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How Coffee Processing Methods Affect Coffee

By Jon Ferguson | October 9, 2020

Over the last few decades, coffee professionals around the world have realized the effect different processing methods have on coffee quality, leading the entire specialty coffee industry to invest and explore these techniques that have been used by coffee farmers for decades. 

But what is “coffee processing” and how does it change the way coffee tastes?  Coffee processing is the overall actions or steps that take place from the time the coffee cherries are harvested off the coffee plant to when the coffee beans are roasted. Growing competition among coffee roasters has increased the interest in coffee processing.

How to Process Coffee Cherries

The processing methods below require removal of the outer layer of cherry pulp and an inner shell layer that covers the seed, known as parchment. With the exception of natural processed (fruit-dried) coffee, the first step following harvest for all other processing techniques is known as depulping.

Removal of mucilage begins as coffee passes through machinery that squeezes and pulls the fruit skin and pulp from the inner parchment seed casing. Removing the sticky mucilage bound to the seed can also be forcibly removed by mechanically scrubbing, or through naturally occurring microbial fermentation processes. These coffee processing methods are constantly evolving and adopting to their markets and environmental surroundings.

Deciding which processing method to use is determined by several factors, including access to water, money for adequate processing equipment, and the market demand for a defined flavor profile from a specified region.  A farmer will choose a processing method based off what is available to him. 

Read: Does Dark Roast Coffee Have More Caffeine?

For example, washed coffee processing is historically common in areas with ample access to water and a recognized demand by international markets for the flavor created by washing coffee.  Water usage differs dramatically between these most common methods used for processing. As shown below, washing coffee to remove mucilage requires more water for separating defects, fermentation, and cleaning of parchment, but in areas where water is not available, natural processed coffees are common.

Natural

Natural processed coffee is the most basic processing method.  When coffee picking is complete, harvesters return to the sorting area near the main processing center. They place all collected cherries, leaves, unripe cherries, and foreign matter on cement patios or beds to begin sorting. Directly after harvesting coffee cherry fruit, producers (or machinery) sift through  sticks, leaves, and other matter, separating the damaged beans from the more complete and ripe ones. Floating coffee cherries are lower quality and separated into lower quality lots, which also improves the selection process.

The coffee cherries are then placed on cement patios or raised beds to be dried to a moisture content of approximately 12%, which protects the beans from mold development during storage. Higher moisture levels change the physical weight of coffee , influencing the price of coffee. Once coffee has reached the proper moisture level, the dried raisin-like covered cherry is then hulled, removing the dried fruit skin and an outer shell.

Specialty grade natural processed coffees started in Ethiopia and gained attention from high end specialty coffee buyers in the early 2000s. Following the popularity of natural processed Ethiopian coffees, Central America adopted the processing technique after discovering the competitive prices this technique was receiving from coffee buyers. Prior to this recent recognition for fruit-dried coffees, parchment-dried washed coffees were the preferred method for producers from 1960-2000 and continues to be the dominant practice today. 

Washed

Fully washed coffees became popular because of a growing demand for cleaner coffee flavors, in addition to the farmer facing lower losses throughout processing stages.

There is a lot of focus within the industry on removing mucilage because this step in the process is known to affect flavor the most. Once coffee is pulped of its skin and fruit, the sticky parchment covered in mucilage remains in a pile often housed in cement storage tanks under water or without water for a period anywhere from six to 72 hours, sometimes longer. 

The amount of fresh water required to properly rinse coffee after the dry or wet fermentation process is an environmental concern because many areas use limited water resources to fulfill demands made by collection stations and exporting agencies. According to a group of coffee experts in Central America, the traditional wet-milling process uses 35-60 liters of water to produce one pound of dry parchment, which equates to approximately one million gallons of water over the course of a typical harvest season (Kline, 2013).

Carbonic Maceration

I would argue that the next closest processing technique to natural processing is known to some as “carbonic maceration,” where whole cherries are harvested and then placed in a sealed container with minimal oxygen, with or without water, and left for a period of time, often overnight, which begins to breakdown attributes of the cherry or fruit that surrounds the seed.  Some producers add water to the container and seal it to prevent any air from being present. This could also be known as “anaerobic fermentation,” which requires the absence of oxygen and is achieved by filling the container with water.

