Coffee Production and Trade

Coffee processing

Wet processing

On-farm pulping - Kenya

Wet processing is more complex than dry processing, requiring specific equipment and (generally speaking) the availability of large quantities of water¹. Green coffee produced in this way is usually regarded as being of better quality and commands higher prices. It is generally used for all arabica coffee, though a small amount of washed robusta coffee is also produced.

Even using careful, selective harvesting methods, unwanted matter will invariably be present amongst the ripe cherries. Preliminary sorting and cleaning of cherries is therefore usually necessary and should be done as soon as possible after harvesting. There are numerous systems available to separate material, generally based on the buoyancy and density characteristics of the matter concerned.

Overripe and undeveloped coffee cherries, sticks and leaves float in water, whereas ripe coffee cherries, green coffee cherries and other material, such as stones, are denser and therefore sink. Flowing water based hydraulic separators are used to isolate these materials from one another.

Green and 'float' cherries are usually then sent directly to a drying terrace, and generally end up for domestic consumption. Sorted ripe cherries are then depulped. Depulping is the key difference between the wet and dry methods, as in the wet method the pulp of the fruit is separated from the beans before the drying stage.

Ripe cherries are squeezed between the fixed and moving surfaces of a pulping machine which removes most of the skin and pulp, leaving behind a slippery layer of mucilage. The flesh and the skin of the fruit are left on one side and the beans, enclosed in their mucilaginous parchment covering, on the other. The clearance between the two surfaces of the pulper is adjusted to avoid damage to the beans. The pulping operation should also be done as soon as possible after harvesting to avoid any deterioration of the fruit which might affect the quality of the beans.

Fermentation tanks - Colombia

Next, the pulped beans go on to vibrating screens which separate them from any unpulped or imperfectly pulped cherries, as well as from any large pieces of pulp that might have passed through with them. From the screens, the separated pulped beans then pass through pre-grading and washing channels where a further flotation separation takes place before fermentation.

Mechanical pulping means that some residual flesh, as well as the sticky mucilage adhering to the parchment surrounding the beans, remains. This has to be completely removed before the mucilage breaks down and contaminates the coffee beans it adheres to. Since the mucilage cannot be readily dispersed in water it is removed by fermentation with enzymes.²

The newly pulped beans are placed in large fermentation tanks in which the mucilage is broken down by enzymes until it can be dispersed by washing. Unless the fermentation process is carefully monitored, 'stinker' beans can develop. For most coffees, mucilage removal takes between 24 and 36 hours, depending on the temperature, thickness of the mucilage layer and concentration of the enzymes. The end of the fermentation is assessed by feel, as the parchment surrounding the beans loses its slimy texture and acquires a rougher 'pebbly' feel. When the fermentation is complete, the coffee is thoroughly washed with clean water.

At this stage the wet parchment coffee, as it is known, has a moisture content (m.c.) of approximately 60%. To reduce the moisture to the optimum 12-12.5% the parchment coffee is dried either in the sun or in a mechanical dryer, or via a combination of the two.

After drying, the wet-processed coffee, or parchment coffee, is stored and remains in this form until shortly before export.

¹ Up to 15 m³ of fresh water per tonne of cherry has been reported for fully washed coffee, though around 6 m³ per tonne of cherry is more usual. It is important to note that most of the water used in the wet process can be recycled, to both save water and avoid environmental pollution. A number of plant design options are available to achieve this. Additionally, some effective low-water consumption pulping machines (e.g. the Colombian BECOLSUB) are also available.

² Brazilian 'descascado' (pulped natural) coffee is one exception to this - after pulping the coffee is sent directly to dry with part or all of its mucilage still attached.