Some of our very favorite foods and beverages are fermented: chocolate, wine, beer, bread, sauerkraut, kimchi, and… coffee? Well, yes and no: While fermentation is involved in the production of a majority of the world’s coffee, coffee itself isn’t a fermented beverage like spirits or kombucha are. Instead, when we talk about coffee fermentation, we’re referring to what the coffee fruit undergoes between the time that the cherries are harvested and the seeds are fully dried and ready for export.

The word “fermentation” describes the process of metabolization of sugars and other compounds by microorganisms like yeasts and bacteria, which will consume those compounds and convert them into fuel for their own use, leaving behind useful by-products like ethanol and different acids. Those converted compounds are absorbed into the cellular structure of the seed and, when heat is applied to those seeds in the roaster, the compounds will be what transform into the flavors we love in coffee: Complex tastes, fruity acids, and other good, or at least interesting, stuff.

Understanding Coffee Fermentation, Processing, and Flavor

While it’s an integral part of coffee’s post-harvest processing and plays a large role in the development of coffee flavor, fermentation is not a very widely understood aspect of coffee’s production cycle, though fermentation does occur in almost every style of coffee preparation. We often use the words “fermentation” and “processing” interchangeably and incorrectly: While they are related, they’re not identical. Fermentation is a natural occurrence that is undertaken by living microorganisms that are in search of energy sources; processing is a purely agricultural set of mechanical and technical steps designed to prepare a crop product for export. While fermentation can be used as part of processing, that distinction is important.

When we think about fermentation, we try to consider many variables, such as: the ripeness of the fruit; the ambient temperature in the environment as well as the temperature in the coffee as it is; say, piled or soaking in fermentation tanks or spread on patios drying, the local population of microorganisms; the water activity and moisture content throughout the drying process; and even more. It is a very difficult process for producers to track, and so most of the ways we discuss fermentation are colloquial, anecdotal, or “layman’s terms.”

The microorganisms that perform fermentation processes are found almost everywhere, and they will begin to consume fuel as soon as they have access to it, which means that fermentation can actually begin before the coffee cherry is even picked. The more fruit material that is exposed to the environment and to the population of microorganisms (when the skin is removed from the cherry, for instance), the faster this process can happen.

Different types of bacteria and yeast populations—which will vary based on location, climate, and health of the local ecosystem—consume different compounds and, so, convert them into different by-products, but generally speaking they will continue to ferment whatever useful material is available to them until they are no longer able to survive. For instance, in a Natural process coffee, they will ferment the fruit material around the seed until either all of the fuel is metabolized or until the environment is too dry for them to live. In a Washed process, the fermentation might happen all the way through the drying process, depending on how much mucilage was left on the parchment after the washing was completed.

Anaerobic Fermentation

To showcase anaerobic fermentation, we visited La Chumeca in Tarrazu, Costa Rica. A special thank you to Martin Urena and His son Emilio who allowed us to follow them through the making of their delicious anaerobic natural coffees! 

Interest in innovative post-harvest processing styles is building across the coffee supply chain. Producers and consumers are seeing benefits like unique flavor profiles that add value to coffee, help producers, processors, mills, farms, and coffees stand out from the crowd, and help diversify offerings. 

 One practice gaining recognition is anaerobic fermentation, meaning coffee is fermented in a low-oxygen environment for some period of time. To ferment in this way, fresh coffee is loaded into a container that is then sealed. Closing the container off to the ambient environment allows the carbon dioxide released from the fermenting fruit to build up and concentrate in the space. No oxygen (O2) is allowed to enter, and whatever oxygen was in the tank at the start is pushed out by the building carbon dioxide. This creates a low-oxygen environment and fosters the development of different microbial species than those common to oxygen-rich environments.

Microbes are the force responsible for how fruit tissue breaks down, the compounds and flavor precursors that are released during fermentation, and nearly all other fermentation outcomes. Therefore, creating an environment for fermentation that favors certain microbe species over others can result in unique flavor profiles – exactly what we see in well-cared-for anaerobically fermented coffees. 

