Is Fermentation Aerobic or Anaerobic? What It Means for Your Beer

The short answer: fermentation is anaerobic. It is how yeast and some bacteria get energy from sugar without oxygen. In beer, yeast ferment sugar into alcohol and carbon dioxide. The twist for brewers is that yeast need a dose of oxygen before fermentation starts, to build healthy cells. After that, oxygen is the enemy of finished beer.

Aerobic respiration vs fermentation

Yeast can make energy from sugar in two ways. With oxygen available and little sugar, they respire. In sugary wort, they ferment.

	Aerobic respiration	Fermentation (anaerobic)
Oxygen	Required	Not used
What sugar becomes	Carbon dioxide and water	Alcohol (ethanol) and carbon dioxide in yeast; lactic acid in some bacteria
Energy per glucose molecule	About 30 to 32 ATP	2 ATP
Where in the cell	Cytoplasm, then mitochondria	Cytoplasm only
In brewing	Only briefly, at the very start	What makes beer

Fermentation yields far less energy per sugar molecule, so yeast have to process a lot of sugar to live on it. That is exactly what brewers want: lots of sugar turned into alcohol and CO₂.

Why yeast ferment even when oxygen is present

Brewer's yeast has an unusual habit called the Crabtree effect: when sugar is plentiful, it ferments even if oxygen is available. Wort is full of sugar, so yeast ferment from the start. This is sometimes called "aerobic fermentation", which is where some of the confusion comes from. The process itself is still fermentation; it just happens with oxygen in the room.

Fermentation vs anaerobic respiration

These are not the same thing, although both happen without oxygen:

• Anaerobic respiration still uses an electron transport chain, like aerobic respiration, but hands the electrons to something other than oxygen, such as nitrate or sulfate. Certain bacteria do this.
• Fermentation skips the electron transport chain entirely. The cell recycles its energy carriers by turning pyruvate into a waste product: ethanol and CO₂ in yeast, lactic acid in lactic acid bacteria.

Beer yeast ferment. They do not carry out anaerobic respiration.

So why do brewers add oxygen to wort?

Because yeast use oxygen for something other than energy. In the first hours after pitching, the lag phase, yeast absorb dissolved oxygen and use it to make sterols and unsaturated fatty acids, the building blocks of healthy cell membranes. Those membranes let the yeast multiply, tolerate alcohol and finish fermentation cleanly.

Under-oxygenated wort causes:

• Slow or stuck fermentations that finish high
• More off-flavours, such as diacetyl, acetaldehyde and sulfur
• Yeast that are weaker to harvest and repitch

How much oxygen, and how to add it

Beer	Dissolved oxygen target
Normal-strength ales	About 8 to 10 ppm
Lagers and high-gravity beers	About 10 to 12 ppm

• Shaking or splashing tops out at about 8 ppm, because air is only 21% oxygen. Fine for small, normal-strength ales.
• An air pump with a stone also reaches about 8 ppm, a little more consistently.
• Pure oxygen through a diffusion stone reaches 10 ppm or more in a minute or two. It is the best option for lagers, big beers and larger batches. Our 0.5 micron stone doubles as an oxygenation stone: connect an oxygen regulator instead of CO₂ and oxygenate the chilled wort in the fermenter before pitching.

Only oxygenate cold wort, before or right at pitching. Hot wort holds very little oxygen and can oxidise, and yeast that are already fermenting do not need it. See our guide to oxygenating chilled wort for the details.

After fermentation starts, keep oxygen out

Once yeast have used the oxygen they need, any more oxygen damages the beer. Oxidation causes:

• Stale, papery or cardboard flavours
• Faded hop aroma, especially in IPAs
• Darkening, very visible in hazy IPAs, which can turn from bright yellow to brownish
• Shorter shelf life

The riskiest moments are dry hopping, transfers and packaging. Ways to protect finished beer:

• Keep the fermenter sealed. An airlock or blow-off tube lets CO₂ out without letting air in, and active fermentation keeps a protective blanket of CO₂ on the beer.
• Dry hop without opening the tank. A HopDrop lets you purge hops with CO₂ and drop them into a pressurised fermenter.
• Transfer under pressure into a CO₂-purged keg with a closed transfer kit, rather than siphoning through open air.
• Ferment in a sealed, pressure-rated tank such as the Unitank, so the beer can be fermented, dry hopped, carbonated and transferred without ever seeing air.

Brewers who take this further, including limiting oxygen on the hot side, call it low-oxygen or LODO brewing. See our LODO guide.

Other fermentations in brewing

• Lactic acid fermentation, by Lactobacillus and Pediococcus, makes the sourness in sour beers. It is anaerobic, like yeast fermentation. See our guide to lactic acid bacteria in sour beer.
• Acetic acid bacteria (Acetobacter) are the exception: they need oxygen to turn alcohol into vinegar. That is why exposing beer or wine to air can turn it sour, and why kombucha is brewed with airflow. See brewing kombucha in a conical fermenter.

Frequently asked questions

Is fermentation aerobic or anaerobic?

Anaerobic. Fermentation is a way of making energy from sugar without using oxygen.

Is alcoholic fermentation anaerobic?

Yes. Yeast convert sugar into ethanol and carbon dioxide without using oxygen. Brewer's yeast will even ferment when oxygen is present, as long as there is plenty of sugar.

Does fermentation need oxygen?

The fermentation itself does not. But brewer's yeast need some oxygen at the start to build healthy cell membranes, which is why brewers oxygenate wort before pitching.

Is yeast aerobic or anaerobic?

Both. Yeast are facultative anaerobes: they can respire with oxygen or ferment without it. In wort, they ferment.

Does fermentation produce oxygen?

No. Fermentation produces carbon dioxide, which is why a fermenter bubbles and why the CO₂ it makes protects the beer from air.

Why use an airlock if yeast need oxygen?

Yeast only need oxygen in the first hours, and you supply it by oxygenating the wort before pitching. After that, the airlock keeps air out and lets CO₂ escape, protecting the beer from oxidation and airborne bacteria.