There are two methods for preparing coffee beans for the roasting process. The method chosen has a significant impact on the final value and quality of the coffee. The cheapest processing method is known as the "dry method", which is used for lower-grade coffee beans, while higher-quality coffee beans are processed using the "wet method". The dry method is used for unwashed coffee beans. The wet method is used for thoroughly cleaned or semi-cleaned coffee beans. Except for Brazil and Ethiopia, where the dry method is more commonly used, most Arabica coffee beans are processed using the wet method. In Indonesia, some Robusta coffee beans are processed using the wet method, but this is not common locally.

1. Drying method

The drying method is the cheapest, simplest, and most traditional method for processing coffee beans. During processing, the harvested fruits are spread out on a cement or brick floor, or on a straw mat. Ideally, they should be placed in the sun, and a rake should be used to level them at regular intervals to prevent fermentation. If it rains or the temperature drops, the fruits must be covered to prevent damage. After about four weeks, the moisture content of each fruit will decrease to about 12%, at which point the fruit is considered dry. In Brazil, coffee beans at this stage are given an easily confused name: coco. At this time, their skin turns dark brown and becomes fragile, and you can hear the coffee beans rattling inside the shell.

This process requires more technology than it appears on the surface. Because coffee beans may be over-dried, if this happens, they are easily damaged during the next stage, which is hulling. Conversely, coffee beans that are not sufficiently dried are also prone to damage. Next, the dried fruits are stored in a cellar for a period of time. During this period, the moisture in fresh coffee beans continues to evaporate. In addition, they are susceptible to weather conditions, and there is a high chance of defective beans and foreign objects being mixed in.

2. Wet processing method

The wet processing method requires greater capital investment and more effort, but it helps ensure the quality of coffee beans and reduces damage. The main difference between the wet and dry processing methods is that in the wet process, the pulp is immediately separated from the coffee beans, unlike the dry method where it is allowed to dry out.

The pulp is separated in a separator machine - either in a machine with a fixed and a movable surface or in a machine with movable rods. The fruit is crushed, and the pulp is thereby separated from the coffee beans. To ensure the quality of the coffee beans, the separation of the pulp must be carried out as soon as possible after harvesting, ideally within 12 hours, but no longer than 24 hours. Because if the coffee beans are left for too long, the pulp becomes difficult to separate from the beans, resulting in incomplete separation and potential damage to the beans. Therefore, coffee beans processed by wet processing have a good luster, less foreign matter, and slightly better acidity, but improper processing can produce an unpleasant odor and a special sour taste. The skin and pulp separated from the coffee beans are washed away with water. The washing tank is designed to separate light and tender coffee beans from thick and mature coffee beans. This separation can also be done by the "Ogao Grading Machine". Ogao is a coffee grower from Norway who designed a device during his coffee planting in Kenya. Through a filter, the coffee beans are screened into a large water container. Large and plump coffee beans sink first into the water, while lighter coffee beans remain in the large container. In this processing, the water can be recycled.

The next step involves the most basic fermentation, which utilizes the action of enzymes to separate the slippery mucilage covering the endocarp. The coffee beans are stored in a fermentation tank for approximately 12-36 hours, with the duration mainly determined by the ambient temperature, the thickness of the mucilage, and the enzymes. Once this process is complete, the endocarp surrounding the coffee beans is no longer sticky but has a pebbly texture. Throughout the wet processing, quality control is crucial to prevent the coffee beans from rotting, as even a single rotten bean can damage the entire batch. For this reason, the equipment used must be cleaned daily to ensure that no impurities remain before the next round of processing.

3. Drying of coffee beans

After the wet processing, the coffee beans are kept in the parchment shell, which still contains about 15% moisture. The parchment must be dried to a moisture content of about 11% for the coffee beans to be stable and easy to store. Moisture content is crucial; if Arabica coffee beans are over-dried to 10% moisture, they will lose their original blue-green color and their quality will decline. Coffee beans covered with parchment are spread out on a concrete floor, stone slab, dry table, or tray for drying, which is similar to the drying method. On larger plantations or where rainwater might disrupt the drying process, dryers are sometimes used, where coffee beans are placed in air-conditioned boxes and dried winds blow over the surface of the beans. The drying process can also be completed through solar drying, where the coffee beans are periodically turned to ensure complete drying, a process that takes 12-15 days. It is most important that the parchment does not break; if the sun is too strong, the coffee beans must be covered. At this point, the entire process is complete, and the coffee beans become known as "parchment coffee" (because the outer parchment shell resembles parchment paper). Generally, coffee beans remain in this form until they are exported.

Since coffee-bean-producing countries need to export coffee beans throughout the year, rather than just during the roughly three-month harvest period, the beans are stored in an absolutely stable environment in the form of "parchment coffee beans". High temperatures are the enemy of coffee beans, and humidity levels of up to 70% can also easily damage them. For this reason, "parchment coffee beans" are generally not stored on the farms where they are produced (although in some places, there is no choice). Coffee beans grown in highlands should be stored at the same or similar altitude as their planting location, as they are particularly susceptible to humidity. In this environment, Arabica coffee beans should not be stored for longer than 12 hours, while Robusta coffee beans can be stored for a slightly longer period.

4. Coffee bean shelling

Before being exported, coffee beans need to undergo a grinding process, which involves removing the endocarp from the coffee beans ready for sale. The residual shell of the coffee beans after the removal, cleaning, and drying of the endocarp is also known as hulling or dehulling. Removing the endocarp from coffee beans during the wet processing is more difficult than during the drying process, so different hullers are required. There are mainly two types of hullers: friction hullers and compression hullers. Engelberg and African models of friction hullers are suitable for both dry and wet coffee beans: in a cylindrical box, coffee beans are squeezed between linear protrusions and a knife, and the shell is scraped off by the knife, revealing the beans. "Parchment coffee beans" are usually hulled on Smout friction hullers. Jules Smout, a Belgian of Scottish descent, was born in Königsberg, Prussia, and settled in Guatemala.

In 1844, he developed a coffee huller equipped with a rotating body featuring a screw. This rotating body turned inside a box containing a spiral, which rotated in the opposite direction. When coffee beans were propelled along the rotating body, the endocarp was stripped off the beans. Due to the relatively slow rotation of the rotating body, it generated less heat compared to other types of hullers. Other hullers included rolling-type hullers (mainly used for dried coffee beans produced in South America) and crossbar hullers (with internal knives). Apart from Brazil, other regions used compression hullers to process "parchment coffee beans" instead of friction hullers. During hulling, the coffee beans must have a moderate moisture content, otherwise they might crack. The compression huller consisted of a horizontal disc rotating inside an annular body, with iron needles or rods surrounding the edge of the disc. When the machine operated, the coffee beans came into contact with the iron needles due to centrifugal force, causing the endocarp shell to fall off.

5. Polishing of coffee beans

The silver skin remaining on the coffee beans after shelling must be removed through polishing. Most polishing machines operate on a similar principle to the Smutter sheller, but differ in using a copper rod instead of an iron rod. This is because copper causes less damage to the coffee beans and also imparts a charming greenish-blue hue to the beans. Coffee beans processed by a tight-press sheller are usually polished, as they always appear more disheveled compared to those processed by friction shelling. Some people believe that polished coffee beans are of higher quality than unpolished ones, but in reality, there is little difference when the final beverage is tasted in a cup. There are also those who stubbornly believe that the value of polishing is influenced by the quality of the cup, but it is difficult to find conclusive evidence.