Beer Brewing – The 3 Stages of Fermentation.

The beer fermentation cycle was a mystery for a very long time. Master brewers knew something was converting sugar into alcohol and Carbon Dioxide but could only guess what it was.

The “mystery” of fermentation is part of the reason why brewing developed so much folklore and many rules of thumb. A master brewer would rely on his “magic stirring spoon” or the dregs of the last batch to start the fermentation cycle.

Today we know that the “magic spoon” by would have contained microscopic beer brewing powerhouses – YEAST.

In fact, the discovery of yeast’s role in beer brewing during the 1800s meant that Germany had to change its “Beer purity law”. Up until then the only ingredients legally allowed in beer production were hops, water and barley.

Beer Brewing

For a beginner brewer the fermentation cycle can be daunting. It is easy to control the quality of the ingredients. Mixing them together is also easy but how do you control billions of yeast organisms?

Once everything is mixed together in the fermenter and the yeast are added, it is natures turn to make your own beer.

Here are the two things you need for nature to do its job properly. Pitch a sufficient quantity of healthy yeast and maintain the correct temperature for fermentation. If you do these two things, your beer brewing fermentation cycle will follow the stages described below.


Beer Brewing – Zero to 18 hours. 

Typically called the Lag phase.

Beer BrewingThis is a nervous time for new brewers. Especially if it is your very first batch. A question often asked during this stage is, “Why hasn’t it done anything yet?” .

The yeast are absorbing as much oxygen as they can. They do this in preparation for growth and metabolizing the beer brewing sugars. You may not see any changes to your beer during this stage but rest assured that there is a lot going on.

The length of the lag phase will vary depending on Oxygen content, temperature and the number of yeast pitched into the beer in the beginning.


Beer Brewing – 4 hours to 4 days.

Growth and Production of alcohol plus Carbon Dioxide.

The yeast get really busy during this period. Multiplying rapidly and converting the beer sugars into alcohol, CO2 and various flavor compounds. You will see foam (krausen) produced on the top of your beer which can range in height from 1 inch to overflowing your fermenter. If you have an airlock on your fermenter, it will be making a lot of noise during this period. 

Note: If your airlock is not making noise but you can see a krausen layer, don’t be concerned. CO2 will escape from your fermenter through the easiest path. If the fermenter lid is not screwed on securely the CO2 often escapes through the threads.

Take a hydrometer reading each day and you will notice the specific gravity of your beer coming down.

During this growth phase the specific gravity can drop quite quickly.


Beer Brewing – 3 days to 14 days.

Conditioning Phase.

Once the yeast have converted most of the sugars into alcohol, CO2 and flavor compounds their growth will slow.The krausen and air lock activity will also decrease.

However, the yeast still have work ahead of them. Now that their main food source is running low, they will reabsorb compounds like diacetyl and acetaldehyde. This is good because these compounds can cause off flavors in your beer.

You can assist this process raising the temperature of your beer by a few degrees during the days before bottling. However, if you are already at the upper temperature limits for your yeast, don’t change a thing.

The conditioning phase is very important for producing delicious homemade beer.

Beer brewing


Notice that the time frames for the fermentation phases overlap significantly. This is because the phases overlap as well. At different times during beer brewing the yeast will be doing different things.

During the lag phase most of the yeast will be absorbing oxygen but some will move progress to growth and alcohol production very quickly. During the alcohol production phase some yeast will begin conditioning your beer.

To make matters more confusing for the beginner brewer. Yeast often behave differently from batch to batch. You can make two batches of the same recipe and use yeast from the same starter culture and one fermenter will go off like a rocket, producing a krausen that wants to escape the fermenter while the other just bubbles away with little or no fanfare.

The best way to determine the phase of your beer is by grabbing your trusty hydrometer and matching the reading to the observed Krausen level. During the lag phase the specific gravity will not change much. During the growth and alcohol production phase the specific gravity drops dramatically and during the conditioning phase the specific gravity will continue to slowly drop until it comes to a complete stop.  Sometimes taking days to drop one or two gravity points.


There you have it. Beer brewing and fermentation 101. The real magic of beer brewing occurs during fermentation and there is no need to be daunted by it.

If your yeast are healthy and the temperature is correct, everything will go smoothly.


Happy Brewing!

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