What happens if you let beer ferment too long?

The most important part of brewing is the Fermentation. This is the stage where the already cooled down wort is mixed with yeast (an action that is know as pitching) which will then “eat” most of the sugars and turn them into alcohol and CO2. Sounds pretty simple, right?

Well, it’s not. Fermentation is a very complex process that requires a lot of our attention if we want to get it right. If we want our beers to go through a healthy and stable fermentation process, we need to be able to manage certain variables while it’s happening.

We need to control the temperature of the fermentation as well as the pitching rate, and we need to give the yeast a rich and healthy wort so that it can do its thing.

If we can manage all of this, we are on a good path towards making a great beer.

In this article, I will be going over what happens if we let the fermentation go on for too long, how long it should generally be for best results, how to know when fermentation is complete, and more!

Let’s get started.

What happens if you let beer ferment too long?

Depending on the yeast strain and fermenting temperature, the whole process will take between 4 to 10 days to complete. After this, fermentation will have stopped and some other yeast related processes will take place, but you can’t really over-ferment beer since once the yeast has consumed all the available sugar, fermentation will subside.

Leaving your beer for a while longer in the primary fermentor can be good for it and it’s generally recommended (more on this later in the post).

You should also know that if you let the beer sit for too long in the same fermentor, a process known as autolysis will kick in, giving some unwanted flavors to your beer.

How long can you leave your beer in the primary Fermentor?

As long as you see fermentation activity going on (bubbling airlock, krausen and decreasing gravity readings), you can and should let the beer sit in the fermentor for as long as it needs to.

Two primary fermantations

After the main fermentation is over, yeast cells will start reabsorbing some compounds that where created during fermentation, being the most popular of these Diacetyl, which gives the beer a sweet/buttery/buttery-popcorn off-flavor, that is easily detected in the finished beer.

The recommended thing to do after primary fermentation, is to leave the beer for a few days in the same fermentor. This step is called Diacetyl rest.

After doing so, cool it down to let any remaining solids precipitate to the bottom (this should take 2 to 3 days) and then transfer to a secondary fermentor or bottle.

How long can you leave your beer in the secondary Fermentor?

If you decided to transfer to a secondary fermentor, pressurized or not, you have to transfer the beer with the least amount of yeast possible. During this maturation stage, all the solids will settle at the bottom, and the remaining byproducts produced during fermentation will be reabsorbed by the yeast.

Depending on the beer style and temperature, you can let the beer sit in the secondary fermentor for a couple of weeks to a couple of months.

Light beers, it doesn’t matter if they are light in color or alcohol content, and hoppy beers, shouldn’t be matured for more than a few weeks.

On the other hand, strong, dark, malty beers can be matured for months.

Are there any risks associated with long fermentation?

Yes, there are! Mostly related with oxygen pick up during transfer or poor sanitation techniques, but there is one other risk that is directly related to the yeast itself, known as Autolysis:

Autolysis is the self destruction, or bursting, of the yeast cell membrane which releases the contents of the cell into the beer, creating an off-flavor that can be felt as “soapy” or “meaty” in the finished beer.

The external factors I mentioned are more so related to poor brewing practices and techniques which usually end up leading to contamination due to improper cleaning and sanitation of the materials used to transfer the beer.

On that same note, oxidation usually happens when we allow for too much oxygen to be picked up during racking and transferring to a secondary fermentation vessel, and oxidation will cause your beer to have a much shorter shelf life.

What variables affect fermentation?

To make sure that we can go through the whole fermentation stage without any surprises, we need to understand what variables affect fermentation, in which way they do so and how to control them.

Ptching yeast to a new batch
  • Type of Yeast: Ale or Lager. That will give us a rough idea on the type of fermentation we are going to go through. Warm, fast and vigorous for ales, and slow, quiet and cool for lagers.
  • Temperature: Yeast dependent, but there’s a simple principle; the colder you ferment (9°C/48°F for lagers and 17°C/62°F for ales) the slower the fermentation will go. This will produce fewer compounds that could be transferred to the beer. On the contrary the warmer you ferment (14°C/57°F for lagers and 24°C/75°F for ales) the faster the fermentation will go.

