What Are Yeast Rafts or Traps (And Are They a Bad Thing?)

If you’ve noticed floating blobs on the top of your beer a few days after pitching yeast, you may be wondering if your brew has gone bad. 

Yeast rafts (or “yeast traps”) are simply clumps of yeast that float to the top of the wort during fermentation. Yeast rafts are a frequently occurring, normal byproduct of fermentation and not a cause for alarm. Before tossing out a batch of homebrew, it’s important to learn to recognize the differences between harmless yeast rafts and infection.

Read on to learn about what causes yeast rafts and how to distinguish them from larger issues.

What is a yeast raft or yeast trap?

Typically the clumps that you see at the top of your homebrew are totally normal.

Yeast rafts develop when clumps of yeast gather together and float on top of fermenting beer. Though they may look alarming, yeast rafts do not indicate dead or dying yeast, quite the contrary.

Because dead yeast sinks, the presence of floating yeast rafts indicates your yeast are viable and alive. 

Yeast naturally clump together as they float around in your beer, and the relatively large surface area of these clumps means they can easily hitch a ride on a bubble of carbon dioxide and float to the surface.

The presence of CO2 bubbles is yet another indicator that your yeast are healthy and fermentation is proceeding as expected.

Is it okay if my homebrew beer has yeast floating in the fermenter?

Yeast rafts are not a problem, and they’re not an indicator of a problem. They will not affect the final flavor or consistency of your brew at all.

The only downside of yeast rafts for some is that they are aesthetically unappealing and may even cause observant friends and family to be wary of your homebrew. 

Yeast rafts can present themselves in any style of beer or with any type of yeast and may or may not ever show up at all. The lack of yeast rafts does not necessarily indicate an unhealthy brew, so don’t worry if you never see them or if you brew in an opaque container and aren’t able to watch the early stages of fermentation.

To ease your fears and thirsty compatriots, remember that fermentation is a quite active process with lots of motion involved. This can kick all sorts of things to the surface of your beer.

As a homebrewer, it’s important to learn the tell-tale signs to look for to determine what is harmless and what is cause to dump a batch down the drain.

How to identify a yeast raft or trap

Yeast rafts typically show up within the first week or so of fermentation.

They look like opaque blobs and can be as small as a sesame seed or as large as a silver dollar with ragged, sort of fluffy edges. They typically are lighter in color than the wort on which they float.

Yeast rafts will rarely cover the entire surface of the beer. Rather, they will float like individual rafts and will almost always disappear on their own once fermentation intensifies and the krausen develops.

Krausen refers to the thick head of bubbles and foam that forms on and above the wort at the peak of healthy fermentation. Krausen, unlike yeast rafts, will not only cover the surface of the beer but also bubble up into the empty space in your fermenter. All of this activity is a sign of a healthy brew. 

Yeast rafts and krausen will smell, well, like homebrew! They should have the sweet, malty aroma of your wort combined with the slightly bready funkiness of yeast. In short, your fermenting beer should always smell pretty much like beer.

Not all floating particles in beer are yeast rafts, though. Some floaties may indicate an infection or unwanted bacterial growth, but these are typically easy to identify by smell and visual qualities that make them quite distinct from yeast rafts. 

Is it yeast rafts or mold?

Most people have experienced the unpleasant discovery of unwanted mold on an old loaf of bread or a forgotten container of takeout in the back of the fridge. The same thing can happen to homebrew. 

Much like yeast rafts, mold typically only develops on the surface of the beer. This is because the alcohol in beer eliminates most types of mold. Mold can be light brown like yeast rafts but more often will appear green or white. Unlike yeast rafts, mold is almost always fuzzy. Its fuzziness is typically very apparent and not questionable. If you don’t see fuzziness, you don’t have mold.

It’s good to remember that mold also takes a long time to develop, much like on your leftovers. If you’re questioning what’s floating on top of your homebrew after a few days of fermentation, it’s doubtful to be mold and much more likely to be harmless yeast rafts.

Some say a little bit of mold can be skimmed off the top of homebrew in the interest of saving the rest of the batch. However, it’s always best to smell and even taste a small sample of your brew if even a small amount of mold is present.

If it smells rancid or off, toss it. If it tastes rotten or unpleasantly musty, toss it. If your brew has been in the fermenter for months on end and mold has taken over, definitely toss it. When in doubt, dump it and start over.

Is it yeast rafts or infection?

It can be difficult to tell the difference between a yeast raft and an infection if you’ve never experienced either in your homebrew before.

The yeast raft will typically develop fairly early on in the fermentation process and will be isolated sections of yeast floating around the top of the wort.

Infection in your homebrew, however, will exhibit at least one of the following characteristics:

  • Oily sheen
  • Development of a biofilm over the entire top of the wort
  • Pellicle (a hard, slimy, or even ropy layer that forms on top of the beer)
  • Mold
  • Unsavory smells

How to tell if your homebrew is infected

So much of learning to homebrew involves repeatedly reading about the importance of thoroughly cleaning and sanitizing your equipment. After a few successful batches, you may feel able to relax your cleanliness standards a bit. Don’t.

The vast majority of unwanted infections in homebrew come from the introduction of bacteria from unsanitary equipment or tools. Every item, from the fermenter it spends weeks in to the hydrometer, which may only be in your beer for a few seconds, should be fully sanitized.

Even with careful sanitation, it is still possible, and sometimes even desirable, for microorganisms to develop in your beer. Sour beers, for example, cultivate the wild yeast Brettanomyces (or “brett” as brewers typically call it) on purpose during fermentation. This provides the pleasant funkiness that is part of the flavor profile of sour beers.

But brett and bacteria like lactobacillus can develop during the fermentation of other beers as well. They will often cause the development of pellicles or slimy, round whitish blobs that grow out of a film on the surface of your beer.

Always thoroughly read your homebrew recipe to determine if pellicles are to be expected. If not, your beer may have been inadvertently infected by wild yeast in the air, oxygen, or unsanitary equipment.

Like mold, these will look very distinct from yeast rafts and will have a notably different, funkier smell. If pellicles grow on your fermenting beer unexpectedly, you may need to dump that batch and try again.

Be sure you are not unnecessarily exposing your fermenting beer to oxygen and that everything it touches has been thoroughly sanitized.