Careful Cellaring, Part 3: The Threat of Light

Another factor that can be damaging to you beer cellar is light. Did you know a beer's flavor can change in minutes in direct sunlight? Even unnatural, fluorescent light can harm your beer. The reason this happens is because the hops in beer are very sensitive to UV light. To explain what happens to the chemistry of beer, we turn again to Jess from our lab. 

Ever wonder why "lite" beers in clear bottles taste better with a lime slice and are skunky without one? It's because of a little nasty compound called MBT or as the organic chemist might say 3-methylbut-2-ene-1-thiol.

The odor and flavor of this compound is often reminiscent of skunks but is commonly referred to as the smell and taste of a "lightstruck" beer. The chemistry that goes on to change your delicious hops to skunkiness is well known and shown in the following graph about "The Lightstruck Reaction" (Graham, 2006):

There is an energy transfer from the light to the Iso-alpha-acid that cleaves the isohexenoyl side chain and leaves a free radical behind. A free radical is a molecule with an unpaired electron making it very reactive. This free radical is then able to bind to naturally occurring mercaptans in the beer, which can often include hydrogen sulfide. In the end, you have a beer with skunk flavor and what is left of the alpha acid.

In the '70s it was discovered that by reducing the carbonyl group or the double carbon bond, the reaction could be prevented. This lead to the use of hydrogenated hop extract, also known as "hydrohop," by some brewers. While some beer manufactures actually consider skunky to be part of the flavor profile of their beer, others prefer to still use clear or green bottles for packaging appearance and avoid the lightstruck flavor by using hydrohop instead of real, quality hop ingredients.

Either way you look at it, living beer is something very easily altered whether it be on purpose or by accident! Though we are not a brewery known for often creating hoppy-tasting beers, we do make some hop-forward beers on occasion, and we certainly use hops for balancing beers with a massive malt bill (i.e. Black Tuesday, which has a surprising 40 IBUs). This is why we use thick, dark glass bottles for our unpasteurized, unfiltered, living beer and recommend storing them in a cool, dark place at a consistent temperature.

Other facts about lightstrike & hops:

• The term "lightstruck" came from the Europeans because they did not know of an animal odor to compare the smell to. This is because there are 11 known species of skunk, all of which live in the Americas, Indonesia and the Philippines. In other words, there are no native skunks in Europe.
• Skunks actually do not produce MBT, they produce several thiols which smell very similar to MBT.
• Only beers with hops can become lightstruck, unless like some breweries they use hydrogenated hop extract, which prevents skunking.
• Lightstruck beer has a very low flavor threshold -- in a range of 2-20 parts per trillion, most humans will be able to perceive this off-flavor
• Molecular oxygen actually inhibits The Lightstruck Reaction, making it one of the only times that oxygen is a good thing in beer.

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Post written by Jessica Davis, our Quality Specialist who makes sure our beer & yeast are healthy. Jess has worked in many a lab, including the one at Stone Brewing Co.

Read more of our cellaring series:

• Careful Cellaring, Part 1: The Quality Assurance Process for Creating Clean, Living Beer 
• Careful Cellaring, Part 2: The Importance of Temperature

Further reading:

• Beer Bottles, The Answer is Not Clear
• When Beer Goes Bad, Light-Struck or Skunked