Should Fermenting Beer Be Kept In The Dark?



If you’ve ever fermented anything or known someone who did, you’ve probably heard a dark environment is best for the task. If you’re new to homebrewing, you may be wondering if this rule is also true for beer. I know I certainly had questions about it when I first started my own brewing journey.

Maybe you’ve got a bit more brewing experience under your belt now and are just curious about whether you’ll do any damage to your brew-baby if you, say, transfer your beer to your secondary fermentation vessel when the room isn’t black as pitch.

Or perhaps you want to ferment in the dark but don’t have the facilities to do so at the moment. Is it pointless to attempt homebrewing in that case? 

Whoever you are and for whatever reason, you’re reading this article, the gist of the answer is this: 

Fermenting beer should be kept from direct UV light as much as possible. Brewing vessel opaqueness matters when choosing a storage location, with glass carboys requiring darker environments or coverings. More opaque containers may be kept in dark corners of open rooms but still out of direct sunlight. 

But again, that’s just the gist. I want to go into the whys behind the answer because the more informed we are, the more we can make the best decisions when it comes to our brewing equipment and methods. 

Frankly, we want beer that tastes good and is capable of making us drunk if called on to do so. Allowing light to hit our beer can put a damper on those plans.

Why should you keep fermenting beer in the dark?


Flavor 

Long story short: direct UV light during fermentation will cause fermenting beer to take on a pronounced skunky flavor. 

Now, maybe some of you IPA fans out there are saying, ‘Nothing wrong with a little skunkiness!’

But that’s the thing: lightstruck beer (as it’s called when direct UV light hits beer) isn’t just ‘a little’ skunky, and the kind of skunky we’re talking about here is not the flavor that reads to your palate as ‘bitter hops’ and makes you smack your lips with pleasure.

Ever smelled a skunk? Evolution gifted them with the ability to spray a sulfur compound at anything that might even possibly kill them. That sulfur stench is a Mother Nature-approved legitimate weapon!    

Do you know why hops give beer its bitter flavor? They contain compounds called isomerized alpha-acids. (Actually, for the sake of technical accuracy, hops contain alpha-acids that get isomerized when we boil our wort.)

Anyway, when beer is exposed to light, these isomerized alpha-acids undergo a reaction and get converted into an off-flavor called 3-methyl-2-butene-1-thiol.

Chemical names being the literal, helpful things that they are, the “-thiol” portion is an indicator for the presence of…. (take a guess!) …. 

sulfur.

The same stuff that has kept skunks on this planet since the late Pleistocene era.

Skunks think sulfur is pretty awesome. Beer drinkers, not so much. 

Yeast health

Also, let’s not forget that yeast, another of the key components in alcohol production, is a fungus. Anyone who’s ever accidentally grown a fungus has probably done so after leaving something in a dark, moist environment for a while (i.e. your foot in a sock, God help you).

Point being, as homebrewers, we want yeast to proliferate. And that means being aware that they’re picky about temperature and that light stresses them out. We could seriously interfere with our fermentation process if we don’t take that into account. 

What to ferment your beer in

There are two main choices for brewing containers: carboys and fermenting buckets. Carboys have a narrower neck and are usually glass or plastic. On the other hand, buckets are—as you may have observed for yourselves—opaque. 

Now that you know the dangers of lightstruck beer, you may be wondering why on earth anybody would want to ferment in a carboy, a vessel that isn’t opaque and thus takes in more light.  

Honestly, there are advantages to both containers, so please check out this article I wrote on the topic. You’ll find that plenty of homebrewers prefer carboys, and tons of other brewers go for buckets. 

I myself like to use different fermentation vessels in different situations. If I’m cooling wort overnight, which I’ve done during the winter months or when my wort chiller was on the fritz, then a tempered glass fermenter is better.

This is because there is less likelihood of leaching of chemicals as might happen with a plastic when adding wort at high temperatures. However, for the most part I’d go for a plastic fermenter as they are less likely to break despite being a tad more difficult to clean.

By far the best models out there for a reasonable budget is the Big Mouth Bubbler from Northernbrewer.

You can now pick this up on Amazon and have it delivered right to your door, check out the latest price here.

One note, though. Plastic carboys can take on a smell after a while, but there are some ways to get rid of odors (which I explore in this article.

Where to store beer while it’s fermenting?

Where you keep your fermenting beer really depends on whether you’re using a carboy or a bucket as your fermentation container, but there are some generalizable rules for any case. 

You want somewhere dark, temperature-controlled, and stable. By ‘stable,’ I mean that the container won’t be moved for whatever reason—not transferred to another table when it gets in someone’s way, not bumped by an unwary passerby or reachable to a toddler hellbent on experimenting with force and gravity.

You get the idea.

Let’s break it down in more detail, looking closer at how you can maintain darkness, temperature control, and stability by brewing vessel type.

Dark

If brewing in a bucket, the natural opaqueness of the container works to your advantage. I’ve heard of homebrewers keeping brewing buckets in, say, a corner of their living room that doesn’t get hit by direct sunlight. Their beer turns out just fine.

I personally used to keep it in at the bottom of a shelving unit with a little curtain covering the fermenter before I upgraded to a temperature-controlled chest freezer (I use this Inkbird ITC -308 temp controller that I got off Amazon).

