The Complete Guide to Beer Fermentation


The fermentation process involves yeast converting the glucose inside the wort into carbon dioxide gas and ethyl alcohol to provide beer with its carbonation and alcohol content. The fermentation process begins after the wort has cooled, yeast is added, and is transferred over to a fermenting vessel.

How long should my beer ferment?

Many beginning brewers wonder how long they should ferment their beer. Truthfully, we have no control whatsoever over the fermenting time. After the yeast is added, it is up to it to do the work.

However, certain conditions can be changed to lengthen or shorten the timeframe- such as controlling the fermentation temperature. It will depend on the strain of yeast you use and what you are looking for in your beer.

How to control the fermentation temperature

A very important component of the fermentation process is temperature control. It can make some of the more significant fermentation process changes, both in terms of the fermentation time and the finished product.

Each strain of yeast has its range of temperature that it works best in. Various temperatures within this range can affect different yeasts fermentation abilities. In general, the cooler that the temperature is, the slower the yeast works, while the warmer it is, the faster it will work during it’s time in the beer fermenter.

Generally speaking, the hotter that the fermentation is – especially when outside of the stated temperature range of a strain of yeast – the more likely it is that you will get unwanted attributes and off-flavors in your beer. When cooler temperatures are being used – especially when outside of the stated temperature range of a strain of yeast- sometimes you can experience extended, stalled timeframes or problems getting to normal fermentation levels. The exception is yeast strains such as Kveik as well as other certain styles such as Saisons and wheat beers.

The general rule for fermentation temperature

You should aim for the lower-middle temperature in the fermentation range of your yeas. For example, if its range is 18-22 degrees C, then you should aim for 19-20 degrees C.

As discussed above, the rule can differ for certain styles like Saisons and wheat beers. More complex fermentation procedures are often required by them.

Kveik is the recent hot trend within the brewing world. It is a ‘super yeast’ that can ferment a beer easily at over 30 degrees C without any of the normal off-flavors being attracted that you tend to get with other yeast strains, which makes it an outstanding yeast when brewing in a warmer climate.

How can I tell when my beer has finished fermenting?

One common mistake that beginner trends tend to make is they use the fermenter’s airlock to gauge the process. However, an airlock is simply a device that does what it describes. It allows built-up CO2 to escape and ensures that nothing can get in the fermenting beer.

While the airlock may be mesmerizing as it makes a glooping sound every couple of seconds, all that tells you is CO2 has escaped from the fermenter. If the fermenter has a non-perfect seal, the CO2 may be escaping while the airlock may stop bubbling.

In the end, there is just one way to tell whether or not your beer is finished fermenting – you need to use either a refractometer or hydrometer. Those devices let you check the wort and beer’s sugar levels.

The general advice on how to tell whether or not the beer is finished and ready to package is having a stable reading on specific gravity over 2 to 3 days. That will ensure that the fermentation process is complete.

Once my beer has finished fermenting, what should I do next?

It is recommended that you allow the beer to rest for a couple of days following the fermentation process. That will give the beer the chance to clear and settle with the yeast on the bottom of the fermenter. If you can somewhat reduce the temperature, we recommend that since it can help to clear the beer.

Once the fermentation process is done, you can immediately package it, allow it to age more, or add other things like oak or fruit. It will depend on the type of beer you brewed.

For a long time, it was thought that the beer should be racked following the primary fermentation inside of a secondary fermenter to eliminate the yeast cake and let it condition better for packaging. Today, the risks of possible contamination and oxidation rarely outweigh secondary fermentation’s benefits. In general, secondary fermentation is only recommended when there will be a secondary fermentation – i.e. keg or bottle fermentation.

While fermenting, what should the beer look like?

The top can be opened so you can see what is happening inside of the fermenter. However, that could change the result, and often for the worse in many cases. See-through or clear fermenters are great when you want to see the progress of the fermentation process without the fermenter needing to be opened and the beer becoming exposed to air.

What your beer is going to look like during the fermentation process is going to depend on the yeast. The following is a rundown of what the yeast does after being added to the wort:

Lag stage (0 to 15 hours)

During this stage, the yeast cells wake up and try to figure out what is happening.

As they wake up, they are looking for morning stimulants such as amino acids, minerals, and oxygen. As they are doing that they begin to realize they are surrounded by food.

No airlock activity occurs during this stage, and natural wort convection inside the fermenter occurs in just small amounts due to whatever temperature stratification remains.

If you are unfamiliar with the lag stage, you can learn more about it here: lag phase | Craft Beer & Brewing (

Growth stage (4 hours to 4 days)

The yeast starts to replicate and starts working on the wort’s sugars.

The Krausen – which is a foamy head made up of sugars and yeast proteins begins forming and growing. High C02 amounts start to be produced and the airlock goes crazy. Also, since heat is produced when alcohol is being produced by the yeast, the wort’s heat convection begins to go up and there will be a gentle rolling over the wort inside the fermenter.

This is when most of the aroma, flavor, and alcohol compounds are produced.

Stationary Stage (3 to 10 days)

By now, all of the easy sugars have been consumed. The creamy white Krausen starts turning yellow (due to the precipitated hop and malt components) and brown (due to the oxidized hop resins).

Many compounds that are considered to be off-flavors such as esters, sulfur compounds, diacetyl, and higher alcohols start to be absorbed by the yeast and they are reformed into ‘nicer’ esters and more alcohol.

The fermenting wort at this point is referred to as green beer. It does have the right balance of flavors yet. The convection and airlock begin slowing down as the yeast starts settling in long term and goes to sleep, which drops it out of the solution as the small amount of remaining food starts disappearing.

Death Stage (several weeks)

Convection stops and airlock activity might stop (or there may be some occasional bubbling). The yeast is asleep for the most part and is hanging out near the bottom part of the fermenter. The beer flavors start to mature and beer begins to clarify.

Read our other articles if brewing is new to you and you would like to increase your knowledge.