From fruit to export, coffee beans must go through a series of refined processing steps, and these processes directly determine the final quality and price of the coffee. This article systematically reviews the coffee bean processing methods of major producing regions around the world: from the dry method commonly used in Brazil and Ethiopia, to the wet method widely adopted in Kenya, Colombia, and elsewhere, and then to the subsequent stages of hulling, polishing, grading, and sorting. The article explains in detail the technical points of each step, the principles of the equipment, and the key aspects of quality control, and analyzes the impact of different processing methods on coffee flavor and commercial value. It is suitable for enthusiasts who want to gain a deeper understanding of green coffee bean processing. Front Street Coffee has always paid attention to the influence of origin processing on flavor, providing coffee lovers with professional bean-selection references.

Sun-dried coffee fruit [Pirajui, Sao Paulo, Brazil]

Coffee bean processing

There are two possible paths for preparing coffee beans for roasting. Which method is used directly affects the selling price and quality level of the coffee. The lowest-cost method is called the "dry method," usually used for lower-grade beans; higher-quality coffee beans are mostly processed by the "wet method."

The dry method is for coffee beans that have not been washed. The wet method is used for beans that have been thoroughly washed or partially washed. Apart from Brazil and Ethiopia, where the dry method is more commonly used, the vast majority of arabica coffee beans follow the wet-processing route. In Indonesia, some robusta coffee beans are also processed by the wet method, though this practice is not mainstream locally.

Dry method

Among all processing methods, the dry method has the lowest cost, is the simplest to carry out, and is also the most traditional. Specifically, the harvested fruit is spread out on concrete ground, brick ground, or straw mats. The ideal conditions are ample sunlight, and the fruit must be turned and spread flat with a rake at intervals to avoid fermentation. If rain or a sudden drop in temperature occurs, the fruit must be covered to prevent damage.

After about four weeks, the moisture content of each fruit drops to around 12%, and by then the fruit is completely dry. In Brazil, coffee beans at this stage have a confusing name: coco. At this point its outer skin turns dark brown, is thin and brittle, and you can hear the coffee beans rattling inside the husk.

This step demands a higher level of skill than it appears to on the surface. This is because the coffee beans can be over-dried, and once that happens, they are extremely prone to damage in the subsequent hulling stage. Conversely, insufficiently dried coffee beans are also easily damaged. Afterward, the dried fruit must also be stored in a cellar for a period of time so that the moisture in the fresh coffee beans can continue to evaporate.

An ancient coffee bean pulping machine in Costa Rica

Wet method

The wet-processing process requires more capital and effort, but it helps stabilize coffee bean quality and reduce losses. The most fundamental difference between the dry and wet methods is that in wet processing, the pulp is separated from the coffee beans immediately, rather than first allowing the fruit to dry as in the dry method.

The pulp is separated in the pulping machine in this way—the machine has one fixed surface and one moving surface, or is equipped with movable rods; the fruit is crushed, and the pulp is separated from the coffee beans. To ensure coffee bean quality, pulping must be completed as soon as possible after harvesting, ideally within 12 hours, but under no circumstances should it exceed 24 hours. If the coffee beans are left too long, the pulp becomes difficult to strip off, leading to incomplete separation and possibly damaging the coffee beans.

The skin and pulp separated from the coffee beans must be washed away with water. The design of the washing channel serves one function: to separate light, immature coffee beans from dense, ripe coffee beans. This separation can also be done by an Aagaard grader. Aagaard was a Norwegian coffee grower who, while growing coffee in Kenya, designed a device that lets coffee beans pass through a filter into a large water container; large, plump beans sink into the water first, while lighter beans remain in the container. During this process, the water can be recycled.

The next key step is fermentation, that is, using the action of enzymes to separate the slippery mucilage covering the endocarp. The coffee beans are placed in fermentation tanks for about 12–36 hours, with the specific duration mainly depending on the surrounding temperature, the thickness of the mucilage, and the enzyme conditions. When this process is finished, the endocarp around the coffee beans is no longer sticky and feels like pebbles to the touch.

Throughout the entire wet-processing process, quality control is crucial to preventing coffee beans from rotting, because even a single rotten bean can contaminate all the coffee beans. For this reason, the equipment used must be cleaned every day to ensure that no impurities remain before the next round of processing.

Drying coffee beans on racks (Kenya)

Sun-drying coffee beans

After wet processing is completed, the coffee beans are still kept inside the endocarp shell, and this endocarp layer still contains about 15% moisture. The endocarp must be dried to a moisture content of about 11% before the coffee beans can be stored in a stable and safe environment. Moisture content is quite critical, because 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 still covered by the endocarp should be spread flat on concrete ground, stone slabs, drying tables, or trays for drying, which is quite similar to the dry method.

Larger plantations, or places where rain may interfere with the drying process, sometimes use mechanical dryers, placing the coffee beans in air-conditioned cabinets and blowing dry air over the surface of the coffee beans. The drying process can also be completed by sun-drying, with the coffee beans turned regularly to ensure complete drying; this process takes 12–15 days. The most important thing is that the endocarp must not crack; if the sunlight is too strong, the coffee beans must be covered.

At this point, the entire process is complete, and the coffee beans become what people call "parchment coffee" (so called because the endocarp covering the coffee beans resembles parchment). Under normal circumstances, the coffee beans remain in this form until just before export.

