In coffee production, post-harvest processing is one of the most influential steps in determining the final green coffee product of any given lot. Each harvested coffee cherry contains the potential to express its flavors in a variety of ways, and post-harvest processing is how coffee producers impart their final influence on the kind of beverage that their coffee is destined to become. With so many processing methods to choose from however, coffee processing is complex and seems to become even more so as producers experiment with innovative developments year after year. At its roots though, what is coffee processing?
In this article we’re going to unpack the basics of coffee processing, defining the most common methods and setting general expectations for what those coffees can taste like, as well as explore some of the growing trends in the industry.
What Is Coffee Processing?
Coffee processing, in its most basic terms, is the means by which coffee cherries are turned into the green coffee used by roasters to make the roasted coffee consumed around the world. Processing takes the fresh coffee fruit and leaves us with dried green coffee ready to be packaged, shipped, and eventually roasted.
A whole coffee cherry, peeled coffee cherry, and the two seeds taken from a coffee cherry on display in Colombia. Photo credit: Aromas del Sur
Most post-harvest processes, regardless if they fall into the Natural, Honey, or Washed categories, share a few things in common:
- Some amount of naturally occurring or intentionally induced fermentation occurs
- The coffee fruit has to be removed from the coffee seeds
- The seeds must be dried to an appropriate humidity, which is typically around 9–11% total moisture content.
Aside from these commonalities, processes can vary widely from one another. Even coffees processed using the same general method—Washed, Honey, or Natural—can include a broad spectrum of specific details by which coffee producers add their individual touch to their lots.
The Three Major Methods
Natural processing—sometimes referred to as dry processing—is the most straightforward process in terms of the number of different steps taken by a producer after harvest. In Natural processing, coffee cherries are dried whole typically on concrete patios or raised beds and the dried fruit is then milled from the coffee seeds after reaching the target humidity. Throughout drying, producers rake the coffee frequently to ensure consistent drying throughout their crop and to avoid the development of mold, over-fermentation, or other potential issues that could cause defects.
Coffee cherries being dried for Natural processing in raised wooden beds in El Salvador
Natural processing requires the least amount of infrastructure or additional equipment that would be needed for other methods, such as a pulping machine or fermentation vessels. This leads to the process being utilized by some producers as a matter of necessity, in some cases due to resource limitations and in others due to climatic conditions, and contributes to the long tradition seen in certain origins like Ethiopia and Brazil of using this process. Natural processing is also a popular choice for some environmentally-minded producers as it requires no water usage and produces no waste water that would need to be processed before disposal.
Natural coffee being dried on a concrete patio with Pulped Natural coffee in the background in Brazil
Compared to the other most common processes—Honey and Washed—Natural coffees will generally have the most body, and will often display the most distinct fruit notes of the three typical processes due to the extended fermentation time that occurs as the coffee fruit slowly dries.
Honey processing comprises everything on the processing spectrum between Natural and Washed processes. In this method, coffee cherries are pulped before being dried and some amount of the cherry’s mucilage is left on the seeds rather than being removed entirely.
Freshly pulped coffee in Nicaragua
Honey coffees go by a number of names depending on their origin and the individual producer’s discretion. Honey, Pulped Natural, and Semi-Washed coffees all share the same processing fundamentals of having the coffee pulp removed before being dried with mucilage still on the parchment coffee. Beyond that, there are also Honey coffees defined even further with names like “White Honey”, “Yellow Honey”, “Red Honey”, and “Black Honey”. These distinctions aren’t strictly defined globally, and instead are also left to the discretion of individual producers. Generally, Black Honey coffees have the greatest amount of mucilage, followed by Red and Yellow, and White Honey will have the least amount of mucilage. White Honey and Semi-Washed are closely related terms in this way, with both typically requiring water usage either during the pulping stage or during a period of wet fermentation which still leaves mucilage on the coffee before being dried.