A Note on Terminology 

When we use the term “anaerobic” in coffee processing, we are only referring to one of the steps the coffee has gone through. We would be more accurate if we said something like, “an anaerobic step for X duration of time,” or, “fermented in an anaerobic environment for X period of time.” 

Fermenting in a low-oxygen environment for any amount of time does not tell the whole story of that coffee’s post-harvest treatment. Going through the anaerobic fermentation step does not change the need to decide on the main processing method to use – coffees fermented anaerobically are still processed as washed, honey, or natural. 

 Saying a coffee is anaerobically fermented does not reveal all the steps the coffee may have been put through before, during, or after the fermentation. There is great variation in this process from farm to farm and country to country. For example, a koji step could have been done before anaerobic fermentation. During fermentation, yeast or other additives could be introduced to the tank. Coffee could also be removed from the anaerobic tank and be fermented again for some time, say, in an open-air tank. 

 Additionally, the fermentation process is technically (chemically) anaerobic, meaning that no oxygen molecules are utilized in the chemical reactions involved in fermentation. Some more technically and chemically minded folks might say that the term “anaerobic fermentation” is redundant – fermentation is already chemically anaerobic. However, if we remember that the space the coffee is fermented in is what we are referring to by using the term anaerobic, we’re all back on the same page. This is just further reasoning why it’s more accurate to use something closer to the phrases mentioned above and refer to the space the coffee is fermented in as being low-oxygen/anaerobic, as well as the time this was done for than to call a coffee “an anaerobic coffee.” 


Different fermentation tank materials have different benefits. Stainless steel is common, as is thick plastic. Plastic bags that tie off and have no holes can also be used. 

 Decisions about which materials to use are made based on several considerations, such as scale, the ability to clean the tanks and processing space (stainless steel doesn’t rust and cleans/sterilizes well), and temperature retention (there’s a big difference in temperature conduction and insulation potential between metal, thick plastic, thin plastic, concrete, tiled concrete, etc.). Much of the decision-making, however, is based on resources available to the person doing the processing. Producers and processors without the purchasing power for specialized tanks might use plastic barrels or buckets with lids that seal well, a common byproduct of food service sold relatively cheaply. Others have luck using plastic bags like GrainPro, designed to restrict oxygen from entering when tied off well at the top.  

 Tanks built for anaerobic fermentation are often outfitted with one-way valves that allow oxygen to escape the vessel but not enter. The valves also release some of the CO2 created during the fermentation process, ensuring the container doesn’t burst with the pressure of the gasses as they build.


Time is one way to measure fermentation. Monitoring the pH, Brix (soluble solids, mainly sugar), temperature, and visible cues like color, taste, smell, and bubble activity are likely even more critical as indicators that demonstrate when coffee is beginning to ferment, when fermentation is well underway, and when it’s nearing its end.


Because anaerobically fermented coffees require so much infrastructure and special monitoring, and tank space is often limited, these coffees are usually sold as microlots. We view anaerobic fermentation as a modifier to processing methods, resulting in coffees that are often tart, creamy or juicy in texture, with high fruit or wine-like sweetness. Anaerobic fermentation is one of the most recent ways that producers differentiate their offerings while expanding the spectrum of flavor attributes we can experience in the cup. This video aims to provide accurate context to a relatively new coffee production practice and celebrate the industry’s evolution through education.  

Fermentation and Processing

On our Processes page, we attempt to outline when and how fermentation occurs during the most commonly found post-harvest processing methods, while also recognizing that there are no hard-and-fast rules in coffee, and your mileage may vary. In our coffee traceability profiles, we attempt to share as much information as can reasonably be gathered and provided by producers, knowing that there is not an efficient and concise way to share all the relevant details.

Click the links below to read more about fermentation in specific coffee processes, and for more background information on our fermentation musings, refer to our blog “Understanding Fermentation and Coffee.”