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Does Fermentation Require Oxygen? (Or Will It Ruin Your Brew?)

How does oxygen affect fermentation in brewing

Alcohol is produced by yeast only under conditions with low or no oxygen present in a process known as fermentation.  Yeast ferments by consuming sugar and converting it into alcohol and carbon dioxide.

Oxygen is not needed for fermentation. In fact, most organism (including yeast) only ferment in the absence of oxygen.

However, yeast strongly prefers using oxygen to produce energy and grow as it makes the process more than ten times as efficient compared to the anaerobic (without oxygen) process.

When brewing, too much oxygen exposure during the brewing process can lead to oxidation, which can ruin the flavor and aroma of your beer, cider, or wine.

And perhaps more importantly, yeast does not produce alcohol in the presence of oxygen!

That’s why it’s important to understand the role of oxygen in fermentation and how to minimize its exposure during the brewing process.

When oxygen is present, yeast will carry out a different process called cellular respiration, which converts sugar more effectively into energy for the yeast without ethanol as a by-product.

It is much easier for the yeast to use oxygen for energy, so it only ferments when absolutely necessary.

Therefore fermentation will only happen in an environment with low or no oxygen. Oxygen can ruin fermentation because the yeast prefers not to produce alcohol when oxygen is available.

Oxygen also allows other microorganisms like acetic acid bacteria to take hold more easily and spoil your brew.

Oxidation of cider can impact its color, astringency and bitterness, and can reduce the total phenolic compounds present. Oxidation can also combine with other compounds, which can impact the flavor and mouth feel of the cider.

Does Yeast Need Oxygen – Introducing Fermentation Biochemistry

During alcoholic fermentation, yeast produces ethanol (alcohol) as a by-product and carbon dioxide. Yeast cells are able to survive and reproduce using the energy generated from this process.

In the presence of oxygen, yeast will prioritize using oxygen for cellular respiration to generate energy rather than producing alcohol, which is a less efficient process for the yeast.

That’s why alcohol is only produced by yeast under anaerobic conditions, ensuring that the yeast will produce alcohol instead of using oxygen for cellular respiration.

I am a geek, so I will explain the biochemistry in a bit more detail, but nothing you really need to know as a home brewer. So read on, or skip, depending on your own geekyness!

Fermentation works on the molecular level by converting complex molecules, such as sugars, into simpler molecules, such as alcohol and carbon dioxide.

The process is catalyzed by enzymes, which are proteins that catalyze chemical reactions. The specific enzymes involved in fermentation depend on the type of fermentation taking place.

Fermentation is a way to replenish NAD+ in cells when oxygen is absent. If oxygen is present, the end product is pure CO2 and water rather than ethanol. Figure borrowed from

In alcoholic fermentation, yeast produces the enzyme zymase, which converts glucose and fructose into ethanol and carbon dioxide.

The overall chemical reaction can be represented as:

C6H12O6 (glucose/fructose) –> 2C2H5OH (ethanol) + 2CO2 (carbon dioxide) + energy

If you have done malolactic fermentation when brewing, made yoghurt, sour dough, or eaten sauerkraut, you will also have experienced another type of fermentation sometimes relevant in brewing:

Lactic acid fermentation. 

Lactic acid fermentation is an essential part of cheese and yoghurt production.

In lactic acid fermentation, lactic acid bacteria produce enzymes, which converts glucose into lactic acid and energy. The overall chemical reaction can be represented as:

C6H12O6 (glucose) → 2C3H6O3 (lactic acid) + energy

In both cases, fermentation is a metabolic process that occurs in the absence of oxygen, in anaerobic conditions.

Here’s a simplified overview of how lactic acid fermentation proceeds:

  1. Glycolysis: The process begins with glycolysis, which occurs in the cytoplasm of the cell. Glycolysis breaks down one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). This process also generates a small amount of ATP (adenosine triphosphate) and NADH (nicotinamide adenine dinucleotide).
  2. Formation of Lactic Acid: In the absence of oxygen, the pyruvate generated during glycolysis is converted into lactic acid. This conversion is catalyzed by the enzyme lactate dehydrogenase. The NADH generated during glycolysis donates its electrons to pyruvate, converting it into lactic acid and regenerating NAD+ (nicotinamide adenine dinucleotide), which is necessary for glycolysis to continue.The chemical reaction is as follows:Pyruvate + NADH ↔ Lactic Acid + NAD+
  3. Regeneration of NAD+: The regeneration of NAD+ is crucial for glycolysis to keep functioning. In the absence of oxygen, cells rely on lactic acid fermentation to regenerate NAD+, allowing glycolysis to continue producing ATP.
  4. Energy Production: While lactic acid fermentation is less efficient than aerobic respiration in terms of ATP production per glucose molecule, it allows for the rapid generation of ATP when oxygen is limited.

These fermentation processes allow microorganisms such as yeast and lactic acid bacteria to generate energy and reproduce.

