What Is Coffee Fermentation?

October 22, 2026

Coffee fermentation is the stage after harvest when yeasts and bacteria feed on the sugars in the coffee cherry's fruit and sticky inner layer. It happens to every coffee to some degree, and how producers control it (oxygen, time, temperature) is one of the biggest reasons two coffees can taste completely different.

Now the longer, weirder version.

Is coffee a fruit?

Yes. The bean you grind every morning is the seed of a small red cherry. Around that seed there's skin, a bit of fruit pulp and a sticky, sugary layer called mucilage. The moment the cherry is picked, something starts happening to all that sugar. That something is fermentation.

What happens during coffee fermentation?

Microorganisms, mainly yeasts and lactic acid bacteria, break down the mucilage (a mix of sugars, pectin and proteins) and turn those sugars into compounds that shape flavor. It's the same basic idea behind bread, yogurt, kombucha and wine.

In coffee, the question isn't whether fermentation happens, it's how much, for how long and under what conditions. Moisture, temperature, time and the microbes present all change the result. Depending on the method, it can last from a day or two to several weeks.

What are the main coffee processing methods?

For most of coffee's history, there were three main ways to process coffee after harvest. Each one handles fermentation differently.

Washed. The skin and pulp are removed, and the beans, still coated in mucilage, ferment in water, usually for about 24 to 48 hours, until that sticky layer breaks down. Then they're washed clean and dried. Washed coffees tend to taste clean, bright and crisp. This is the classic Colombian style.

Natural. The whole cherry is dried in the sun with the fruit still on. Fermentation happens slowly over the two to four weeks it spends drying, so the fruit has plenty of time to influence the seed. Naturals tend to be fruity, sweet and heavier.

Honey. Somewhere in between. The skin comes off, but some of the mucilage stays on during drying. The name comes from how sticky the beans look, not from actual honey.

In recent years, specialty producers started treating fermentation less like a step to get through and more like a tool. That's when coffee got really interesting.

What is anaerobic coffee?

Anaerobic coffee is fermented with restricted oxygen, usually in sealed tanks. Producers ferment whole cherries or depulped beans, with or without water, then finish them as washed, natural or honey coffees. One version studied by researchers, self-induced anaerobic fermentation, has been linked to high cupping scores when combined with selected yeasts. And the yeasts at work in coffee fermentation produce compounds tied to fruity notes like banana and pineapple.

What is carbonic maceration coffee?

Carbonic maceration means fermenting whole coffee cherries in sealed tanks filled with carbon dioxide instead of oxygen.

The technique was borrowed from wine. It's best known in France's Beaujolais region, where whole bunches of Gamay grapes go into CO2-filled vats and fermentation starts inside the intact fruit, giving soft, fruity wines. Coffee producers applied the same idea to whole cherries, and it went from niche to famous in 2015, when Saša Šestić won the World Barista Championship in Seattle with a carbonic macerated Sudan Rume from Colombia.

What is co-fermented coffee?

Co-fermented coffee is fermented together with other ingredients, like fruit, so their flavors and microbes become part of the process. This is where you get coffees that taste unmistakably like watermelon or tangerine. (Our Rebel is one of them.) It's also the most debated corner of specialty coffee: some people love the intensity, others prefer coffee that only tastes like what the farm and the variety give it.

Does fermentation always make coffee taste better?

No. Great fermentation can turn a good coffee into an unforgettable one: more fruit, more florals, more sweetness, more of that "wait, what?" feeling.

Bad fermentation shows up fast too. Some bacteria produce off-flavors, including a potato-like taint, that bury everything else. Getting it right takes clean equipment, ripe cherries, constant monitoring and careful drying. And for small farms, a batch gone wrong isn't a fun experiment. It's a real financial loss.

That's why the process name on a bag only tells you part of the story. "Anaerobic" doesn't guarantee wild flavor. The result depends on how carefully the fermentation is managed. The producer's skill is what makes the difference.

Why fermentation matters to Raro

Fermentation is a big part of why Raro exists. It's where producers take risks, get creative and make coffees you won't find anywhere else. When we cup samples, the process is one of the three things a coffee has to nail to make the cut, and it has to show up in the cup, not just on the label. (Here's how we select our coffees, step by step.)

The farms we work with cover the whole range, from strictly classic processing to lab-grade anaerobic fermentations and co-ferments. Meet the farmers behind Raro.

Curious what fermentation tastes like? Come find out. Shop Raro

FAQ

Is all coffee fermented?
To some degree, yes. Every processing method gives microbes a chance to act on the fruit's sugars. Washed coffees ferment to remove the mucilage, and naturals and honeys ferment while they dry.

Is washed or natural coffee better?
Neither. It's taste. Washed coffees tend to be cleaner and brighter, naturals fruitier and heavier. Try both and see which one you reach for.

What does anaerobic coffee taste like?
Often fruit-forward. The yeasts at work in these fermentations produce compounds linked to notes like banana and pineapple. But every lot is different, and the producer's skill makes the biggest difference.

Why does some coffee taste like fruit?
A mix of the variety, where it's grown and how it's processed. Fermentation is one of the biggest levers producers have to bring fruit flavors forward.