For those of you new to the craft beer world, or maybe still in transition from the big guys, beers such as Stella Artois or Heineken are commonly found options that are considered to be a bit more obscure than your Budwiser or Coors offering. If you’ve ever been a regular drinker of these brands, or their green bottled brothers, you may have noticed something different about them. The first beer you had, tasted as you expected, However, that last beer you brought back, that was sitting on the cooler after a day spent pool side, now tastes awful. When this happens, your beer has become skunked.
This phenomenon known as skunked beer has been blamed on many factors, the most common being refrigeration practices (allowing your beer to go from cold to hot to cold). Though that can make beer stale by increasing the rate of oxidation, it's not the culprit for that skunky taste.
Skunked beer is caused by a specific chemical reaction triggered by exposure to light, as explained by the American Chemical Society. This is known by many brewers in the craft beer industry, and explains the push away from green, or jeebus forbid, clear glass bottles, towards cans and brown bottles.
The name “skunk” fits this process perfectly. Beer’s primary source of bitterness comes from the addition of hops. They're added to the wort, or not-yet-beer, during the brewing process. When boiled, hops release iso-alpha acids into the liquid. If beer is exposed to sunlight, the sun's power breaks down those iso-alpha acids. The result: compounds bound with proteins which contain sulfur. This creates a new chemical that is almost exactly identical to the one found in skunk spray.
People can taste this chemical in concentrations of one part per billion.If you filled an Olympic-sized swimming pool with beer, one eyedropper of this stuff would change the way it tasted.
In short, keep your beer out of the sun, and lets hope the move towards canning continues.