Exactly how that happens is less important than doing it well. The pulp and mucilage are high in water and sugar content—two attractive resources to microorganisms whose overabundant presence during drying is suspected of negatively impacting the cup quality of the coffee. Minimizing or eliminating their growth is a key aspect of cherry processing.
Ultimately, individual farmers decide how to process the cherries depending on the available resources, cost of processing, the climate at the time of processing, the potential of a price premium, and/or the desired taste outcome.
There are three common methods of cherry processing: natural, pulped natural, and washed. There are variations on these but to go into them all is overwhelming. We’ll stick to these three.
In the natural process, also known as the full natural or the dry process, the entire fruit remains intact while the seeds are dried. The seeds are not removed until every layer, including the seeds, has been dried. On farms where coffee is harvested mechanically, many cherries are already dry when the coffee is harvested. These cherries, sometimes called raisins, can be separated and sold as natural coffee.
The pulped natural process is one step removed from the natural process. The cherries are pulped (the skin and fleshy pulp removed) and the seeds, still covered by the parchment and mucilage, are dried. This process sometimes goes by alternate names, but
“honey” is the most common.
Did you know?
On average, about 100 gallons (378.5 L) of water are required to produce 20
grams (0.7 oz) of roasted coffee, enough to brew about one 11-ounce (325 ml) cup of coffee.
The washed process (a.k.a. the wet process) removes not only the skin and pulp but also the mucilage before drying down the coffee. There are several ways of doing this.
Traditionally, the mucilage is removed by fermentation, either by covering the coffee with water until the mucilage is degraded or simply leaving the coffee to sit and ferment without water (known as dry fermentation). The term “fermentation” is used because microorganisms, naturally occurring on the coffee or in the environment, consume the mucilage and degrade it via metabolic fermentation processes, though microbial enzymes also play a role. When the mucilage is completely degraded and removed, we deem the fermentation process complete. The fermentation process takes as few as six hours and as many as forty-eight to complete, though typically it lasts twelve to thirty hours. The time required depends on the volume of coffee, ambient air temperature, and temperature of the water (if present) used for soaking.
An alternative method uses a demucilager/demucilator to mechanically remove the mucilage just after pulping, eliminating the need for any kind of fermentation before drying. A demucilager forces the coffee into a small space, causing the seeds to rub and push against each other and the sides of the container. The pressure liquefies the mucilage, allowing it to be washed away in a few minutes by the small amount of water added to the process. Since water is used to rinse the coffee seeds upon completion, we call these coffees “washed coffees.” Whether a washed coffee is fermented or demucilaged, the cup quality tends to be similar.
It is well accepted by both the coffee industry and scientists that processing affects the cup profile. A generality on perfectly pampered and accomplished processing on farms where hand-harvest methods are used is that going from washed to pulped naturals to full naturals creates an increasing intensity of sweetness, fruitiness (ferment to some), acidity, and body. Some people suggest that the coffees become increasingly complex through this progression.
On farms where coffee is mechanically harvested, the results of perfect dry processing on cup quality aren’t as predictable. Natural processed coffees from these farms can be more acidy and fruity than washed coffees, or they can be earthy and/or spicy.
“Coffee is a language in itself.”
A big question that is largely unanswered is, how does cherry processing affect coffee quality? What is happening, biochemically, to create such organoleptically noticeable changes in the same batch of seeds? Many people in the coffee industry proffer that the sugars and “fruitness” of the mucilage and pulp diffuse into the seed. Unfortunately, this hypothesis lacks any scientific data to support or refute it.
There is not much data to address what is going on with flavor as a result of processing.
Moreover, there is as yet no data linking specific coffee chemistry (green or roasted) to organoleptic quality. So, even when changes in coffee bean chemistry are demonstrated, there is no evidence to support that those differences are causing the tastes we experience.
The same coffee processed by different methods will present different amounts of a variety of cellular molecules, dependent upon the processing method. Also, differences in coffee bean metabolism have conclusively been shown between washed and full natural coffees. It is not unreasonable to hypothesize that pulped naturals might fall somewhere in the middle of these differences.
Two metabolic responses have been demonstrated. One is that the seed begins its germination sequence almost immediately after being picked. If the presence of germination-specific molecules (isocitrate lyase and β-tubulin) is measured in coffee shortly after picking and daily until the seeds reach 12 percent moisture, differences are seen between seeds that are fermented and seeds that are naturally processed. In washed coffees, the amount of these molecules peaks a couple of days after harvesting and drop significantly in about a week, whereas in natural coffees, the quantity of those molecules peak a week after harvesting and slowly decline for another week or so. Two factors explain these patterns. The first is that coffee pulp has inhibitors that slow down the germination process. Second, washed coffees dry quickly and, consequently, quickly reach a state of cellular quiescence. Full naturals, with greater mass and higher water content, require more time to dry down to that quiescent state.
There are three common methods of cherry processing: natural, pulped natural, and washed. Notably, there is little data addressing how processing affects flavor.
The second response is related to water stress. Natural processed coffees accumulate a much larger amount of γ-aminobutyric acid (GABA), a molecule known to occur in water-stressed plant cells. As explained earlier, this disparity exists because the natural processed coffees remain metabolically active for a longer time than the washed coffees.
These responses indicate a significant amount of metabolic activity that is captured by just a few molecules, and the actual changes within the seeds go much farther than just these molecules. It is reasonable to hypothesize that the differences in flavor from different cherry processes stem from these metabolic processes. Yet, until more research is done, we can only hypothesize as to whether the flavor comes from seed metabolism, a migration of compounds into the seed from the mucilage and fruit, or both.
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