But why even make these molecules such as alcohol or lactic acid when they are toxic to the yeast or bacteria at moderate amounts?

Well, they do it because it is the most energetically efficient way of getting rid of the electrons donated by the sugars they eat.

You see, electrons are not nice too keep loose in the cell, so you want to donate them to some molecule.

Normally, under aerobic conditions, oxygen accepts these excess electrons, but when no oxygen is present, some other molecule has to!

The breakdown product of the digested sugars (acetaldehyde) works as an alternative electron acceptor instead of oxygen, but leaves behind a relatively big by-product – ethanol.

Ethanol cannot be broken down further without oxygen, so the yeast will have to live with its own waste product as long as oxygen is absent.

Do yeast cells grow faster with or without oxygen?

Yeast cells grow much faster when oxygen is present. This is why I always recommend stirring your juice or wort well just after adding the yeast, but before the actual primary fermentation has stated.

The graphs below show how the presence of oxygen leads to increased growth rate and sugar consumption or fermentation/respiration rate of different popular yeasts used in wine and cider brewing:

The performance of five popular wine yeasts in the absence and presence of oxygen. Yeast generally grows faster and uses more sugar, and produces more CO2 when oxygen is present. From PhD thesis of BS Chidi 2016.

Not only does the yeast work faster when oxygen is present, it also grows faster and can achieve up to twice the total biomass (measured here as OD/optical density of the culture) when oxygen is available.

This is because, as I wrote in the beginning of this article, that aerobic respiration is much more efficient compared to anaerobic respiration.

Do fermentation vessels need to be airtight?

Fermentation typically occurs in an anaerobic environment, meaning without air. This makes the formation of ethanol much more efficient.

However, some forms of brewing, such as those that occur in open crocks or barrels, can allow for limited exposure to air.

This is because, as explained in the previous section, that during active fermentation, the CO2 will create an excess pressure inside the vessel that pushes out any oxygen before it has a chance to enter!

So a fermentation vessel does not have to be completely airthight as long as active fermentation is taking place.

The key is to limit the amount of oxygen present to prevent the growth of unwanted microorganisms.

Fermentation jars can be closed tightly without being airtight using certain fermentation lids.

The container used for fermentation should be sealed as much as possible to prevent the entry of air, but should also allow for the release of gases produced during fermentation.

Can you open the lid during fermentation?

It is generally not recommended to open the lid during wine or cider fermentation. Allowing oxygen to enter will slow down the fermentation process and increase the risk of contamination.

You should only open the lid in the beginning (before fermentation) or when transfering to a new vessel (racking) or when bottling.

Additionally, opening the lid can also introduce unwanted bacteria or wild yeast into the fermenter, which can also spoil the final product. It is best to leave the lid sealed and monitor the fermentation process through the airlock only until ready for bottling.

However, in the beginning of the brewing process, where the yeast has still not grown to its full biomass, there can be a benefit of letting in some oxygen to encourage more efficient yeast growth.

This is usually done in the beginning before making the container airtight.

After the fermentation vessel has been airtight, and ethanol has formed, there is a risk of ethanol oxidation and growth of unwanted bacteria if oxygen is let in again.

During active fermentation, when bubbles appear vigorously in the airlock, it is not such as big deal to open the lid as the large amounts of CO2 produced at this stage will quickly displace the oxygen that have entered.

Therefore it is recommend minimizing the times the lid is removed at the end stages of fermentation.

Do you need an airlock for fermentation of cider?

Yes, an airlock is necessary for fermentation of cider. Airlocks are used to allow carbon dioxide produced during fermentation to escape without allowing oxygen or other contaminants into the fermenter.

However, there are many ways to make an airlock and it does not have to be the traditional S-shaped plastic tube you might be used to.

An airlock can simply be a tube with one end through an airtight lid in your fermenter and the other end into a bucket or glass of water!

A semi-tight lid can also be a type of airlock as pressure will build up and escape, but air will not be let in.

In fact, this is how I made my first cider. Simply by leaving a rubber ring under a loosely screwed on lid! No fancy equipment needed for that…

Can you remove airlock during fermentation?

It is not recommended to remove the airlock during fermentation, but again, if it is done for a short time period during active fermentation, it might not be a big problem.

The airlock serves as a barrier to prevent oxygen from entering the fermenter while allowing the carbon dioxide produced during fermentation to escape.

Removing the airlock can allow oxygen to enter the fermenter, which can disrupt the fermentation process and potentially spoil the wine or cider.

Additionally, it may also introduce unwanted bacteria or wild yeast into the fermenter. It is best to leave the airlock in place and monitor the fermentation process through it.

Can you ferment in a closed container?

Yes, but it can be quite dangerous! Whereas secondary fermentation (bottle carbonation) is performed in closed containers, you should never perform primary fermentation in a closed container.

