Palm oil comes from the fruit of the oil palm Elaeis guineensis Jacq, and can be made either from the fruits themselves or from the nuts. Chemically, palm kernel oil (PKO) is quite similar to coconut oil (Wiener et al. 1989). The edible form is flavourful even when used raw, and palm oil is also used cooking and for the manufacture of soaps, margarine, grease and candle.

In Nigeria many of the vegetable oil industries now rely on palm oil, because of the scarcity of oils from other plant sources, such as groundnuts. Most of the Nigerian palm oil is produced by traditional methods, which involve the fermentation of the oil palm fruits before processing. Here fermentation means storing the fruits for one or more days after harvesting, to allow them to loosen out of the bunches.

As elsewhere, fruits are sometimes processed without fermentation in Nigeria. This takes place in oil palm mills where the bunches are sterilized by steam in order to loosen the fruits. While this results in a high-quality oil, there are very few modern oil palm mills in Nigeria. Many of them are obsolete, inefficient, and in oil palm plantations, and thus out of reach of the majority of palm farmers. This is one reason for the need to improve the traditional methods of processing oil palm fruits.

The traditional methods of palm oil production, especially in the oil palm belts of Eastern Nigeria, differ only at the extraction level. The previous stages in the process, from sterilization to digestion, is almost the same, invariably involving the fermentation of the fruits before processing. The different methods of extraction are as follows:

• hand pressing;
• rinsing the digested mash in water, allowing the oil to float to the surface;
• using the screw press.

In a 1992 study carried out by Chukwu in the seven states which lie in the oil palm belts of Eastern Nigeria, it was found that while the majority of the farmers ferment their fruits for 4 to 5 days before processing (see the appendix below), others allow fermentation to continue for 7 days or even longer. However, many of the palm farmers are not aware of the effect of over-fermentation on the oil they produce.

The number of days the fruits are fermented has an effect on the quantity and quality of the oil produced. Experimental results show that on average oil palm fruits contain 18% palm oil, about 90% of which can be extracted mechanically. In Nigeria, however, only about 40% of the available oil is extracted using traditional methods (Usoro 1974). Irvine (1979) found that using traditional methods just under half of the available oil is extracted. If the screw press (an improved traditional method) is used, slightly more than half can be extracted. By contrast, in a palm mill equipped with a sterilizer and hydraulic presses, the efficiency of extraction is often as high as 75-85%. The Nigerian Institute For Oil Palm Research (NIFOR) has estimated that the annual losses due to the low efficiency of the traditional methods amount to some 250,000 tonnes of palm oil (NIFOR 1987).

It is more difficult to arrive at exact figures with respect to the quality of the product, as there are no internationally accepted quality standards for palm oil. However, one way to assess quality is to measure the amount of free fatty acids (FFA). For example, palm oil is traditionally bought on the basis of an FFA content of 5% by importing countries, with financial penalties for exceeding this percentage. The FFA content of mature fruits, intact in the bunch, is on the order of 0.1% (NIFOR 1989). The oil from fresh ripe fruits contains very little more than this 0.1%, but when bruised and crushed fruits are used, the FFA content increases rapidly (Loncin 1952). The NIFOR (1989) noted that palm fruits are particularly susceptible to deterioration and that the lipolytic enzymes are so active that even under the most favourable conditions, palm oil seldom has a free fatty acid content under 2-3%. Under crude conditions of processing, the FFA content may be 20%, 40%, 60% or even higher.

It is commonly accepted that a great deal of oil produced by traditional methods is of poor quality, due to the high level of free fatty acids (FFA) and the high moisture content (mc). This is largely the result of over-fermentation and bruising sustained during handling. The NIFOR (1986) found that the quality of the oil produced depends to a large extent on the correct timing of harvesting. The best time to harvest the fruits is when they start to loosen and can be dislodged; this is often indicated by the presence of 2-3 loose fruits on the ground at the base of the oil palm tree. If harvested earlier, the fruits will not be ripe and will not have reached their full oil content, resulting in oil of poor quality. If harvested too late, the FFA content of the oil will increase, which likewise causes the quality of the oil to decline. In addition, there is a danger of bunch rot, which also results in a deterioration of the oil. This has been confirmed by Heartley (1988), who observed that postponing harvesting until after the natural loosening of the fruits from the bunch causes the FFA to increase, with the resulting decline in quality. Thus it is clear that the precise moment of harvesting is crucial. A high moisture content combined with high temperatures during digestion also increases the rate of free fatty acid formation (Jacobsberg 1971).

