5.0. Conclusions and Recommendations.
It was noted in chapter 4 that in many studies that have been carried out, coffee originating from African coffee producing countries is more contaminated with Ochratoxin A (OTA) than coffee from other producing areas. It can also be seen from Annex 4 that, the percentage of coffee exports from most African producing countries are low (less than 80%), and bearing in mind that coffee consumption is low in these countries, most of the unexported (unexportable) coffee is due to low quality (rejected lots). Most of these rejected coffee may find its way to the local market, which poses a healthy risk to the residents because some of it may be contaminated with OTA.
As mentioned earlier, most coffee consuming countries are becoming keen on quality of green coffee and soon there is likelihood of a legal OTA limit being set together with other quality standards.
The importance of coffee in most of the African and other developing countries has been mentioned and in order for these countries to survive and compete favourably in a liberalised coffee market where no quotas are in force and the best quality coffee fetches better prices, they must address the issue of coffee quality with a high level of importance.
Both at the farm and post farm levels, the recommended procedures of handling coffee should be followed strictly in order to obtain top quality coffee. Below are some of the recommendations and suggestions to be followed.
5.1. Ecological Factors
As mentioned earlier, ecological conditions affect the growth of coffee trees and this is ultimately reflected in coffee quality. It is therefore important to grow coffee in the ideal ecological zones in order to maximise on quality of the beans as well as quantity.
5.2. Nutritional factors
Coffee bushes require constant application of nutrients for better yields and quality of beans. Proper agronomic practices are essential and will lead to better yields and quality. The countries concerned must ensure proper education to farmers on these practices by employment and training of enough agricultural extension workers and availing all the resources required for proper extension work.
The issue of credit to farmers for purchase of farm inputs like fertilisers must be addressed.
5.3. Insect and fungus attacks
Attacks by berry borers, bugs (antestia) etc weaken the bean, reduce cherry density and can result in insect damaged beans with an unpleasant taste. Certain insects including the fly (Ceratitis coffeae) would appear to introduce bacteria which makes the coffee to produce potato taste, which can be detected in certain Arabicas from Burudi, Kenya, Rwanda and Tanzania. The molecule responsible for this taste has been identified as 2-methoxy-3-isopropyl-pyrazine.
Coffee berry disease causes fruits to dry out, leading to damaged and dry beans found in green coffee.
Proper and timely application of insecticides and disease control chemicals will minimise this damage.
5.4. Effect of pesticides and fertilisers
Some pesticides can transmit unpleasant tastes and odours to coffee and can be controlled by knowledge and application of the correct ones and at the correct time.
Excess potassium fertiliser can lead to harsh beverages (see coffee tastes, 3.05). This problem is encountered in Kenya, where mulching is widely practised, through the excessive introduction of potassium-rich pennisetum purpurem. Too much nitrogen fertiliser increases the caffeine content of beans, leading to a more biter beverage.
5.5. Harvest quality
Harvesting is an essential stage in coffee quality. In the great majority of producer countries, coffee is hand picked. To ensure a good quality coffee, the cherries should be picked one by one; ensuring only ripe cherries are harvested. However, this requires a large work force, which is often difficult to find, so strip picking is too often carried out. This consists in stripping off all the cherries on a branch irrespective of ripeness: unripe green cherries, yellow cherries, ripe red cherries and overripe brown cherries. The green coffee obtained by such harvesting is heterogeneous and its liquor is very hard.
The number of physical defects is defined by an increasing linear function of the percentage of unripe beans. For example, 15% green cherries in the harvest corresponds to 120 defects in the commercial coffee sample of 300 grams and is reflected in beverage quality. This 15% threshold should not be exceeded.
The existence of black beans after drying (which give hard, bitter and woody beverages) is largely attributable to harvesting quality:
If the cherries picked are too ripe, they give unpleasant, fruity flavours to the beverage. Finally, irrespective of the harvest, if drying is too short, the layers of cherries are too thick or they have not been covered overnight or in wet weather, there will be substantial proportion of black beans.
5.6. Primary Coffee Preparation
As mentioned earlier, two types of preparation methods lead to two types of commercial green coffee:
The amount of pulp around the beans usually determines the primary preparation method: wet process or dry process.
The pulp has a high water content. If it is thick it will make direct drying difficult. It therefore has to be removed, using the wet process. This is the case for most Arabicas. Robustas have thin pulp and can be processed directly by the dry process.
