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

As producers consider the effect of fermentation more and more on the quality and profile of their coffee, they are adopting different and interesting techniques to their repertoires in order to diversify their offerings. One method that’s becoming more popular is fermenting coffee in a controlled anaerobic environment, meaning that the coffee is held in a vessel without any presence of oxygen during some of all of the duration of fermentation.

The fermentation process itself is already anaerobic, meaning that the yeast and bacteria that do the work of fermenting a coffee cherry’s sugar content do not need oxygen to be present in order to successful complete their mission or transform the organic material. (This is why we specify that it’s the environment that’s anaerobic in these cases.) One of the main benefits of holding the coffee in an oxygen-depleted environment, then, is to slow the fermentation process, which allows a totally different spectrum of flavors to express. Where in a Washed process the controlled fermentation might last 12–24 hours on average, anaerobic environment fermentation can take 96 hours or longer, depending on the thermal retention inside the tank.

Different fermentation-tank materials will have different thermal retention, and producers choose wisely based on the desired effect. Stainless steel is a common material, as is thick plastic. The tanks will contain a one-way valve that allows the producer to remove oxygen from the vessel and release CO2 created during the fermentation process; careful monitoring of the coffee fruit’s pH as well as the temperature inside the tank is pivotal to ensuring the success of the process.

After fermentation, the coffee’s processing can be completed in any number of ways: The fruit can be fully removed as in a Washed process, the pulp can be removed but the mucilage left on as in a Honey process, or the cherries can be dried whole as in a Natural. The length of drying time will vary based on this last step as well as the environment around the drying area.

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.”