The Lab

Post-Harvest Coffee Processing: Washed

The washed processing style was largely made popular by Costa Rica in the mid 1800s. At this time, coffee was largely produced using the dry, natural process that was easily susceptible to defects. Europeans and the English in particular favored the clean and clear cup produced in this style. Washed coffee comes from most of Central and South America as well as parts of Africa and Indonesia.

First, coffee is picked, either by hand or mechanically. The picked cherries might be “floated” in water to remove any over-ripe cherries, leaves, sticks, or other debris. Ripe and unripe cherry will also float if there has been any damage like in the case of berry borer disease. If the seeds did not develop properly, this will also make the cherry less dense causing it to float.

The “sinkers” are sent to the depulper which is designed to remove the cherry’s fruity flesh or pulp from the seed. Traditional drum and disc pulpers will remove the fruit of any cherry that is fed into it. They operate based on abrasive surfaces which scrape the pulp from the seed. Some operations, especially those that harvest mechanically, will use screens to remove the smaller, under-ripe cherry. Newer equipment uses pressure to sort out the under ripe cherries before pulping. Inside these depulpers are small screens just large enough for only the seeds to pass through. The pressure at which the depulper is calibrated to will decide which seeds are pushed out of the cherry through the screen and which are left whole and expelled through a different channel. Under ripe cherries are harder and would require more pressure to be pulped. Fully ripe cherries are softer and require less pressure. It is up to the miller to regulate this pressure to pulp the desired quality of coffee.

Empty fermentation tanks in Burundi

After pulping, the coffee seeds are still covered in a sticky mucilage. The mucilage is removed by soaking in tanks for 8-48 hours. During the soaking period, yeasts, bacteria, and other microbes produce enzymes which breakdown the polysaccharides in the mucilage. The fermentation of polysaccharides produces ethanol as well as lactic, acetic, and other acids. The rate at which this breakdown occurs is tied to the temperature. Colder, higher altitudes will require longer fermentation periods to achieve complete breakdown of mucilage. Traditionally, a producer will feel the coffee in the tank and end the fermentation when the surface of the seed is rough and not covered in the slimy pectin. This approach is not exact and many producers today will measure the pH of the water and empty the tanks when the water becomes acidic at a pH of 4-4.5.

Fermentation tank being emptied in Kenya

After soaking, the coffee may be pushed through channels using rakes or paddles which will loosen and remove the last of the mucilage. This will further sort the coffee by density since lower quality beans will be carried to the end of the channel while higher density beans will remain near the top.

The fully washed coffee will be dried on patios or raised beds or in mechanical driers until the moisture level is 10-12%. When on patios or beds, the coffee will need to be turned consistently for even drying. If the coffee is not evenly spread out, there will also be uneven drying.

Coffee Drying on patios in Guatemala

Coffee is different from some other plants in that it begins to germinate after the seeds are separated from the cherry. The plant likely evolved this way to spread its seeds through consumption by animals. The hard parchment layer protects the seed from the stomach acids of the consuming animal, and germination will start when the animal passes the seed. Recent studies have shown it is this germination that is largely responsible for the flavors of a washed coffee. Complex sugars are consumed during the metabolic activities related to germination, but amino acids and other compounds are created. The germination is halted when the coffee is dried thoroughly.

Variations

Raking coffee

Double Washing or Double Fermentation

The double washed process is perhaps the most well known variation on the traditional process. It can be found across Africa in Burundi, Rwanda, and Kenya where it has gained the most recognition. After pulping, the coffee is left to ferment dry without addition of any water for 8-12 hours. It is then washed through a channel that sorts by density. The sorted beans are then soaked in tanks for a second, wet fermentation and raked through a channel again. The seeds are typically dried on raised beds for 1-2 weeks. This processing style yields exceptionally clean and vibrant coffees likely because of the extended period of germination for the seeds and the second sorting of the highest density coffees.

Mechanical Washing

Mechanically demucilaged coffee is increasingly popular because of the low amount of water that is required. In this method, the traditional soaking in tanks is replaced by equipment that removes the mucilage using high speed rotors and water. Adjusting the rotor speed and water pressure will affect the level of demucilation up to a certain point. The flavors of this processing style are similar to a washed coffee in cleanliness but generally have a more mild acidity. The seeds will be dried for approximately 1-2 weeks on patios or raised beds or may be dried mechanically.

Washed coffees produce approachable and clean coffees with generally increased complexity. Royal NY offers a variety of washed coffees from around the world.

Patrick McKeown

Patrick began his coffee journey on Long Island at a small coffee bar and roasting at home. Since then he has been a barista, manager, and craft roaster in NYC.