Fermenting in carboys has become extremely common these days. They’ve gotten much cheaper and more readily available than they used to be, and they seem to be included in so many brewing kits. For that reason, I find myself (and others) constantly warning against their use when I see people using them on /r/homebrewing, /r/mead, and /r/prisonhooch.

While you technically can use a carboy or other narrow-necked vessel for primary fermentation, it is strongly discouraged. For a number of reasons, active fermentation requires plenty of extra headspace. Carboys are better suited for long-term bulk aging of your beer, mead, or wine.

Carboys do look cool. And they’ve always been quite popular amongst homebrewers – at least for as long as I’ve been doing this.

And while they were a lot more expensive and difficult to find when I first got started, it’s very easy to get them for quite cheap on both Amazon and MoreBeer.com.

But despite looking cool and seeming better for homebrew, you’re actually much better off with even a simple food-grade bucket for primary fermentation.

While your brew is undergoing active fermentation – complete with bubbling and foam – you want there to be plenty of headspace within the vessel, and a wide mouth doesn’t hurt.

All it takes is one gusher for a new brewer to learn this lesson – but take it from me, it’s a lot easier to just avoid the lesson altogether. Cleaning your brew off of your ceilings is no fun. 

However, that doesn’t mean you shouldn’t buy carboys. They’re actually great – for bulk aging in secondary.

Of course, this is not a discussion on the pros and cons of glass. I’ve already written about that. This article is a discussion on the narrow neck, and why it’s not an ideal form factor for primary fermentation. 

Why You Shouldn’t Ferment in a Carboy

As I’ve said many times in my articles: fermentation is messy.

Like I already said, all it takes is one gusher for you to learn how important it is to use a wide-mouthed vessel with plenty of headspace during primary fermentation.

1. Narrow-Necked Vessels With Limited Headspace Can Cause Blowouts

When fermentation gets vigorous, it can foam up, filling the headspace with yeast rafts, krausen, and other gunk. Particularly aggressive fermentations can get really foamy, and push that gunk out through the airlock!

You don’t want to have beer, mead, or wine spraying all over your ceiling and walls!

That’s why I always use a fermentation vessel with ample headspace and a wide mouth – and then I give myself some extra security by using a blowoff tube (at least until the most aggressive phase of fermentation is complete).

2. Narrow-Necked Vessels are Impossible to Clean

But even if you could (somehow) predict that a particular batch won’t be very aggressive, you still wouldn’t want to use a carboy (or any other narrow-necked vessel).

They’re impossible to clean!

The krausen, yeast clumps, lees, and any other sediment gets absolutely caked onto the sides and bottom of the interior of whatever vessel you use. It only gets worse the longer it sits!

Narrow-necked vessels like carboys don’t allow you to reach inside to scrub the interior. Sometimes, you can get it decently clean using PBW or OxiClean, and they do make bendable brushes specifically for cleaning carboys, but neither of those methods are truly reliable or consistent.

That’s why I always look for wide-mouthed fermenters like the Little Big Mouth Bubbler, Demon Brewing’s BrewDemon, or even just a simple plastic bucket. They provide plenty of headspace for foaming during fermentation, and when it’s time to clean them out, I can reach my whole arm inside to scrub every inch until it’s completely clean.

Why Do People Want to Ferment in Carboys?

Homebrewers often want to ferment in carboys.

I remember when I first got started, it’s what all the cool people were doing on forums like HomebrewTalk. Carboys just looked so much cooler than plastic buckets; but back then, you couldn’t get them as easily and cheaply as you can today, so I had to “settle” for plastic buckets.

When I did finally get to try them out (around 2018, I think, was when I finally bought my first one), I quickly realized the downsides to fermenting in them, and soon switched to other, more useful vessels.

But it’s not hard to see why people want to use them – at least until they learn better.

I highly doubt this is an exhausted list, but these are a few of the more common reasons people attempt to brew with carboys.

1. You’ve Seen Your Favorite YouTuber Using a Carboy

Honestly, the biggest reason people continue to use carboys for primary fermentation is probably because they see other people doing it.

You expect someone who’s brewing for show on YouTube or other social media to be knowledgeable, and whatever equipment they’re using is probably a pretty good choice.

And it sure does look good to see them all lined up on a shelf behind your favorite BrewTuber, staged nicely with perfect lighting.

But consider this: those who are actually using carboys to ferment might be opting for what’s coolest looking on camera rather than what’s actually the best choice.

However, in many cases, these guys (and gals) aren’t fermenting in carboys. They likely have something better – often even high-end – and the carboys you see lined up on the shelf are actually aging batches.

