we know that light affects beer and produces a skunky smell and taste.  But how?  One of the main ingredients of beer is hops.  Hops plants (scientifically called Humulus lupulus), contain acids called isohumulones.  Isohumulones, also known by brewers as iso-alpha acids, are the compounds found in hops that impart the bitter flavor to beer.  Various chemical variations of iso-alpha acids exist, including humulone, adhumulone, cohumulone, posthumulone, and prehumulone.  During the brewing process, humulone present in hops is converted to isohumulone.  Isomhumulone is the chemical responsible not only for the bitterness of beer, but also is the chemical responsible for skunking.  Here is the chemical structure of humulone, the most prevalent alpha acid in hops, and isohumulone, the bittering chemical found after brewing wort at high temperatures.

 

The bitterness of beer is measured according to the International Bitterness Units (IBU) scale, with one IBU corresponding to one part-per-million of isohumulone.  This means that isohumulones have a very low flavor threshhold.

But what happens to isohumulones when you expose them to UV radiation?  With the help of other chemicals in wort, light interacts with hop isohumulones to almost instantaneously produce a chemical called MBT (3-methyl-2-butene-1-thiol).

MBT (3-methyl-2-butene-1-thiol)

You don't have to be a chemist to notice that a distinctive feature of MBT is the presence of the letters "SH".  "S" stands for sulfur.   The sulfur used to make MBT is derived from various amino acids and other chemicals present in the wort.  And herein lies the answer to the mystery of the skunky beer. The sulfur compound MBT is very easily perceived as a skunky smell and taste in beer.

In summary, hops contain alpha-acids.  During the brewing process the predominant alpha acid humulone is converted to isohumulone.  This provides beer with bitterness units.  UV radiation (sunlight, fluorescent lighting) can react with isohumulone.  More chemistry then occurs between isohumulone and other compounds found in wort and the result is the production of MBT.  And hence your beer is perceived as skunky.

But let's not forget about the skunk shown in the first picture.  What does this animal have in common with a skunky beer?  Skunks produce sulfur-containing chemicals in their anal glands to ward off predators.  Hence, a beer that contains MBT does in fact smell like a skunk's ass!

References Cited:
Skunked beer mythbuster
Home Brew Talk MBT
Hops Chemistry
Sulfur Compounds in Beer
Professor Beer
Coppertail Brewing: The Skunk