one of the old brewing kettles

Beer fermentation process

Blub, blub, blub, blub. That’s what our huge (25l) vessel of ‘beer-to-be’ sounds like when it’s becoming beer. It’s the gas escaping from the ‘brewing bucket’ through a ‘water lock’. It’s a good thing the gas escapes, else the bucket would probably burst. But why is that gas actually formed during beer brewing?

It’s all part of the fermentation process which is an essential step of making a decent beer. Without fermentation no alcohol, no degradation of sugars, no proper flavour development. In other words, no beer. So we’ll take a deep dive in beer fermentation, might be good to dig into the introduction to fermentation first.

Beer fermentation requires yeast

For beer fermentation to occur, you need yeast. As described in the in-depth post on yeast fermentation processes, yeast converts glucose into carbon dioxide and ethanol during fermentation. This fermentation doesn’t require any oxygen.

Yeasts are living micro organisms and need to be nurtured properly for them to thrive. The temperature needs to be just right and there has to be plenty of food (glucose) for the yeast to do its work. Therefore, the first few steps of beer making are mostly developed to create this great environment for the yeast to thrive in. (There’s also some flavour development where hops are added, these hops aren’t necessary for the yeast to do its work.)

Gathering ‘food’ for the yeast

In my post describing the beer brewing process you can learn all the different steps involved with beer making (be it a simplified version). The first few steps, especially mashing and sparging, are essentially steps in which ‘food’ for the yeast is gathered.

beer brewing process

In these steps glucose is extracted from the grains that are used to brew beer. The mashing and sparging assure that enzymes can break up polysaccharides into the smaller glucose molecules and makes sure that these are released from the cells.

Since the fermentation process requires glucose to occur, glucose actually is the ‘food’ that yeasts use to obtain energy.

Beer fermentation process

Once enough ‘food’ sits in the wort and the first flavour development (by the addition of hops) has finished, the fermentation process can start. Yeasts are added to the liquid and they can start consuming glucose.

However, fermentation does not always start immediately. At the start oxygen can still be present allowing the yeast to respire instead of ferment. When respiring yeasts use oxygen to break down glucose. No ethanol is formed here, instead glucose is completely broken down into water and carbon dioxide, getting out all possible energy from the glucose molecule.

However, since the yeast used for beer making really likes fermentation it easily switches over to fermentation when oxygen concentration is low(er) or simply when there is so much food it doesn’t have to get all energy out of the glucose molecules.

The basic beer fermentation process is the same as a normal yeast fermentation process. Glucose is converted into ethanol (alcohol) and carbon dioxide (a gas). However, a lot more happens during fermentation than just the formation of these two molecules!

Yeasts are alive

Since yeasts are living micro organisms, they don’t only ferment. They can produce a lot of different molecules that each contribute to the flavour profile of a beer. This is where experience and knowledge and understanding of the process and yeasts comes to play.

Each type of yeast produces different molecules and thus a different flavour profile of the beer. Also, the temperatures, glucose concentration and various other parameters influence how the yeast behaves and thus which molecules are formed! Some examples of type of molecules that are formed are:

  • Esters: these contribute to the ‘fruity’ aroma of a beer.
  • Ketones (e.g. diacetyl): some of these molecules can give the beer a honey or butterscotch flavour
  • Acetaldehydes
  • Sulfur volatiles
  • Organic acids
  • Fatty acids

And so are other micro organisms!

In the air around us there are a lot more micro organisms who would love access to a liquid with a high concentration of glucose! These micro organisms can infect the beer and result in a lot of off-flavours. That is why beer production has to take place in a properly sanitized environment.

brooklyn brewshop starter kit

Carbonation of beer

Even though beer can nowadays be carbonated using specially designed carbonation equipment, it can also be done by just using yeast.

As discussed above, yeast fermentation results in the formation of carbon dioxide, a gas. After the main fermentation stage though, beer is bottled and shipped and the carbon dioxide can escape from the beer again.

However, some beer types are left to ferment in the bottle. For that to be possible enough active yeast and glucose have to be present. But if they are, the yeast will again ferment glucose and form new carbon dioxide. This carbon dioxide will stay inside the beer until the bottle is opened, resulting in a carbonated beer!

The foam on top of a beer

A foam is nothing more than air bubbles dispersed inside a liquid or a solid. The foam on top of your beer is essentially carbon dioxide with a layer of water and proteins surrounding the air bubbles. This foam forms on top of the beer because of the sudden release of pressure of the beer bottle. All of a sudden the carbon dioxide can float freely. Unfortunately for the gas it is withheld by the liquid (mostly the proteins), causing this foam.

But why the ‘blub blub blub’?

The reason our home brewed beer blubs is the escape of gas during the fermentation process. A too high carbon dioxide concentration can be disadvantageous for the yeast activity. Also, you don’t want too much pressure build up inside your brewing vessel. That is why the vessel is made in such a way that carbon dioxide can escape, without any air coming in.

This is done by placing a ‘water lock’ on top of the vessel. Gas can go through the water to go outside, but nothing else will come in!


The brewer’s handbook, The complete book to brewing beer, Ted Goldammer, 2nd edition (has a lot more in-depth information on brewing beer, flavour profiles, etc.)

Lehninger principles of biochemistry, D.L. Nelson & M.M. Cox, 4th edition

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