Anaerobic Fermentation for Coffee Processing: The Flavor Impact

Fermentation is a common step in food production. It uses microorganisms and their enzymes to trigger a chain of biochemical changes in raw ingredients—often changing flavor, texture, and even shelf stability. Typical participants include bacteria and yeast, which metabolize sugars and other compounds and generate alcohols, organic acids, gases, and aroma-impacting metabolites (such as esters). You’ll see fermentation everywhere in daily life, from yogurt and vinegar to beer and wine.

In coffee processing, fermentation can happen “naturally” as part of the post-harvest workflow, or it can be intentionally controlled. During fermentation, factors like microbial activity, temperature, time, pH, and gas exchange interact—and those conditions can influence aroma, sweetness, acidity, and the overall direction of the cup.

In recent years, low-oxygen (often referred to as “anaerobic”) fermentation has become a more common tool in specialty coffee processing design. This article explains what anaerobic fermentation coffee is, how it’s used in coffee processing methods, what flavors are often associated with it, and how results can differ when anaerobic fermentation is paired with natural process coffee, washed process coffee, and honey process coffee.

Fermentation in Coffee Processing

From coffee fruit to green coffee ready for roasting, producers typically work through steps such as removing skin/pulp/mucilage (depending on the method), fermentation, drying, and later hulling (removing parchment) during dry milling. The exact sequence varies across coffee processing methods.

Within coffee, fermentation may be:

• Naturally occurring as part of drying or mucilage breakdown, or

• Deliberately controlled using vessels, timing, temperature management, and monitoring

What Is Anaerobic Fermentation Coffee?

Anaerobic fermentation refers to placing coffee cherries or pulped coffee (often still with mucilage) in a sealed or near-sealed environment with limited oxygen exchange. By controlling gas exchange, time, and temperature, producers aim to guide fermentation metabolites and aroma precursors in a way that can shift flavor expression.

In practice, “anaerobic” in coffee doesn’t always mean absolute zero oxygen from start to finish. More commonly, it means an oxygen-limited, sealed environment—sometimes with controlled venting—where oxygen levels drop as fermentation progresses and CO₂ builds up.

Carbonic Maceration Coffee and Why 2015 WBC Helped Popularize It

It’s difficult to credit anaerobic processing to a single origin story or one person. 

That said, one high-visibility moment that helped bring the concept into broader specialty coffee discussion was the 2015 World Barista Championship, when Sasa Šestić used and discussed a wine-inspired approach commonly referred to as carbonic maceration coffee. 

In this approach, whole cherries are fermented in a sealed tank under CO₂-rich conditions, creating a low-oxygen or near-anaerobic environment.

Today, “anaerobic coffee” and “carbonic maceration coffee” are often discussed together. Carbonic maceration is frequently treated as a specific expression under the broader umbrella of anaerobic fermentation methods in coffee processing. 

Aerobic vs Anaerobic Fermentation in Coffee: What’s the Real Difference?

In coffee processing conversations, “aerobic” versus “anaerobic” is mainly used to describe how much oxygen exchange is happening during fermentation.

• More open fermentation environments allow more gas exchange with ambient air.

• Sealed, low-oxygen environments restrict oxygen exchange and may include one-way valves or controlled venting.

Because oxygen can vary by vessel design and even by depth within the same container, the focus is more on practical control: sealed conditions, limited oxygen exposure, and managed release of gases.

These differences can affect which microorganisms dominate and which metabolic pathways are emphasized. That may influence how components in pulp/mucilage (especially sugars) break down and react—and how organic acids and aroma precursors are produced or retained—shaping the intensity and “style” of fermented notes, fruit expression, and sweetness–acidity structure.

Sealed systems can also be easier to standardize when well managed: time, temperature, and gas exchange can be monitored more deliberately, which can help repeatability across batches. Even then, key conditions (time, temperature, pH, venting/pressure state) still need close tracking to keep results aligned with the target profile.

What Is Double Anaerobic Fermentation?

“Double anaerobic fermentation” typically refers to a two-stage, sealed (low-oxygen) fermentation plan for the same coffee lot.

