Skunked! How Sunlight Steals the Soul of Your Beer

Ah, beer. A refreshing drink enjoyed around the world, brewed with care and then, occasionally, damaged by something as ordinary as light. Leave the wrong beer in strong sunlight for long enough and its fresh hop character can be replaced by a sharp, sulphurous aroma usually described as skunky or lightstruck.

That much is true. What I am less convinced by is the way some beer reviewers talk about clear bottles as though every beer inside them is doomed before the cap has even been lifted. I have drunk a lot of beer from clear glass over the years and can remember very few that were genuinely skunked. I would estimate comfortably fewer than a quarter of 1%, or fewer than one bottle in 400, and probably considerably less than that.

Clear glass certainly increases the risk, but risk is not the same thing as certainty.

What actually makes beer go skunky?

The main reaction involves the bitter compounds created from hops during brewing. When wort is boiled, hop alpha acids are converted into iso-alpha acids, which provide much of a beer's bitterness. Beer also contains riboflavin, or vitamin B2, which acts as a light-sensitive trigger.

When suitable wavelengths of light reach the beer, the riboflavin helps break down those iso-alpha acids. The resulting fragments can react with sulphur-containing compounds to create 3-methyl-2-butene-1-thiol, (that makes me look clever, but I had to Google that!), these are generally shortened to 3MBT or MBT.

MBT is extremely potent. Only a tiny concentration is needed before the human nose can detect it, which is why a badly lightstruck beer can announce itself as soon as the bottle is opened. The smell is most often compared with skunk spray, although British drinkers who have never met an annoyed skunk may find descriptions such as sulphurous, rubbery, catty or cannabis-like more useful.

Wet cardboard is sometimes included in lists of lightstruck aromas, but I think that muddies the water a little. Cardboard and papery flavours are actually more closely associated with oxidation and stale beer. A beer can be both old and light-damaged, of course, but they are not the same fault.

Does clear glass automatically ruin beer?

No. Clear glass offers very little protection from the wavelengths that cause the reaction, while brown glass blocks far more of them. Green bottles sit somewhere between the two, offering limited protection but considerably less than brown. Cans provide the most complete barrier because light cannot reach the beer at all.

However, the bottle is only one part of the story. A clear bottle kept inside a closed cardboard case, moved into a shaded shop, stored in a fridge and drunk indoors may receive very little damaging light. A brown bottle left on a sunny pub table is better protected, but it is not surrounded by a magical force field.

This probably explains why my experience differs from the impression given by some reviews. Most bottled beer spends much of its life boxed, stacked or refrigerated. It is not normally sitting uncovered in a conservatory for three weeks. A clear bottle does not necessarily spend its journey bathing in direct sunlight.

There is also a danger of tasting with our eyes. When somebody already believes that clear glass equals skunked beer, every herbal, sulphurous or unusual hop note can become evidence for the prosecution. I would rather smell and taste the beer first, ideally without letting the packaging make the decision for me.

Have modern hops been changed to prevent lightstrike?

Not exactly. It would be misleading to say that today's hop varieties have simply been bred or altered so they cannot skunk. The more important development is the use of specially processed hop extracts.

Brewers can use reduced or hydrogenated forms of iso-alpha acids, including products based on rho-, tetrahydro- and hexahydro-iso-alpha acids. These provide bitterness but are much more resistant to the reaction that creates MBT. They are often described as light-stable hop products and can be especially useful for beers packaged in clear or green bottles.

For the protection to work properly, the beer's bitterness needs to come from those light-stable products rather than ordinary iso-alpha acids. It is not a case of adding a small amount at the end and making every other hop compound invincible. The brewing process and recipe have to be designed around it.

This is another reason why a beer in a clear bottle may be perfectly sound. The packaging might look vulnerable, but the brewer may have reduced the risk through the bittering ingredients used. That does not mean every clear-bottled beer is light-stable, so sensible storage still matters, but it helps explain why skunked beer is not an everyday occurrence.

How quickly can lightstrike happen?

Under strong direct sunlight, the reaction can begin quickly. That is why leaving a freshly poured lager on a bright outdoor table is not ideal (to be fair if the weather is hot, these beers tend not to stay in my glass for long!), particularly when it came from a clear or green bottle and uses conventional hop bittering. Indoor lighting can also contribute, although intensity, distance, exposure time and the type of light all make a difference.

The practical answer is not to panic, but not to be careless either. Keep bottled beer in its box until needed, store it somewhere dark and cool, and avoid displaying clear or green bottles in a sunny window. Once poured, drink it normally rather than treating every ray of daylight as an emergency.

How can you tell whether a beer is genuinely lightstruck?

The aroma is the clearest sign. A properly lightstruck beer usually has an unmistakable sharp, sulphurous or skunk-like note that sits above the intended malt and hop character. The fresh aroma may seem flattened underneath it, but dramatic colour changes and sudden extra bitterness are not reliable tests on their own.

One useful experiment is to buy two bottles from the same pack, keep one in darkness and deliberately expose the other to direct sunlight before tasting them side by side. Pour them into identical glasses and ask somebody else to mix up the order. That tells you far more than looking at a clear bottle and announcing that it must be faulty.

Lightstrike is real, well understood and worth preventing. Brown bottles and cans are safer packaging choices, and no brewer wants their beer unnecessarily exposed to sunlight. I just do not accept that clear glass makes a beer automatically bad. My own drinking experience suggests genuine skunking is rare, and when it happens the difference is usually obvious rather than something that has to be imagined into existence.

So, protect your beer from strong light, especially for prolonged periods, but judge what is in the glass. The bottle tells you about the risk. Your nose and taste buds tell you whether the damage actually happened.