Honey coffee being dried in El Salvador
At the most basic level, this method requires a pulping machine and a drying space for the mucilage-covered coffee. Similar to Natural processing, Honey requires producers to monitor the drying process closely as the sweet mucilage can leave coffee vulnerable to over-fermentation or mold development leading to defects. Producers who choose Honey processing often find elevated sweetness, body, and fruit flavors in the cup compared to Washed processing. Others may choose this method to conserve water in their operation while producing a cleaner overall cup than they would achieve with Natural processing.
Washed processing—sometimes referred to as wet processing—requires the greatest number of different steps of the three major processes. Washed coffees are defined by having the fruit and all of the mucilage removed from the coffee seeds prior to being dried. This can be achieved in a variety of ways, either by mechanically scrubbing the coffee clean with a demucilager during pulping, or by fermenting the coffee which loosens the mucilage from the parchment itself so that it can be more easily washed away with water. Around the world Washed processing has developed into several distinct variations consisting of different steps and styles, with some origins even becoming synonymous with certain practices such as Double Washed coffees frequently being referred to as “Kenya-style Washed”. Regardless of the variety of ways that producers go about this method, all coffees that are dried in their parchment with no fruit or mucilage left on the seed are defined as Washed.
Washing green coffee in a washing channel, fully washed parchment coffee ready to be dried in Nicaragua
Washed processing was long held as the traditional method of coffee production in several regions of the world, and many producers still hold to that tradition. Washed coffee is easier to dry than Natural or Honey, both because they dry relatively quickly by comparison and because there’s lower risk of unintended or defective fermentation to occur once the mucilage is removed. This means that despite the additional steps needed for Washed processing, the results are more predictable for producers and carry a lower chance of ending in a lost crop.
Washed coffee being dried in Ethiopia
In the cup, Washed coffees are generally defined by their clean and delicate body and their accentuated perceived acidity. This style of processing will typically involve the least amount of fermentation, as the sweet mucilage which provides energy for fermentation has the least amount of contact with the parchment coffee. Due to this lower level of fermentation, Washed coffees are often thought of as having higher flavor clarity and less-prominent fruit flavors compared to Honey or Natural coffees.
Beyond the Basics
Fermentation Styles in Coffee Processing
Throughout recent years, industrious coffee minds around the world have invested themselves into experimenting, exploring, and innovating in coffee processing, leading to a boom of new terms seen on coffee bags from every origin. These innovative processes can help producers access premium markets, increasing cup scores and potentially driving up sale prices for successful lots. Perhaps most notably, “Anaerobic Fermentation” and “Carbonic Maceration” have become recognized as highly sought after differentiated specialty processes since the mid-2010s. However, understanding what makes these coffees different from a more traditionally processed lot isn’t always easy without a little further explanation. To start, we should first look at “traditional” fermentation in coffee processing.
Pulped coffee being added to tiled tanks for wet aerobic fermentation in Brazil
Aerobic vs. Anaerobic Fermentation in Coffee Processing
Scientifically, aerobic processes relate to, involve, or require the presence of free oxygen molecules. In contrast, anaerobic processes require the absence of free oxygen. Historically in coffee processing, the fermentation stage is carried out in the open air either with or without water. Even in wet fermentation, during which the coffee is submerged under water, fermentation tanks are left open and oxygen in the atmosphere plays a role in the ongoing fermentation process. The presence of oxygen results in a relatively quick fermentation, often ranging from 12–72 hours, and produces results in the cup that most coffee drinkers will recognize as the standard due to the significant majority of coffees being fermented this way regardless if they’re ultimately destined for Natural, Honey, or Washed processing.
Anaerobic fermentation in coffee processing is defined by extended fermentation times completed in sealed environments without the presence of oxygen. This can happen in several kinds of vessels, such as metal barrels, plastic containers, and vacuum packs, as long as the container has a means for oxygen to be removed or escape from the environment. Most commonly however, this style of fermentation happens in metal or plastic barrels fitted with a one-way valve. As fermentation takes place, carbon dioxide is produced which forces oxygen out through the valve, creating the anaerobic environment which results in the production of different organic compounds than would be created in an aerobic environment. Coffees produced with anaerobic fermentation commonly display accentuated fermentation flavors, intense acidity, and something of a “boozy” or alcohol-like flavor in the cup.
