Wet Processing

Reception and floatation

Cherry arrives and is immediately assessed and separated. Floatation in a reception tank is the first mechanical control: ripe, dense cherry sinks; over-ripe, insect-damaged, dried and hollow cherry floats and is skimmed off. Hand sorting catches what floatation does not.

Everything downstream is limited by what happens here. A station that accepts mixed-ripeness deliveries cannot recover the resulting cup later, at any stage.

Pulping

A pulper strips the skin and most of the pulp from the seed by pressing the cherry between a moving surface and a fixed one, sized so that the seed passes and the skin does not. Correct clearance setting is the whole job: too tight and the seed is bruised and cut; too loose and unpulped cherry passes through into the fermentation tank.

• Cut and bruised beans are a pulper setting problem and appear as a defect class in the green.
• Unpulped cherry in the tank ferments differently from pulped parchment and produces localised over-fermentation.
• Pulping should follow reception quickly. Cherry held overnight before pulping begins fermenting inside the fruit.

Fermentation

The mucilage is a pectin layer bound to the parchment. Fermentation uses the naturally present microbial population to break it down so it can be washed off. Duration is commonly 12 to 36 hours in East Africa, driven by ambient temperature and altitude — cold high-altitude stations run long, warm low-lying ones run short.

Washing, grading channels and the soak

Washing removes the broken-down mucilage. In East Africa this is frequently done in grading channels — long troughs in which the parchment is agitated and moved with water. Denser parchment settles and moves slowly; lighter parchment floats forward. The channel is therefore a density separation as well as a washing step, and the separations it produces are often kept apart through drying.

The soak follows in Kenya and commonly in Burundi: the washed parchment is held submerged in clean water for a further 12 to 24 hours. It is not a second fermentation — the mucilage has already gone — and it is associated with the clarity those origins are known for.

Drying

Parchment goes to raised beds or patios and is dried down to a target moisture, commonly around 10 to 12%. The controls that matter are layer depth, turning frequency, protection during the hottest part of the day, and covering overnight against dew.

• Drying too fast cases the outside of the bean and traps moisture inside, giving uneven results at hulling.
• Drying too slowly extends the window in which mould can establish.
• The final days are where over-drying happens, because the coffee looks finished before it is.
• Parchment should be rested after drying before hulling. Hulling straight off the bed gives more damage and a less settled cup.

Water and effluent

Traditional wet processing uses a lot of water, and the fermentation effluent is acidic and high in organic load. Water availability is a genuine constraint on where washed processing is viable, and effluent management is now a standard part of station design — soak pits, settling ponds, recirculating pulpers and mechanical demucilagers all exist to reduce the load.

For a buyer this is not only an environmental question. Stations under water stress make processing compromises, and those compromises show up in the cup.