2.2.2 Oil seeds

The service mill

In West Africa, oil seeds are generally processed to sell the product on the weekly market. In that case it is a great advantage, if a reasonable quantity can be processed at any one time. This is possible by using the services of the local grain mill, which operates on the principle of a rotating disc. As discussed above, the crushing stage in the traditional process is the most labour-intensive and exhausting one. A women processor can have her oilseed crushed for a fee, if a grain mill is available and if the miller is prepared to crush the oilseed concerned (which is not always the case, because the mill becomes dirty with sticky material). A drawback is of course that a great deal of the added value has to be paid to the miller.

A typical example is the groundnut processing into oil and tunkusa (see 2.1.2). It was observed that a quantity of 18.1 kg shelled groundnuts was processed into 3.7 kg oil and 15.7 kg tunkusa (giving 14.6 kg kuli-kuli) by 3 women in only 8 working hours, using the service mill for grinding the groundnuts. Crushing by hand would have required about 20 hours alone. Service mills of the type suitable for grinding oilseed can be found over the whole of West Africa. Oil seeds which can be crushed in such mills include: groundnuts, palm kernels, and shea nuts. Coconut can be grated by service mills equipped with a grater.

The use of mechanized size reduction replaces not only heavy and exhausting work, but also improves oil recovery, as a much larger number of oil-containing cells are opened to produce oil.

The ghani

On the Indian subcontinent, one will come across the ghani oil mill as the village level processing system.

The ghani consists of a mortar and pestle in which the seed is crushed. Pulverization and oil expression are carried out at the same time by rubbing the seed between the pestle and the wall of the mortar. Water has to be added to realize a preparation process, called “cooking", by which the oil emulsion in the cells is broken, and the micro oil droplets are combined into larger ones. The ghani process requires much mechanical energy. A ghani operated by one bullock (the equivalent of 0.35 kW) can process 5 kg oilseed in about one hour. Hence, 0.35/5 or 0.07 kWh are required to process I kg of an oilseed into oil. This energy consumption is about equal to the maximum amount of energy required by small oil expellers.

(Oil expellers require between 1.5 and 2.5 kW to process 35 kg of an oilseed per hour, or between 0.04 and 0.07 kWh per kg oilseed.)

Figure 12: Power Ghani

Besides, the ghani requires a lot of maintenance and repair, as the surfaces of the mortar and the pestle suffer much abrasion. The mortar can be recut 2-3 times but has then to be replaced. The pestle needs to be replaced at regular intervals as well. Every 48 hours these surfaces should be checked on wear. Also the bearing at the top has to be checked regularly.

The introduction of a mechanized version into Tanzania, the so-called "power ghani" has had disappointing results, since special skills seem to be required for their operation and maintenance. In India, these mechanized versions are quickly replacing the original animal-driven ones. Also modernized versions are available, equipped with a turning mortar instead of a turning pestle as is the case with the traditional ghani. Ghani oil is highly appreciated because of its special flavour, particularly in the case of mustard seed oil.

The hand press

Although the wet process can be improved by mechanized size reduction using a motorized disc-mill or a motorized grater, it can be of advantage to introduce the dry process, using a hand press.

Figure

Figure 13: Palm Nut Cracker (KIT/UNATA)
Figure 14: Cocos Grater (KIT)

Figure 15: Roller Mill (KIT/UNATA)

As no motorized equipment is required, it is particularly suitable as a women's activity, enabling the maximum of the value added to be retained by the processors themselves. The dry process consists of the five process-steps shown in flowsheet 5.

Flowsheet 5 Dry Process for Processing Oil seeds (General Flowsheet)

1) decortication: to prepare a rawmaterial for further processing with the highest possible oil content,

2) size reduction: to obtain a well crushed material, with still some coherence, to facilitate filtration during pressing but to avoid the pressing-out of fine material with the oil. The best way to crush is between rollers into very fine flakes, preferably thinner than 0.1 mm. Particles to start with should be smaller than 5 mm diameter.

3) cooking: to prepare a mass in which the oil is present in a form that it can relatively easily be pressed out. During cooking the following processes take place:

- weakening of the cell walls,
- enaturing of proteins, destabilizing the oil emulsion, the original form in which the oildroplets are present in the cells,
- coalescence (flowing together) of micro oil droplets into larger ones, - diminishing of viscosity because of higher temperature.
Cooking is a process in which temperature, moisture and time play an important role.

4) pressing: to separate the oil from the rest of the seed. To be able to use a press with the lowest possible maximum pressure, size reduction and cooking should be carried out with great care. For most oil seeds optimal conditions are achieved by pressing at about the same moisture content as that of the original rawmaterial. In that case, the maximum pressure should be 60 kg/cm². Higher pressures have, in that case, little effect, as the mass will be extruded through the holes in the press cage. Oil recoveries are good, while equipment remains simple. At higher maximum pressures either cage contents become inefficiently small or the equipment becomes much too complicated and too expensive.

