Author: Jake Huolihan

With some notable exceptions, brewers who package their beer in bottles generally prefer them to be amber in color because it blocks out more UV light, which interacts with isomerized hop compounds to produce 3-methylbut-2-ene-1-thiol (3-MBT). The telltale sign of a beer with higher levels of 3-MBT, as anyone who has drank from clear or green bottles likely knows, is a distinct skunk character, also known as lightstruck.

For the most part, lightstruck beer is an issue viewed as being packaging related, as the rest of the brewing process is largely free from exposure to light, especially on the commercial scale where fermentation tanks are usually made of stainless steel. However, some of the most commonly used fermentors by homebrewers are made of clear glass or plastic, materials that readily allow any environmental light to touch the beer. While fermentation chambers are a good option for avoiding light exposure, those who don’t have the luxury of such an enclosure are cautioned to keep their fermentor covered to avoid producing this off-flavor.

As someone who has used clear fermentation vessels extensively and started out by fermenting in a windowed basement, I’ve always been curious as to the effect exposure to sunlight during fermentation has on my beer. The mechanism for beer skunking is relatively well understood and accepted in the brewing community, though I was still curious if the methods I’ve employed to keep sunlight out of my fermenting beer were worth the effort.

| PURPOSE |

To evaluate the differences between a pale lager fermented in a clear vessel that was covered and the same beer fermented in a vessel exposed to regular sunlight.

| METHODS |

Since the vector for lightstruck flavor is the photodegradation of isomerized acids from hops, I went with a hoppy American Pilsner for this xBmt.

Pepe

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 5.5 gal 60 min 45.0 IBUs 3.3 SRM 1.049 1.013 4.8 % Actuals 1.049 1.014 4.6 % Fermentables Name Amount % Weyermann Pilsner Malt 8.562 lbs 85.09 Weyermann Vienna 1.5 lbs 14.91 Hops Name Amount Time Use Form Alpha % Hallertau Magnum 6 g 60 min Boil Pellet 14 Loral 11 g 30 min Boil Pellet 10.3 Perle 11 g 30 min Boil Pellet 8 Saphir 11 g 30 min Boil Pellet 3.5 Loral 24 g 5 min Boil Pellet 10.3 Perle 24 g 5 min Boil Pellet 8 Saphir 24 g 5 min Boil Pellet 3.5 Yeast Name Lab Attenuation Temperature Global (L13) Imperial Yeast 75% 46°F - 56°F Notes Water Profile: Ca 58 | Mg 0 | Na 8 | SO4 60 | Cl 61 Download Download this recipe's BeerXML file

The night before brewing, I collected the full volume of water for a 10 gallon batch and adjusted it to my desired profile before milling the grains.

I started off the next morning by turning on the element to heat my strike water then proceeded to measure out the kettle hop additions.

When the water was slightly overheated, I transferred it to my mash tun and allowed it to preheat for a few minutes. I was impatient and mashed in a little early, which led to a slightly higher than expected mash temperature. No biggie.

I returned every 15 minutes to briefly stir the mash over the course of an hour long rest.

I stole a small sample of wort to ensure the mash pH was where I expected it to be.

At the completion of the mash, I ran off the sweet wort to a kettle.

I turned the element on and added the first wort hop addition. Once the wort was boiling, I set a timer for 60 minutes and added hops at the times laid out in the recipe.

When the boil was finished, I hastily chilled the wort.

A refractometer reading showed my OG was spot on.

The wort was then evenly split between 2 identical fermentors.

My Colorado basement stays fairly cool this time of year, so I decided to forgo fermenting in my chamber and instead controlled temperatures using heating pads since cooling wouldn’t be needed. One of the full carboys was immediately placed in a place that gets no direct sunlight and covered with a large box, while the other was placed next to a large window that receives direct sunlight about 8 hours per day. I began to experience a slight feeling of regret at this point.

Using leftover wort, I made vitality starters with Imperial Yeast L13 Global, pitching one into each batch after a 4 hour wait.

Controlling the beers to 66°F/19°C, I noticed airlock activity the following day with the uncovered carboy bathed gloriously in the rays of the morning sun, which is much closer in Denver than many places in America.

Signs of fermentation were absent after 5 days, so I took hydrometer measurements confirming both beers had reached FG.

Moving both carboys to an area free of direct sunlight so as to prevent UV exposure to the covered batch, I proceeded with kegging.

I skipped fining with gelatin and let the filled kegs lager on gas in my keezer for a couple weeks. When it came time to collect data, the beers were clear and nicely carbonated.

| RESULTS |

A total of 20 people of varying levels of experience participated in this xBmt. Each participant was served 1 sample of the beer fermented under cover and 2 samples of the beer fermented in direct sunlight in different colored opaque cups then asked to identify the unique sample. At this sample size, 11 tasters (p<0.05) would have had to identify the unique sample in order to reach statistical significance, which is exactly how many were able to do so (p=0.038), indicating participants in this xBmt could reliably distinguish a Pilsner fermented in a dark environment from one that was exposed to direct sunlight during fermentation.

The 11 participants who made the accurate selection on the triangle test were instructed to complete a brief preference survey comparing only the beers that were different. A total of 5 tasters reported preferring the beer fermented in the dark, 4 said they liked the beer fermented in direct sunlight more, and 2 tasters reported perceiving no difference.

My Impressions: During fermentation, I smelled a cannabis-like aroma from the lightstruck beer that was absent in its covered counterpart, which had me convinced they’d be different once finished. Sure enough, I was consistently able to tell the beers apart, correctly identifying the odd-beer-out in 8 out of 10 semi-blind triangle tests. Interestingly, the differences in aroma seemed to fade as the beers sat out, leaving me to rely more on taste and had me wondering if the lightstruck character had dissipated. As much as I enjoyed the version of this beer fermented under cover, it took everything in me not to dump the lightstruck batch before I finished collecting data. Yuck!

| DISCUSSION |

A simple task: pour yourself a glass of pale beer, something cheap and evil is fine, or a tasty homebrew works just as well. After taking a refreshing sip, place the glass of beer in direct sunlight and start a timer for 45 seconds. While waiting for the timer to beep, pour a similar amount of the same beer into another glass, except leave this one out of the sun. When the timer goes off, grab your sunbathed beer and sample it next to the other sample. Smell and taste a difference?

Skunking is real. Through both organoleptic sensory evaluation as well as objective lab analysis, we know that UV exposure produces perceptibly high levels of 3-MBT in beer dosed with hops, hence some larger breweries rely on hop extracts to circumvent this risk. While the most commonly discussed vector for UV exposure is packaging, the fact tasters in this xBmt were able to tell apart beers fermented in either a dark environment or in direct sunlight validates the concern that lightstruck character can develop during fermentation.

Given my personal experience with these beers, I was surprised more people weren’t able to tell them apart, as the skunk aroma known to be present in green bottled quaffers was rather easy for my biased palate to detect. I did some research and found some talk of people having different sensitivities to 3-MBT, which could help explain the split preference for the beers in this xBmt. Although, I guess it’s possible some people just like that lightstruck character.

Even if this xBmt had returned non-significant results, there’s no way it would have influenced my approach of fermenting in an enclosed dark environment. For those who may not have a fermentation chamber and ferment in clear vessels, I have no problems throwing my support behind the recommendation to keep your beer covered.

If you have any thoughts about this xBmt, please do not hesitate to share in the comments section below!

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