Sullivan bakery's whole wheat bread, left in closed dutch oven, right open tray

How the Fermentation Process Works – Yeast & Its Byproducts

Science is everywhere when you’re in the kitchen. Just think of your bread dough bubbling away and expanding while proofing. Or a beer in the works.

You might not even recognize the science, so common the processes are. The processes described above both involve yeast and fermentation. The so called fermentation process is pretty common one in food which is why it’s time for a deep dive.

In this post we’ll be discussing fermentation reactions, the products that are formed during fermentation and what exactly fermentation is. In separate posts you can learn more about two common uses of yeast & fermentation: beer and bread.

Fermentation, yeast?

Let’s take one step back before diving deep into the fermentation process and products. Why discuss fermentation and yeast? What are fermentation and yeast?

Yeast has been used for millenia, even though we might not have known it were yeasts at play. Yeasts were used for making bread and brewing beer. Yeasts are part of the kingdom of fungi, being a specific type of fungus: a unicellular one. That is, they are made up of only one cell. They grow by splitting themselves in two.

In food yeasts have several functions, for example: production of ethanol in beer brewing or the production of carbon dioxide in bread making. The most common yeast used for these processes is the so-called Saccharomyces cerevisiae.

A side note here, fermentation can also occur without yeasts being present. A common example is lactic acid fermentation (as happens with sauerkraut). Lactic acid bacteria cause fermentation in that case.

Anaerobic vs. aerobic conversion

In order for yeasts to grow and survive they can convert carbohydrates (e.g. glucose) into energy, just like people. This energy is then required to keep all sorts of processes in the yeast going. We do this conversion mostly with the aid of oxygen, which is why we breath. Yeasts however can do so pretty ok both with and without oxygen.

When there is no oxygen the environment is called ‘anaerobic’, if there is oxygen it is called ‘aerobic’. So, yeasts, can convert carbohydrates into energy both in an anaerobic and aerobic environment.

Fermentation definition

When oxygen is used to convert carbohydrates into energy this is called ‘respiration’. Fermentation on the other hand is nothing more than the opposite: this is the process that occurs in the yeast when it converts carbohydrates into energy without using oxygen.

The interesting thing about yeasts used for beer and bread making is that they can do this conversion without oxygen even though oxygen is present. This is interesting because actually the reaction that does use oxygen is a lot more energy efficient!

What happens during fermentation?

Fermentation is nothing more than a chemical reaction. It all starts with glucose. This glucose might already be present (e.g. sugars), but the yeast might also have to convert another larger carboyhydrate into glucose.

This molecule of glucose then reacts in a series of chemical reactions, ultimately releasing ethanol and carbon dioxide! The overall (simplified) reaction scheme looks as follows:

C6H12O6 → 2 C2H5OH + 2 CO2

You can see that 1 glucose molecule (C6H12O6) is converted into 2 ethanol (C2H5OH) and 2 carbon dioxide (CO2) molecules. You can see that no oxygen is required for the reaction to occur.

Going too fast? – If you’re not familiar with chemical formulas read my post on the topic first or follow week 3 of my food science basics course.

seeded sourdough bread slices
No airy bread without fermentation!

Fermentation – chemical reaction equations

In reality, this reaction does not occur in just one step. Instead, a series of reactions leads to this overall equation. There are other molecules that are formed in between. But, these are converted again until only ethanol and carbon dioxide are left. Let’s discuss those individual steps.

Step 1: Glycolysis

First of all the glucose (remember, the molecular formula of glucose is: C6H12O6) is converted into 2 individual pyruvate molcules (C3H3O3).

Wondering what happened to those H-atoms of which there were 12 in glucose and only 2×3 in the pyruvate molecules? These haven’t disappeared but have moved on to larger molecules in the cell.

Glycolysis again is more complex than is looks. The conversion consists of 10 consecutive steps. During the reaction energy is released, which is what the overall process is all about. This energy is caught and stored by the cell for use in other processes.

Step 2: Fermentation

The glycosis process is not limited to fermentation only. Once the pyruvate has been formed it can still be converted with the use of oxygen (what we call respiration) or travel through various other reaction pathways. If the yeast is fermenting, the pyruvate will go into the fermentation process.

This next step is relatively simple, it only consists of two steps, so we’ll show them all here

C3H3O3 (pyruvate) → CH3COH (acetaldehyde) + CO2

CH3COH (acetaldehyde) + NADH + H+ → C2H5OH (ethanol) + NAD

Both these steps are catalyzed by enzymes. You might have come across enzymes before on this blog. Enzymes catalyze a lot of reactions in food, e.g. the browning of pesto or that of bananas. Again, energy is released which is stored in the process.

Notice the sudden occurence of NADH and protons (H+)? The NADH are important structures in cells that facilitate a whole range of reactions,  including this one.

Using fermentation in food production

Now that we’ve covered the basics we’ve learned that yeasts convert glucose without any oxygen (which is what we call fermentation) into ethanol and carbon dioxide. But why do we want that to happen?

Let’s stick with the two examples from the beginning again:

  • Beer: of course, we want alcohol (=ethanol) to be formed during beer production!
  • Bread: it’s all because of the carbon dioxide, carbon dioxide is a gas and this is what leavens our breads!

Controlling fermentation processes

Despite the advantages, fermentation is a challenging process since we’re using a living organism to perform the chemical reactions. When dealing with a living microorganism it is important that the circumstances are as ideal as possible for the yeast. Yeasts can die or stop fermenting if the conditions aren’t ideal.

So if you want fermentation to stop, or to continu, slow down or speed up, there are a lot of ways to influence the process!

Influence 1: Temperature

This is probably one of the most important variables. Yeasts will die at too high temperatures (most don’t survive temperatures above 50°C). Also, their growth rate depends on the temperature. If it’s too low they will barely grow, the same goes up if it’s too high. The ideal temperature (range) differs per yeast type though.

Influence 2: Alcohol content

Most yeasts can only handle a certain maximum alcohol content. If the content goes over this value they will simply stop growing or die. This is the reason drinks with a high alcohol content are generally not fully made through fermentation.

Influence 3: ‘Food’ supply

In other words, the food supply, if not enough food is present for the yeast they won’t be able to grow and thrive. This is a reason that when bottling home-made beer you often add an extra shot of sugar to the mix (after the 1st fermentation). This assures that the yeasts can ferment again and produce gases which will remain in the bottle.

It’s not only the fermentation reaction

Since yeasts are living organisms, there’s never just one reaction that takes place. During fermentation a whole lot of other processes can take place which result in the formation of flavours, aromas, etc. It’s also what makes beer and bread making and all those other processes so interesting, not only in a factory but also at home!

Good luck with your future fermentation endeavours.


Some of the sources I’ve used for writing this article: Lehninger Principles of Biochemistry, chapter 14, 4th edition; Merriam-Webster, Brittanica and

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    • Hi Molly, glad to hear you liked it! The author would be Julie, or as I refer to it in the blog, science chef.

  1. I have never read an article by you that I did not find comprehensible! You are doing a great -and valuable- job!

  2. Bakers use yeast to make bread can i get bread before alcohol formation step has reached becoz alcohol is not permitted plz any wayout

    • Hi Rutba,

      Yeast will produce some alcohol during bread fermentation, there’s no way around this, unless you make bread without yeast (e.g. use baking soda, or make flatbreads without yeast such as paratha). Keep in mind that in bread only a very small amount is made and that this will most likely evaporate during baking.

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