Coffee Lab: Fermentation

What it does—and does not do—for coffee


When it comes to coffee, fermentation is, at its most basic, a means to an end. Whereas the metabolic process is key to developing flavor in foods as varied as kimchi, preserved lemons, or chocolate, fermentation in coffee serves a different, but equally important, purpose: freeing the coffee seed from the surrounding sticky mucilage (or fruit pulp).

Until the advent of demucilaging machines, fermentation was the only way to separate the fruit from the seed.

Fermentation in Coffee 

For as long as coffee (the beverage) has existed, farmers have fermented coffee cherry, without even labelling the process as such. They’ve placed cherry, whose skins have been removed, in tanks to ferment without water, or submerged them in water to soak for a few hours or a few days. In the case of natural processing, farmers have laid whole cherry out to dry in the sun. Rather than relying upon scientific tools to monitor microbial activity, seasoned growers use sensory cues as their guide to signal when the process is done. The microbial flora—bacteria, yeasts, and fungus—present on the cherry, in the air, or in the fermentation tanks begin a process whereby microorganisms transform the insoluble pulp consisting of pectins and sugars into a soluble material that can be washed away from the seed.

Until the advent of demucilaging machines, fermentation was the only way to separate the fruit from the seed. Whether coffee is wet, natural, or honey processed, adequate fermentation is the means by which the clinging pulp separates from the coffee bean. If local farming practices dictate an exceptionally long soak in water because chilly nights suppress the rate of fermentation, or call for a post-ferment soak (as is done in Kenya), then farmers may do that, too. 

Modern Methods


Technical advances are a boon for farmers who might not have access to enough water to wet process, but whose climatic conditions are also not conducive to air-drying whole cherry outside. This is the case for the Iwacu Coffee Washing Station in the Nyanza District of southern Rwanda. A Penagos, a kind of demucilaging machine, allows the small washing station, located in a far drier region than the river-fed western part of the country, to produce coffee of equal beauty. It works by using high pressure and a just little water to remove the cherry's skin and fibrous, sticky mucilage. 

Using a Penagos can, if desired, eliminate the need for fermentation. If farmers opt for this technology, they gain control in other ways, too, as they are no longer at risk of over-fermention, which can impart overripe fruit notes to the finished coffee.

New Frontiers for Fermentation 

At the same time that technological advances are giving farmers alternative tools, fermentation is also coming under new consideration as an area of the coffee process where scientific knowledge lags far behind its practical applications. Exciting new work, like that of coffee specialist Lucia Solis, reveals that producers can actively choose how to ferment their coffee by selecting specific strains of yeast. 

Just as cheesemakers select bacteria, or a brewer safeguards the yeast, Brettanomyces, to develop tartness in sour beer, coffee producers may yet discover the ability to manipulate flavor (or other outcomes) through a better understanding of fermentation. One thing is for certain: More control in fermentation presents an advantage for farmers whose infrastructure, such as the number of fermentation tanks, is limited. 

The reasons why a coffee tastes a certain way is complex. While fermentation in coffee does not penetrate the seed’s composition, like it does in the cacao bean for chocolate, it certainly plays a role in the final taste, and for now, it remains the necessary means by which most producers around the world wrestle the coveted coffee bean free—bringing it one step closer to admirers like us.