From the Field, From the Lab

Wednesday, September 4, 2019

Coffee Processing Primer

Coffee processing is no simple task. It requires a mixture of scientific knowledge, intuition and superb timing. But the supports and resources available to farmers are slim. This often leaves farmers in a catch-22 wherein they are expected to “maximize the potential of their coffee and provide a portfolio of coffees,” as the Q Processing Level 1–Generalist Student Workbook explains but simultaneously experience a dearth of information about the science and mechanics behind a variety of processing methods.  

Learn more about coffee processing in this free course from The Center. 

A lack of readily available information is less surprising when considering how comparatively recent the variety of processing methods we have today are. Until the 1850s, natural processing was the only processing method. It was with the dawn of the Industrial Revolution in the 1850s that the washed process was developed (we’ll cover the differences between these methods and more a little later in this post).  

Back to the Basics  

As residents of 2019 and beyond, we can benefit from the vast reserves of knowledge built up in the past 20, 150 and 300 years in coffee processing. But, let’s begin with the basics.  

Each coffee cherry is made up of five main layers. Processing is all about removing all these layers over specific amounts of time (including time for fermenting and drying) to eventually get down to the bean itself.  

  1. Skin/peel (exocarp)  
  2. Pulp/mucilage (mesocarp)* 
  3. Parchment (endocarp) 
  4. Silverskin/chaff (spermoderm) 
  5. Bean (endosperm) 

*The difference between pulp and mucilage is that pulp is typically the portion of the mesocarp removed during pulping while the mucilage is the portion that remains attached after pulping.  

Now that we understand the composition of a bean, let’s get down to nitty-gritty of how processing works. Then, we’ll explore some of the components of different processing methods. 

By Sun or by Machine?  

Drying coffee beans is essential to bringing out the oft-exquisite flavors within beans. Perhaps more importantly, it also prevents mold growth or fermentation during storage and transportation. Processing, which, in all its forms, boils down to drying beans, is about instilling potential value in the beans for the mid- to long-term. In drying/processing beans, we are preserving their flavors for an extended period of time and thus enabling their global transport and enjoyment. But drying beans is a very delicate game. Dry the beans too quickly and they taste flat. Dry them too slowly or not enough and they often develop a moldy or fermented flavor.  

Today, producers can choose between using natural sunlight to dry coffee, as it has been done for thousands of years, or taking advantage of modern technologies and drying beans by machine. Sun drying takes multiple days and is dependent on good weather, but it allows more fruity notes to develop in naturals. Machine drying, on the other hand,  is much faster, potentially taking only a few hours, but this fast drying time can stymie the full expression of flavors in a bean and can potentially shorten the shelf life of the coffee. At the same time, however, many farmers are using a careful, calculated form of machine drying to produce specialty coffees. This usually involves drying the beans low and slow, with the temperature not to exceed 40 degrees Celsius and intentional rest periods between dryings to stabilize humidity.   

To Ferment or Not to Ferment?  

Because we now have mechanical demucilation that can remove the mucilage from beans without fermentation, we need to view fermentation as “a tool for achieving cup characteristics as opposed to mere demucilation,” explains the Q Processing Level 1–Generalist Student Workbook (32).  

So what exactly is fermentation? And why should we think of it as a flavor-enhancer? During fermentation, ⅔ of the simple sugars in mucilage are consumed by microorganisms and broken down into other aromatic and flavor compounds. Lactic acid, acetic acid and ethanol are produced, enhancing the natural acidity of a coffee. These microorganisms are so busy that the pH of the coffee drops from 5.5 to 3.5 due to the production of acid. For more information on the science of fermentation, visit our recent article on the chemistry of processing.

Coffee Processing Methods  

Natural Processing 

Naturally processed coffee is the oldest form of coffee processing. The process for natural coffee is fairly simple, at least on paper. Producers take harvested cherry and, after sorting or pre-cleaning them (optional), they lay them on a patio, mats or raised drying beds (really any flat surface) to begin drying. Cherry must be turned frequently to prevent over-fermentation, especially at the beginning of the drying process. Once cherries reach around 12% moisture, they are usually stored as drieds pods until ready for sale. Then, they are sent to the dry mill for final milling before being packaged and shipped.  

Natural processing is often chosen for its low water usage, because it produces less biowaste and wastewater, and for its high potential to produce fruity flavors. The downsides of choosing natural processing are the increased drying time and the added labor needed to repeatedly turn  the cherry to ensure uniform drying. The very high risk of over-fermentation or development of mold is another potential drawback to natural processing.  

Not so very long ago, the intention and outcome of naturals was quite different from today’s naturals. The distinctly fruity – sometimes boozy – notes we’ve come to love about natural-processed coffees were once considered defects–a sign of over-fermentation–and naturals were intended to taste as similar to washed coffees as possible. Only in the last 5 to 10 years have the fruity flavors of naturals become prized in their own right.   

