Structure of the coffee fruit

There are two distinct processing methods, dry and wet, and both methods are used in Hawaii. The dry method is used at a few of the small farms in Kona, and at the more recently established large coffee operations, on raisins or fresh cherries, which are dried whole without pulping. They are milled when thoroughly dried to remove the dehydrated pulp, parchment skin, and silver skin in one operation. The product is green coffee, ready for grading and bagging, and is often called "natural coffee."

The wet method is used throughout the state, and particularly in Kona. It involves the removal of the pulp, fermentation of the mucilaginous material covering the parchment skin, rinsing, and then drying. Most countries producing mild coffee use the wet method. A variation of the wet method is mechanical demucilization. The Fukunaga (CTAHR) hydro-pulper is an example of a machine for this purpose (see Fukunaga 1957).

The first step in wet processing is to remove the outer skin of the cherry and the pulpy flesh beneath it. This flesh and the fruit skin are collectively called the "pulp" after their removal in a process called "pulping."

Beneath the fruit flesh is a layer of slimy, mucilaginous matter, and beneath it is the parchment skin, a very tough, almost shell-like coating that is difficult to remove. The mucilaginous layer is removed after the harvested cherries are pulped, either by natural fermentation during soaking or by mechanical means. Coffee beans when dried and ready for milling are covered by their parchment skin and are referred to as "parchment coffee."

Beneath the parchment skin and covering the coffee beans is a thin membrane called the "silver skin"; a small portion of this skin is usually found on the cleft of the bean after hulling. After hulling the bean is referred to as "green coffee."

The coffee cherry normally contains two beans (its seeds), which are flat on one side and develop in the center of the fruit with the flat sides facing each other (Fig. 5). When only one bean is produced in the fruit, it is round and called a "peaberry." In rare instances, three beans develop that are roughly triangular in shape.

Cherries are processed as soon as possible after harvesting to avoid deterioration of the bean. Another reason for rapid processing is to control loss of bean weight. After the cherry is picked, it begins losing moisture. If the cherry is sold by weight and it is not delivered quickly, then weight and therefore dollars are lost. The beans are also losing weight due to physiological respiration. The rate of weight loss is greatest during the fermentation process, when from 3 to 4 percent of the weight of green coffee is lost. If the coffee is completely processed within 24-36 hours instead of over a period of several days, as is the case in "dry processing," as much as 6 percent of the bean weight can be saved.

Care of harvested coffee before pulping

One of the chief causes of quality deterioration in coffee in Hawaii is decomposition of the cherry pulp before pulping. Detrimental effects occur when harvested cherries are left in bags or boxes for more than 24 hours. If the mucilaginous coating protecting the bean decomposes before pulping, the bean can be mechanically damaged in the pulping process. Without mucilage, the beans will not slip through the pulper grooves and may be nicked, scratched, or chipped by the rough surface of the pulper drum. Beans with mucilage stuck behind ones that are not slippery can also be damaged. The mucilage layer deteriorates when the cherry pulp is allowed to ferment by becoming overripe on the tree or by delaying pulping after harvest.

Also, heat generated by fermentation of the pulp causes the bean to respire and ferment, resulting in weight loss and discolored, sour beans. The sour characteristic of fermented beans is one of the worst defects.

Flotation is recommended before pulping to separate immature, dried, and overripe cherries, which will float, from the better quality ripe berries, which sink. The floaters are pulped separately.


Drum-type pulpers are commonly used in Hawaii; other types are disc and rubber-breastplate pulpers. The drum pulping machine consists of a drum about 11/2-2 ft in diameter that revolves at high speed. The drum exterior surface is roughened. As the drum revolves, coffee cherries are forced between the rough surface and a fixed plate; the cherry is squashed, and the beans are separated from the pulp. The pulp is then ejected from the pulper. The space between the drum and the plate, through which the pulp is carried out of the machine, is not large enough to accommodate the beans, however. Accordingly, grooves are provided in the plate through which the beans can move, facilitated by the slippery, mucilaginous coating on the parchment skin, which is abundantly present when the cherry is fully ripe. The beans that slip through these grooves are ejected out of the opposite side of the machine from the pulp.

