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What Are Yeast Rafts And How Do You Prevent Them?

What Are Yeast Rafts And How Do You Prevent Them

Home-brewing is sometimes a very tricky business with countless pitfalls – one of which is yeast rafts. 

Fortunately, yeast rafts don’t necessarily mean that you’ve got a bad batch of beer, so don’t dump it just yet! 

What Are Yeast Rafts?  

What Are Yeast Rafts  

Yeast rafts (or yeast traps) are clumps of yeast that band together and float to the top of the wort during fermentation. They happen frequently during the brewing process and are perfectly normal. 

Yeast rafts often occur when a bunch of yeast has hitched a ride on a bubble of CO2. CO2 is also a good sign for your beer, and so again, yeast rafts are perfectly fine and nothing to worry about. 

Does Yeast Sink Or Float?

Does Yeast Sink Or Float

It depends on whether the yeast is dead or alive. Dead yeast doesn’t float but instead sinks to the bottom of your wort, whereas living yeast does not. As noted, sometimes it hitches a ride up the top on a CO2 bubble. This means that if you see yeast rafts, your wort is hale and hearty, so don’t fret!

Should I Be Worried If I See Yeast Rafts In My Brew?

Should I Be Worried If I See Yeast Rafts In My Brew

As stated – not at all. Yeast rafts are nothing to worry about and are actually indicators of a good batch of home brew. However, that’s not to say that a lack of yeast rafts means that there’s anything wrong. Their appearance is completely random. 

However, some brewers do not like them because they are aesthetically unappealing, and may even put friends and family off from trying your beer. Obviously, that’s something to be avoided.

There’s no one kind of beer that’s at lesser or greater risk of developing yeast rafts – it can happen in a stout, a lager, or anything in between. 

How Do I Identify A Yeast Raft? 

How Do I Identify A Yeast Raft 

Yeast rafts (or yeast traps) tend to start showing up within the first week of the fermentation process. They vary in size but are typically opaque blobs with uneven, fluffy edges. They are usually of a lighter colour than the wort in which they float. 

Yeast traps will generally disappear once the krausen kicks in. 

What Is Krausen And How Is It Different To Yeast Rafts? 

What Is Krausen And How Is It Different To Yeast Rafts 

Krausen is the foamy head that builds on top of your wort when fermentation hits its peak. It is a normal part of brewing and usually happens 12-36 hours after your yeast is pitched. Krausen covers the entire surface of your beer (unlike yeast rafts, which are sporadic) and will bubble up (again, unlike yeast rafts, which are static). 

Yeast rafts and krausen smell pretty much the same – like beer. 

How Do I Distinguish A Yeast Raft From An Infection? 

How Do I Distinguish A Yeast Raft From An Infection 

It’s difficult to categorize what constitutes an ‘infection’, since one brewer’s infection is another’s intended effect. For instance, sour beers rely on the introduction of specific bacteria that can cause an ‘infection’, but it doesn’t mean the beer is bad. 

One thing that you might mistake for a yeast raft is a pellicle. It’s a layer that forms on top of the beer and can vary in appearance, but is often slimy or greasy, off-white in colour and may have spider-web-like strands connecting the filmy bubbles that form on top. 

It’s important to note that a pellicle does not mean that your beer has gone bad, necessarily. It’s likely that wild yeast or bacteria has ‘infected’ your wort, but that needn’t be the end of the world. It may simply be that you’ll end up with a sour beer, instead. 

The only way to really be sure if a beer has gone bad, ultimately, is to smell and/or taste it. If it smells or tastes bad, then it’s time to dump it. 

How Do I Distinguish Yeast Rafts From Mould? 

How Do I Distinguish Yeast Rafts From Mould 

Mould is unambiguously a bad sign – there’s no wrangling a nice sour beer out of a batch that’s got mould on it, and if you see mould then it’s not a good sign (though not necessarily a sign that you should dump the whole batch). 

Mould takes a while to develop, and so if it’s been a few days, it’s unlikely that it’s mould. Mould also looks very different to yeast traps – it’s white or green and fuzzy, unlike fluffy-edged yeast rafts. 

If there is only a little mould, you can skim it off and taste the beer to see if it tastes OK. If the beer is fine, eliminating the mould on the surface should take care of the problem (mould cannot survive in an atmosphere with alcohol, so there is no chance of any below the surface).  

How Do I Aerate My Yeast? 

How Do I Aerate My Yeast 

It’s important to aerate your yeast during primary fermentation. This is because yeast is aerobic (requires oxygen) during this period to grow, multiply and ferment. 

Aerating your yeast is a straightforward process – you can simply stir the wort after adding the yeast to ensure that the mixture is oxygenated. It’s also possible to use a pump or introduce pure oxygen, if necessary. 

How Can I Prevent Yeast Rafts? 

How Can I Prevent Yeast Rafts 

There’s no sure-fire way of preventing yeast rafts from forming. They will often drop off on their own, and by the time your krausen forms they are no longer an issue. The best thing is to treat them for what they are – a sign of living, thriving yeast that means your fermentation process is coming along nicely! 



Though yeast rafts might not be the most aesthetically pleasing sight when you open up the carboy to check on your wort, they are in no way a sign that anything is wrong. Though there may be legitimate concerns that you’re dealing with an infection rather than yeast traps, this is quickly and easily ascertained with a few checks. And remember – as long as it doesn’t taste or smell bad, then there is nothing at all to be concerned about. 

Bottom line: yeast rafts are perfectly normal, nothing to worry about, and quite likely inevitable. Don’t worry about them and wait for your delicious brew to be ready! 

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Miranda Sharp

Miranda Sharp

I'm an Editorial Assistant based in South East Asia having travelled all over the world. I mostly cover the LATAM timezones managing the content side of things here. On weekends, you will find me watching Grey's Anatomy and plethora of Netflix soppy dramas or munching on dishes I would have doled out from MasterChef

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