In both methods of processing, the following precautions must be taken:
5.6.1. Wet process
This process requires considerable care, as very unpleasant flavours can be caused by certain preparation errors:
5.6.2. Dry process
Drying is an important and tricky stage in coffee preparation. It contributes considerably to end product quality and can prove detrimental to quality if not carried out correctly.
Coffee left to dry under poor conditions (layers too thick, inadequate homogenisation, rehydration) can be subject to quality deterioration:
Coffee, whether wet or dry processed, arriving from farms and estates for hulling is not always sufficiently dry, and it is often necessary to dry certain batches further. Apart form providing storage stability, a uniform moisture content of 11% enables husk and parchment to be removed more easily.
Dry parchment coffee, and particularly dried cherry, may contain many impurities, e.g. pebbles, pieces of metal and other foreign bodies. These need to be removed by a preliminary cleaning to protect the processing equipment. Cleaning can be carried out by use of a hopper with screens to remove large and medium-sized impurities, followed by magnetic separator to remove metal pieces and a cleaner-separator which combines the effect of sifting and pneumatic dust removal. An air float separator can be used to sort particles with the same diameter but different densities.
Husk coffee and parchment coffee is processed to give commercial green coffee. When the coffee reaches this stage, all its intrinsic qualities have already been acquired. The different sorting operations to which it is subjected can only reduce its percentage of defects.
After cleaning and stone removal, the coffee is hulled (husk or parchment removal). At this stage it is important not to heat the beans during hulling because this will affect the final colour and taste. The hulling machines must be well adjusted to avoid breakages of the beans or passage of un-hulled coffee. Hulling may or may not remove a final layer closest to the beans, called the silverskin. If removal of silverskin is required, it may require separate equipment following hulling, called polishers.
The green coffee obtained is then sorted according to size, density and colour.
5.10. Bean size sorting
The bean size classifications is because of the following reasons:
Machines for size sorting use the sieving principle. Two types are available, one using screens mounted in drums and the other a vibrated flat bed of rectangular shape. Apertures of the screens are either round or oblong for peaberies.
5.11. Density sorting
Green coffee at this stage will still include dust and other light particles, together with beans of light density, or which are deformed, discoloured or insect-attacked. Separation is carried out by use of vibrating screens or a Catador. The Catador is based on the principle of pneumatic separation by the use of an upward current of air created by a radial blade fan rotating at about 550rpm within a vertical chamber and can separate the beans into two categories, heavy and medium heavy, from other material.
5.12. Electronic colour sorting
This final sorting operation is to separate beans with an undesirable colour, such as black, white or brown beans.
Manual sorting which is tedious and labour intensive, is being increasingly replaced by electronic sorting using photoelectric cells. Electronic sorters are capable of eliminating:
Green coffee, following its preparation by the methods described earlier, will inevitably be stored under different conditions for different periods of time at different locations, and indeed its transportation from the producing country to the consuming countries involves a period of enforced storage, which may be many weeks. During storage some changes in the quality and appearance of the green coffee can be expected (including the potential development of moulds).
Ship transportation to the ports of consuming countries may involve the problem of moisture release (as in containerised shipments) as a consequence of a change to a lower humidity rather than temperature and this moisture may deposit locally and cause mould growth. There may be storage at the port of the consuming country, before road or rail transport to the point of roasting or extended storage before sale in the spot market.
Storage of coffee in a producing country can be in the form of dried cherry, or dry parchment coffee, or, cured green coffee. Storage conditions need not be exactly the same, since both husk and parchment provide a good protection against insects and also a barrier against moisture transfer.
As already mentioned, the moisture content of coffee for satisfactory storage should not be over 11 %. At this level, mould growth and enzymatic activity is minimal.
A traditional storage building is suitable for storing green coffee in sacks, as long as it is heat insulated to prevent variations in temperature and perhaps also ventilated. Sacks should be stored on raised pellets placed some 0.5 m minimum distance from walls, and all openings should be equipped with devices to prevent insects from entering (e.g. vents should have thin wire mesh covers).
Coffee can also be stored in bulk, as is done for buffer stocks in consuming countries. This is done in metal or concrete bins.
Coffee needs to be transported at different times; belt conveyers, bucket elevators, screw conveyers and pneumatic conveying are all used. For green coffee, pneumatic conveying has to be applied carefully by avoiding bends in pipework and high velocities, because this will cause coffee breakages. Robusta coffees tend to be more brittle than Arabica, and the percentage of broken beans is higher.