Pulping systems used with mechanical harvesters generally remove raisin cherries by flotation and use a classifier-type pulper that separates hard, immature cherries from mature ones. The cherries pass through a cage with bars spaced to permit beans but not cherries to pass between. Soft, ripe cherries are forced through the bars, but hard, immature cherries pass through the cage and exit at one end.

The pulper should be thoroughly cleaned and overhauled every year. It is advisable to adjust every operating part of the machine to avoid bean damage. For example, if one defective point in the pulper were to nick or otherwise damage one bean per revolution, the pulper could damage six pounds of parchment coffee per hour in a small-capacity pulper (Goto and Fukunaga 1956).


After pulping, the mucilage covering the parchment must be removed before drying. The slimy mucilage is insoluble in water and very difficult to dry. When mucilage-coated beans are dried in an oven or on a drying floor, the mucilage will reabsorb moisture and become sticky afterward. Sun-drying mucilage-coated beans takes a long time because the coating picks up moisture each night. Drying without removing the mucilage is advocated by some equipment manufacturers in Brazil, where the humidity is lower.

Two methods are used in Hawaii to remove mucilage: natural fermentation and mechanical demu-cilaging. Natural fermentation is the most common. Freshly pulped coffee is placed in a fermentation vat where bacteria and fungi decompose the mucilage to a soluble material that is readily washed off with water. This method works best at a temperature of between 80o and 90oF. Too high a temperature will ruin the quality of the coffee, while too low a temperature will retard fermentation.

The fermentation vat is usually made either of wood (generally redwood), concrete, fiberglass, or stainless steel. A benefit of wood is that it holds more of the heat generated by the fermentation process. The temperature gradually rises without artificial heating, but steam can be introduced into the vats as a means of warming the fermenting beans.

The vats are usually 4 ft deep, 4 ft wide, and 6 ft long. They should be cleaned and washed thoroughly after each batch of coffee is fermented, in order to remove all undesirable odors. As the coffee comes out of the pulper, it falls into the vat. It is important that a relatively large volume of coffee be placed in the vat, so that heat from fermentation will not dissipate as soon as it is generated, in which case it will take longer to complete the process. When the vat is half full, about 14-18 hours will be required to complete the fermentation. The length of time required also depends on the prevailing temperature, and the process is faster when the nights are warm. Experience will teach the processor to judge the approximate amount of time it takes to complete fermentation of a given amount of coffee in the vat under familiar temperature and other conditions.

The fermentation must be stopped as soon as the mucilage is completely broken down. This can be determined by a very simple test: wash a handful of the beans from the vat in clean water. If they are no longer slippery, the fermentation is complete.

In the fermentation process, the action of certain bacteria and fungi breaks down the mucilage into simple sugars and proteins. After completion of this breakdown, putrefying bacteria begin to multiply on the dissolved protein compounds, and the beans should be removed and washed because they will absorb odors if left in the vat too long. The putrefying bacteria also can affect the proteins in the bean itself, and their access is made easier when the bean has been damaged by the pulper. This can result in discolored beans with a foul odor, known as "sour beans," which can ruin the quality of an entire batch. Therefore, the beans should be washed with clean water as soon as the fermentation to remove mucilage is complete. The water is drained off the beans by a "shaker" machine equipped with a framed screen bottom. As the beans are being shaken, they move toward the drying platform or dryer. Clean water is sprinkled on them as they move, washing them as clean as possible.

The pulper, the fermentation vats, and the shakers should be completely cleaned of coffee beans after their operation is completed. Any beans left will ferment and become sour before the next batch is processed. One sour bean per pound is a serious defect.

Mechanical demucilaging

Several types of commercial coffee demucilaging machines are available. Hydropulpers use water under high pressure, while other types use moving plates or brushes and the action of the beans moving past each other to wear off the mucilage. The mucilage on the coffee bean can be removed even more rapidly if the demucilaging machine is adaptable to using heated water (110-120°F).

The "Fukunaga" demucilaging machine was developed at the CTAHR Kona Research Station in the 1960s, but it was never commercialized. However, new pulping and demucilaging equipment is continually being developed and sold by various coffee equipment manufacturers. For example, the coffee research organization of Colombia's coffee industry, CeniCafe, recently developed a low-water-use demucilaging system, which uses only 2-5 percent of the water required for the standard wet method in Colombia. CTAHR is in the process of evaluating it with grower-processors in Hawaii to determine the effect on cupping quality.


