A stuck fermentation can be one of the most frustrating aspects of the brewing process for homebrewers and professionals alike. In this article, we are going to explain exactly what a stuck fermentation is, what causes it, how to tell if your fermentation is stuck and if it is how to fix it…
What is a Stuck Fermentation?
A stuck fermentation is when the fermentation after having started correctly has now stopped before reaching your final gravity. Meaning the yeast has basically stopped doing its job of converting all of the available sugars in the wort over to alcohol and CO2. If you did nothing to restart or fix the issue and proceeded to bottle your beer it would taste quite sweet, be at a much lower ABV than you intended, and not taste very appealing.
The good news is that there are methods you can use to restart the fermentation, but before doing so it is important to make sure that you do indeed have a stuck fermentation on your hands…
How to Tell if Fermentation is Stuck?
The only way to confirm that you have a stalled or stuck fermentation is to take gravity readings with your hydrometer. Take your hydrometer readings for 3 days in a row if the readings are the same each day or very close, meaning the specific gravity is not dropping or is barely doing so then you have a stuck or very sluggish fermentation.
This is one of the reasons a hydrometer is a very important tool for brewing. Another reason is to make sure you hit your OG (original gravity). It is important that you are taking an original reading of your wort. You might believe you hit your target OG, but in many cases, you can overshoot it and end up with a higher OG than you planned. If you are not aware this happened then you might mistake a stuck fermentation for one that actually finished fermenting but at a higher final gravity (FG) than the recipe called for because your OG was higher.
Is It Actually Stuck? Or Just Finished on Non-Fermentable Sugars?
Before you go trying to restart anything, it is worth making sure you actually have a problem to fix. A lot of the time a fermentation that looks stuck is really just… done.
Here is what trips people up. Your wort is not made up of one single type of sugar. Some of those sugars, like maltose, your yeast gobble right up. But your wort also contains longer sugar chains called dextrins, and standard brewing yeast simply does not have the enzyme needed to break those down. The yeast eat everything they can, and then they leave the dextrins behind. That leftover sugar is exactly what gives your beer its body and mouthfeel, and it is also part of why your final gravity lands where it does.
So if you pitched healthy yeast, held a good temperature, and your beer has parked itself around 1.008 – 1.012 and refuses to budge, there is a very good chance it is not stuck at all. It has hit its natural final gravity and finished. This is completely normal, especially if you mashed on the warmer side.
That is the big tell. If you have already warmed the fermenter up, given it a few extra days, and the gravity still will not move, that usually points to non-fermentable sugars rather than lazy yeast. A truly stuck fermentation will almost always respond to warmth and fresh, healthy yeast. A finished one will not, because there is nothing left in there that your yeast is capable of eating.
What If You Want It To Finish Drier?
Let us say you have confirmed the beer is finished, but it landed higher and sweeter than you wanted. You have a couple of options.
The best fix is to prevent it next time, and it starts at the mash. Your mash temperature controls how fermentable your wort is:
Mash cooler, around 148 – 150°F, and you create more simple sugars the yeast can actually ferment. The result is a drier, more attenuated beer with a lower final gravity.
Mash warmer, around 154 – 158°F, and you create more of those unfermentable dextrins. The result is a fuller-bodied, maltier beer that finishes at a higher final gravity.
So a lot of the time, the finish you want is something you build right at the mash rather than something you fix later.
But what if the beer is already in the fermenter and you want to dry it out anyway? This is where an enzyme called glucoamylase comes in. Glucoamylase does the one thing your yeast cannot. It breaks those big dextrin chains down into simple, fermentable sugars, which the yeast can then ferment as normal. Add a measured dose to the fermenter and you will often see gravity start dropping again, sometimes all the way down toward 1.000.
This is the same tool commercial breweries use to make light beers and bone-dry brut IPAs. A word of caution though. Glucoamylase is powerful and it does not know when to stop. It will keep converting dextrins into fermentable sugar for as long as it is active, so if you overdo it or leave it too long you can end up with a thinner, drier beer than you bargained for. Follow the dosing recommendations, and understand that you are trading away body for dryness.
For most everyday batches you will never need it. But it is a great tool to have in the back pocket for the day you want to take a beer drier than your yeast can manage on its own.
What Causes a Stuck Fermentation?
The bottom line is there is something wrong with either the yeast itself or its environment is not providing it with ideal conditions to perform its work effectively.
