What Happens If You Let Beer Ferment Too Long (Can Beer Over-Ferment?)

Homebrewing beer can be a rewarding experience for beer enthusiasts, but sometimes the time to produce that one perfect gallon of beer can be difficult to pace. You know that it’s important to avoid under-fermenting your beer, but is it possible to over-ferment?

Homebrew beer cannot over-ferment because once the yeast has consumed all sugar, fermentation will end, a process that normally takes between 1-3 weeks. However, leaving beer inside the fermenter for many weeks or months after completion of fermentation can produce off-flavors and increase the chances of infection.

Keep reading to learn how to monitor the fermentation process and tell when it’s complete, so you don’t spoil your homebrew.

How long can beer ferment before going bad?

Yeast’s primary fermentation time generally lasts between three and seven days.

After four days, the initial fermented state is complete, but the beer is not yet ready to be consumed. At this point, the result is “green,” or young beer.

This beer still needs time to mature to develop the right flavor profile. Young beer can require one to two weeks in secondary fermentation, during which unwanted byproducts settle out. These products are not harmful or toxic to ingest, but they can make your beer smell or taste bad. It is important to allow this process an adequate amount of time.

Smaller quantities of beer are stored soon after fermentation because the process is quicker. Still, some variables can extend the process for some beers, even in smaller batches.

Variables affecting the process of fermentation

Every beer style, recipe, and even batch takes a unique amount of time to ferment.

Some of the variables that can affect the fermentation process include the:

  • type of yeast
  • fermentation temperature
  • alcohol content (ABV)
  • type of beer
  • type of fermenter

Type of yeast

Yeast comes in either dry or liquid forms. Dry yeast is more common than liquid yeast because it is easier to store and use and contains nutrients for proper fermentation.

Liquid yeast is somewhat more delicate, but not without some benefit. Certain yeast strains only come in liquid form, which gives certain beers their unique flavor.

If you are using liquid yeast, consider that you may need to wait a longer fermentation time. If the dry yeast takes three or four days to ferment, liquid yeast can last up to ten days.

Remember also to take one more week of conditioning into account.

Fermentation temperature

Another crucial factor is temperature.

Since yeast is a living being, its reaction is influenced by the fermentation temperature. Yeast tends to react faster at higher temperatures. On the contrary, it has slower activity at low temperatures.

A common ale beer is fermented at around 21°C (69.8°F). Many homebrewers do not control the fermentation temperature very closely, accidentally allowing some fluctuation. Without a professional setup, it is normal to have higher or lower temperatures. At a higher temperature, yeast can finish fermentation in up to 18 hours.

However, faster is not always better. Be sure to note the recommended temperature range for your yeast. If you ferment at higher temperatures, the yeast can produce excess esters, a chemical compound that brings an undesirable flavor to the beer.

Alcohol content (ABV) 

Alcohol by volume (ABV) measures the amount of alcohol in a certain volume of liquid.

Beer with a high ABV takes more time to ferment.

Generally speaking, the most popular beers are 4-7% ABV, with the average being about 5%.

Type of beer

Strong beer is made of wort with a high original gravity (OG). This high OG results from a high content of sugar in the wort.

The yeast has to consume these sugars to make alcohol and leave the beer with a much lower final gravity (FG). As it takes time for the yeast to process the sugars, it makes sense that a strong beer requires more time to ferment.

Type of fermenter

The most effective fermenter is a tall vessel with a conical bottom. 

A wide fermenter with a flat bottom is likely to take longer because the pressure on the yeast cells is lower in spacious fermenters.

What is yeast autolysis?

Leaving beer in the fermenter for too long increases the chance of autolysis, a process in which the yeast cells’ vacuolar membranes disintegrate and release hydrolytic enzymes, causing the cells to burst open, releasing the content into your beer. Sounds unappetizing, right?

Check out this resource for more information on autolysis!

How long should beer ferment in the primary?

Ale can be left for a week in the primary fermenter, whereas lager requires more time. 

The difference between the two types of beer is that lager beer ferments at low temperatures, which produces a lighter drink. Their fermentation timelines are different because lager yeasts work better at around 10° Celcius (50° Fahrenheit), so the fermentation takes more time. A common lager has a total fermentation period of 3 to 4 weeks, of which 10 to 14 days in primary.

How long can you leave beer in the secondary fermenter?

The secondary fermenter is used to clean the beer from the sediment from primary fermentation, including dead yeast cells, hop sediment, and bitter malt sediment (tannins). An excess of these leftovers (or “slurry”) can negatively affect the final taste of your beer.

The beer is left to mature for another seven days in the secondary fermenter before distributing in bottles. During this process, you have to handle the wort with care, to minimize the risk of introducing excess oxygen. Over-oxidation can change the flavor of the beer.

Proper use of the secondary fermenter when brewing at home mitigates the risk of yeast autolysis. In commercial beer preparation, brewers may take additional steps to avoid this issue.

When is fermentation complete?

The best way to avoid overdue fermentation is to monitor the process consistently.

Here are some of the easiest ways to tell when your fermentation is complete: 

  • measure the wort gravity
  • taste the result
  • check the airlock

For a detailed explanation on how to tell when fermentation is done, visit this article.

Measure the wort gravity

The most accurate way to check if fermentation is complete is by using a hydrometer to measure the gravity of your brew.

If the gravity has dropped to a stable final reading, the yeast has completed its work as indicated in your recipe.

Taste the result

If you do not have a hydrometer on hand, you can try a taste test.

If your wort has limited or no sweetness, it means that the yeast successfully converted the sugars during primary fermentation.

Check the airlock

Once fermentation is complete, the yeast ceases to produce carbon dioxide.

During fermentation, the airlock bubbles as gas escapes. When complete, the liquid in your airlock should be nearly or entirely still.  

Once the yeast is added to the wort, it can take up to 12 hours for activity to start, after which you will notice activity in the airlock over the next few days. As the yeast nears the end of fermentation, it slows down and settles to the bottom of your fermentation vessel.