Processing of coffee is the method of converting the raw fruit of the coffee plant (cherry) into the commodity green coffee. Coffee beans are the seeds of fruits, which resemble cherries, with a red skin when ripe. Beneath the pulp, each surrounded by a parchment-like covering, lay two beans, flat sides together. Underneath the parchment the beans are covered in another thinner layer, the silver skin. Coffee beans must be removed from the fruit and dried before they can be roasted; this can be done in two ways, known as the dry and the wet methods. When the process is complete the unroasted coffee beans are known as green coffee.
Most specialty coffees are processed by the “wet” or washed method. After being picked at the peak of ripeness, the coffee beans are “squeezed” out of the cherry-like skin by a pulper and separated with the aid of water. The beans are then allowed to ferment for 36 to 72 hours and washed with clean water. This process removes the mucilage or fruity layer before the coffee beans are put out to dry for about 2-3 weeks. The beans are turned several times a day to ensure that all the beans dry evenly. After an additional “conditioning” period of about 6-8 weeks, the coffees are then milled (to remove the parchment or pergamino layer) before being sorted to remove imperfections and graded for size, weight, and most importantly – liquor or cup quality. In the dry or natural method, after being picked, the whole cherries are placed directly on drying patios and left to dry in the sun. The dried cherries are later milled, sorted, and graded for export.
Even after careful harvesting, a certain number of partially dried and unripe cherries, as well as some stones and dirt, will be present 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.
As with most agricultural products, not all lots are the same. Therefore, it is up to the coffee roaster to buy, blend, and roast each coffee in such a manner that you, the consumer, can expect the same product each time you buy a pound of your favourite coffee from your retailer.
The dry method (also called the natural method) is the oldest, simplest and requires little machinery.
This method involves drying the whole cherry. There are variations on how the process may be carried out, depending on the size of the
plantation, the facilities available and the final quality desired. The three basic steps, cleaning, drying and hulling, are described below.
First, the harvested cherries are usually sorted and cleaned, to separate unwanted fruit, leaves, soil and twigs. 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 ensure even drying. It may take up to 4 weeks before the cherries are dried to the optimum 12.5% moisture content, depending on the weather conditions. 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. A coffee that has been over dried will become brittle and produce too many broken beans during hulling (broken beans are considered defective beans). Coffee that has not been dried sufficiently will be too moist and 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 95% 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. Decaffeinated Processing
Most decaf coffees are made using a chemical process first used in Europe. This process involves soaking the beans in water and then “washing” them in methylene chloride to absorb the caffeine from the bean. After this, the beans are rinsed clean of the chemicals, dried and shipped to the coffee roasters. The advantage of this method is that it provides decaf coffee with more flavour than the Swiss water processing. Although there is virtually no trace of any chemicals left in the bean after roasting, some people are uncomfortable knowing that the coffee they are drinking was chemically processed.
Swiss Water Process
Decaffeinated coffee has come a long way since its invention in 1903. Unfortunately though, some of us are still drinking in the dark ages. You see, most decaffeination processes employ chemical solvents such as methylene chloride or ethyl acetate to strip caffeine molecules from the green coffee bean.
At the Swiss Water Decaffeinated Coffee Company, they use water from the coast mountains of British Columbia to gently remove the caffeine until the coffee beans are 99.9% caffeine-free, while maintaining the bean’s distinctive origin and flavour characteristics.