Identifying Off-Flavours in Beer: Light-struck

Roughly a decade ago I was teaching and living overseas in the mountainous country of Armenia. The beautiful, hot summers were filled with delicious barbeques and shish-kabobs, bushels of fresh apricots, tiny cups of coffee, and drinking warm, light-struck beer on patios. It wasn’t that I was actively seeking out sub-par brew, or that I even enjoyed the taste—there just wasn’t much of an option. And as the old adage goes, “when in Rome…”

The issue with the beer came down to packaging and handling. The beer in the region was almost all packaged in 0.5 litre clear or green bottles, and shipped to stores in milk crates. These beers would arrive to restaurants, outdoor cafes and corner shops, often being left to sit outside in the sun. Sometimes you would be lucky and get a fresh batch that hadn’t had time to turn, or you learned which places refrigerated the beer for foreigners (many locals preferred ambient temp beer). That was a while ago and things have probably changed since my visit, but needless to say, in my adventure abroad I developed a keen, discernable sense of what it means for beer to be truly light-struck.

Alpha acids and riboflavin

Light-struck beer is described as a mild skunky or sulphury (dare I say fart-like?) aroma and flavour. This occurs when light hits beer and reacts naturally with alpha acid hop compounds and riboflavin. This can happen at any point in a beer’s life once hops are added, including fermentation, cellaring, bottling, aging, and even imbibing. It can be easy to mis-identify this off-flavour and mistake it for the subtle DMS sulphur compounds that come from beers brewed with pilsner malt, mis-attributing oxidative qualities, or even mis-identifying the taste of an infected tap-line that wasn’t cleaned properly. It just takes practice. Doing an off-flavour training panel can help a lot with discerning the subtle differences.

Some beer enthusiasts like to cite many European or Latin American commercial import lagers that are packaged in green or clear bottles as go-to examples for light-struck beer. And indeed, this was the case for quite a long time. However, brewing science has come a long way in the past couple of decades and the majority (if not all) of these breweries use modified and treated hops or hop extracts that eliminate these skunk-causing compounds, thus allowing them to continue using their signature bottles without the risk of light-struck beer. If people are still drinking one of these beers and claim they can taste the light-struck quality, they are probably either confusing the DMS compounds mentioned above or noticing possible oxidation of the product. In all honesty, though, it is most likely a simple psychosomatic perception because they believe and expect the off-flavour to be there because that is what we have all been taught for so long.

The most likely places you will experience a light-struck beer these days is either from a homebrew that wasn’t properly cared for or sitting on the patio and drinking a beer at your local craft brewery.

How to mitigate and avoid light struck beer

As a homebrewer it’s not likely you will have access to the specialty hop products that the macro folks are using, but there are some easy steps to avoid drinking a skunk bomb later. The important thing to note is that once you have light-struck compounds in your beer, there is no going back.

Fermenting and aging beer – If fermenting or aging in a glass carboy or other clear vessel, keep the beer in a dark room or closet and try and wrap it in something that will protect it from any stray light. A blanket or a brown paper bag work great. Letting your beer sit exposed in a well-lit room will ensure you end up converting those light-sensitive compounds before you even have a chance to choose what bottles to put it in.

If you are brewing on a larger system with tanks that have a sight glass, you need to know that any beer in that sight glass is compromised (the exception being UV-level food-grade glass or tubing). This especially goes for craft brewers. Once you transfer into a brite tank and you are able to get the fill level, close off the site glass. Whatever you do, do not drain the tank and allow the beer in the sight glass to mix in the tank. Once you know your volume, you should be logging and tracking your kegging or bottle/can fills off of the tank and you should not need to rely on the visual fill level of the site glass. True, the volume of light-struck beer that would mix back in would most likely be below the flavour threshold, but you are compromising the longevity of your beer and quality of the final.

Bottling – The most common advice you will hear as a homebrewer is to bottle your beer in brown bottles. Unless you are able to experiment with some fancy hop extracts, or you made a gruit that is devoid of all hops, do not bottle in clear or green glass. This also goes for growlers—do not buy, use, or fill clear growlers with beer. They are only to be used as decorations.

If you did end up bottling into some green Grolsch style bottles or something fun, simply keep them away from any light as much as possible and consume within a month or two. They will not be good candidates for aging or sending to competitions.

Enjoying a beer outside – Drinking a beer in a glass is the best way to experience the full flavour and aroma. However, when enjoying your beer outside keep your glass in the shade as much as possible. Sunlight can convert enough compounds in the span of 10-15 minutes for a trained palette to notice a difference, and some beers can be fully compromised by the end of an hour. The answer here is not to chug your beer (please, drink responsibly), but instead opt for smaller glasses, stick to bottles or cans if shade isn’t available, or choose a beer style that has fewer hops.

Notes on infected beer

Light-struck aroma and flavours should not be confused with the sewage qualities or really dank fart-like smells and tastes that come from infected batches. Drinking one of these beers probably won’t make you sick, but you will likely experience bloating, bad gas, or indigestion, so it’s best to dump it out. If you order a beer from a restaurant or brewery that has any of these qualities, it is completely okay to return the beer, let them know of the issue, and get a different beer. If you notice this happening consistently from certain locations or from multiple taps, it probably speaks to the standard of cleaning and maintenance of tap-lines and it is advised to avoid these places all together. If you have a kegerator or a home tap system, make sure you don’t fall into this trap either. As long as you are cleaning and sanitizing regularly, you should have nothing to worry about.

David Boggs has been homebrewing for over 10 years and currently works as a professional brewer in Edmonton. He holds a certificate in Applied Craft Brewing and is a Recognized BJCP Judge. He is a fan of Belgian ales and anything barrel-aged.

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