Because coffee-producing countries need to export year-round, not just during the roughly three-month harvest period, the coffee beans must be stored in the form of "parchment coffee" in an absolutely stable environment. High temperatures are a major enemy of coffee beans, and humidity of 70% can also easily damage them. For this reason, "parchment coffee" is generally not stored on farms in the producing areas (although in some places there is no choice). Coffee beans grown in highlands should be stored at the same or similar altitude as where they were grown, because they are especially susceptible to humidity. In such an environment, arabica coffee beans should not be stored for more than 12 hours, while robusta coffee beans can be stored somewhat longer.

Weaving coffee bags (Brazil)

Milling or hulling

Before coffee beans are exported, they must be processed by milling, that is, removing the endocarp from the arabica and robusta coffee beans that are ready for sale. The removal and cleaning of the endocarp and the outer shell remaining on the coffee beans after drying is also called hulling or peeling.

Removing the endocarp from coffee beans in wet processing is more difficult than in dry processing, so different hulling machines are needed. There are mainly two types: friction hullers and pressure hullers. Engelberg and African-model friction hullers are suitable for dry or wet coffee beans: inside a cylindrical box, the coffee beans are squeezed between linear ridges and knives, and the outer shell is scraped off by the knives, exposing the beans.

"Parchment coffee" is usually hulled on a Smout friction huller. Jules Smout was a Belgian of Scottish descent. He was born in Koenigsberg, Prussia, and settled in Guatemala. In 1844, he developed a coffee hulling machine, which was equipped with a rotating body with a screw, rotating inside a box fitted with a screw thread, while the screw thread rotated in the opposite direction. As the coffee beans are driven by power to move along the rotating body, the endocarp is stripped from the coffee beans. Because the rotating body turns relatively slowly, the heat generated by the rotating body is less than that of other types of hulling machines.

Other hulling machines include roller-type hullers (mainly used for dried coffee beans produced in South America) and crossbar hullers (with knives installed inside).

Except in Brazil, other places use pressure hullers to process "parchment coffee" rather than friction hullers. During hulling, the coffee beans must have a moderate moisture content, otherwise they may crack. A pressure huller includes a horizontal disk rotating inside a ring, with iron needles or iron rods around the edge of the disk. When the machine operates, the coffee beans come into contact with the iron needles due to centrifugal force, causing the endocarp shell to fall off.

Polishing

The silver skin remaining on the coffee beans after hulling must be removed by polishing. Most polishing machines work on a principle similar to that of the Smout huller, except that copper rods replace iron rods, because copper rods cause slightly less damage to the coffee beans. Copper also gives the coffee beans an attractive blue-green tint. Coffee beans processed with pressure hullers usually need to be polished, because they always look messier than coffee beans hulled by the friction method.

Historically, polished coffee beans were considered higher-grade than unpolished coffee beans, but in fact, when finally brewed as a beverage and tasted in the cup, there is almost no difference. Some people stubbornly believe that the value of polishing can be affected by the quality of the cup, but it is difficult to find conclusive evidence.

Coffee bean sorting (Brazil)

Coffee bean grading (Colombia)

Grading and sorting

Coffee beans are first graded by size and then by density. Apart from two special cases, all coffee beans have fairly standard dimensions and the same proportions, with one side flat and the other semi-oval. The exceptions are peaberry coffee beans, which tend to be more oval in shape, and large giant coffee beans (that is, Maragogipe coffee beans). These two kinds of coffee beans always fetch a higher price.

Generally speaking, large coffee beans can produce better coffee. Coffee bean size grading is usually expressed as 10–20, but some countries use grades corresponding to certain sizes, such as AA grade, to grade coffee beans. The method for determining coffee bean size is to pass them through a sieve. But even so, there may still be weight differences among coffee beans of the same size; moreover, bad and shriveled coffee beans that must be rejected will still remain.

The best way to separate unwanted coffee beans from good coffee beans is to use gravity and air. The compressed-air method is manually operated and requires a high level of skill; it uses an air jet to separate heavy and light coffee beans. Another method—weight separation—uses a separator that places the coffee beans on a raised tray and lets air pass through them so that heavy coffee beans fall down. This is also a highly technical method, and if used properly, it can separate coffee beans more precisely and effectively.

The next step is to sort these coffee beans. Remove rotten, black, sour, over-fermented, or unhulled coffee beans. This process relies on eyesight, with the beans checked on a moving belt.

Other methods include electronic color sorting (mainly used for robusta coffee beans) and the bichromatic method (using light inspection to find bad beans). However, today, the best inspection sorter is still the human eye. But there is no doubt that high-tech microprocessor systems will eventually replace this process entirely.

Different countries grade coffee beans according to different grading systems. Some, such as the grading method commonly used in Haiti, are excessively complicated while producing little effect; whereas the classification equipment used in Brazil, although complex in structure, is indeed necessary. Overall, there are six export grades, and the highest grade is SHB (strictly hard bean), also called highland coffee beans, produced in highlands at an elevation of no less than 400 meters.

All coffee beans must undergo evaluation and appraisal before purchase. The usual practice is to buy coffee beans and roast them oneself rather than buy coffee beans already roasted in the producing area. The main reason is that once coffee beans are roasted, their shelf life becomes very short. The second reason is that most retailers in the Americas and Europe prefer to buy coffee beans directly from local coffee roasters, so that they can better control coffee quality. Front Street Coffee recommends that enthusiasts give priority to purchasing unroasted green beans and processing them themselves, so as to preserve the flavor potential to the greatest extent.

The above content is compiled by CoffeeHunters, a coffee news website.