Mechanisation was introduced to Cameroon in the 1930s through the importation of Colin palm oil expellers. The Colin is a low-pressure, continuous-feed expeller made in France. It has two 6’ (2 m) diameter coaxial counter-rotating screws that turn horizontally or vertically in a perforated cage. The discharge end is fitted with a backpressure cone. As the cooked palm fruit is fed into the expeller it is pushed forward by the spiral flights (worms) against the backpressure of the end cone. The oil is forced out through the perforated sides of the cage. The remaining fibre and nut are released at the end of the cage through the gap between the end cone and cage body. The ability to simultaneously de-pulp and press is a major advantage of this type of press.
Small expellers may be manually operated or motorised. These expellers have been the dominant–if not the exclusive equipment–used by small-scale palm oil processors in Cameroon.
In Ghana and Nigeria the earliest equipment introduced was the Stork manual hydraulic press. The impression was created that, for economic reasons, the only operation that needed mechanisation was oil pressing. In colonial days farm labour was cheap and easily available. Hence there was no attempt to mechanise the digestion operation. Thus, in the British colonies, early attempts at mechanisation had to focus on complementing the presses with mechanical digesters. Two types of digesters were developed: horizontal digesters based on the dry process technique; and the vertical digester, which adopts the wet process technique.
In the wet system, sterilized fruits are poured into the digester. As the fruits are being macerated, hot water is continuously poured into the digester (at a regulated rate) to wash off the released oil. The resultant mixture of water and oil is filtered and then clarified.
Another attempt at mechanising the maceration process resulted in the development of the manual digester for women. This digester consists principally of a large wheel (connected to the differential system of a car axle), and a vertical shaft carrying some beater arms that rotate inside a conical shaped metal trough. The ratio of rotation of the wheel to the vertical shaft is 1:7. It takes between 12 and 15 minutes to digest a 30 kg load of fruit.
The mechanical digesters currently in use consist of a cylindrical shell and a system of beaterarms driven by a 6 hp. diesel engine through a speed reducer (where necessary). The speed reducer steps down the speed of the motor (engine) to 125 rpm – the running speed of the digester. The digester is capable of macerating over 250 kg of fruits per hour and has the singular attribute of macerating thoroughly either the Dura or Tenera fruit or a combination of both without breaking any nut.
Pressing
The traditional method of oil extraction consists of:
• steeping the pounded fruit mash in hot or cold water;
• removing fibre and nuts in small baskets and hand squeezing;
• filtering out residual fibre from the oil/water emulsion in perforated metal colanders or
baskets;
• boiling and skimming palm oil from the oil/water mixture;
• drying the recovered oil.
Standing by the open fire during this operating period is not only a health hazard but is inefficient, as a lot of oil is left trapped in the mixture as an emulsion. It was long realised that pressing is a bottleneck in small-scale palm oil processing. The process is usually conducted slowly to avoid the huge loss of oil that might result from inadequate pressing. The economic importance of this process was therefore long recognised and has received the greatest attention for mechanisation. Presses developed over the years have included models such as:
• Manual vertical screw-press
• Stork hydraulic hand press
• Motor-jack press
• Motor-jack/cantilever press
• NIFOR hydraulic hand press
• Combined screw/hydraulic hand press
• mechanical screw-press
The manual vertical screw-press, the stock hydraulic hand press and NIFOR hydraulic hand press enjoyed the highest patronage in Nigeria for a long time, even though oil loss/fibre ratio for these presses range from 18-35 percent. This should be expected as the operation of these presses depends on the strength of the operator.
In Ghana efforts to deliver a low-cost press to the smaller village processors, led the Technology Transfer Centre (of the University of Science and Technology, Kumasi) in the early seventies to come up with an inexpensive manually-operated spindle press. The presses delivered low pressures and relied on manual labour for pressure development. The throughput was about 50 kg per hour or 1.5 tonnes per day. For the really small-scale extractors in villages with small patches of oil palm farms these screw-presses gained widespread preference. Here, the traditional mortar and pestle was used to pound (digest) fruits and then the mash was taken to a press operator who extracted the oil for a fee.
The manual spindle-press was affordable and was bought by individuals and groups. In the Kusi area of the Eastern Region the use of the press was rented to the whole community. This was to signal the beginning of community-based service palm oil milling.