2. Fundamentals and State of Knowledge
2.3 Palm Fruits and Palm Oil Extraction
It is reported that the Oil Palm (Elaeis guineensis) originated in the tropical rain forest region of West Africa. During the 14th to 17th centuries some palm fruits were taken to the Americas and from there to the Far East [53]. Nowadays, the oil palm is grown as a plantation crop in most countries with high rainfall in tropical climates within 10° of the equator, because of its economic importance as an high-yielding source of edible and technical oil [54]. Among the producers, Malaysia is the biggest one with production of 11.41 millions tons per year (contribution is 54% of total global palm oil).
According to Poku [53], the palm bears its fruit, ranging from 6 to 20 g, in bunches varying in weight from 10 to 40 kg. The fruits (Figure 2.7) are made up of an outer skin (exocarp), a pulp (mesocarp) containing the palm oil in a fibrous matrix; a central nut consisting of a shell (endocarp); and the kernel, which itself contains an oil, quite different to palm oil, resembling coconut oil.
Figure 2.7: Palm fruits: a) Exocarp, b) Mesocarp, c) Endocarp, d) Kernel.1
1 Modified figure from source: www.uncleharrys.com/images/red-palm.jpg
a) b) c)
d)
b) The composition of palm oil
Triglycrides constitute the major component of palm oil (90%) [54]. Monoglycerides and diglycrides follow with 5%. The fatty acid composition of glycerides is presented in Table 2.5. C- 16 and C-18 are the main fatty acids. It could be observed that palm oil has a balanced fatty acid composition in which the level of saturated and unsaturated fatty acids are almost equal (50%
saturated, 40% monounsaturated and 10% polyunsaturated fatty acids). As a consequence of high polyunsaturated acid content palm oil is a good oil with the ability to reduce blood cholesterol and the risk of coronary heart disearse. Besides, palm oil contains ca. 3% of free fatty acids and 1% of other minor components. The minor constitutents of palm oil include carotenoids, tocopherols, sterols, phosphatides, triterpenic, and aliphatic alcohols (Table 2.6). Among them, the most important are carotenoides and tocochromanols.
Table 2.5: Fatty acid composition of Malaysian palm oil (after [55]).
% of total acids
Acid Range Mean
12:0 0.1-1.0 0.2
14:0 0.9-1.5 1.1
16:0 41.8-46.8 44.0
16:1 0.1-0.3 0.1
18:0 4.2-5.1 4.5
18:1 37.3-40.8 39.2 18:2 9.1-11.0 10.1
18:3 0.0-0.6 0.4
20:0 0.2-0.7 0.4
Crude palm oil contains carotenoids, ca. 600 ppm, mainly in the form of alpha- and beta- carotenes (Figure 2.8), the precursor of vitamin A. The presence of these carotenes plays an important role in oxidative protection to the oil. On the other hand, crude palm oil contains ca.
800 ppm tocopherols and tocotrienols (the whole group called tocochromanols) (Figure 2.9). The major portion of total tocochromanols in palm oil is alpha-tocopherol, known as vitamin E, and gamma-tocotrienol. These compounds are also antioxidants and provide some natural oxidative protection to the oil.
Table 2.6: Minor components of crude palm oil (after [55]).
ppm
Carotenoids 500-700 Tocopherol and tocotrienols 600-1000
Sterols 326-527 Phospholipids 5-130
Triterpene alcohol 40-80
Methyl sterols 40-80
Squalene 200-500
Aliphatic alcohols [7] 100-200
Aliphatic hydrocarbon [14, 15] 50
a) alpha-carotene b) beta-carotene
Figure 2.8: Strucstrures of carotenes
Figure 2.9: Tocopherols and tocotrienols.
It is obvious that the combined effects of properties of carotenoids, tocochromanols and the high portion of unsaturated acids give palm oil a higher oxidative stability compared to many other edible oils. However, these carotenoids are thermally destroyed during the deodorization
stage. Commonly refined palm oil retains about 50% of the tocochromanols. A lot of new and improved methods for palm oil production have been investigated and applied during the last decades.