Yeast is a brewer’s best friend however they have to be treated right and given the right conditions to do their job correctly. If the right conditions are not present they may not go into action converting the sugars or they may become too stressed doing so. If either of these things occurs you can end up with a stuck fermentation.
Typically the problem will come down to one of the following issues:
Not enough yeast was pitched into the wort.
Too much yeast was pitched, which resulted in excessive activity at the start of fermentation, creating a large amount of krausen that blew out through the airlock taking a large percentage of yeast with it.
The yeast used was unhealthy or even dead.
The fermentation temperature is too cold resulting in the yeast remaining dormant (not waking up) or sluggish.
The fermentation temperature was too hot, which causes the yeast to finish its work much too quickly and also creates off-flavors.
You pitched the yeast into hot wort. It is important to cool your wort to the temperature range indicated on the package of yeast. If you pitch the yeast when the wort is too hot you can kill it.
How to Prevent a Stuck Fermentation
Basically do the inverse of what we just covered.
Make sure the yeast you are using is still within its expiry date, do not use old and expired yeast.
Store your yeast properly until brew day. We recommend keeping it in your refrigerator.
Use the right amount of yeast. Tools like brewer’s friend can help you calculate the proper amount, however, for a 5-6 gallon batch of average strength beer 4 – 6% ABV, one package of yeast is all that is needed.
Pitch the yeast only after you have cooled the wort to the correct pitching temperature as indicated on the yeast package. Usually well below 80 degrees F and in most cases under 75 degrees F.
Keep the fermentation temperature at ideal levels. Typically between 65 – 75 degrees F. The manufacturer of the yeast will list the temperature range on the package.
How to Fix a Stuck Fermentation
To restart a stuck fermentation you can try the following:
The simplest and typically most effective method to restart your fermentation is to move your fermenter to a space that has a room temperature between 68 – 75 Degrees F. The majority of the time when it comes to ales the problem is the temperature is too low. Lagers are a different story altogether. Here is a post on how to brew a lager.
The second most used method is to add additional yeast to the fermenter. You can try adding a new package of yeast, however, this will not always work. Alternatively, you can implement a method called Kräusening. This is where you make a small yeast starter in a separate 1 – 2 quart container. When the starter has reached high Kräusen you add all of the contents of the starter into your fermenter. This method should get your fermentation unstuck very quickly.
You will hear some brewers recommend the following two methods but we do not recommend them as they can create off-flavors in your beer.
Some brewers will open the lid on the fermenter and stir the yeast with a sanitized spoon. The belief is that adding oxygen and putting the yeast back in suspension will revitalize the yeast and get them working again.
Other brewers will add yeast energizer to the fermenter. When they do so they again give it a good stir which oxygenates the wort and can also allow for other bacteria to infect the batch.
Frequently Asked Questions
Should I stir my homebrew during fermentation?
No, it is not recommended. When you open the lid of your fermenter you run the risk of allowing bacteria and other contaminants to get into your brew. The other thing that stirring does is oxygenate your beer, which you do not want to have happen. The only time oxygen is good for your beer is at the very start of the fermenting process when you pitch the yeast, any other time and it can infect your beer and cause off-flavors.
Why is my homebrew not bubbling?
There can be a host of reasons it is not bubbling, the first of which is you have a stuck fermentation as we talked about in this article. Another very common reason is that you have a leak in your fermenter that is allowing the CO2 to escape out instead of the airlock. Let your beer finish fermenting and then between brews check for leaks. You can tell if your beer is fermenting by taking hydrometer readings.
How can I speed up my slow fermentation?
It depends on what you mean by slow fermentation. If it is stuck or sluggish follow what we outlined in this article. If your yeast is good and you have your fermenter sitting in a space that is within the correct temperature range then you will just need patience. Some beers take longer to ferment. If your gravity has not moved 3 days in a row you can be assured fermentation has ended. Having said that two weeks in the fermenter is what we always recommend as it not only assures all fermentation will be completed but it also allows your beer time to condition properly.
Additional Reading:
How long to ferment beer?
Pressure fermentation explained
How to ferment beer
The Final Word
A stuck fermentation is something that every brewer will eventually have to deal with. Knowing how to deal with it is the key. The first step is to take hydrometer readings to make sure that it is actually stuck. If it’s stuck and you are sure you used healthy yeast, try moving your fermenter to a warmer location in your home, 9 times out of 10 this will fix the problem
P.S. If you have not done so yet be sure to take advantage of getting copies of the recipes for my personal top 5 best-selling beers from my brewpub. Details are on the side of the blog or at the bottom if you are on your smart device.