Coffee : small-scale processing
Practical Action
centrifuge.
Drying
The extracts can be dried in a spray drier, freeze drier or a drum drier.
Spray drying Spray drying requires that the concentrated coffee solutions extracted are
'atomised' to form droplets (10-200 micrometers in diameter) and then sprayed into a
current of heated air (150-300ºC) in a large drying chamber, this is usually a concurrent air
flow. Complete and uniform atomisation is required. For coffee drying, a centrifugal
atomiser is usually used. The liquid is fed to the middle of a rotating bowl (peripheral
velocity of 90-200m/s). This produces droplets with diameters of 50-60 micrometers in a
uniform spray. The droplets are dried very rapidly (1-10 seconds) due to the very large
surface areas. The dry powder is removed from the base by a screw conveyor or a pneumatic
system.
Freeze drying Freeze drying takes place by sublimation (ie a solid material transforming
directly to vapour without passing through a liquid stage). The coffee solution is slowly frozen
in conventional freezing equipment. The frozen material is put under reduced pressure and
heated. As long as the pressure in the freeze drier is below the vapour pressure at the surface
of the material, it will sublime directly to vapour without melting. The vapour needs to be
removed by a vacuum pump and is condensed on refrigeration coils. The final drying stage
involves evaporative drying (desorption). This is achieved by raising the temperature to near
ambient whilst retaining the low pressure.
Coffee needs to be frozen as a foam (by gas inclusion) to prevent formation of a glassy
vitreous frozen material.
Drum drying This is rare nowadays. It involves the extracts being dried by direct contact
on a heated cylindrical surface.
Packaging and Storage
Materials Packaging of coffee (especially if it is ground) requires polypropylene. Polythene
cannot be used as the flavour components diffuse through it. The use of laminates is popular
but more expensive. Glass is also a suitable container.
Simple sealing Polypropylene bags can be sealed using simple hand-operated electric
sealing machines.
Storage The optimal conditions for storage are low temperature, low humidity and pest-free
in a shaded area. To prevent pests entering the store, the roof should be completely sealed.
Mosquito netting should be placed over the windows, and the doors should be close-fitting.
Processing Waste
Coffee pulp If not treated, this will give rise to unpleasant odours and attract flies and
insects. It can be converted into animal feed, soil conditioner, or used for caffeine extraction
and biogas production.
Waste water Care needs to be taken to prevent water from the processing, polluting local
rivers or lakes.
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