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The Science of Coffee: Processing Methods

The Science of Coffee: Processing Methods

How do you take your coffee? With honey? Dry? Shaken not stirred? Do you want it washed, dried and folded? Or just au Natural in its birthday suit?

…There seems to be a fair amount of confusion and a general lack of education when it comes to coffee processing. Many of our customers are unaware and sometimes uncomfortable inquiring when they see the terms Washed, Honey, and Natural on the menu, or hear terms like Carbonic Maceration and Double Fermentation. It can be intimidating, for sure! We would like to amend that and answer your questions.

Let’s start with the most common type of processing that all coffee drinkers are familiar with, whether they know it or not.

Washed Processing

Most coffees are processed in this style and because it is so common they will often not be labeled so. The process involves removing the entire fruit (de-pulping and demucilation) of the coffee cherry from the pit (bean) before drying. By that token, it’s fair to say that flavor from washed processed coffees are instilled in the bean during growing rather than during processing. The pit will absorb various amounts of sugars, nutrients, and other chemicals, either present in the environment or the developing coffee cherry. This also means that the flavor is primarily influenced by terroir, farming practices and varietal.

First, coffee cherries are immersed in water, encouraging the cherries to either float or sink. Bad or unripe fruit will float, and healthy cherry will sink. While still immersed, the coffee will be pushed through a screen where some of the pulp is removed. Afterwards, the rest of the pulp and mucilage is removed by one of two ways:

It is either mechanically removed by specialized machinery, or fermented off over the course of 24 to 36 hours.

Fermentation time is dependent on temperature, thickness of the mucilage layer, and concentration of microbes/enzyme. It also may require a fair amount more water whilst creating more pollutants. Coffee waste water is unusable and produces an extremely foul odor. This is less of an issue than it has been in the past due to better technology, and an effort to be more environmentally conscious by coffee mills. Fermentation is also unpredictable, especially when variables are poorly managed. This can introduce unwanted sour notes and other unwanted "off" flavors. Upon completion of the fermentation process, the beans are washed thoroughly with clean water in tanks or special washing machines.

Fermentation, albeit sometimes difficult to control, is preferred by many mills, as it is one last chance to impart flavor in the bean. The beans are basically steeping in their own super-flavorful coffee berry juices during this stage.

Machine-assisted removal is accomplished via a scrubbing process that requires less water and less time. In spite of being more economical and predictable, this fails to maximize flavor potential.

After washing, these coffees are left to dry on patios or beds under direct sunlight.

Washed processed coffees are prized for the level of clarity that they can produce. They are true to the region from which they are sourced in a way that other processes cannot achieve. 

This next process can be thought of as almost the antithesis of the first…

Natural (Dry) Processing

Formerly known as a dry processing, natural processing is a traditional Ethiopian practice. Unlike washed processing, the fruit is left intact from picking all the way through completion of the drying process. Coffee cherries that are to be natural processed are often separated by a special method. Instead of separating coffee cherries based on buoyancy, the traditional agricultural process of winnowing is performed. Other times an immersion separation process as in washed processing is utilized. Because Natural processing does not require much if any water, it is more eco-friendly than washed processed coffees.

After selection of the usable cherries is complete, the coffee is off to the drying patios or beds. The coffee will then be sun-dried with the cherry intact. Workers will rotate the cherries, raking and turning by hand, at regular intervals to avoid any mildew or molding. This will also ensure that each cherry reaches the targeted moisture content. The process can take up to a full month, depending on conditions, rotation intervals, and the particular coffee.

During drying the coffee cherry is gradually fermenting; imparting a milieu of flavors into the bean.

Natural processing has received a bad rap throughout the history of coffee production. This is primarily the result of poor and inconsistent methods.

As more producers have gained expertise in natural processing, it’s become apparent that Natural Processed coffees actually have some of the most unique and robust flavor profiles of any coffee. Ranging from pleasant tartness and heavy fruit notes, to luscious chocolate and incredible sweetness.

Next up is a sort of middle road between a Washed and Natural process` that allows us to further customize what we have in the cup...

Honey (Pulped Natural) Processing

Honey Processing involves removing only the coffee cherry flesh, leaving behind what is known as the mucilage. Different levels of mucilage left on the pit to ferment and dry result in different flavors being imparted.

The mucilage is the sticky substance left on the pit after mechanically removing the skin and pulp off the cherry. This stickiness is in part how this method got its name. 

