|Small Scale Processing of Oilfruits and Oilseeds (GTZ, 1989, 100 p.)|
|0.2 Technical aspects|
In addition to the distinction made between traditional and modern methods, the processes for oil seeds should also be divided into so-called wet and dry extraction methods.
Of the traditional wet processes, the extraction of coconut oil from fresh coconuts is the best known. It starts with grating the meat, after which the oil as well as the proteins and impurities are extracted as a milk from the fibrous residue by pressing (by hand or foot) and rinsing with fresh water. The milk is left to stand to form an oilrich cream on top. The cream is boiled to separate the oil from water and other impurities. The oil can be skimmed off. It still contains a protein- rich residue that can be filtered off after drying and used for human consumption.
Other oil seeds, like groundnuts, palmkernels and sheanuts are roasted and crushed as fine as possible (e.g. first by pounding, followed by crushing between stones or a stone and an iron bar). The crushed mass is mixed with water, and the oil is obtained by cooking the mixture, causing the oil to float. The oil is finally skimmed off and dried by heating. Sheanut oil is often obtained by beating air into a mixture of crushed seeds with some water using a hand-operated buttermaking process. The milk or cream floating on top of the beaten mass at the end of the process is then cooked to evaporate the water and dry the oil.
The weak points of these processes are the grating or crushing steps. They are timeconsuming and exhausting work, yet crushing is generally not fine enough. Thorough crushing can improve the oil recovery considerably. In many areas, engine-driven discmills are used by women in small commercial enterprises to get their seed crushed.
All the traditional dry processes, as well as the modern dry-extraction methods, consist of three essential unit operations:
- size reduction,
- conditioning by heating and
- eparation of the oil.
The difference between the dry processes is the way by which the oil is separated. With respect to this difference, the following traditional methods can be distinguished:
- without pressure,
- with a wedge press,
- with a screw press,
- with a beam press or
- with a ghani (which combines the above unit operations).
In historical perspective, the use of pressure in the process seems to indicate a society with a higher technical level of achievement. Wedge, beam and screw presses have already been used by the Egyptians, Romans and Chinese. The beam press, which took up a lot of room, was soon superseded by wedge and screw presses, which work fairly satisfactorily. The animal-driven ghani is mainly used on the Indian sub-continent, from where it originates.
Traditional dry processes are very labour intensive and improvements seem appropriate, at least for any kind of marketoriented production. The potential for improvements would best be tapped by the introduction of simplified versions of the modern technologies (see below); e.g. by
- crushing the seed in a roller mill,
- "cooking" in a directly fired pan,
- pressing with an unsophisticated spindle press, a hydraulic press or an enginedriven oil expeller.
In India, the productivity of ghanis has been drastically improved by the introduction of motor-driven versions, which are fast replacing the animal-driven ones.
As mentioned, the modern dry processes consist of the same unit operations as the traditional extraction methods. First, the shells or hulls are separated from the nutor seed kernels to obtain a mass with a maximum oil content. Palmnuts from the African oilpalm or American palms are cracked. Seeds, such as groundnut, sunflower, cotton and kapok are decorticated. The oil-containing kernel material is then milled between rollers to obtain a wellcrushed material in
the form of flakes. The crushed mass is cooked" in a set of steam heated pans in a humid atmosphere and subsequently dried.
The dry mass is then pressed, a process which generally is applied twice; i.e. prepressing and deep-pressing. The extraction generally takes place by means of oil expellers. Finally, the oil is filtered.
The modern processes, as opposed to the traditional methods,
- apply higher pressures and gain higher yields,
- use power-driven size-reduction equipment and therefore have a higher power consumption per kg of oil,
- are less labour intensive, but require higher initial investments,
- involve less variable, but more constant costs.
The most modern process is the solvent extraction. In this process, the reduced seeds are chemically extracted with a nonpolar solvent (usually hexane). In contrast to the modern dry processes with expellers, the solvent extraction cannot be carried out economically on a small scale.
Nevertheless, the process has a number of advantages, such as
- high extraction yield (95 - 99 %),
- high capacity continuous process.
For the purpose of a small-scale production, the main disadvantages, such as:
- large initial investment capital needed, - large maintenance
- need for skilled labour
are decisive, in particular for most projects in developing countries.
Modern wet processes have been developed for coconut and groundnut processing, but are not likely to become economic. Therefore, the modern wet processes will not be further discussed in this publication.