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What is a Stuck Fermentation?

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Has your latest fermentation come to a screeching halt? Or did it never really get started? Put down the champagne yeast… we’re about to cover the causes and solutions for stuck and failed fermentations.

What is a Stuck Fermentation?

A stuck fermentation is when a fermentation starts and stalls or never gets started in the first place.

If you’ve taken gravity readings over the course of several days and you’re not seeing a drop in gravity, you’ve got a stuck fermentation.

Fermentations will sometimes never get started or stop under poor conditions. A stuck fermentation is, by definition, a failure on the yeast’s behalf to lower the gravity of a beer to its potential final gravity.

What is a Stuck Fermentation?

What a Stuck Fermentation Isn’t

Let’s go over a few things you shouldn’t confuse for a stuck fermentation.

Big Beer Syndrome

If you’ve ever brewed a big beer you may know that the FG can sometimes be quite high. I once brewed a Pastry Stout that ended at 1.069! It started at 1.166. If you do the math, that’s 12.7% ABV.

The above example is extreme. But it shows that sometimes a yeast can’t chew through any more sugar. In this case, it was likely a result of alcohol intolerance, along with a lot of unfermentable sugars.

Small Gap Syndrome

I don’t know what’s wrong? My software told me the FG would be 1.012 but the beers at 1.016 and has been for two weeks.

That’s not a stuck fermentation. The beer is done. It could be your yeast strain, software calculations, wort composition, mash schedule…keg it and relax.

Saison Stall Isn’t a Stuck Fermentation

The Saison Dupont strain is infamous for stalling. It will get to about 1.035 and sit there. The classic advice is to warm the fermentor and wait for it to finish. This can take a long time!

I say avoid it to begin with and open ferment that sucker. This isn’t just an anecdote anymore. There is plenty of science behind why certain yeast strains perform better with open fermentation. Grab some foil and do it.

How to Check if it’s Stuck

Do a forced fermentation. This may not be practical but I’ll tell you how to do it anyway.

Take a sample (250 mls or so) from the fermentor. Pitch a crap ton of yeast (an eighth of a pack or vial). This is obvious but you need to use the same strain as you did for the main batch. Ferment it warm, say 70-80 F, until finished. This will tell you what the actual FG of your beer is…or should be.

What Causes a Stuck Fermentation?

Yeast Health

Yeast health is paramount. It’s more important than how much you pitch, within reason.

The best way to ensure yeast health is top notch is to brew with the freshest yeast possible. If you’re working with some old yeast, use a starter. Actually, just use a starter anyway.

Don’t pitch a single pack or vial of 12 month old yeast into a 1.066 starting gravity and expect great results. It could stall or just make crappy beer.

Yeast Amount

Pitching the right amount of yeast is important. If you really underpitch, your lag time will be really long. Anything over 24 hours is certainly cause for some concern. Pitch more active yeast.

In order to figure out about how much yeast you should use, try the old calculator from mrmalty.com. It’s a classic and gets the job done.

Most of the time, if you underpitch a beer it will still finish. Just expect a long lag time, long fermentation, and a less desirable yeast profile.

Nutrient Poor Wort

If you made a wort with a ton of adjuncts, you may encounter a sluggish or stalled fermentation. To avoid this, add yeast nutrient to your wort in the last five minute of the boil.

To remedy this, use a yeast energizer. Energizer is not the same as a yeast nutrient. Energizer can be added directly to the fermentor in order to get the yeast going again.


I’m stating the obvious here, the temperature you ferment your beer at has a huge impact on the characteristics of fermentation. Here’s a quick example. If you pitched an ale strain at 50 F, you probably just need to warm it up to a more normal ale temp.

Sometimes just bumping up the temp by a degree or two is enough to get things rolling.

Lack of Oxygen or Lipids

Yeast need oxygen in order to function. The truth is that they can use other things, like lipids, in place of oxygen. A wort that is rich in trub (with a lot of nutrients) will require lower amounts of O2.

Most of us have been told to leave the trub out of our fermentors. So, at a minimum you should shake your carboy before you pitch your yeast.

How to Fix a Stuck Fermentation

We already covered some ideas on how to avoid a failed or stalled fermentation. Let’s look at how to fix one that is stuck.

Agitate the Fermentor

This is probably the number one piece of advice given to a young padawan with a stalled fermentation. Gently swirl up the yeast in your carboy. The idea is to get them back in suspension and in an environment where there is sugar and nutrients available for them to “do their thing”.

It’s easy enough and worth trying.

Raise the Temp

Be careful with this one. Timing is critical. If a beer has failed to start, only raise the temp as high as you’d be ok fermenting. For example, raising the temp from 66 F to 68 F is ok for an ale strain but don’t go to 75 F and expect a great tasting beer.

It is ok to raise the temp a bit more aggressively if your beer has stalled after 3 or more days of “normal” fermentation.

Temperature is very strain dependent. Work within the recommended parameters for your strain.

Pitch More Yeast

If you can get your stalled or failed fermentation to start or start again, try pitching more yeast. This time make sure you use a nice healthy and active starter. I highly recommend using the shaken-not-stirred (SNS) method for producing a yeast starter. It’s simple, cheap, and very effective.

Enter Brett and Company

If all else fails, call on Sach’s freaky cousin Brettanomyces. Things will get funky but Brett will get the job done and you’ll end up with a fully fermented beer. You may also choose to add some lactobacillus or pediococcus while you’re at it.

Final Thoughts

If there are no signs of fermentation you can wait up to 24 hours. Most aren’t comfortable waiting that long. After that it’s time to take action.

Employ one or a combination of the fixes we’ve covered. There are a lot of ways for a fermentation to go astray, but with the tools above you can get back on the road to good beer.

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