Coffee cherries in a stainless steel fermentation barrel, anaerobic process cherries ready to be dried in Costa Rica
Learn more about Anaerobic Fermentation in our blog - Producer Insights: Anaerobic Fermentation in Coffee Processing
What is Carbonic Maceration?
Carbonic maceration in coffee is a form of anaerobic fermentation, sharing many of the same environmental and infrastructure requirements for producers looking to adopt these processes. As in anaerobic processing, carbonic maceration occurs in a sealed vessel with extended fermentation times compared to a more standard aerobic fermentation. However, carbonic maceration goes a step further than typical anaerobic processing as the vessels are injected or flushed with CO2 at the beginning of the process in order to create a CO2-rich environment immediately rather than allowing the fermentation to produce those conditions naturally over time. This added step makes a noticeable difference in the cup; carbonic maceration processed coffees are known for being exceptionally complex, and commonly have a distinctly silky texture, delicate acidity, and profound sweetness.
Stainless steel tanks ready for anaerobic and carbonic maceration processing in Costa Rica
Additives in Coffee Processing
As coffee processing has developed, some producers have found success in adding a variety of different ingredients in the fermentation stage of processing to achieve a number of different outcomes. These additions can produce more consistent or efficient results, can naturally influence the final potential flavor profile of a coffee, or can contribute brand new and otherwise impossible-to-achieve flavors to a production batch. These additives can be broken down into four broad groups:
- Commercial yeasts and other microorganisms
- Natural yeasts and other microorganism starters from the farm
- Sugar, molasses, and other energizing ingredients for fermentation
- Flavoring agents like spices, herbs, and fruits
Each group influences the final product in its own way, but all of them share the same goal of giving producers greater control over the final flavor experience in the cup.
Open air and sealed plastic fermentation barrels used for producing infused coffees in Colombia
Commercial Yeasts and Other Microorganisms
Fermentation in coffee processing occurs as yeasts and other microorganisms break down the sugars in the coffee fruit, producing organic acids and other compounds which help define the flavor potential of a coffee. Commercial additives are used by producers to ensure these necessary components are standardized across all of their processes, leading to increased batch consistency. While there are always naturally occurring microbes present in freshly harvested coffee cherries, these commercial additives are typically powerful enough to dominate the fermentation process and can help define a standard fermentation profile regardless of potential environmental variations that occur over the harvest and processing period.
Natural Yeasts and Other Microorganisms
While some producers opt for commercial additives, others choose a more local approach when adding microorganisms to their production batches. Each farm, based on its location and other environmental factors, has its own distinct collection of naturally occurring yeasts and microbes found around the property. These can be cultivated intentionally, either by culturing them independently ahead of time or by using the microbe-rich fermentation byproduct sometimes called mossto in Spanish. These cultures are somewhat like sourdough starters, containing all of the necessary components for fermentation in large concentrations, and being unique in their compositions from producer to producer.
Filtering mossto to be added to a fermentation batch in Costa Rica
Sugar, Molasses, and Energy
While coffee cherries contain their own sugars which fuel fermentation, some producers choose to add additional sugars to the process to ensure there is ample energy for yeasts and other microbes to perform their function. This addition is another way to achieve consistency in processing, as the total sugar content in coffee cherries varies from farm plot to farm plot, variety to variety, or harvest year to harvest year. By adding fuel directly to the fermentation by way of sugar, molasses, and other sweeteners, producers can be certain there is plenty of energy available for fermentation to complete its role in processing.
Flavoring Agents and Infused Coffees
While the majority of coffee processing is completed to prepare coffee for roasting and to influence the naturally occurring flavors in the beans, some producers choose to intentionally add new and otherwise unachievable flavors to their coffee by including flavoring agents during fermentation. These flavor agents are commonly fruits, herbs, and spices, and frequently are sourced from the local area and sometimes from the coffee producer’s farms themselves.