Wet Processing: An In-Depth Look
Back for more? I certainly hope so! This article dives deeper than the primer into fully washed processing and explores some of the science behind fermentation and other reasons this type of processing is used other than flavor profile. It’s going to get a bit technical, but I hope you hang in there because if you do, you’ll have a seriously strong grip on the fully washed process—but first, some recap on processing.
Historically, coffee processing can be divided into two large categories: wet and dry. Dry processing produces coffees typically referred to as naturals. For now, we’re going to just focus on wet processing. Wet processing has more variability and encompasses three broad subcategories of coffees: fully washed, pulped natural (honey coffees), and semi-washed. This article will focus on fully washed coffees, while pulped natural and semi-washed will be covered later in the series.
Let’s start off with a sweet flow chart. This will help us visualize the three different paths pulped coffee can take within wet processing.
All wet processed coffees are pulped. Pulping coffee is when the outside skin of the coffee cherry is removed from the coffee. This leaves a sticky, slick coating on the coffee bean. This coating is known as the mucilage layer. How the mucilage is removed after pulping determines where in the wet processing spectrum a coffee falls. A coffee’s mucilage layer thickness varies greatly with altitude and can have a huge impact on the coffee’s overall flavor profile, which what process is used to remove it, or not remove it, is so important. I highly recommend trying to grab a coffee tasting series where one coffee is processed multiple ways. This will really give you a great picture into how processing effects the coffee’s flavor profile.
Fully washed is unique because it employs a fermentation step. Here we’re not talking about the production of alcohol, which is what most of us think about when hearing the word “fermentation,” but the breakdown of the mucilage layer into weak acid. Pulped coffee is dumped into large soaking tanks and then partly or fully submerged in water. Submersion activates the coffee’s pectinase, an enzyme already present in the coffee seed itself. I like to think of the pectinase as a sort of mucilage eating Pac-Man finding and going around chomping on pectin dots. The pectinase then begins to chow down on the mucilage layer and dissolves it through hydrolysis. This process can take between 12-48 hours and is directly dependent on temperature and altitude. Additionally, some mills may use a double fermentation process.
This variation on the fermentation stage uses “dry” fermenting. Dry fermenting is fermentation without adding more water to the soaking tank, so the only water present is the water used from pulping the coffee. This technique causes the soaking water to become acidic within a few hours, which prevents fungi from forming. After the soaking water drops to a certain pH, either more water is added to the coffee to start normal fermentation or the coffee is moved to another tank where more water is added to begin the second stage of fermentation.
As the mucilage layer dissolves, a significant drop in the fermentation water’s pH occurs and becomes acidic. The pH of fermentation water is in the range of 3.5 to 5.5 when fermentation completes. In my own research, I have observed a pH drop from 7.5 to 5.5 over a twenty-four-hour period. Well that’s neat, but why does that matter? For the average coffee consumer, it probably doesn’t, but the rate at which the pH drops is an indication of the rate of the fermentation reaction. This could be particularly helpful in determining how long coffee should sit in the fermentation tank. There is also some debate on whether or not pH of fermentation water could be correlated to cup quality, but for now the jury is still out. What is probably more important to cup quality is ensuring that fermentation goes to completion and that the coffee does not over ferment.
After fermentation is completed, the water is drained from the vats and the coffee undergoes a final rinsing. At this point the coffee is ready to be dried. Drying the outside of the coffee quickly is critical to prevent continued fermentation on the surface of the coffee. This phase can be considered pre-drying and takes about 24 hours. Once the outside of the bean is dry, the coffee is moved to drying beds where it continues to dry until is ready to be packed up
Three Forces Driving Fully Washed Coffee
So why even use the fully washed process? You could just skip fermentation and go the drying phase or use a mechanical demucilager and save time. Well, there are at least three driving forces for fully washed coffee.
First, cup quality. If you survey finalists in tasting competitions or look for the highest scoring coffees around the web, an overwhelming amount of them are fully washed coffees. I’m not saying natural or other wet processed coffees cannot be excellent, and I have had quite a few that were exceptional, but on average, fully washed coffees score higher than their semi-washed and pulped natural brethren—which will typically correlate to a higher selling price at auction. And if you’re a farmer depending on that coffee for your livelihood, this can be a pretty strong incentive for pursuing fully washed coffees, but using the fully washed process does not guarantee high scoring coffees.
