How Coffee is Processed

Processing Coffee
Once coffee beans have been harvested and sorted, they must be processed. This means the fruit needs to be dried and hulled and the "beans" sorted, graded, and bagged. There are a few different ways this can be done.

Dry Process
Considered the most natural process, dry processing is when the coffee cherries are sorted then laid out on raised beds to dry in the sun. The cherries are raked many times throughout the day to ensure even drying and to prevent mildewing. This allows the excess moisture to bake out of the fruit. In some cases, the cherries are placed into a machine to dry further after a few days of sunning. When space is at a premium, the cherries may only be dried in machines. The goal of this process is to get the moisture content to 10%.

After they are dried, the cherries are stored in silos until they are sent to a mill to be hulled. During this procedure a machine removes the outer layers of the cherry in one step, leaving behind two beans with the parchment layer—a paper-like coating—sometimes referred to as parchment coffee. Most arabica coffee from Ethiopia, Haiti, India, and Brazil is processed using this method while almost all robusta coffee is.

Semi-Dry Process
In areas where it is too damp to dry process, coffee must be semi-dry processed. Also known as wet processed, wet-hulled, or semi-washed, this process is believed to increase body and reduce acidity.

In this method, the skin of the cherries is removed, and the cherries are stored for up to a day with the pulp or mucilage still on them. The mucilage is then washed off leaving behind the parchment coffee. At this point, the beans are spread out on beds or mats to dry in the sun. The beans are raked a few times per hour to make sure the beans dry without becoming mildewed or growing bacteria. After drying, the beans are sent to be hulled.

Polishing, Cleaning, Sorting, Grading
After the coffee beans have been hulled of the fruit and parchment, whether by dry or semi-dry processing, they are polished, cleaned, sorted, and graded.

Polishing removes any parchment or “silver” skin left from the hulling machine. This optional process essentially separates the “chaff” from the beans. Once polished the beans are cleaned and sorted. As the beans move through the sorting machines, which sort by size or density, any sticks, clumps of dirt, rocks, or other non-coffee bean items are also removed.

After cleaning and the initial sorting, the beans move on to be sorted by color. Though there are machines that can do this, they are expensive to purchase and maintain. Also, hand-sorting provides much-needed jobs in the areas where coffee is grown and processed. Some of the very best coffee has been hand-picked two or even three times to ensure only the finest beans are chosen.

Once the beans are polished, cleaned, and sorted, they move on to be graded. Coffee is categorized by things like where it was grown, at what altitude, the size of the bean, how it tastes, and other such criteria. And if the coffee comes from a specific location, think a region or an estate, there may be even more quality aspects beans are tested for. The higher and more consistent the quality of the coffee, the higher the price that can be commanded.

Additional Processing
Beyond the basic steps for processing outlined above, some coffees may undergo other steps. Two examples are decaffeination and aging.

Decaffeination removes most (not all) caffeine from green coffee prior to it being roasted. There are three methods to decaffeinate coffee. The first uses ethyl acetate to moisten the green coffee which removes most of the caffeine. The beans are then taken out of the ethyl acetate and steam-stripped to remove any trace of the ethyl acetate. The caffeine can then be removed from the ethyl acetate which can be reused to decaffeinate more beans.

A second method to decaffeinate coffee is the Swiss Water Process. During this process, green coffee beans are soaked in a green coffee extract (GCE) which is a solution of all the water-soluble components of green coffee, with the exception of caffeine. Because the extract has no caffeine, the caffeine molecules from the green coffee migrate to the extract while all the flavor and other water-soluble components stay in the green coffee due to those being equal in both the green coffee beans and the GCE. If this process sounds vaguely familiar, that’s because we learned about it in middle school science class! The method is similar to diffusion, the scientific phenomenon whereby molecules of a certain substance (caffeine) from a highly concentrated solution of the substance (green coffee) move to a region of lower concentration (GCE).

The third method of decaffeinating is the most common, supercritical carbon dioxide (CO2) extraction. This process takes green coffee beans that have been moistened and puts them in contact with CO2 which is maintained at 4,000 pounds of force per inch squared and at temperatures between 194 deg F and 212 deg F. This removes approximately 97% off the caffeine. The CO2 can be reused after being run through an activated carbon system to remove the caffeine.

The last processing method is aging. Aging coffees is not a widely used method of processing. Shipping coffee worldwide once took months and exposed the beans to weather and other aspects of travel along the way. The most popular of these aging methods is “monsooning”. As coffee was traveling from Yemen to Europe it spent months at sea being exposed to the salty ocean air. When it arrived in Europe the coffee beans were swollen and yellowed having absorbed a great deal of moisture. This gave the coffee a pungent aroma and wild flavor while greatly diminishing the amount of acidity in the beans.

Today, the monsooning process is recreated by storing the coffee beans in open warehouses near the coast. The sixteen-week process affects the coffee beans in the same way, producing swollen, yellowed beans with a unique flavor profile.