Pulp Natural or “Honey Process”

Similar to natural processed coffees, pulp natural processing does not require washing or fermentation, so it requires little to no water.  The drying stage of coffee processing follows the removal of mucilage. Understanding the necessary environmental conditions and having enough space to dry coffee influences the decision on which drying method is best for an operation.

The pulped-natural processing method adds an additional step to natural processing. Cherries are passed through a de-pulper which removes the outer skin and fruit surrounding the parchment layer of the coffee seed. There is very little water used in this process, allowing for a varying amount of sticky mucilage to remain. The process is relatively new, starting in Brazil during the early 1990s. The processing method was effective for separating unripe cherries from ripe selections that were commonly mixed during mechanical harvesting and strip picking (Wintgens & others, 2004).

The dry mucilage on the outer layer of parchment affects the flavor of coffee, tasting similar to naturally processed coffee. Coffee producers have found this process adds unique fruit flavor attributes to otherwise common profiles. Coffee buyers have also found pulp-dried coffees blend well with either washed or natural coffees in the roasting process, providing greater flexibility in application (Wintgens & others, 2004).

Wet-Hulled

While most of the coffee processing practices share three similar techniques, removing the outer protective shell when it has not yet fully dried is less widely practiced. Indonesia is well known for this method, also described as wet-hulled, and more regionally known as Giling Basash. The climate in Indonesia during harvest is typically wet and humid with significant cloud cover. These climatic conditions make it difficult for producers to quickly dry coffee. In comparison to other coffee growing countries where drying coffee may be achieved within 20 days, many parts of Indonesia reach 30 days or more (Owen, n.d.).

Most coffee producers in Indonesia pick their own red cherries and pulp them at home, leave it in a bucket overnight to ferment and wash it clean with water in a pan the following morning. Coffee is typically put into a bag for delivery and taken to a hulling station where parchment is removed. What differentiates this process is an unusually expedited removal of the parchment.

While the parchment is still wet, producers spread it across a plastic tarp for a few hours until it reaches 50% moisture content.  Once the coffee is delivered to a wet-hulling site, the parchment is removed and exposes the raw green coffee seed where moisture content typically rests above 25-30%.  At this stage, coffee will be laid out once again on cement patios for several more days, bringing moisture levels closer to 20 percent. During this time, coffee seeds are exposed directly to the elements, such as rain, human activities, and other contaminants. Thereafter, the coffee is collected in burlap sacks and taken to a final export warehouse station to dry and sort.  This style of coffee processing and drying is commonly used for lower-grade Robusta qualities because of the very low production costs, ease of processing, and higher commodity volumes, and typically receives minimal prices within the market.

Wet-hulling adds several distinct flavors to coffee. Although these attributes are sought after by a set of regional markets, many professional U.S Specialty coffee drinkers have a negative response to these qualities, describing such flavors as showing mustiness or earthiness (Davids, 2015).

Drying and Storage

One of the most important after harvest processing that establishes cup quality is the drying stage. As mentioned in several of the processing methods, coffee seeds share an international standard for achieving a moisture content of 10% to 12% to retain safe storage of raw green coffee.

Benefits to each type of coffee drying technique are often measured by climatic conditions, economic situations, and cultural tradition. For example, many producers have adopted the use of raised drying beds that were first used in parts of Africa, typically made of a wooden frame and a fine mesh base for holding wet parchment well above the ground. This technique allows air to flow underneath the bed of drying coffee, while also allowing the sun to dry. The parchment is moved by hand, turning often to help maintain consistency in drying.

Producers from all over the world have started to adopt drying coffee on raised beds. Many producers simply use plastic tarps that are placed over the ground for drying parchment. At the end of the day, coffee is collected in large sacks, stored inside of a room, and placed outside the next morning to continue drying.

Read: What is Espresso?

As plantation size, investment, and volume increase, mechanical drying is often used. With mechanical drying, farms dry coffee faster and more consistently.

The Future of Coffee Processing

As coffee farmers continue to share ideas in online videos, professional seminars, and industry friendships, the industry explores new methods to meet the demand, whether that is driven by environmental sustainability or economic prosperity.

Learn more about Arbor Day Coffee here.

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