Small-scale oilseed processing
Water assisted. Here the finely ground oilseed is either boiled in water and the oil that
floats to the surface is skimmed off or ground kernels are mixed with water and
squeezed and mixed by hand to release the oil.
Ghanis. A ghani consists of a large pestle and mortar rotated either by animal power or
by a motor. Seed is fed slowly into the mortar and the pressure exerted by the pestle
breaks the cells and releases the oil.
Manual pressing. Here oilseeds, usually pre-ground, are pressed in manual presses.
Some pre-heating and the addition of water can improve yields.
Expelling. An expeller consists of a motor driven screw turning in a perforated cage. The
screw pushes the material against a small outlet, the "choke". Great pressure is exerted
on the oilseed fed through the machine to extract the oil. Expelling is a continuous
method unlike the previous two batch systems.
Solvent extraction. Oils from seeds or the cake remaining from expelling is extracted
with solvents and the oil is recovered after distilling off the solvent under vacuum. This
process is usually only carried out in large scale production plants and is not suitable for
small scale production.
Most small enterprises will find that small expellers are the best technology choice. Methods
such as water extraction and manual pressing only produce small amounts of oil; the extraction
efficiencies (the percentage recovered from a possible maximum) are low and labour
requirements high. Solvent extraction while highly efficient involves very substantial capital cost
and is only economic on a large scale. There is also a health and safety risk from using
The basic steps involved in processing oilseeds by expeller are shown in the flow diagram below.