There are some others factors that are good to take into account, and although they don’t affect fermentation directly, they can help us decide how to ferment our beer properly:

  • Alcohol content: not all the yeast strains tolerate the same levels of alcohol, so we need to choose the right type of yeast to avoid unhealthy and uncompleted fermentations. This is of huge importance when we are brewing strong beers like Tripel, Quadrupel, Barley Wines or Doppelbocks.
  • Type of Beer: some beer styles require a specific strain of yeast. For example, a Bavarian wheat beer (Hefeweizen or Weissbier) needs a specific yeast strain that produces some aromas that are typical in this style of beer (bananas and clove)and the best way to achieve this is by fermenting at higher temperature ranges (24°C/75°F).
  • Fermentation vessel: depending on the fermentor we have at home, we can decide on either transferring it to a secondary vessel, to do primary and secondary in the same vessel, or to ferment under pressure.

All of these variables have to be taken into consideration when brewing. Most of the times, when you change one of this variables, it affects the others.

As an example we can use the Hefeweizen style that we used above:

For this style you need to use a specific yeast strain that will produce the specific aromas I mentioned. Temperature has to on the warmer ranges (24°C/75°F), and as far as the fermentation vessel, the Hefeweizen requires a secondary fermentation in the bottle, so we need to choose a fermentor that will let us do it this way (a pressurized vessel, like a keg, is out of the question).

How to know when fermentation is complete

There’s a few ways to monitor fermentation: you can look at the airlock and see if it is bubbling or not, or you could take a look at the Krausen, the foam that forms during fermentation, However, the best way of monitoring your fermentation is by taking gravity readings during the whole process.

First we need to take the first reading when we pitch the yeast; This reading will give us what is called “original gravity” (OG). After doing that, we can take readings every day or every two days, and if everything is going according to plan, the gravity in those readings should be decreasing and it’s a sign that the sugars in the wort are being consumed by the yeast and that the fermentation is running its course.

To know if the fermentation is complete, we need to get two different readings with the same gravity. Ideally, these readings should be taken at least 24hs apart from each other, and they will give us what is know as the final gravity (FG).

A densimeter helps the gravity readings during fermentation
A hydrometer will help us take a gravity reading from the beer.

A good practice as well is to taste every sample that you take. This way you will know if there’s something strange going on since they should be really sweet in the beginning but should lose that sweetness towards the end of the fermentation

FAQs

Getting to the end of the post now and I would like to go over some FAQs just to clarify some doubts that you may have, answered in a quick and simple format.

Does fermenting beer longer make it strong? The fermentation process ends once the yeast has consumed all of the available sugars. So, leaving the beer for longer in the fermentor doesn’t increase it’s time fermenting nore make it stronger. What will determine how string the beer is, are the Original Gravity (OG) and the yeast used for that specific brew.

Can you ferment beer longer than 2 weeks? If it’s a lager yeast fermenting at low temperatures (9°C/48°F), it could take up to 10 days or 2 weeks to complete, and if you under-pitch the yeast, basically giving the wort less yeast than recommended, the fermentation process can take even longer than two weeks.

Can you ferment beer in 3 days? There’s a relative new yeast strain called Kveik that can ferment a full batch in less than 3 days, but the fermentation temperatures required for this yeast are in the range of the 35°-40°C or 95°-104°F, which is not that easily achieved at home.

Can you speed up fermentation? You can control some variables, like the pitching rate and temperature, to make the yeast work a little bit faster, but it’s not recommend since there’s the risk of getting some off- and unwanted flavors in your beer if you do so.

Conclusion

Fermentation is a really complex and sensitive process that requires a lot of our attention if we want to get a great end-result.

A small batch of beer fermenting

We can accelerate or slow down the fermentation by controlling the temperature and by choosing a specific strain of yeast for the type of beer style we want.

But once the yeast is pitched and the fermentation begins, there’s not much we can do. We just need to monitor it and wait.

Hoping that all of this information is used towards making your next brew taste even better!

Cheers!

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