Coffee Processing

Coffee beans are the seeds of fruits that resemble cherries, with a red skin when ripe. Two beans, flat sides together, lie beneath the pulp and each is surrounded by a parchment-like covering. When the fruit is ripe a thin, slimy layer of mucilage surrounds the parchment. Underneath the parchment the beans are covered in another thinner membrane, the silver skin. Each cherry generally contains two coffee beans; if there is only one it assumes a rounder shape and is known as a peaberry. Coffee beans must be removed from the fruit and dried before they can be roasted, which can be done in two ways: the dry and the wet methods. When the process is complete the unroasted coffee beans are known as green coffee.

Dry Method
The dry method (also called the natural method) is the oldest and simplest, and requires little machinery. It involves drying the whole cherry. The process varies, depending on the size of the plantation, the facilities available, and the final quality desired. The dry method has three basic steps: cleaning, drying and hulling.

First, the harvested cherries are usually sorted and cleaned, to separate the unripe, overripe, or damaged cherries and to remove dirt, soil, twigs and leaves. This can be done by winnowing, which is commonly done by hand, using a large sieve. Any unwanted cherries or other material not winnowed away can be picked out from the top of the sieve. The ripe cherries can also be separated by flotation in washing channels close to the drying areas.

The coffee cherries are spread out in the sun, either on large concrete or brick patios, or on matting raised to waist height on trestles. As the cherries dry, they are raked or turned by hand to insure even drying. Depending on the weather conditions, it may take up to four weeks before the cherries are dried to the 12.5% maximum moisture content. On larger plantations, machine drying is sometimes used to speed up the process after the coffee has been pre-dried in the sun for a few days.

The drying operation is the most important stage of the process, since it affects the final quality of the green coffee. Coffee that has been over-dried becomes brittle and produces too many broken beans during hulling. Coffee that has not been dried sufficiently becomes too moist, and is prone to rapid deterioration caused by the attack of fungi and bacteria.

The dried cherries are stored in bulk in special silos until they are sent to the mill, where hulling, sorting, grading and bagging take place. All the outer layers of the dried cherry are removed in one step by the hulling machine.

The dry method is used for about 90 percent of the Arabica coffee produced in Brazil, most of the coffees produced in Ethiopia, Haiti and Paraguay, as well as for some Arabicas produced in India and Ecuador. Almost all Robustas are processed by this method. It is not practical in very rainy regions, where the humidity of the atmosphere is too high or where it rains frequently during harvesting.

Wet Method
The wet method (also called the washed method) requires the use of specific equipment and substantial quantities of water. When properly performed, it insures that the intrinsic qualities of the coffee beans are better preserved, producing a green coffee which is homogeneous and has few defective beans. Hence, the coffee produced by this method is usually regarded as being of better quality, and commands higher prices.

Even after careful harvesting, a certain number of partially dried and unripe cherries, as well as some stones and dirt, remain among the ripe cherries. As in the dry method, preliminary sorting and cleaning of the cherries is usually necessary and should be done as soon as possible after harvesting. This operation can be done by washing the cherries in tanks filled with flowing water. Screens may also be used to improve the separation between the ripe and unripe, large and small, cherries.

After sorting and cleaning, the pulp is removed from the cherry. This operation is the key difference between the dry and the wet methods, since in the wet method the pulp of the fruit is separated from the beans before the drying stage. The pulping is done by a machine which squeezes the cherries between fixed and moving surfaces. The flesh and the skin of the fruit are left on one side and the beans, enclosed in their mucilaginous parchment covering, on the other. The clearance between the surfaces is adjusted to avoid damage to the beans. The pulping operation should also be done as soon as possible after harvesting to avoid any deterioration of the fruit which might affect the quality of the beans.

The pulped beans go on to vibrating screens which separate them from any u-pulped or imperfectly pulped cherries, as well as from any large pieces of pulp that might remain. From the screens, the separated pulped beans then pass through water-washing channels where a further flotation separation takes place before they are sent to the next stage.

Because the pulping is done by mechanical means it normally leaves some residual flesh as well as the sticky mucilage adhering to the parchment surrounding the beans. This has to be completely removed to avoid contamination of the coffee beans by products resulting from the degradation of the mucilage. The newly pulped beans are placed in large fermentation tanks where the mucilage is broken down by natural enzymes until it is dispersible, when it can be washed away. Unless the fermentation is carefully monitored, the coffee can acquire undesirable, sour flavors. For most coffees, mucilage removal takes between 24 and 36 hours, depending on the temperature, thickness of the mucilage layer, and concentration of the enzymes. The end of the fermentation is assessed by feel, as the parchment surrounding the beans loses its slimy texture and acquires a rougher “pebbly” feel.

When the fermentation is complete, the coffee is thoroughly washed with clean water in tanks or special washing machines. At this stage, the wet parchment coffee consists of approximately 57% moisture. To reduce the moisture to a maximum of 12.5% the parchment coffee is dried either in the sun, in a mechanical dryer, or by a combination of the two. Sun-drying is done on extensive flat concrete or brick areas, known as patios, or on drying tables made of fine-mesh wire netting. The beans are laid out in a layer of 2 to 10 cm, and turned frequently to insure even drying. Sun-drying should take from 8 to 10 days, depending upon ambient temperature and humidity. Coffee dries more quickly if raised on tables because of the upward draught of warm air. The use of hot-air drying machines becomes necessary to speed up the process in large plantations where, at the peak of the harvesting period, there may be much more coffee than can be effectively dried on the terraces. However, the process must be carefully controlled to achieve satisfactory and economical drying without any damage to quality.

After drying, the wet processed coffee, or parchment coffee as it is commonly known, is stored and remains in this form until shortly before export.

The final stages of preparation of the coffee, known as “curing”, usually take place at a special plant just before the coffee is sold for export. The coffee is hulled, to remove the parchment, then passes through a number of cleaning, screening, sorting and grading operations which are common to both wet- and dry-processed coffee. Electronic sorting machines may be used to remove defective beans that cannot be distinguished by eye.

The wet method is generally used for Arabica coffees, with the exception of those produced in Brazil and the Arabica-producing countries mentioned above as users of the dry method. It is rarely used for Robustas.

Semi-Dry Method
In the semi dry-process, coffee beans are pulped as in the wet process. The coffee beans “with parchment and some mucilage still attached“ are then dried, instead of the fermentation performed in the wet process. After drying, the coffee beans are dehulled (separated from the parchment), sorted, and placed in burlap sacks for export. The semi-dry process is common in the islands of Sumatra and Sulawesi, and is sometimes used in Brazil.

Copyright Coffee Research Institute