If brewing in a carboy, you’ve got to be more careful. You definitely want to go as dark as possible with these: a dim corner of a basement, for instance, or a closet. 

If there is an uncovered window anywhere near that carboy, you’ll need to get it (the window or the carboy, but preferably the carboy) covered up ASAP. (More on carboy coverings later).

Temperature-controlled

The wrong temperatures can sabotage fermentation. If brewing an ale, make sure your temps stay roughly within 68 to 72 °F (20 to 22 °C). Lagers need 45 to 55 °F (7 to 13 °C). 

However, different yeast strains will have slightly different temperature ranges, so always check any instructions you have on the packet and aim for around the middle number.

I’m currently brewing a SMaSH beer using Safale s-04 which gives me an ideal range of 59°F-68°F (15-20°C), so I aimed for 63.5°F (17.5°C) with a 2°F (1°C) range on my Inkbird, so the cooling/heating kicks in when it reaches 61.7 °F (16.5°C) at the low end and 65.3°F (18.5°C) at the high end.

And don’t forget that the fermentation process itself is exothermic (i.e. heat-releasing), so it can raise the temperature inside your brewing container by up to 8 °F (4 °C) higher than it reads on the outside.

Buckets and carboys should both be stored somewhere you can control the temperature. In either case, a fermentation chamber (more on those here,) can be an extremely helpful piece of brewing equipment, especially in a climate where the ambient temperature isn’t brew-friendly. 

Stable

The reason we don’t want our fermentation vessel to be moved is because that could shake the fermenting beer inside. 

Why is shaking bad? Well, think about what happens if we beat eggs with a whisk or shake up a beverage in a bottle—we’re causing aeration.

The only time we want to allow oxygen into our beer is after we’ve boiled the wort but before we’ve pitched the yeast. Any other time and we’ve got trouble. 

There’s more to it, so check out this article I did on the topic for the full story. 

Ways to reduce the light exposure of your fermenting beer

What can you do if you’re brewing in a place with no dark storage area—no closet, basement, etc.? 

Worry not! You’ve got options! 

First, why not brew in a fermenting bucket? It’s opaque, so half the work is done for you. Even if you don’t want to buy a dedicated brewing bucket, you can go along to your local Costco and find food-grade containers that will do exactly the same job (and they tend to be cheaper- just need to drill an airlock hole).

Barring that, cover your carboy with a blanket or t-shirt, or even a cardboard box. 

If you want something a little more long-term, get light-blocking drapes for your brewing room, if that’s an option.

There’s also a trusty beer cooler. Those are nice and dark inside. 

Should you switch from a primary to secondary fermenter in the dark?


If you’re doing secondary fermentation and it’s time to switch containers, there’s no need to do it in the dark. That’s overkill. But try to avoid doing it outside, moving the fermenters around (could stir up the dead yeast) and splashing (could cause oxidation).

I’ve more to say on the topic of switching to secondary fermenters, but I already did so here, so why not pop over to have a look? 

Do artificial lights affect fermentation?


A good rule of thumb is to remember that any light in the UV range—even artificial—can skunk beer, and the Sun is obviously the biggest offender. 

Not every type of artificial light will have this effect, though. 

It’s all about the wavelengths. 

The range of light wavelengths that tend to produce skunkiness is around 350 to 550 nanometers. At 350 nm, you’re in the high end of the UV range. At 550, you’ve reached the visible range.

Halogen and incandescent lights tend to have longer wavelengths that don’t cause beer to skunk, but be aware that they do give off some skunk-level wavelengths. Still, these lights are less likely to produce off-flavors in fermenting beer.

However…LED or fluorescent lighting generally has peaks at around 400 nm. Beer is more likely to take on a skunky flavor when exposed to these two.

So if you’re fermenting in a carboy, be sure to cover it up if the room is lit by LED or fluorescent lighting. 

Where to store bottled beer after fermentation?

We should store our bottled beer somewhere dark and temperature controlled. 

Right after you’ve capped the bottles, keep them somewhere at fermentation temps for two weeks for the carbonating process. If you have a fermentation chamber, which really did change my brewing effectiveness, then ideal put the bottles in there.

I literally stacked my beer, when I bottled it, in the chest freezer and put cardboard on the lower levels to get all 60 odd bottles in.

After that, keep them in a cool and dim place. If you want to find out more about whether or not you need to refrigerate your beer after you’ve finished fermenting it, check out my article here.

How long can you ferment beer?

It normally takes 7-14 days, but you can age higher ABV beers in a secondary fermenter.

Now, most homebrewers nowadays don’t even use secondary fermenters but there is something to be said for the fact that in some cases fermenting in primary for too long because that can cause grassy off-flavors.

It’s a personal choice you need to make and the real issue is oxidation (from transferring the beer) versus off-flavors from autolysis.

A lot of homebrewers like to start fermenting right before they take a vacation. Is that okay? Read about that here.

Can light make your beer stop fermenting?

Light won’t stop fermentation, but other things will: issues with temperature, lack of oxygen before fermentation, and old yeast are the main culprits there. 

If your beer isn’t fermenting, there are some things you can do to fix it.

Phil - BeerCreation

Hey, I'm Phil. I'm passionate about all things beer. I love making it, drinking it and best all, learning about it!

Recent Posts