2. Fatty Acid Composition of Palm Oil, Palm Oil Fractions and Palm Kernel Oil

Palm Oil

The fatty acid chains present in the palm oil triglycerides could vary in the number of carbons present in the chain (chain length) and in structure (presence of double bonds, i.e., unsaturation). It is the variations in the structure and number of carbons in these fatty acid chains that largely define the chemical and physical properties of palm oil. Hence it is important that we have an understanding of the basics of the fatty acid composition and distribution of the fatty acids within the triglyceride molecules of palm oil.

Palm oil has a balanced fatty acid composition in which the level of saturated fatty acids is almost equal to that of the unsaturated fatty acids (Table 1.1).

Palmitic acid, C16 (44–45%) and oleic acid, C18:1 (39–40%) are the major component acids along with linoleic acid, C18:2 (10–11%) and only a trace amount of linolenic acid, C18:3. The low level of linoleic acid and virtual absence of linolenic acid make the oil relatively stable to oxidative deterioration.

The fatty acid compositions of palm oil are shown below.

The TAG (triacylglyceride) profile of palm oil has been characterised by carbon-number gas. The TAG of palm oil consists of C46 to C56 molecules in a near normal distribution, the major TAGs being of C50 and C52.

These carbon numbers represent the number of carbon atoms in the three acyl chains and exclude the glycerol carbon atoms. A more detailed profile of the TAGs is seen in Table 3.3. Palm oil has high contents of disaturated (POP and PPO) and monosaturated (POO and OPO) TAGs. Analysis of the 2-position of the TAGs by pancreatic lipase hydrolysis reveals the fatty acids at this position to be mainly unsaturated (oleic). However, it is unique among vegetable oils because a significant amount of the saturated fatty acids (10 to 16%) are in the sn-2 position of its triglycerides. It is also distinguished from other oils by a very high level of palmitic fatty acid (44%). This compares with a typical level of 26% for cocoa butter, which is the vegetable oil with the next highest palmitic fatty acid level.

P –Palmatic, O – Oleic, S – Stearic

TAGs can be further divided into four component categories: trisaturated (SSS) TAGs, disaturated-monounsaturated (SSU) TAGs, monosaturated-diunsaturated (SUU) TAGs, and triunsaturated (UUU). TAGs.

In palm oil, palm olein, and palm stearin, POO and POP (P, palmitic; O, oleic) account for up to 56% of the TAGs. In addition to POO and PPO, palm stearin also predominantly contains PPP, a completely saturated TAG. Thus, in palm stearin, SSS and SSU constitute more than 75% of the total TAG.

Every fat and oil possesses a natural FA distribution among the glycerides, which affects a product’s consistency as either a solid or a liquid. SSS TAGs provide structure, SSU TAGs provide both structure and lubricity, and the lower-melting unsaturated TAGs provide lubricity only.

Palm oil is unique among vegetable oils in having a significant amount of saturated acids (10–15%) at the 2-position of its TAGs. The appreciable amounts of disaturated (POP and PPO) and monosaturated (POO, OPO and PLO) are apparent as high-melting and low-melting fractions in the differential scanning calorimetry (DSC) thermograms.

The oil can be easily separated into two products, palm olein and palm stearin. Figure 3.2 shows the products obtained from multiple fractionations of palm oil. A wide range of fractions with different properties to suit requirements of the food industry is available through dry fractionation.

Palm Olein

Palm oil, a semi-solid at ambient temperature (25–30°C), may be fractionated into a liquid fraction (olein) and a more solid fraction (stearin). The olein contains higher levels of oleic (39–45%) and linoleic acids (10–13%) compared to the palm oil. Palm olein remains clear at ambient temperature of 25°C.

Further fractionation of the olein produces a more unsaturated fraction, often called super-olein or double fractionated olein. These have higher levels of oleic and linoleic acids, ranging from 43–49% and 10–15% respectively, resulting in iodine values of 60–67 and with lower cloud points of about 2–5°C. In contrast, oleins with IV of less than 60 have cloud point of 6–10°C.