Originated and perfected in Costa Rica, this method has had innumerable variations—each resulting in a unique flavor profile and tailored to the coffee being processed. Different degrees of mucilage will yield a different color on the outside of the bean as it dries.

Here’s a quick rundown of the different honey processes typically used and their respective characteristics:


Black Honey 

Mucilage: 0% removed

Drying: Covered; quick drying

Character: Sweet and full bodied with fruity/pulp depth

Yellow Honey 

Mucilage: 50% removed

Drying: Dried uncovered

Character: Floral, light body, apricot

Golden Honey 

Mucilage: 75% removed

Drying: Dried uncovered

Character: Crisp, citrusy

Red Honey 

Mucilage: 25% removed

Drying: Dried uncovered

Character: Sweet and syrup

White Honey 

Mucilage: 100% removed

Drying: Dried uncovered

Character: Clean, balanced


This will vary from mill to mill but it gives you an idea of the flavors imparted by different degrees of mucilage. Your main takeaway should be that the more mucilage that’s left on the pit, the greater the sweetness and depth of flavor a particular coffee can attain. True to its name, this sweetness can sometimes be reminiscent (but still distinctly different than) the flavor of honey.

Honey processing is evolving rapidly, and new variations continue to arise. Processing in general is exponentially evolving as technology allows for more complex methods and accurate measurements.

Next up are a few newer and more experimental processing methods that we are very excited about. 

Washed Carbonic Maceration

Similar to a typical washed process but with the benefits of a controlled fermentation, Carbonic Maceration is common in wine making and has only recently been adapted for coffee fermentation.

In this process ripe coffee cherries are washed and placed in a hermetically sealed stainless steel tank where they will undergo anaerobic fermentation. In this method all oxygen is pumped out by flooding it with carbon dioxide (i.e. hermetically sealed). The temperature and pH is periodically measured and maintained. After the prescribed amount of time (depending on varietal and preference), the coffee is removed from the tank and left to dry in temperature-controlled greenhouses.

This process yields coffee with intense aromas that is consistent from one batch to another.

Yeast Fermentation (Inoculation)

This is another newcomer to the coffee world. Yeast fermentation is utilized in a wide range of foodstuffs, including wine, beer, kombucha, bread, and cheese. Wild yeasts are present pretty much everywhere, and can find their way into any coffee fermentation. What’s different is that selective yeasts are now being used intentionally in a controlled environment.

The first introduction of this method took place in El Salvador when Green Coffee Buyer for Counter Culture Coffee, Peter Giulliano brought a few select strains to world renowned coffee producer and Cup of Excellence winner Aida Batlle. Batlle has been a close friend of ours for years and if there’s one thing to know about her, she’s a relentless innovator.

The first rounds were admittedly unremarkable, with only a slight improvement in cupping score. But, this method theoretically offered promise for controlling a fermentation that at times may go inexplicably awry.

A few short years later, Batlle was approached by Scott Laboratories—the primary supplier of yeast to US wineries. They had been working on yeasts specifically cultivated for coffee fermentation. Originally they had designated 3 specific yeasts for coffee fermentation, 2 of which were of the sacharomyces cerevisiae species of yeast—the same yeast you will find in many beers. This was just the beginning.

The process has evolved and has numerous benefits for coffee producers and drinkers alike. Yeast inoculation can add cup complexity, reduce cup defects, increase the shelf life of green coffee, create consistency batch-to-batch and day-to-day, reduce water usage by effective demucilation and offers labor predictability. It’s not to say that yeast inoculation in coffee fermentation is a magic bullet, but it can only enhance an already well produced coffee.

When in season, we will carry a handful of yeast-fermented Aida Battle Select coffees. We suggest you try the Brazil Fazenda California, and our newest edition El Salvador Finca Panorama. Alos, be on the look out for our up and coming addition from Claudia Mathies Rank, Finca Paraiso.

Coffee processing probably has the greatest influence on the end-product (or at least on par with terroir and farming technique, however, a poorly processed coffee can thwart even the most talented farmer’s efforts.)

We’ve covered a fair amount of ground here, but keep in mind that there are infinite variations on the methods discussed above. Processing will always be somewhat unique to the mill, coffee, and region.

Next time you order your morning cup of Joe or beans for your home think about the flavor you enjoy in your coffee and look to see what process suits your palate. You are now armed with a new arsenal of coffee knowledge that won't only impress you're barista, but will also give you more purchasing power.


  • Cup of Joe? Give up repeating the tired old cliches. Otherwise a good explanation.

  • Thanks about your journey and research

    Andebet kassa

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