A common workflow looks like this:

1.  Anaerobic fermentation with whole cherries.

2. After depulping (removing skin and pulp), the coffee—still with some mucilage—undergoes a second anaerobic fermentation.

3. Drying begins, followed later by hulling (removing parchment) during dry milling.

Producers may use a two-stage approach to help steer aroma direction and mouthfeel structure because the material being fermented differs between stages (whole cherry vs. depulped coffee with mucilage). Whether the cup becomes more overtly “fermented,” more juice-like, or more complex in sweetness–acidity structure still depends heavily on time, temperature, microbial management, and drying control.

How the Anaerobic Process Coffee Method Works

Because anaerobic fermentation relies on a low-oxygen environment, it typically requires sealed vessels or specialized containers that make conditions easier to manage. When anaerobic fermentation is applied within different coffee processing methods, it often includes the following steps:

1. Load coffee into the fermentation vessel

Ripe cherries (or depulped coffee) are placed into a sealed container to reduce oxygen exchange and support monitoring. Common vessel materials include stainless steel and food-grade plastic.

The form of coffee placed in the tank depends on the base method:

• Whole cherry for anaerobic natural-style workflows

• Depulped coffee (often with mucilage) for anaerobic washed or anaerobic honey workflows

2. Reduce oxygen / build a CO₂-rich environment

To create low-oxygen conditions, producers may:

• Seal the tank and let fermentation naturally consume oxygen while CO₂ accumulates, or

• Actively inject CO₂ to displace oxygen (gas flushing/blanketing)

One-way valves or controlled venting are also used to release fermentation gases while limiting outside air from flowing back in.

3. Monitor and control fermentation

During fermentation, producers monitor conditions such as temperature and pH to manage fermentation progress toward a target flavor result.

There is no single standard duration. Depending on the mill and the flavor goal, fermentation commonly runs from about 18 to 96 hours, with 24–72 hours often cited as a frequent range. Longer time can intensify fermentation character, but “better” still depends on balance—sweetness, acidity, perceived alcohol/ferment intensity, and cup cleanliness.

When CO₂ is injected or builds up rapidly, pressure can rise. Controlled venting (via one-way valves or staged release) helps reduce safety risk from excessive pressure.

4. Continue with drying (and washing or mucilage removal, if applicable)

After fermentation, coffee is removed from the sealed vessel and moves into drying. Depending on the processing method, producers may wash to remove mucilage (washed process coffee) or proceed directly to drying with mucilage retained to a chosen degree (honey process coffee). Parchment is typically removed later during dry milling after drying is complete.

Flavor Profile: What Anaerobic Coffee Often Tastes Like

Under low-oxygen conditions, microbial activity can produce acids, alcohols, esters, and other metabolites while sugars and other components in the fruit/mucilage are transformed. Those changes can influence aroma direction and mouthfeel.

That said, final flavor is still shaped by multiple variables—variety, growing environment, processing details, roast level, and more—because those factors influence the starting material (precursors) and the chemical changes that follow through roasting and brewing. As a result, two coffees labeled “anaerobic” can taste completely different.

In broad terms, anaerobic fermentation coffee is often described as having more distinctive, high-impact character. Common descriptors include:

• winey fermented impressions

• tropical fruit tones

• smoother, softer-feeling acidity

• sometimes higher perceived sweetness and a fuller mouthfeel

With double anaerobic fermentation, the shift can feel even more aromatic or more obviously “fermented,” but the outcome still depends on time/temperature control, microbial management, and drying quality.

Pros and Cons of Anaerobic Fermentation Coffee

Advantages

• More vivid, recognizable aromatics and textures may emerge—stronger fruit expression, wine-like notes, or a different sweetness–acidity structure.

• Flexible pairing with core coffee processing methods (natural process coffee, washed process coffee, honey process coffee), including extended variations like double anaerobic fermentation.

• Greater potential for repeatability when oxygen exchange is limited and key variables (temperature, pH, time) are monitored—assuming the process is well managed.

