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Stuck Fermentation

There is a problem everyone encounters in homebrewing: Stuck Fermentation.

What is it?

Fermentation is said to be stuck when primary fermentation slows and stops well before the intended final gravity. For example, a batch of Pale Ale starts out at a gravity of 1.056 that quits fermenting at 1.026. This is way too high. You'd expect it to finish somewhere around 1.014 or so. It ends up being much sweeter than intended. Much of the sugar in the batch doesn't ferment. It is still a decent beer and drinkable but let's look at how to avoid stuck fermentation in future batches.

Is it really stuck?

A stuck fermentation can have many causes but before applying damage control, make sure that it really is stuck. If your airlock activity has stopped, don't assume the worst. Perhaps...

You're hydrometer jar is too narrow.
If you test your beer and the CO2 bubbles in the beer raise the hydrometer, giving you a false reading (shake off the bubbles and it sinks again), using a larger diameter jar may resolve that problem.
Your fermenter is leaking CO2.
When you use a plastic pail for a fermenter, you may discover that the gasket leaks. It may be just enough to cause your airlock to go quiet. Use a glass carboy and rubber stopper for your primary fermenter so you can trust your airlock.
Your beer is actually finished.
Just because the airlock is quiet two days after you pitched your yeast, don't assume it's stuck. We've had a couple of batches ferment completely in 18 hours. Both of these beers involved warmer fermentation with lots of yeast.

Assuming you've eliminated these false alarms, you might really have a stuck fermentation. A genuine stuck fermentation can be summed up with a simple statement: The yeast has lost its appetite. Ever lose your appetite? Why?

The yeast is ill.
Just as your own appetite disappears when you get the flu, yeast won't ferment your wort when it's under the weather. If you've left too much sanitizing solution in your fermenter, the yeast could be literally poisoned. All sanitizer solution should be allowed to completely drain from your fermenter and transfer hoses. Here's a tip: sanitize the top of a can of cheap beer and use it to rinse out your fermenter. The beer is pasteurized and free of impurities and it won't hurt the taste.
The yeast is cold.
Many ale yeasts will go dormant if they fall below a certain minimum temperature. In the wintertime, your basement is probably too cold for fermentation if you're in the North. At cold temperatures your yeast can hibernate and the fermentation is stuck.
The yeast finds your wort unappetizing.
Many yeasts are particular in their diet. They crave a well-rounded diet. Your wort may be lacking something they need. The most frequent deficit is free amino nitrogen (FAN). Many extract only recipes lack adequate FAN for happy fermentation.
Your yeast thinks it has done its job.
Yeast strains are like humans. They specialize in different jobs. Some yeasts are strong swimmers that stay in suspension for a long time, happily fermenting away. Other strains have a strong tendency to sink to the bottom of the fermenter (flocculate) and drop out of solution. Some strains, such as Whitbread variants are a mixture of several strains. The intention is to have the flocculators drag the swimmers to the bottom to clear the beer. Those who re-use your yeast by repitching might be selecting the sinkers in favor of the swimmers.
You didn't use enough yeast.
A batch of beer is a huge feast. If you don't invite enough guests to the party, you'll have leftovers. Yeast is a mysterious beast. It reproduces through budding asexually. For this to occur, it must metabolize things like sterols, fatty acids, oxygen, and FAN. Should your wort be lacking in these nutrients, your yeast may stop reproducing, become satiated, and step away from the table.
Your yeast is short of breath.
There has been quite a bit of discussion on the role of oxygen in healthy fermentation. Ask about the role of oxygen in a room full of brewing chemists and you'll likely start a brawl. When you pitch your yeast, your wort should have a nice supply of dissolved oxygen in it. The yeast will use this oxygen to be happy. Insufficient oxygen in your wort can lead to sluggish fermentation. Once the fermentation is underway, you should avoid adding any more oxygen or the beer will be prone to becoming stale.
Your yeast has been taken over.
If your sanitation isn't up to par, wild yeasts and bacteria may have taken over the feast. This is usually a fatal problem. Sanitize, sanitize, and sanitize!

How to avoid stuck fermentation

It follows that you should avoid all the causes listed above. Here's a list of suggestions:

Keep your equipment sanitary.
Everything that touches the wort below the boiling point should be sanitized well. Be sure to allow the sanitizer to drain away or rinse it with pre-boiled water or cheap beer.
Watch the fermentation temperature.
Use a thermometer to check on the fermentation temperature often. Watch out for drafts and maintain the optimum temperature for the yeast you are using.
Give your yeast the diet it likes.
Brewers of all-grain batches usually have no problem with this. Extract-only brewers should try to use high-quality extracts that are fresh as possible. Use a commercial yeast energizer in batches. A yeast energizer is like a multi-vitamin, chock full of the stuff your extract wort lacks.
Be careful if you re-pitch.
Some strains repitch better than others do. Avoid Whitbread and other multi-strain yeasts in favor of single strains. Limit yourself to one repitching.
Make a yeast starter.
If everyone made a healthy yeast starter, we'd have fewer stuck fermentations. For a five-gallon batch of beer, you'd like to have at least two fluid ounces of clean, healthy yeast slurry. The only way to generate this much yeast from a pouch of liquid yeast or a packet of dry yeast is to grow up a starter.
Aerate your wort.
If you are a low-tech brewer, spend ten minutes vigourously shaking out your fermenter before pitching. If you are more high-tech, you can rig an aeration stone to an aquarium pump and HEPA filter and pump air into your wort. Using a five-second blast of pure oxygen through an aeration stone can make a dramatic difference.

How to Fix a Stuck Fermentation

Since it's hard to diagnose some fermentation problems, start with the easier solutions first.

  • Warm the beer up a few degrees and wait a day.
  • Rouse the yeast by rocking and sloshing the carboy around and see if this works. Sometimes, you can nurse fermentation along by frequent rousing of the yeast.
  • Pitch more yeast. If the beer is still stuck, make up another yeast starter from a fresh pack of the same yeast. When this starter has grown a collar of foam, pitch it into your beer and wait for a couple of days. This has fixed many problems.
  • Pitch different yeast. If it won't budge below a certain point, call out the heavy artillery: Champagne yeast! Pitch a starter of the stuff and fermentation will resume with gusto. Keep in mind that Champagne yeast is very alcohol tolerant and your beer might ferment too completely. The result might be a very alcoholic beer lacking in body and mouthfeel. This is a last-ditch resort.

Be patient! Homebrewing beginners are sure to encounter some setbacks. Even experienced homebrewers are prone to the occasional stuck fermentation. Remember, if you think you've found yourself at an impasse, simply take a step back and reassess the situation. Search the web forums, or contact your local homebrew store for advice. When you finally get your next batch of successful homebrew, pour it into one of our custom beer glasses! Custom beer belongs in custom beer mugs. You're taste buds will thank us!