"Skunked" Beer, "Light Struck" Beer, and the Impact of Temperature Changes

Jake Wrote:

On multiple occasions in the last year, I have heard people use the term "Skunked" to describe an off flavor that they perceive in a beer. I have also been asked by friends if the myth that cooling a beer, letting it come back to room temperature, and then cooling it again will cause the beer to get "Skunked". I read a really informative post on the Binny's (Chicago store) Blog and I want to re-post it to make sure that it gets preserved. For reference, Adam Vavrick ("Vav") is one of the most respected guys in Chicago craft beer and is the primary beer buyer for the Binny's Lincoln Park location that is about to expand to 50,000 square feet of beer, wine, and liquor.

The Blog starts with a customer question:

I just bought a case of Petrus Aged Pale Ale. I opened it up when I got home and noticed it was cold. I thought this was probably a mistake since most people buying in bulk would want it warm so they can cool it at their leisure. I called to see if I could exchange it, and the guy said could but the beer would be fine. Basically it’s a myth about the warming/cooling of beer that leads to aged taste or skunkiness, and exposure to light matters. I was confused, having thought that was a factor.

- D.G.

And then Vav provides the answer with a side of great customer service: 

Hi D.G.

The beer is absolutely fine for a number of reasons. Bear with me. There’s a lot of science coming up.

“Skunking” is a specific occurrence in beer, not a catch all for when beer goes bad.

Beer uses hops as its bitter component to balance the sweetness of malt. When hops are added to the beer during the boiling, they release Iso-Alpha Acids, which are the main components in hop oil. During the boil alpha acids isomerize, or transform into different molecules, which are very bitter. These, along with other hop oils, remain in the beer, contributing hop aroma. Some of the oils are quite volatile, and will break down when exposed to ultraviolet light. The reaction is quite fast: literally a matter of minutes and the isohumulone will break down into a chemical similar in makeup and aroma to the chemicals that come from a skunk’s anal scent glands. This is why beers from certain big European breweries smell skunky: green glass offers almost no protection from light. Clear bottles offer no protection. Brown bottles offer great but not perfect protection. A fun experiment is to grab a four pack of either Pilsner Urquell or Beck’s in cans. Pour a couple ounces into a clear glass, and put it on a sunny windowsill for two minutes. Then pour a few ounces into a second glass and smell the beer in both glasses. The difference will shock you.

Some beers like Corona and some Miller products use hop extracts that have been chemically altered to prevent skunking, even in clear bottles. So that’s where skunking comes from. In fact, we also call it Light Struck.

Still with me? Go grab a beer and then come back. There’s more…

The other part of your question is temperature variations. There is some truth to temperature fluctuations affecting beer, but it’s negligible. Basically, beer’s biggest enemies are time, light and oxygen.

Beer is partially a solid. Around 2% of beer is solid: carbohydrates, sugars, and more. This  makes up the body and head of a beer. These solids, given a long enough time, will settle out of solution to the bottom of the container. Some beers hold up longer than others; it depends on the makeup of the beer, like how much residual sugar is left after fermentation, or how attenuating (thorough, basically) the yeast was, and so on. When beer is stored cold, it stalls the aging process. So purely as an arbitrary example, a Pale Ale with, say, a 6 month shelf life, might have a 5 month shelf life when stored at room temperature, a 4 month shelf at 85 degrees, 3 month at 95, etc. These aren’t exact numbers, but you get the idea. The warmer the beer is stored, the shorter the shelf life.

One way to accelerate the aging process is drastic temperature swings from near freezing to near boiling. In fact, many breweries do just that in their labs, subjecting their beer to harsh conditions to see just how durable they are as part of their quality control procedures. The negligible amount this happens in the real world when you take a beer out of the cooler, allow it to warm, and then re-chill it is is absolutely harmless. And even if it did speed up aging, it certainly wouldn’t cause skunking.

Petrus Aged Pale Ale (good choice, by the way) is a sour ale. In fact, it is a pale ale that uses a very small amount of hops, which then goes into 200 hectoliter oak tuns, called foudres, where it sits for upwards of 3 years. In general, sour ales require less hops because you WANT infections from various little funky bugs and yeasts. Hop oils, in addition to bittering, also have preservative qualities. By using less hops, it allows those funky bugs and yeasts to go work unimpeded. Additionally, most sour ales, particularly lambics, use aged hops, most typically hops that have been aged for three years or more. This aging drives off many of the oils that contribute to both the preservative effects and aromatics, leaving only the bittering potential.

So there you have it. Your beer is just fine. If you’re still uncomfortable, of course we’ll take them back, but rest assured that they are just fine.

So, next time your friends say that they want to buy beer warm because they are going to transport it, or say that something "Skunked" their beer, do us all a favor and point them to this post. I wish someone had done that for me in my early days of craft beer drinking.

Full disclosure: Petrus Aged Pale is one of my all-time favorite beers