Given the need to improve traditional methods, a study was carried out with the following objectives:

• To compare three traditional methods of palm oil extraction;
• To determine the effect of the fermentation period of oil palm fruits on the quantity and quality of the palm oil produced;
• To determine which of the methods is most efficient.

Each of the three traditional ways of palm oil extraction was examined. For each method, some 23 large, fresh bunches of fruit were collected, which had just been harvested from a plantation of the NIFOR where improved types of Tenera are grown. The choice fell on Tenera because it is fast replacing the wild Dura type, thanks to its high yields. The bunches were separated into spikelets for faster fermentation. The fruits were then divided into nine piles of 50 kg each. The central stalks, which were removed when the bunches were separated into spikelets, were weighed and divided into nine portions with an average weight of 19.5 kg. This meant that each pile contained 69.5 kg of fresh raw material. Each pile was covered with plantain or banana leaves and stored in an open shed, to protect it from the rain. The covering of leaves served to increase the temperature, thus accelerating the rate of fermentation. The piles were numbered 0-8, and each day one pile was processed, starting with the pile numbered ‘0’ on the day of harvesting and ending with the pile numbered ‘8’ on the 8th day of fermentation.

Sterilization, stripping and digestion

On the day of harvesting, the fruits had not yet begun to loosen out of the spikelets. They were boiled or sterilized in a 120-litre drum for 2½ hours over a wood fire (see photo). The fruits were stripped and re-sterilized for an additional 1½ hours, to raise the temperature before pounding (i.e., digestion). After pounding, the hot mash was removed for oil extraction. On the next day, the first day of fermention, the second pile (number 1) was processed. Since the fruits did not loosen out well, the same procedure was employed as on the day of harvesting. The fruits started loosening out on the 3rd day (2nd day of fermentation); they were stripped before boiling. Moulds started to develop on the spikelets on the 5th day of fermentation and by the 6th day they were seen on the outer skin of the fruits as they were stripped.

Oil extraction

Method 1. Hand pressing After the nuts were removed, the digested mash was squeezed between palms and fingers, and the oil collected and weighed on a scale sensitive to within 0.01 grams.

Method 2. Rinsing out the digested mash in water The digested mash was mixed with a large volume of water and the fibre washed out in such a way that the oil floated to the surface. It was then skimmed, collected and weighed.

Method 3. Screw press method Nuts were removed from the hot mash after digestion, after which it was re-sterilized for one hour and fed into the screw press. The oil was collected and weighed.

After extraction, the palm oil is referred to as crude, because it contains a certain amount of water and impurities such as sludge. In order to purify it, the crude oil was brought to a boil in a 30-litre aluminium pot. After boiling for some time, a drop of water was introduced into the boiling oil. If a loud cracking noise was heard, the oil was free from water and boiling was stopped. The oil was allowed to cool and then decanted, leaving sludge and other impurities in the bottom of the pot. This ‘clarified’ oil was then weighed.

Tables 1, 2 and 3 show the quantity of palm oil in kg which was extracted by hand pressing, rinsing the pulp in water, and using the screw press method. The quantity of oil extracted by hand pressing is poor on the day of harvesting and the first two days of fermentation. Comparing these values with those of the rinsing out and screw press methods, the quantities were low.

The amount of oil extracted by the three methods increased between the day of harvesting (zero day of fermentation) and the third day of fermentation, when peak values were recorded. From the fourth to the eighth day quantities continued to decline (see Figure 1).

The reason that the highest amount of oil was obtained on the third day of fermentation was also investigated. This apparently had to do with the fact that the fruits were thoroughly digested, i.e., pounded, so that the mash bore no traces of the outer skin. The nuts were very clean, and no mesocarp adhered to them. It was only on this third day of fermentation that the amount of oil extracted by hand pressing approached that obtained using the screw or mechanical press. Rinsing in water contributes to the efficiency of oil extraction on the third day of fermentaion. Table 4 shows the percentage of oil recovered by the three methods of extraction: hand pressing, rinsing the pulp in water, and using the screw press method. In the case of all three methods, the highest oil recovery rate occurred on the third day of fermentation.

