The process includes
- Removal of pulp and mucilage followed by washing to obtain clean wet parchment.
- Drying of the parchment coffee.
- Removal of the parchment and film by hulling followed by grading to obtain the desired grades (sizes) of the clean coffee.
Wet processing can be done for both Arabica and robusta coffee. Arabica is produced at high altitudes (over 1,500m above sea level) in the Mount Elgon areas in the East, the highland areas of Nebbi in northern Uganda and the mountainous areas of Kisoro and Rukungiri in the Southwest.
The coffees produced are generally described as ‘mild’. The harvest often includes unripe, immature cherries, dried cherries, twigs and leaves. These are lighter than mature ripe cherries and can, therefore, be removed by a floatation process.
The process can be done in a simple vat or mechanically in a washer separator, which floats off the impurities and also washes the ripe cherries.
The cleaned cherries are then pulped — a process in which the wet beans are squeezed from the cherries leaving the pulp. Pulping can be done using a hand-pulper with a capacity of 300kg/hr of fresh cherries. The capacity may be increased by the incorporation of an electric motor or a diesel/petrol engine.
Larger units of up to 4.0 tonnes per hour are available at central pulping stations. The wet parchment beans have a mucilage layer around them that is removed by bio-chemical enzyme activity through controlled fermentation to give ‘fully washed’ coffees.
If the mucilage is mechanically removed, the coffees produced are referred to as semi-washed.
After the mucilage is degraded, it is removed by washing in a channel or vat filled with water. The density of the parchment coffee is slightly higher than the water and the beans will sink to the bottom of the vat.
It is, therefore, necessary to continuously stir the beans using rotary stirring rods or manually using spades in the washing channel.
In a mechanical mucilage remover, mucilage degradation and washing are done in a single operation.
The wet parchment free of mucilage at moisture contents of 50 – 60 % is then dried on suitable raised drying tables to the required 12 % to ensure their conservation.
Mechanical driers can be used to hasten the drying regime after draining off some of the water.
This takes place in two stages:
- Drying of the cherries (usually under the sun)
- Removal of the dried coverings (husks) in a mechanical operation (hulling). Smallholder farmers can access hullers from local artisans around the country for as low as sh450,000. Motorised hullers cost over sh1m, depending on the capacity.
The harvested cherries are usually not sorted before commencement of the drying regime.
Careful harvesting to exclude immature cherries and extraneous matter e.g. stones is essential. Further, do not leave twigs and leaves in the harvested coffee because they are also considered contaminants.
The drying regime should begin immediately after harvest to avoid the development of undesirable taints and moulds. The cherries are spread out to dry in the sun on suitable drying surfaces, for example, raised trays or tarpaulins.
- The coffee must be frequently stirred to achieve uniform drying. The coffee should not be rewetted at any time during the drying regime. Do not dry your coffee on bare ground because this causes contamination and lowers the quality of the product.
- Drying will be complete when the dried cherries (kiboko) have attained moisture content of between 13% – 14%.
- Stop drying when the coffee makes a rattling sound when you shake it. This can take one week to 10 days depending on the amount of sunshine.
- Do not heap wet or partially dry coffee for more than 12 hours because this creates heat that seems to ‘cook’ the coffee rather than dry it. This affects the aroma. Spread the coffee on a flat surface, with a thickness of not more than four inches.