Off Flavours: Lightstruck / Skunk

Ah, skunk. A not so humble American native that always reminds me of a ratel, except that instead of being a bad-ass die-hard, its best defence is an offence.

An offensive offence at that, spraying a repulsive odour from anal scent glands. Come to think of it, it is a bad-ass after all. Quite literally.

But wait, did you click on a nature & wildlife article, or were you reading about off flavours in beer?

As it turns out skunk is an off flavour in beer, similar in aroma to what our furry friend uses to fend of bears. A terrible thing to find in a beer, skunk is never appropriate to any style.

What both a terribly off beer and skunk have in common, are sulfur containing chemicals. Think rotten eggs, hot-water springs, the Caltex refinery when you’re down-wind… they all contain sulphur in some shape or form. A compound to which us, delicate beer-enjoying humans, are unfortunately quite sensitive to.

But why does beer get skunked?

You can guess from the term lightstruck that it has to do with exposure to light. Beer can be skunked by both visible and ultraviolet light from a particular range of the spectrum.

The clearer the glass in which your beer is stored, the more light from this range the beer is exposed to. Clear glass offers virtually no protection, green glass precious little, and brown glass offers the best.

But how? Strap on your science helmet, it’s going to get rough!

But wait, before we put on our propeller hats, have a look at a simplified explanation:

So, we’re blaming alpha acids. Alpha acids are the stuff in hops that gives a beer aroma or bitterness, collectively called humalones.

The longer you boil hops, the more humalones are isomerised, the process of rearranging the same atoms to transform one molecule into another. In this case to isohumalones, aka iso-alpha acids. It’s like a light switch – same thing, different position – and it’s that different position that switches on the bitter.

When the skunking light gets to these iso-humalones, part of the molecular structure is energized to a point where it comes apart. Where it does, it leaves a hole in the molecule that is attractive to sulfhydryl radicals – free-floating compounds that are there because of the alcohol. When the sulfydryl moves in on this ‘broken’ isohumalone, a flavour compound called 3-MBT is formed.

It’s the buildup of these 3-MBTs, and just a tiny bit is needed, that is the source of the skunky odour.

Sulfhydryl itself is part of a compound called thiol, which bonds strongly with mercury compounds, and so in 1832, the term mercaptan was coined from latin, meaning capturing mercury. Mercaptan, along with sulfidic, are terms also used to describe lightstruck beer.

How to Prevent Beer From Getting Skunked

Keep your beer out of light, and not just sunlight either, but artificial light too. That might make you think twice about beer on the shelf, next time you go shopping. Choose beer stored away from light, especially sunlight and opt for cans, brown-glass bottles, kegs or bottles that or otherwise completely encased.

That said, I have personally never encountered skunk (the animal or the off flavour), so I experimented with some of my home-brew. I left it in direct sunlight for almost an hour… and nothing. Perhaps I didn’t leave it long enough, or perhaps the hops I used were a type of pre-isomerised hop that apparently prevents this type of thing.

Let me know in the comments if you’ve ever had a skunked beer… maybe you can describe the flavour to me.

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