Anaerobic and anoxic fermentation in coffee

Also known as: Anaerobic fermentation, Oxygen-free fermentation, Sealed-tank fermentation, Carbonic maceration

Anoxic coffees are fermented in sealed bags, tanks or barrels limiting the oxygen, before drying as a washed, honey or natural. Controlling oxygen, time and temperature lets producers shape fermentation precisely, often creating intense, layered flavours that traditional processing can't reach.

How it works

  1. Ripe cherries are picked and sorted
  2. Whole cherries or pulped seeds are sealed in tanks, barrels or bags
  3. Oxygen is pushed out, by CO₂ produced during fermentation or by flushing with CO₂
  4. The coffee ferments for a set time, often 48 to 200+ hours, with pressure, pH and temperature monitored
  5. Some producers add selected yeasts, or rinse in hot then cold water (thermal shock) to finish
  6. The coffee is then dried as a natural, honey or washed coffee
  7. Dried coffee rests before hulling, sorting and export
Coffee fermenting without oxygen (anoxic) in blue sealed barrels on El Vergel Estate, Colombia

How it affects flavour

Anoxic fermentation tends to amplify flavour: deep tropical and red fruit, and sometimes a winey or liqueur-like character, with a dense, syrupy body. Well-controlled lots stay clean and complex; longer fermentations become bolder and more divisive.

The full story

Fermentation happens in every coffee process; the difference here is control. In an anoxic fermentation, coffee is sealed in an oxygen-free environment, usually a tank or barrel fitted with a one-way valve. The microbes working on the fruit sugars change what they produce when oxygen is absent, and that shifts the flavour compounds that end up in the seed. By adjusting time, temperature and whether the cherry is whole or pulped, producers can push a coffee in very different directions.

Anoxic or anaerobic? The two terms are often used interchangeably. Strictly, "anoxic" describes the environment (no oxygen), while "anaerobic" describes how microbes behave without it. Because fermentation itself doesn't require any oxygen, "anoxic" is the more precise term for a sealed, oxygen-free fermentation environment. That's why we use it on our labels.

Variations you'll see on our coffees:

  • Carbonic maceration: borrowed from winemaking. Whole cherries ferment in a sealed tank flushed with carbon dioxide, which often brings vivid red fruit and a winey character.
  • Thermal shock: after fermentation, the coffee is soaked in hot water and then cold, which is said to help lock in aromatics. It's associated with experimental Colombian producers.
  • Yeast inoculation: selected yeast strains, sometimes from wine or beer, are added to steer fermentation towards particular flavours.
  • Extended fermentation: some lots ferment for 100–250 hours for maximum intensity.

These methods have helped producers earn premiums for distinctive lots and experiment with varieties like Gesha, Sidra and Pink Bourbon. They're also debated: some drinkers feel heavy fermentation can mask a coffee's origin. We look for lots where the processing adds complexity while the character of the farm and variety still comes through.

How the processes compare

ProcessFruit removedTypical cup
WashedBefore dryingClean, bright, high clarity
HoneySkin only; mucilage left onSweet, rounded, medium body
NaturalAfter dryingFruity, heavy body, sweet
AnoxicVaries; sealed fermentation firstIntense, layered, spiced fruit

Coffees to try

FAQ

What does anoxic coffee taste like?

Usually intense and fruit-forward, with a winey edge and a syrupy body. The exact profile depends on time, temperature and the drying method used afterwards.

Is anaerobic coffee the same as anoxic?

Yes. "Anoxic" is the more precise term for fermentation in a sealed, oxygen-free tank.

Are flavours added to anoxic coffee?

Not in genuine anoxic fermentation; the flavours come from the fermentation itself.

What is carbonic maceration in coffee?

A type of anoxic fermentation borrowed from winemaking, where whole cherries ferment in a tank flushed with carbon dioxide.

Explore other processes

Want to go deeper? Processing is covered in detail in our SCA Green Coffee courses.