5) drying of the oil: to prepare a dry and pure oil, fit for long-term storage. Oil should be dry and free of impurities. The oil has therefore to be heated to 130°C to remove all traces of moisture. After leaving the oil to stand for a few days, the impurities will have settled and pure oil can be decanted. The oil should be filled to the top in clean and dry bottles or tins, and stored in a dark and cool place. Oil treated in this way has a shelf- life of at least 6 months.

KIT has developed unsophisticated equipment to carry out the dry process, completely by hand. It can be used to process oil seeds at the village level as: groundnuts, sunflowerseed, palm kernels, coconuts, sesame seed, rape- seed, castor seed and shea nuts.

Figure 16: Heating Oven (KIT)

The equipment consists of the following (hand-operated) machines:

- sunflowerseed decorticator or palm nut cracker (see Figure 13), to be rebuilt into a hammermill
- winnower
- roller mill (see Figure 15)
- "cooking" furnace (see Figure 16)
- hand press (see Figure 17)

Table 9: Typical Performance of KIT/UNATA Hand-operated Equipment:¹,²

1 Other seeds that can be processed following the KIT/UNATA hand-operated system include sesame seed, rape and castor seed.

2 Performance according to field experience.

3 Seed as described: groundnuts (decorticated), sunflowerseed (not decorticated), palm kernels (after cracking of palm nuts), coconuts (as such).

4 Dried for two days ( =30% moisture).

5 Oil content and recovery are not given as a percentage, but as kg per 100 nuts!

A grater for grating fresh coconut (a drill type) and a different type for half dried copra (disc type, see Figure 14) are under development. Originally, hydraulic hand presses were used, based on hydraulic lorry jacks.

Figure 17: Spindle Press (UNATA)

It appeared, however, that already at a maximum pressure of 60 kg/cm² good oil recoveries could be obtained and therefore the UNATA spindle press was adopted. The press was redesigned to meet the requirements and has been put into production (see Figure 17). Only for shea nut processing does this press not give a good oil recovery, as a maximum pressure of 120 kg/cm² is required. Up to now, only the hydraulic press, designed for the GTZ/GATE project in Mali, can be efficiently used to process shea nuts.

However, a spindle press for shea nut processing is under development. The performance of the KIT process, using the UNATA spindle press, is given in Table 9.

In Tanzania, IPI has developed equipment for processing sunflowerseed, following the same process. The equipment is described in Chapter 5.1.2.

IPI has carried out detailed research on the effect of the different process steps described above on the overall press efficiency; i.e. the oil recovered as a percentage of the oil present in the rawmaterial. The results are illustrated in Figure 18.

Figure 18: Oil Yield as Determined by the Number of Machines Used in the Process for Sunflower Seeds (IPI)

Legend for combinations tested:

- P: Press (alone) - DP: Decorticator + Press
- RSP: Roller mill + Scorcher (cooking furnace) + Press
- DSP: Decorticator + Scorcher + Press
- DRSP: Decorticator + Roller mill + Scorcher + Press

For its hand-operated system, IPI advises boiling the crude oil with salt (2 %) and water (10 %), decanting and filtering through a cloth. The performance of the IPI system is summarized in Table 10.

Table 10: Typical Performance of IPI Hand-operated System

In Chapter 3, details are given on the sunflowerseed processing project in Zambia and on the GTZ/GATE project in Mali. The economic aspects of these activities are worked out in Chapter 4.

Oil-Expellers

The dry process cannot only be executed by mechanical or hydraulic presses, but also by continuously operating screw presses, generally named oil expellers. An oil expeller consists of a perforated cage in which a tapered screw turns. The screw is tapered in a way that the free space between the centre of the screw and the cage gradually becomes smaller to the end of the cage. With this system, very high pressures can be exerted on the material to be pressed.

The rawmaterial has to be prepared in principle by decorticating, crushing between rollers and cooking. Depending on the type of machine, however, some oil seeds can be processed even when not decorticated. Expellers are usually driven by petrol or diesel engines or electric motors but can also be run on animal or water power.

The performance of an expeller developed especially for processing on a small scale is presented in Table 11. Other small expellers give comparable results.

Table 11: Typical Performance of Oil Expeller MINI 40 ¹,²

1 Manufactured by Simon-Rosedowns, Hull, England.

2 Figures from the manufacturer for one pressing only. More oil can be produced by pressing a second time.

The performance of an expeller developed for processing in one step (so-called "deep" pressing) is given in Table 12.

Table 12: Typical Performance of Oil Expeller MRN (AP VII) ¹,²

1 Manufactured by Maschinenfabrik Reinartz, Neuss, F.R.G.

2 Figures from the manufacturer. Even higher recoveries can be obtained with proper preparation including heating.

In the above tables, all data on performances are those given by the manufacturers and may have been determined under favourable conditions. After some time of intensive use, the oil recovery of expellers usually declines until an overhaul becomes necessary. Furthermore, it should be kept in mind, that oil expellers require considerable maintenance and repair, for which expensive spare parts are required. Generally, conditions in villages are such that it is very difficult to run expellers economically, if they can be kept running at all! Expellers are generally found in towns, e.g. in East Africa, where they are sometimes made available as service mills.

More details on expellers as alternative oilseed processing equipment are given in Chapters 3, 4 and 5.