2. The Wine- or Mead-Making Kit That You Purchased Came With a Carboy

Many of us get our starts with a homebrewing kit. Oftentimes, a fledgeling brewer might not have even thought about homebrewing – until a friend or family member gifted them a kit!

However, not all of these kits are very good. Many of them are super cheap, providing low-quality equipment and ingredients to cut costs.

If you’re on the lookout for your first kit, please do your homework and get something good! I’m not very experienced with kits, but I am experienced with equipment – and have a couple of recommendations. You might try this one if you’re looking to make beer, or this one if you’d rather make mead or wine.

But, of course, do your own research!

Or better yet, buy what you need piecemeal, selecting high-quality equipment and ingredients. You’ll have a better time and make a better product, and may even save some money that way. I laid out in a previous article everything you’d need to buy for your first batch.

However, it is possible that the kit isn’t low-quality, and that the creators of the kit chose to provide carboys with one purpose – which we’ll touch on in the next segment – to give you one single vessel in which you can both ferment and age a batch.

But still, I’d suggest opting for a kit (or buying a vessel separately) that has a wide mouth.

3. You Want to Avoid Racking, and Hope to Bulk Age in Your Primary Vessel

If you only want to purchase one fermenting vessel and use it for everything, and you do plan on aging your brews in secondary, it seems it would make sense to prioritize the qualities that make a vessel best for bulk aging. After all, the batch is most vulnerable during that stage.

The most basic qualities you’re looking for in a good secondary vessel are:

• Constructed from a non-porous material – such as glass.
• Reduced headspace, to minimize potential contact with outside air – a narrow neck will solve this.

(I’ll expand on this in the next section)

However, you probably want to balance that against your needs during primary fermentation.

For this reason, if you only plan to get one fermenter, I’d strongly suggest getting a wide-mouthed vessel.

There are plenty of wide-mouthed vessels made from glass, such as the Little Big Mouth Bubbler. With a glass vessel, you’ll be able to leave the brew in it even after primary fermentation is completed – no matter how long you intend to age it.

And if you’re worried about headspace? Just don’t open the vessel until you’re ready to bottle or keg! As long as the vessel remains sealed (with a reliable airlock) the CO₂ produced during fermentation will remain trapped inside, protecting your batch from oxidation as it ages – even for long periods of time.

After all, if you don’t plan on racking the liquid, there should be no reason to open up the vessel anyways.

What Are Carboys Good For?

All of this is not to say that carboys are bad, or that you shouldn’t buy them.

In fact, I probably have more carboys than any other vessel for homebrewing!

That’s because they’re by far the most useful vessel for bulk aging in secondary.

After fermentation is complete, you want to do everything you can to prevent (or at least minimize) contact with the outside air. Avoiding oxidation is the biggest hurdle we have as homebrewers to go from making mediocre beer, mead, and wine to making something great.

That’s where the narrow neck is extremely useful: if you’re able to fill the vessel up all of the way to the neck with liquid, it’ll result in very little headspace and a small surface area. Even if the vessel is left open for a period of time, this minimizes the amount that can enter the headspace or come into contact with the liquid.

If you rack to secondary while residual CO₂ is still in solution and the batch is off-gassing, then the tiny headspace will fill with CO₂ and push any oxygen out – further reducing any chance of oxidation from the transfer.

Furthermore – carboys are typically made from glass. I’ve gone into the pros and cons of fermenters made from glass and other materials in a previous article, but in summary: for bulk aging, you kind of have to use glass or stainless steel (not plastic!).

Glass is non-porous, which means that the liquid cannot seep into the material, nor can air penetrate from outside.

If you were to age a batch in a plastic vessel, it would actually oxidize a bit over time because air can slowly penetrate the material. For some recipes, this actually isn’t a problem at all, and can produce wonderful flavors – in fact, it’s one of the characteristics of wooden barrels.

However, typically, you would not want that, especially for any batches you intend to age for an especially long time.

Conclusion

Narrow-necked vessels such as carboys should not be used for primary fermentation.

The form factor of a carboy (specifically the minimal headspace and narrow neck) make them prone to blowouts, and it makes them difficult to clean after all of that fermentation gunk gets caked onto the inside.

But the exact features that make carboys terrible for primary fermentation are what makes them perfect for bulk aging in secondary!

So hopefully I’ve caught you before you had to learn this lesson the hard way. Go ahead and keep any carboys you own – you’ll definitely use them – but get something better to ferment in!