What can go wrong, will go wrong. The net result of sorting is a broad range of potential defects to match the wide spectrum of events experienced by the red, ripe cherry before, during and after picking and the bean as it moves through the various washing, drying and cleaning stages of on-farm processing.
The term, defect is used in the coffee industry to describe quality-related factors that determine the proportion of defective beans and the presence of extraneous (foreign) matter of coffee and non-coffee origin. The type of defect and potential problems caused may relate to the presence of hard abrasive objects that damage machinery, downgrade by buyers due to non-pleasing appearance (shape and color) and lack of uniformity and chemical factors that interfere with taste and aroma development during roasting.
There are some 20 different bean defects in green coffee that must be removed before roasting. Common occurring defects classified and categorized by bean color, size, shape or aroma/taste aberration and presence of foreign matter will include:
Foreign matter - includes a wide range of coffee and non-coffee materials related to harvest and on-farm processing, including small stones, glass, metal, soil, dust, dry cherry, pulp, parchment, leaves, twigs and fragments of wood
Coffee well sorted
Sorting for coffee is a long, continuous process of preparation which starts with re-drying insufficiently dried coffee beans arriving from the farm to an 11% moisture level. An optimum bean moisture content ensures that outer coverings are removed more easily during the hulling and de-husking of, respectively, wet and dry processed coffee, thus reducing the risk of residual parchment or husk finding its way into cleaned and bagged green coffee.
Consignment of dry parchment coffee and specially dried coffee cherry, arriving from the estate, will almost certainly contain a wide range of dense foreign objects including stones, glass and metal with potential to damage coffee bean hulling machinery. Initial cleaning is a three-part process involving a screen-fitted hopper to sieve out medium to large objects, a magnetic separator to remove metal objects and a cleaner-separator which integrates sifting and pneumatic forces for dust removal.
Bean size grading after hulling is important to maximize financial return on larger beans, including peaberries, and to ensure a uniform roast. Furthermore, it makes subsequent sorting of beans by differential density much easier. Size grading is carried out using a flat screen grader, that retains beans of the required size on a vibrating screen, or a cylindrical drum grader where the different size grades of beans pass through the revolving screen at successive points along the drum.
Sorting by density
Green coffee at the post hulling and size grading stage will still contain a significant amount of dust and other very low density particles, along with varying proportions of light-weight beans and other defective beans which are misshapen, discolored or damaged by insects and mold fungi.
Sorting by differences in density-in order to remove light beans- is carried out by using pneumatic (air pressure) or gravimetric (gravity flow) methods. This is the first stage in the final sort before roasting and depends on differences in density between normal, healthy beans and those which are mechanically damaged, deformed, insect damaged or unhealthy. (Density is a comparative measure, which relates the mass [weight] of two objects having the same volume or size). Density sorting is efficient to a certain degree. Many defective beans, and especially those caused by external color differences or bad internal chemistry, have virtually the same density as normal beans.
Pneumatic sorting relies on two physical parameters of the coffee bean: its density and diameter, but size grading is essential prior to pneumatic sorting. The basic laws of physics show that two spheres (approximate shape of a normal coffee bean) with different diameters can in fact have the same settling velocity when their density is different, which would prevent any separation of heavy and light beans on the basis of density.
Pneumatic sorting is embodied in the catador, dual-purpose coffee cleaning and grading equipment with its roots in the Brazilian coffee industry. The catador uses moving air to differentiate between and separate beans of similar size but different density or between beans with the same density but different size and shape. Put simply the catador can be used to weed out normal looking beans with defective contents (light beans), broken or damaged beans and to separate normal beans of various densities into specific quality grade batches. For this reason, it is most successful with pre-size graded coffee.
The catador comprises a vertical shaft that is divided length-wise into two columns (shafts) with an aspirating radial blade fan, operating at 550 rpm, at the base of each shaft. Beans are fed into a hopper on the side of the first column where the heavy beans fall to the bottom, while the lighter fraction is carried upwards in the stream of air. The air, carrying dust and other fine particles, is expelled through a screen while the remainder passes into the second column, where the air is less forceful. A second separation into medium weight and light beans is made in this column.