As soon as the beans are demucilaged and washed, they are dried either by sun-drying or in artificial dryers. A combination of the two is most popular.


On small coffee farms in Kona, drying is often done on platforms called hoshidanas. The beans are spread evenly over the drying area and turned over periodically. Provision is made to protect the beans from rain, usually with movable roofs over the platforms. Operations in the newer coffee-growing areas of Hawaii use fixed or movable clear plastic roofs or tunnels with fans for ventilation. In Kona, it takes from four to six days to completely dry coffee beans when sunny weather prevails. If the weather is cloudy, it takes much longer. The thickness of the layer of beans on the drying area also affects drying time. A layer 1-11/2 inches deep should be turned three to four times a day.

Artificial drying

The use of mechanical dryers is becoming increasingly popular. The beans can be placed in the dryer right after washing. Alternatively, sun-drying and artificial drying can be combined, first drying the beans on a drying floor for 24-48 hours or longer, then finishing them in the mechanical dryer.

The rotary drier is the most popular of several types in use. It consists of two concentric, perforated drums mounted horizontally on the same shaft. The outer drum is closed at the ends, while the inner drum is open at one end. Beans are placed between the two perforated drums. Hot air is forced into the inner drum through the open end while the drums are rotating. The hot air enters the coffee chamber through the perforations of the inner drum. The heated air picks up moisture and escapes through the perforations in the drums.

Stationary dryers have enclosed, shallow trays with screen bottoms. Coffee is placed on the tray, and heated air is forced through the coffee via the screen bottom. Drying towers or vertical dryers steadily move the coffee down through shelves as dry air moves up through the coffee.

Air to dry coffee can be heated by solar or fuel-based methods. Heating may be either direct or indirect. In direct-type heaters, the heated air is blown directly through the coffee. Only efficient, high-pressure, nozzle-type burners should be used for this type of heating. Gas is a good fuel for direct-type heaters, because gas normally burns clean and without objectionable combustion products. In indirect heaters, the heat is extracted by a stream of air in a heat exchanger. With these, any type of burner can be used, because the coffee is not contacted by directly heated air and fumes. However, good design of the heat exchanger is important to its efficiency, otherwise considerable heat may be lost up the chimney.

The temperature and air volume are very important factors to consider. The most important principle in artificial drying is that heat should be introduced at a low temperature at first and gradually increased, particularly if an early model of rotary dryer is used. When possible, sun-dry for a day before using a dryer; otherwise, use a starting temperature of 95°F and gradually raise it to 140°F.

Overheating ruins the quality of the coffee. If the beans are heated to above 150oF, the coffee loses its aroma. Another danger in overheating, especially when the moisture content of the bean is fairly high at the time it is subjected to excessive temperature, is a condition called "case-hardening," when the outer surface of the bean dries too fast, causing a hard shell to form. The interior of the bean then becomes difficult to dry, because moisture has difficulty in escaping through the hardened shell. Such beans, even when they look completely dry, often cannot be milled (hulled) because the interior of the bean is still soft. Even if milling were possible, spoilage would likely occur.

To be safe, the temperature of the incoming air should be less than 150°F. Some coffee dryers can be operated at higher temperatures, and it is important to check the manufacturer's operating instructions.

The volume of air introduced to a dryer should be sufficient to prevent any condensation on the walls of the dryer or on the beans near the air intake. Condensation at the outlet is common. Insufficient air causes "sweating," condensation of moisture on the surface of the bean, which results in poor quality coffee.

The moisture content of the beans after washing varies from 50 to 55 percent, depending upon the amount of surface moisture. Dry beans should have a moisture content of about 12 percent, roughly, from 11 to 13 percent. Beans with a moisture content higher than 13 percent will turn an opaque white during storage. Such beans are poor quality and will be downgraded.

When parchment coffee is just removed from the fermentation tank, the beans are soft and light green. As they begin to dry, their color turns to dark amber. As they dry further, it turns back to a light green, hence the name "green coffee." The beans should be hard at this stage. They may be tested for hardness by biting. If they are not quite dry, they will "give" when bitten. If they are dry, they will break if bitten hard enough (although teeth will probably break first!).

