Washed Processing

Washed Processing – an Overview

The wet mill at Finca San Jerónimo Miramar

 Sometimes called wet processing, washed coffees are known for being clean tasting with a lot of clarity in the cup. Though they are often thought of as the purest expression of a region’s terroir (the particularities of climate, soil, altitude, and latitude that inform a coffee’s flavor), this isn’t necessarily the case. When done well though, washed processing produces delicate, complex cups that are highly sought after.

 There are a few drawbacks to washed processing though that may lead producers to either choose natural (dry) processing or to send their cherries to a larger mill to be processed. First, it requires an enormous amount of fresh water – that for the most part can’t be reused. In a typical wet mill (washed processing facility), it takes 40 cubic meters of water to process one metric tonne of coffee! So, without even considering equipment costs or construction, a producer without abundant access to fresh water won’t be able to wet process their coffees. Additionally, equipment and the facility itself can be cost-prohibitively expensive.

 There are a lot of variances in how coffee is washed, but a few steps will always happen first. A freshly picked batch of coffee cherries must be cleaned, sorted, and separated before processing can begin. This can be as easy as throwing a basked of cherries into the air and letting the wind carry away any twigs or leaves – but no matter the method, growers must ensure that their product is 100% coffee. Sometimes coffee is presorted for ripe/unripe cherries or visible defects after this step. This can result in high quality lots, but it is labor intensive. Next coffee is floated in vessels varying from large, specially designed trenches to simple 55-gallon drums. Ripe and overripe cherries will sink, while unripe cherries will float and be skimmed off. These aren’t thrown out – rather they’re processed as lower quality or commodity-grade lots.

Cherries being de-pulped at FJM

 Next, coffee cherries are mechanically “pulped,” that is, the skin and much of the fruit (mucilage) is removed. This can be with a simple bicycle-operated machine with a turning drum, or it can be with large scale mechanical de-pulpers. This is a delicate step, because if the parchment layer of the bean is cut (the innermost layer), microbial attack can rapidly advance. Machines must be set to just the right size for the appropriate amount of mucilage removal, and regularly calibrated to avoid damaging the beans.

 Coffee is then fermented in large, usually open-air tanks. These tanks are typically made of concrete and can be covered with ceramic tile, stainless steel, or paint. Fermentation can occur with or without water – though it’s most often done soaking in water. If the coffee is dry-fermented, a roof will be necessary to prevent the top layer from drying out. Fermentation is discussed in detail in a separate post, but briefly: microbes including bacteria, yeasts, and filamentous fungi that are (usually but not always) naturally occurring in the air or on the cherries themselves act on the mucilage, consuming some of it, and creating an acidic environment that causes the mucilage to breakdown. Coffee is held in these tanks until the mucilage is completely gone.

Julio Cuy Xar, head of the cooperative at La Armonía Hermosa. To the left you can see a small mechanical de-pulper and a fermentation tank.

 After this, some coffees are washed to remove fermentation residue. If this occurs, it is the most water intensive step in the process. In some instances, particularly in Kenya, coffees are soaked in fresh water at this point, a kind of secondary fermentation. It is believed that this leads to a vast improvement in flavor.

 Finally, all coffee, no matter how it is processed, must be dried. The three main drying methods are raised beds, patios or rooftops, and mechanical dryers. The objective of drying is to achieve a stable, safe, and equal moisture content – historically about 12%, though new research suggests 11% is better. Producers have a de-incentive to excessively dry, because it will reduce the weight of the beans and thereby reduce their profits.

Georgio Bressani, one of the owners of FJM, standing in front of fermentation tanks in their wet mill.

 After drying both washed and natural processed coffees must be dry milled. Typically, coffee is stored in the parchment layer after drying. This helps protect the bean and keep the moisture content stable. It can be kept in this manner for a maximum of six months – after that, the embryo will die. Usually after the coffee is sold, the parchment is removed – the beans are “hulled.” They are often screened for size after hulling to make life easier for roasters and to remove any unwanted material.

 Now, the coffee is ready to be packed and shipped to the roasters!