The catador is simple, practical and efficient. But the basic laws of mechanics, relating to the flow or air over beans and on which its operation is based, conspire to give imperfect separations. Because coffee beans are not perfect spheres, drag coefficient is variable and when subjected to turbulence, the bean may present itself differently in the flow of air. The velocity of the air is lower near the walls of the column, the adjustment mechanism can cause deflection of the air flow and the velocity and flow rate of the air are affected by the resistance offered by the surface of the bean.
The result of these scientific imponderables is that uncontrollable variations in air-flow can shift the dividing line for separation of beans into light, medium and heavy grades. This problem of imperfect separation has been overcome, to some extent, by making differential density sorting an integrated process combining the use of pneumatic separators like the catador with the use of gravity flow separators.
Gravity separators harness the force of moving air but in this case, they help to fluidize an inclined bed or deck of green coffee beans. The force of the air combined with reciprocating vibration of the deck causes the dense particles to move progressively higher up the inclined bed, while the lighter (less dense) fraction floats downwards.
Modern gravity separators are extremely versatile. With appropriate adjustment of deck speed, air volume, deck type and angle, as well as bed depth of green coffee, they will accurately achieve a variety of separations. These include separation of highly dense objects such as stones from the bulk of coffee beans, separation of defects from normal beans and sorting/re-sorting of normal beans into separate quality grades on the basis of differential densities.
Sorting by density has virtually no effect on the proportion of discolored beans, unless external color defects are associated with internal defects that result in a reduced density, lighter bean. Discolored and defective beans have been traditionally removed by hand and still are where the size and/or profitability of the operation allow. Over the last 50 years the human eye has increasingly been replaced with the ‘electronic eye’ of the more economic to operate colorimetric sorter.
Monochromatic machines were the first to use reflection of white light to detect and remove black beans that reflect a much lower proportion of light. Monochromatic sorters generally fulfilled the sorting needs of Robusta origins in which weeding out of black and dark colored beans is usually all that is required.
Sorting discolored defects in Arabica beans is more demanding. It requires a bichromatic machine that uses a two color combination, usually green/red or blue/red, selected by the operator to pin-point and take out the over-fermented and reddish beans, dark colored/black beans, grassy green, immature beans, and yellow beans. Defects eliminated by monochromatic sorters include black beans, gray or dull beans, and some beans attacked by insects. With bichromatic sorters the user can eliminate black, gray, brown, foxy, unripe beans that were damaged by insects, as well as, dried cherries.
Many argue that the well-trained human eye is at least as effective as an electronic sorter. There are situations, where very small operations producing a very high quality and high value niche origin, where labor is no constraint, for which separation by eye/hand coordination is still the most effective method of sorting.
That said, neither the monochromatic or bichromatic electronic sorter, nor the human eye can deal with what has rapidly become the most destructive defect in coffee. The stinker bean of wet processed coffee resulting from over-long fermentation and/or use of dirty water, and caused by build up of volatile chemicals in the bean, is by far, the most damaging defect of green coffee.
The putrid odor becomes apparent during roasting and just one stinker in a cylinder can come through to affect the whole batch and completely ruin cup quality. Electronic sorters that deliver a beam of ultra violet light have become the answer to this problem. Stinker beans passing through the beam fluoresce trigger a mechanism in the sorter (compressed air gun) which deflects the bean into a reject channel.
Sorting capability and capacity of electronic sorters can be adjusted to eliminate more or fewer defect types and numbers of defective beans. The adjustments required will depend on the various types of defects present, the number of beans affected and also the profitability of the operation.
The losses incurred by removal are, at least compensated for by the extra value of the sorted product.
At the end of the day, proof of the quality control achieved by electronic sorting, especially for stinker bean, is in the cup of coffee. But buyers dare not wait until this stage to find out whether a roasted batch of beans has been denigrated by defects. Problems can be pin-pointed much further back along the line by comparative chemical testing in the laboratory of unsorted, sorted and rejected samples. In this way, specific chemicals may be attributed to particular cup quality problems.