Explained · On the label

Coffee fermentation, explained.

Every coffee ferments after picking: yeasts and bacteria break down the sweet fruit around the bean. What the label calls washed, natural or anaerobic says how the farm steers that process. The newer methods shut out the oxygen, add cultures or end the fermentation with hot and cold water. The coffee comes out fruitier, sometimes wilder, and the processing tastes louder than the origin.

The classics

Washed: the fruit comes off before drying. Clean, clear, bright acidity
Natural: the whole cherry dries in the sun. Fruity, sweet, often like berries or wine
Honey: the skin comes off, some of the sticky fruit stays on. In between, with a lot of sweetness

The new words

Anaerobic: fermented in a sealed tank, without oxygen
Carbonic maceration: whole cherries under CO₂, borrowed from wine
Thermal shock: a warm bath, then a cold one, to stop the fermentation
Co-ferment: fermented together with fruit, spices or cultures

Anaerobic

Whole cherries, or beans still covered in sticky fruit, go into a sealed drum or tank. As they ferment they release CO₂. A valve, often just a hose ending in a bucket of water, lets the gas out and keeps oxygen from getting in. Without oxygen, other microbes take over, lactic acid bacteria among them, and they make other flavours: more fruit, more body, sometimes something boozy. It can take days, at some farms much longer.

Carbonic maceration

A kind of anaerobic fermentation, borrowed from winemaking, where Beaujolais is made this way. In the strict sense, whole, intact cherries go into a tank filled with CO₂, so they ferment from the inside. Saša Šestić won the 2015 World Barista Championship with a carbonic maceration coffee from Colombia and made the word known in coffee. On bags today it is used more loosely, sometimes for depulped beans as well.

Thermal shock

After fermenting, the beans go into a warm bath, usually 40 to 50 °C, and then into cold water at 5 to 15 °C. The jump in temperature stops the fermentation on cue, before it turns vinegary. Diego Samuel Bermúdez of Finca El Paraíso in Cauca, Colombia, made the method known. Thermal shock coffees taste clean and very aromatic.

Example	What happened to it	Note
DAK, Milky Cake (Castillo, Finca El Paraíso)	Three days anaerobic in water with a milk culture, then a thermal shock wash	Vanilla cake, pistachio, cardamom
Nas Jaafar, World Brewers Cup 2026	Geisha from Finca Nuguo, Panama, anaerobic natural	Brewed with the winning Switch recipe

Co-ferment

Here the farm adds something to the tank: fruit, spices, or cultures of yeast or bacteria. The flavour then comes partly from outside the bean. Some drinkers love it, others find it is no longer about the coffee. If you care, look for the word on the label or ask the roaster.

Taste it yourself

The easiest way to learn fermentation is to taste the same coffee processed differently. That is exactly what James Hoffmann and Lucia Solis did with the Fermentation Project: one lot from Guatemala, processed four ways, sold as tasting sets by over a hundred roasters. The live tasting was on Hoffmann's YouTube channel on 3 October 2026. With your own coffees, cupping side by side does the same: a washed and an anaerobic coffee from the same country next to each other show the difference fastest.

FAQ

What does anaerobic coffee taste like?

Usually fruitier and heavier than a washed coffee from the same farm, often with notes of tropical fruit, wine or something boozy. How strong depends on how long and how warm it fermented.

What is the difference between anaerobic and carbonic maceration?

Anaerobic only means without oxygen and covers many setups. Carbonic maceration is one of them: whole cherries in a tank filled with CO₂.

Why are these coffees more expensive?

Tanks, temperature control and the extra work cost the farm money, and not every batch works out. DAK notes on Milky Cake that the producer raised the price significantly.

Sources: anaerobic and carbonic maceration follow Royal Coffee New York and Ozone Coffee, thermal shock specialtycoffee.id (July 2026), the example DAK, the Fermentation Project Sprudge and KW Coffee Collective, the champion's coffee Roastopedia.