2.3.2 Palm oil processing
According to Shahidi [56], palm oil extraction includes following steps:
- Fruit Reception: Palm fruit should be handled with utmost care to minimize the damage to the fruit
- Sterilization: This step is carried out by exposing the fruits to a steam pressure of 3 kg/cm2 (143°C) for approximately 60 min. The objectives of sterilization are: prevention of further rise in the free fatty acid (FFA) of the oil due to enzymatic reaction (i), facilitation of mechanical stripping (ii), preparation of the pericarp for subsequent processing (iii), and preconditioning of the nuts to minimize kernel breakage (iv).
- Stripping: The objective of this step is the separation of the sterilized fruit from the bunch stalks.
- Digestion: Sterilized fruits are reheated to a temperature between 95 and 100°C for approximately 20min. The objective of this step is to loosen the pericarp from the nuts and to break the oil cells before passing to the oil extraction unit.
- Oil Extraction: Commonly a continuous screw pressing system is applied. The products from the press included a mixture of oil (66%), water (24%) and non-oily solid (10%), and a press cake containing fibers and nuts.
- Clarification: The crude oil is separated from fibrous materials by screening and water by settling and centrifugating. The oil is then processed in a vacuum dryer and finally a cooler before going to the storage tanks.
- Oil storage: Storage tanks should be internally coated with epoxy material to prevent iron pickup. The temperature is maintained between 32 and 40°C during storage and transit. The unloading or loading temperature is between 50 and 55°C. The crude palm oil will be refined later by chemical or physical methods, which is well described by O'Brien [57].
Up to now, recovery of oil from the mesocarp by using a screw pressing system is the most commonly used method. However, a significant quantity of carotenoids (4000-6000 ppm) and tocochromanols (2400-3500 ppm) remain in the residual oil (5-6% on dry basis) in the palm press fibers [58].
2.3.3 SFE of palm oil and derivative products – State of the art
Supercritical fluid technology has been proven to be a modern technique for edible oil processing. Recently, supercritical CO2 has been applied in purification and fractionation of crude palm oil [59-62]. Extraction of palm oil direct from palm pulp or kernel under supercritical condition has been conducted [63-67]. Besides, the waste from palm oil processes like pressed palm fiber [68-71] were also investigated.
a) Fractionation and purification
Markom et al. [60] fractionated crude palm oil using SCCO2 at temperatures of 40, 50 and 60°C and pressures of 110, 140 and 200 bar. It was found that the concentration of carotenoids increased as a function of amount CO2. Adding polar cosolvents such as ethanol did not affect the extraction of carotenoids from crude palm oil.
Crude palm oil can be refined by continuous SCCO2 [61]. The refined oil can have less than 0.1% free fatty acids, higher carotene content, and low amount of diglycerides. It was proven that a co-solvent can improve the refining process of palm oil.
Carotenoids and tocohromanols can be purified from crude palm oil, processed with SCCO2. A well developed process had been proposed at VTII department, TUHH. More details can be found in the literature reported by Chuang and Brunner [62] or Gast et al. [72].
b) Extraction
SCCO2 has been used as a solvent to extract oil from the pulp or the pressed residue. The main focus is the yield of extract material. Pressure, temperature and flow rate of CO2 are commonly investigated parameters. The extraction performance of an investigated system is usually evaluated by overall extraction curves.
Bednarski [63] extracted palm mesocarp by SCCO2 at 30 MPa + 40°C, 40 MPa + 60°C and 50 MPa + 80°C. The total extraction curves were studied, and carotene and tocopherols were also investigated. However, the residual oil and its minor concentration in the residual fiber after the SCCO2 extraction were not reported.
Suitable pre-treatment methods for raw material have been also studied. Bisunadan [65]
reported extraction of oil (triglycerides) from sonicated-dried, cooked-wet, sonicated-wet, cooked-dried, wet and dried palm fruits at pressures of 30, 40 and 50 MPa and temperatures of 40, 60 and 80°C. Lau et al. [64] also extracted oil from dried mesocarp. The applied conditions