Airtight and oxygen-free is not necessarily the same! Never tightly close a fermenting vessel.

Fermentation forms CO2 and although much less than that of respiration, it is definitely enough to create a high pressure that will lead to rupture of even the sturdiest of fermentation containers!

For secondary fermentation, it is only safe because you have calculated exactly how much priming sugar to add so that you get the perfect CO2 formation and no more than that!

What does oxidation do to cider? Can oxygen ruin fermentation?

Oxygen mostly just allows the yeast to “cheat” us by not producing the alcohol we want in our fermentation, but it can also lead to unwanted oxidation and growth of spoilage bacteria and mold.

Oxygen can ruin fermentation by promoting the growth of unwanted microorganisms, such as aerobic bacteria, unwanted yeasts and mold/mildew because many can only really grow with oxygen or are able to outcompete the brewing yeast in the presence of alcohol.

Mold (filamentous fungi) is an example of organisms that grows only in the presence of oxygen and will therefore typically appear on the surface of a liquid fermentation.

Therefore, the presence of mold patches on top of your cider/wine/beer is a sign that too much oxygen has been let in!

mold growing on cider
If you see some hairy growth like this on your brew. Chances are that it has gotten exposed to too much oxygen!

These organisms can produce off-flavors, spoil the fermented product, and prevent the desired microorganisms from carrying out the fermentation process.

Worst case, they can be directly dangerous by producing toxins (some molds and bacteria do!).

  1. Oxidative spoilage: Oxygen can cause the wine or cider to become stale and lose its freshness and aroma by reacting with some of the flavourful compounds in the brew.

  2. Growth of unwanted microorganisms: Many bacteria require oxygen to survive (well) and all molds will only grow when oxygen is present. Oxygen also encourage the
    Therefore, you might risk pushing your fermentation off-track by letting in oxygen or even ruining it completely.

  3. Formation of acetic acid: Oxygen can cause the growth of acetic acid bacteria, which can lead to the formation of acetic acid and spoil the wine or cider. Prolonged oxygen exposure can also lead to direct oxidation of alcohol into acetic acid.

  4. Formation of other acids: Oxidation of brewing metabolites can also cause the formation of other volatile acids, which can lead to off-flavors and aromas in the wine or cider.

  5. Interference with the yeast metabolism: Oxygen can cause the yeast to produce unwanted by-products such as diacetyl, which can lead to off-flavors and aromas in the wine or cider.

  6. Premature aging: Oxygen can cause the wine or cider to age faster than desired, resulting in a loss of complexity and flavor.

Oxidation can affect the taste of cider due to the presence of oxygen. This can cause the cider to taste sour or bitter, and in some cases, like wet cardboard. Oxygen can enter the cider during bottling, depending on the technique used, which can cause oxidation flavors to develop.

Oxygen can also cause the cider to age more quickly than intended, which can result in a sour taste. In order to avoid this, it is important to use an appropriate bottling technique, such as a bottle filler, to minimize the amount of oxygen that can enter the cider.

To avoid this, the goal is to maintain an anaerobic environment during fermentation and limit the exposure of the fermented product to air.

However, some fermentation processes like wild fermentation, kombucha and sourdough bread making, require (limited) exposure to air.

The microorganisms that are used in these fermentations are able to ferment the sugars even in the presence of oxygen, but create mostly acids instead of alcohol as a by-product.

Does oxygen kill yeast?

No, oxygen does not directly kill yeast. In fact, yeast requires oxygen to grow and reproduce. Oxygen is essential for the aerobic respiration of yeast, which is a process that allows the yeast to generate energy from glucose and other sugars.

Without oxygen, yeast can still produce energy, but through a less efficient process known as anaerobic respiration, which produces ethanol and carbon dioxide as byproducts.

However, exposure to too much oxygen can have negative effects on yeast, such as producing reactive oxygen species that can damage the yeast’s cellular components and reduce its ability to grow and ferment.

Additionally, oxygen can also promote the growth of other microorganisms, such as bacteria, which can compete with yeast for resources and cause spoilage of the fermentation process.

In summary, while yeast requires oxygen for growth, too much oxygen can be harmful and can negatively affect the fermentation process.

What does oxidized cider taste like?

Oxidized cider will often have a sour taste, similar to wet cardboard. It can have a distinct metallic flavor, with notes of sherry or vinegar.

It may also have a hint of caramel or port-like sweetness. The longer the cider is aged, the more intense the oxidized flavors will become.


It is important to understand the role of oxygen and how it affects fermentation in order to produce quality beer, cider or wine.

Fermentation occurs in anaerobic conditions and oxygen can ruin brewing of especially wine, cider and beer by encouraging the growth of unwanted microorganisms, leading to off-flavors, spoilage and direct oxidation of alcohol into acetic acid.

Therefore, oxygen should be limited during the brewing process and appropriate bottling techniques should be used to minimize oxygen exposure during aging.

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