Challenges

• Higher monitoring and record-keeping demands (time, temperature, pH, venting/pressure). Poor control can increase the risk of off-target results or failure.

• More equipment needs and maintenance cost (sealed tanks, valves, monitoring tools).

• Scaling can be harder due to vessel capacity limits, batch management complexity, and risk control.

• Even with similar inputs and protocols, results can still vary by farm, mill, and micro-conditions, so “anaerobic” is best treated as a flavor clue—not a guarantee.

Anaerobic Fermentation and Coffee Processing Methods: Natural, Washed, and Honey

Anaerobic fermentation is a fermentation approach inside the overall coffee processing flow. Coffee still must move through fruit removal (to varying degrees), fermentation, drying, and later hulling. Below are three common pairings.

Natural Process: Anaerobic Natural Coffee

Natural process coffee (also called dry process) is one of the oldest and simplest coffee processing methods: whole cherries are dried in the sun, fermenting naturally as moisture drops. Because skin, pulp, and mucilage remain on the seed for much of the process, fruit sugars and related metabolites can strongly influence the cup—often showing richer fruit tones, higher perceived sweetness, and a heavier body.

Anaerobic natural typically ferments whole cherries in a sealed, low-oxygen vessel first. After fermentation, the cherries move into drying, and the dried outer layers are removed later.

Often associated cup direction: more vivid aromatics and stronger fermentation character, with pronounced sweetness, fruit, and sometimes wine-like impressions. Actual results still depend on fermentation and drying management.

Washed Process: Anaerobic Washed Coffee

Washed process coffee generally starts by removing skin and pulp. The coffee—often still coated with mucilage—then ferments in a tank (sometimes in water, sometimes as a “dry” fermentation without added water). After fermentation, mucilage is washed off, and the coffee is dried. Washed coffees are often described as cleaner, brighter, and more acid-forward, with higher clarity.

Anaerobic washed coffee applies fermentation in a sealed, low-oxygen environment after depulping. After fermentation, the coffee is washed to remove remaining mucilage and then dried.

Often associated cup direction: the clarity and brightness typical of washed process coffee, combined with added fruit/fermented aromatics that can read as wine-like or more intensely fruity—without necessarily losing the “clean” structure when well executed.

Honey Process: Anaerobic Honey Process

Honey process coffee removes skin and pulp but retains some mucilage during drying. Fermentation continues naturally during drying, and the amount of retained mucilage can vary by producer.

Anaerobic honey process shifts the fermentation stage into a sealed, low-oxygen environment before drying. Depulped coffee with a chosen amount of mucilage is fermented in a sealed tank, then dried.

Often associated cup direction: honey-like sweetness and syrupy texture (often described as honey/caramel-like), alongside more pronounced fruit-wine aromatics and a different expression of acidity and mouthfeel—making the profile feel more vivid and layered.

What’s The Difference Between Anaerobic Natural, Anaerobic Washed, and Anaerobic Honey?

The biggest differences come from:

• the coffee’s physical state during fermentation (whole cherry vs. depulped coffee with mucilage, and how much mucilage remains), and

• what happens after fermentation (washing/mucilage removal or direct drying, plus drying style).

Those workflow differences help explain why two “anaerobic” coffees can present very different flavor structures even when the fermentation approach is similar.

Why Consistent Grinding Matters 

Anaerobic fermentation coffee can be expressive—sometimes intensely so. That also means grind inconsistency or residual grounds left behind from a previous coffee can have a bigger impact on perceived clarity and balance. 

If you want to taste what a specific anaerobic coffee process is actually offering—its fruit character, sweetness–acidity shape, and fermented aromatics—repeatable grinding and easy cleanup become especially useful.

Femobook’s electric grinders are designed around straightforward daily workflow, with a focus on quick, practical cleaning to help reduce retained grounds that could carry over into the next brew. 

Depending on the model, features like timed grinding can keep your routine efficient, while a wide adjustment range helps you dial in for different brewing styles—pour-over, espresso, or immersion—so you can explore how the same anaerobic coffee expresses itself across methods.