Two factors were used in determining the quality of oil extracted: the free fatty acid content and the moisture content of the palm oil. Tables 1, 2 and 3 show the free fatty acid (FFA) values and moisture content for all samples produced using the three means of oil extraction.

The FFA content was low, at any rate for hand pressing and screw press, because whole fresh bunches were used, with negligable bruising. The range of values, especially up to the 3rd day of fermentation agrees with the NIFOR figures (1989), which stated that for mature fruits, intact in bunches, the FFA is on the order of 0.1%. There the quality of oil produced, especially by means of hand pressing and the screw press, met the recommended world market limit of less than 5%. This was due largely to the type of bunches used. When the mash was rinsed in water, the FFA rose rapidly from the third day of fermentation on. This may have been due to the water in which the oil was extracted. According to Jacobsberg, hydrolysis must have take place (Jacobsberg 1971); he found that at high temperatures the moisture content increases with each day of fermentation.

Table 5 shows the bacteria counted in oil samples for the two methods of extraction which make use of a press. Sanitation was a problem here, and the presence of bacteria may have negatively affected the results. For example, oil from fruits of the day of harvesting should not have any such bacteria, because they were not fermented and there were no bruises. The high bacteria count must have been due to the fact that as a rule the pounding mortars are not cleaned before use. The equipment should actually be washed beforehand with boiling water. In oil palm mills, the utensils are always sterilized with steam before the start of operations. It was noted that the research results compared unfavourably with those recorded in the literature. However, the latter may have been obtained under ideal conditions. For example, on the day of harvesting oil fruits should not display a bacteria count, since the fruits showed no bruising and were not yet fermented. In practice, however, there was a high bacteria count. This must have been due to the fact that the pounding mortars are not usually washed after they are used. The need to sterilize equipment cannot be stressed too strongly. In mills, sterilizing equipment by steam is standard procedure before any new use.

Conclusion

The proper timing of the fermentation of oil palm fruits has been shown to improve the quantity and quality of the palm oil produced by traditional methods, especially when the fruit bunches are fresh. Three days of fermentation is seen to be ideal, regardless of which traditional method is used. When 50 kg of fruits in spikelets were processed, the method which involves rinsing the mash in water resulted in the highest quantity of oil, 11.3 kg, followed by the screw press method with 8 kg and the hand press with only 7 kg. This period of fermentation (three days) is therefore recommended for farmers. However, the quality of oil produced by washing the mash in water was not as high as that produced by the other two methods. This may have been due to the high moisture content in the oil. More time and energy was also required for the clarification of the oil by water extraction, suggesting that this is not a suitable method for processing large amounts of oil palm fruits, especially in the rural areas where water may be scarce. Moreover, the hand pressing method has the lowest FFA. This might have been due to the absence of previous micro-organisms in the extraction equipment as obtained in the screw press. However, hand pressing is tedious and slow and the least efficient of the three methods. Therefore, the screw press method is recommended, provided the equipment is kept clean. It produces a sufficient quantity of good quality oil in an efficient way.

E.U.U. Ituen
Department of Agronomy
University of Uyo, Uyo
Nigeria

Dr I.V.O. Modo
Department of Sociology
University of Uyo
P.M.B. 1017
Uyo, Akwa Ibom State
Nigeria

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Babatunde, O.O., O.O. Ajibola and M.T. Ige (1988) ‘A modified process for low cost palm oil extraction’, Journal of Food Science and Technology Vol. 25, No.2: 67-71.

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Irvine, F.R. (1979) West African crops. Oxford: Oxford University Press.

Jacobsberg, B. (1971) La production d’usine Luile de palm de’ Lante qualite’, O. Leagineaux 6, 26, 781.

Loncin, M. (1952) L’hydrolyse spontanée autocatalitique des triglycerides, O. Leagineaux 7, 695.

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NIFOR (1989) History, activities and achievements.

Usoro, E.J. (1974) The Nigerian oil palm industry, government policy and export production (1906-1965) Ibadan: University of Ibadan.

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