Pulped Natural/Honey Processing  

Pulped natural processing comes from Brazil. It was originally called “pseudo-natural..” Today, the process is also called: mucilage-dried, honey and semi-dry.  

To make pulped natural processed coffee, a producer sorts and pulps harvested cherry. The beans, with some mucilage still attached, are then dried on a flat surface  until reaching around 12% moisture content. Like naturally processed beans, pulped natural processed beans need to be turned frequently to prevent uneven drying and unwanted fermentation or contamination from occurring. The beans are then stored as dry parchment until ready for sale, when they are transported to a dry mill.  

Though following the same procedures as pulped natural, “honey” processed coffees originated in Costa Rica. In 2008 an earthquake left many of the Costa Rican wet mills without access to water. This led many people to experiment in Costa Rica with water-free processing methods including the pulped natural process. During this time, Costa Rican producers popularized the term “honey” to evoke the sweetness of the bean.  

Honey processed coffees come in a number of ‘colors.’ Though the definitions of what makes each ‘color’ is hotly contested, in general color depends on a number of factors including: the initial amount of mucilage, sugar content (°Bx), in-cherry fermentation and partial drying.  

A significant benefit of honey processed is that it can create a wide range of flavors and profiles, adding complexity and variability to a coffee. It does come with challenges, however. One such risk is that of over-fermentation or flat out failure. Another issue is that initial moisture levels and weather conditions affect drying times, making drying time unpredictable and often dependent on the weather.  Further, mucilage tends to absorb moisture from the air (hygroscopic), meaning that humid climates might be especially prone to honey processing failures. At the same time, traditional solutions to humid or wet climates–using a mechanical dryer–are difficult to utilize because of the stickiness of the beans.   

Demucilaged Process  

Historically, demucilation took place after fermentation but this method, developed in the 1990s in Brazil was promoted as an ecological alternative to washed process that creates less pollution. Cherry is sorted and pulped before being mechanically demucilaged. Then, the beans dry to 10-12% moisture content before being stored in parchment until ready for dry milling.  

The taste characteristics of demucilaged coffee are hotly contested with some arguing that because the fruit is removed from the bean so early and no opportunity for fermentation is given, demucilaged coffee tastes flatter than other processing methods. However, others argue that such claims are biased and based on opinion rather than on specific sensorial evidence.  

Washed Process 

Washed processing was developed in the 1850s in Jamaica and was originally called “West Indies Process” according to the Q Processing Level 1 handbook.  Today, it’s also known as parchment dried and fully washed. In washed processing, harvest cherry is pulped and then fermented to ‘loosen’ remaining mucilage. The remaining mucilage is then washed off and the beans are dried to approximately 12% moisture content before being stored as dry parchment until ready for dry milling. Washed process produces much more waste water. Washed coffees are also higher in trigonelline, a bitter alkaloid that breaks down into pyridines and nictonic acid during roasting, both important aromatic compounds. 


Also known as seed dried or Giling-Basah, this method is mainly used in Indonesia. After harvest and sorting, the cherry is pulped. Fermenting and washing the mucilage away is optional. Then the beans are dried to 25-40% moisture content before being hulled. Then the bean is dried to 10-12% moisture and stored before dry milling. One potential problem with wet-hulling is that in the second drying stage the beans are more likely to pick up flavors from the drying surface. If the drying surface isn’t pristine, the beans may be infused with funky, earthy notes. The faster drying time allows farmers to get money for their crop more quickly, as they often sell their beans at the 25-40% moisture stage. Beans dried using the wet-hulled process often have a distinctive blue-green hue.  

What To Do With the Pulp?  

Many of the methods described above involve pulping the cherry and removing much of the fruit. But what happens with all that pulp once it’s been removed? Farmers can do a few things with the pulp. They can use it as natural compost and fertilizer for their fields. Many farms reuse their pulp in this way. Another, fairly recent industry for pulp is in drying it and selling it as “cascara.” Cascara, which means “skin” in Spanish, is often sold to cafes to be turned into tea, syrups and other drinks that are gaining in popularity across the world, especially for summer drinks. However, in order to dry the pulp for sale as cascara, they need to calibrate their pulper to make sure it doesn’t grind the skin into pieces that are too small, which would make it very hard to dry.  

Looking Towards the Future 

In order to ensure quality processed coffees in the future in the face of climate change, declining producer populations and more, we as roasters and importers must continue to express interest in well- and/or inventively-processed coffees. Further, we must help nurture and expand a knowledge support system for farmers to access the information they need to learn new or improve their existing processing methods. This may take the form of new certifications or classes for processing, more online articles paired with increased internet access among farmers and more.  

Learn more about coffee processing in this free course from The Center.