The second and third forces driving fully washed coffees are location dependent—climate and availability of water. The washed process was developed in regions that have constant rainfall during the harvesting season. Since there is an abundance of rainfall, water for wet processing and fermentation is readily available. However, the wet climate during the harvesting season makes using the natural (dry) process impractical. The natural process requires extended periods of dry weather for the production of high quality coffee, a condition not present in equatorial regions where it can rain daily during the harvesting period.
Fully Washed Coffee and the Environment
Personally, I love fully washed coffees, but since I am an environmental engineer I couldn’t finish this article without mentioning some of the environmental issues that arise from this process.
Obviously, this process requires a lot of water, but how much is a lot? A typical number quoted is that it requires 140 liters or 37 gallons of water to produce one 12-ounce cup of coffee (1). However, most of this water is used for growing the coffee tree and not processing. It is estimated that only 0.37% of the water required for producing this cup of coffee is attributed to the fully washed process, but this small fraction can have a large environmental impact. This is because fully washed processing water is typically pulled from surface water, such as streams, rivers, or lakes. Diverting this water for processing can lead to water shortages downstream, but this is entirely dependent on location. If the mill is in a remote location, far from the villages or towns, more than likely the water diverted from the stream will be replenished from rainfall draining back into the waterway. In my opinion, what is of more concern than amount of water taken from the stream is the water put back into the stream.
The fermentation stage produces some pretty intense wastewater. This wastewater is characterized by high concentration of organic compounds, which causes an oxygen sag in the waterway where it is dumped. An oxygen sag in a river is where oxygen in river water is sucked out of the water and used by micro-organisms to break down the wastewater’s organic compounds. It can get pretty technical, but what is important is that this oxygen depletion can wreck river ecosystems that may not recover in the dry season (2). Additionally, this small, but potent amount of wastewater can render river water undrinkable for humans and even make us sick (3). Three years ago, the SCA reported that 7,000 families in Nicaragua were without potable water for two weeks due to coffee processing wastewater (4). So this wastewater can be kind of a big deal, which is why some countries have and others are beginning to develop regulations for coffee mills. There are also watershed management tools being developed for specific use in coffee growing regions.
Wet processed coffees come in three broad flavors: semi-washed, fully washed, and pulped natural. What determines where a coffee falls into these three categories is how the sticky mucilage layer is removed from the coffee. Fully washed coffee undergoes a fermentation step where enzymes remove this layer. The fully washed process is typically used in areas where the wet season coincides with the harvesting season and water is readily available. Because of fermentation, this process has the potential to yield some seriously delicious coffee with a clean flavor profile.
Maybe you really want to dig into coffee processing, but can’t find enough info on the web. I highly recommend the “Handbook of Coffee Post-Harvest Technology” by Dr. Flavio Borem. It’s pricey, but it was easily the best $90 I have ever spent on coffee related items and has been invaluable in furthering my coffee knowledge.
Sources and Further Reading
Chapagain, A. K., and Hoekstra, A. Y. (2007). “The water footprint of coffee and tea consumption in the Netherlands.” Econ., 64(1), 109-118.
Beyene, A., Kassahun, Y., Addis, T., Assefa, F., Amsalu, A., Legesse, W., Kloos, H., and Triest, L. (2012). “The impact of traditional coffee processing on river water quality in Ethiopia and the urgency of adopting sound environmental practices.” Monit.Assess., 184(11), 7053-7063.
Haddis, A., and Devi, R. (2008). “Effect of effluent generated from coffee processing plant on the water bodies and human health in its vicinity.” Hazard.Mater., 152(1), 259-262.
Flores, M., Stubblefield, A., and Hicks, P. (2013). “Coffee Clearwater Revival: Protecting Water Resources in Coffee Producing Areas of Central America.” Presented at SCAA Event 2013 in Boston, MA.