For new homebrewers, it’s always a surprise to see active fermentation for the first time. It can get gross, and without prior experience, it can be difficult for a new brewer to figure out what’s a normal part of fermentation, and what is the result of an infection. One common question you can find on homebrewing forums and subreddits is, “is my beer/mead/wine infected?” with a picture of some sludge floating in it. Is this sludge normal? Is it safe?

If you see clumps of white, tan, or brown sludge floating on top or in your wort or must, your batch is not infected or contaminated. These are yeast rafts: they are a perfectly normal, and even desirable, part of fermentation.

Fermentation is messy, and while it’s beautiful and tasty and aromatic when it’s done and the liquid has cleared, it can be pretty offensive to all of your senses while it’s actively fermenting.

Part of this is that the yeast cells, while microscopic and invisible at the individual level, will tend to collide with each other (and any other solid or haze-inducing particles in solution) and merge together into mushy, disgusting-looking clumps of sludge that homebrewers have lovingly dubbed “yeast rafts.”

You may find these yeast rafts floating atop the liquid or swimming around under the surface. It might be one giant clump that spans nearly the entire surface, or a bunch of little clumps floating around independent of one another.

But these clumps are NOT BAD!!

In fact, they’re very, very good.

If you see yeast rafts forming and swimming around in the batch, it’s a good visible indicator that fermentation is actively happening. The yeast are alive, healthy, and have grown into a large enough colony that these yeast rafts are possible.

(The inverse is not true, however: if you don’t see yeast rafts, it doesn’t mean there’s a problem or that fermentation is not happening.)

But if you see yeast rafts in your homebrew – relax, don’t worry, and just let it do its thing!

What Are Yeast Rafts?

Yeast rafts form when individual yeast cells collide with each other (and other solid particles floating in the liquid) and then stick together, merging to form gross brown, white, or tan sludge that can float on or under the surface of your fermenting wort or must.

That’s really all it is – clumps of yeast!

Yeast rafts can also stick to or merge with any other solid particles floating in the liquid – hazy proteins from grain (adjuncts like wheat and oats will contribute heavily to this), hop particles (dry hopping causes this), or solid bits of fruit.

Some fermentations can be really disgusting-looking, with a ton of sludge floating around in the liquid, while others may not end up with yeast rafts at all. There are a lot of variables that go into it, and every batch is different.

Certain yeast strains are more likely to stay in solution and not flocculate (which reduces the likelihood of the cells sticking together and forming rafts in the first place). This often results in a much cloudier or muddier fermentation, but can have very few, small yeast rafts – or none at all!

Additionally, recipes that don’t have a lot of haze-inducing ingredients like adjuncts or late-addition hops can result in less clumping and fewer (or no) yeast rafts, since there are no extra solid particles for the yeast cells to collide with.

There are surely a number of other variables I haven’t mentioned that can impact the degree to which yeast will clump up and form rafts in your brew!

How to Determine if You Have Yeast Rafts (or Is It a Real Infection?)

Yeast rafts are very distinct formations, and are easily discernible from other formations like pellicles and mold, both of which can be indicative of contamination in your brew.

The easiest description for determining if what you’re seeing is a yeast raft (compared to anything else that might be bad or unexpected) is if it’s sludgy.

If the formations you’re seeing look like sludge, mud, or lumps of clay, and are kind of oozy, then you’re just looking at yeast rafts, and it’s a perfectly normal part of homebrew.

Additionally, if it’s floating in the liquid, under the surface, then it’s safe. Mold and pellicles only form on the surface of the liquid. Yeast rafts can also form on the surface (and they often do!) but they look decidedly different.

If the formation looks solid, like a white crust or film of some kind has formed on the surface, then you have a pellicle – which is indicative of a contamination, likely wild yeast or Lactobacillus. Click on the links for previous articles with more information on those infections!

Alternatively, if it looks like mold, then it’s mold. Mold is very obvious – it looks the same as any mold you’d see on fruit or bread. If it’s fuzzy or furry, you have mold. This is pretty much the only formation you really have to worry about. For more information on mold, check out this other article.

Are Yeast Rafts Safe?

If you find yeast rafts in your homebrew, it is perfectly safe!

Those clumpy, sludgy formations are a great visible indicator that something is happening and you’re getting alcohol.

If you see yeast rafts, don’t be afraid! Just let it ride. They will flocculate and settle out after fermentation is complete, forming a thick layer of sediment at the bottom of the fermenter and leaving behind the clear, delicious liquid that you can easily siphon off of the yeast cake.

Clarifying solutions like bentonite, gelatin, and others will help speed this process up. Cold crashing helps, as well – just make sure to do it safely!

It’s important to take note, however: just because yeast rafts are a good indicator of fermentation, the inverse is not true! Just because you don’t see yeast rafts, it doesn’t mean fermentation is not happening. As stated above, every fermentation is different, and there are a bunch of variables that can result in the yeast not clumping together and forming rafts.

Unless you see anything really problematic, like pellicles or mold, or otherwise have a measurable issue, such as the gravity not changing, you don’t have anything to worry about.

Just relax. You’ll be drinking delicious homebrew in no time!

Conclusion

If you see clumps of white, tan, or brown sludge floating on the surface of or inside of your beer, mead, or wine during fermentation, you have nothing to worry about!

These formations are a normal part of active fermentation and are nothing to worry about.

Give it time, and they’ll settle out – and you’ll have a finished homebrew, ready to enjoy.