The dryness of the bean, whether dried in the sun or by artificial means, should be checked from time to time. This may be done by removing a handful of the parchment coffee and rubbing it between your palms. If the parchment does not rub off, the bean is not sufficiently dry. It is a good practice to peel the parchment off a few beans to more accurately examine their condition. Electronic moisture meters can determine moisture content of both parchment and beans in a few minutes. However, the most accurate reading is obtained five hours after removal from the dryer. By then, the beans have cooled, and the moisture content throughout the bean is uniform. By recording the difference in moisture soon after removal from the dryer and then five hours later, you can determine how much to adjust the first reading. Otherwise, follow the manufacturer's instructions.

Underdrying will cause quality loss. Overdrying will result in weight loss. However, it is safer to over-dry than to store underdried coffee. Properly dried parchment coffee can be stored for from several months to years depending on the humidity and temperature. In the past, there was demand for "aged" coffee, parchment coffee that had been stored for a few years before roasting, during which time the green color faded. Today, many specialty coffee buyers reject faded coffee.

Milling and grading

Coffee is processed only to the parchment-coffee stage on some farms. The parchment coffee is then sold to a larger plantation or to a miller who mills (hulls) the coffee. Some farms today have their coffee "contract milled" and "contract roasted" and sell their green or roasted coffee as "estate coffee." Large farms frequently do all their processing, including roasting, although most coffee is sold as green coffee.

During hulling, the parchment and silver skin covering the green bean are removed. Several types of machines are available for hulling. After the parchment skin (hull) is removed, considerable amounts of silver skin are still attached to the green bean. The amount of silver skin remaining on the bean depends on the coffee cultivar and its growing and processing conditions. In Kona, for example, silver skin on coffee grown at lower elevations is generally harder to remove than the skin on coffee grown in the upper areas. Coffee is frequently "polished" to remove all the silver skin and give the coffee a more attractive, smooth, shiny appearance.


The hulled coffee beans are separated according to size with mechanical graders. The peaberry type beans are also separated from the flat beans. Peaberry coffee generally commands a higher price in the specialty coffee trade, because it is believed to have superior flavor. Some people claim that each peaberry has twice the flavor of a single bean because peaberries occur when only one seed instead of two develops in the cherry. Peaberries may result from poor pollination, boron deficiency, or when one seed dies.

' Grading is done by first separating the beans according to thickness. This is done by "screening" the beans through parallel bars that are closer together at one end than the other. The coffee is dropped into the narrow end of the screen, and the flatter beans fall through first, while the thicker ones fall last. Peaberries also drop out last, together with the largest beans. After grading through parallel bars, the beans are further separated into sizes through screens with round-holed openings of various diameters. Peaberry coffee beans generally are of smaller diameter than the flat beans, and they go through the screen with the smallest holes.

The number of grades into which the coffee is separated varies. In Hawaii in the 1990s, the coffee is separated into five grades defined by State of Hawaii regulations: Extra Fancy (screen size >18), Fancy (screen size >17 but <19), No. 1 (screen size >15 but <18), Prime (screen size >14 but <17, No. 3, and Off-Grade. Hawaii's grading is based on bean size and the number of imperfections in a pound of coffee. Grading by size will indirectly determine the quality, because the most imperfections are found in the smaller grades. Grading is done under permit and certified by the Hawaii Department of Agriculture, Commodities Branch, which can be contacted at P.O. Box 22159, Honolulu, HI 96823, telephone 808-973-9566, for the most recent standards and rules.

To raise the quality of size-graded green coffee, various procedures are used to separate out the imperfections. Imperfections, including shells, "quakers" (immature beans), stones, etc., have different specific gravity (heaviness, or weight per unit of volume) than normal beans and can be separated mechanically. Various types of machines are used to cull out the lighter and heavier beans, but air separators are most common. Imperfections such as sour beans and black beans cannot be separated by specific gravity differences, but electronic optical sorters can separate the coffee according to color.

After hulling and grading the coffee, it is bagged and shipped. Coffee is usually not roasted before long-distance shipping, because roasted coffee will not store as long as green coffee. However, one-way gas valves on foil bags have greatly increased the shelf life of roasted coffee. Green coffee can be stored for several years if storage conditions are cool and dry.

Processing summary

Because harvesting and processing are, after proper cultural practices in the field, the key to quality coffee, the following summary is presented with the intent of emphasizing essential steps in both of these key production areas.