As the iodine value increases, the cloud point decreases, though not linearly. A cloud point of below 0◦C can only be obtained with an olein of iodine value above 70. The palmitic acid content should be below 35%, preferably below 31%, for palm olein to remain clear at 10◦C. Fractions with iodine above 70 and a cloud point of −4◦C are described as top-oleins. This olein can satisfy the cold test in which the oil must remain clear after 5.5 h at 0°C.

The differences in TAG composition between olein with iodine value not exceeding 60 and those above 60 are detailed in Table 3.3. The ratio of POP/POO influences the crystallisation of palm oil as shown in Table 3.5

Saturated TAGs such as PPP, MPP and PPS are the seeds of crystallisation. Other crystallisation inducers are diacylglycerols such as dipalmitoylglycerol. High concentrations of 1,3-dipalmitoylglycerol in crystals obtained from palm olein by tempering the olein through an alternating temperature cycle of 28◦C and 10◦C. It is notable that diacylglycerols are preferentially distributed into the olein phase during fractionation. A higher concentration of diacylglycerols is found in more unsaturated oleins.

Palm Stearin

Palm stearin, the harder fraction of palm oil, contains the more saturated fatty acids and TAGs. A much harder stearin is also available with as much as 79% palmitic acid. This stearin has a tripalmitoylglycerol (PPP) content of 60% and is used as hard stock for soft margarines and in infant fat formulas. Advances in crystalliser designs, cooling programs and filtration technology have enabled a wider range of stearins to be produced. Another stearin, produced from a second fractionation of the olein, is called palm mid-fraction (Figure 3.2). This oil contains high C50 (POP) TAG (Table 3.6) and is utilised in manufacture of a cocoa butter equivalent.

The use of high-pressure membrane filtration has helped to improve the quality of palm mid-fractions. Products of iodine value around 33–35, previously only available through solvent fractionation, can now be produced from dry fractionation processes.

Palm Kernel Oil

Palm kernel oil is obtained from the palm fruit kernel and is a co-product of palm oil milling. Palm oil and palm kernel oil are different in characteristic and properties even though they are derived from the same fruit. The kernel oil is characterised by its high level of shorter and medium fatty acid chain lengths (C6–C14). Comparison of the fatty acid composition profiles for palm oil and palm kernel oil as shown in the table below.

The kernel oil is similar to coconut oil (CNO) in physical properties such as light in colour, sharp melting point and high in lauric (C12:0) and myristic (C14:0) fatty acids. However, palm kernel oil has a lower content of short-chain FA (C6:0−C10:0) and a slightly higher oleic (C18:1) acid content than coconut oil. In palm kernel oil, the saturated fatty acid content accounts for almost 90% of the total fatty acids and the remaining 10% of the fatty acids are mono-unsaturated and poly-unsaturated fatty acids.

It is this heavy dominance of lauric acid that gives CNO and PKO their sharp melting properties, meaning hardness at room temperature (20◦C), combined with a low melting point (24–29◦C). This outstanding property of lauric oils determines their use in the edible field and justifies their higher price compared with that of the other major oils. Because of their low unsaturation, the lauric oils are also very stable to oxidation.

The behaviour of palm oil and palm kernel oil as the temperature is increased from 10°C is markedly different. Palm oil gradually increases in liquid oil content until it melts at about 36°C. Palm kernel oil, on the other hand, is still a hard solid at 25°C, but then melts very abruptly below 30°C. Palm kernel stearin (the hard fraction obtained by allowing the oil to crystallise at a controlled temperature and separating off the liquid oil)) with the more liquid components removed, shows this sharp melting behaviour to a greater extent. After hydrogenation， the shape of the melting curve is little changed， but it moves to a higher temperature by 4 or 5°C. By judging the amount of hydrogenation of the stearin carefully, an almost identical melting behaviour can be obtained, with applications in confectionery and other food uses.

Fats melting sharply just below mouth temperature leave a clean, cool, non-greasy sensation on the palate, impossible for any of the common non-lauric oils to match. Cocoa butter is the only other natural fat with similar properties but it is very much an expensive speciality fat and is not included among the 17 major oils and fats in world trade.

PO – Palm Oil
PKO – Palm Kernel Oil
CB – Cocoa Butter
PKS – Palm Kernel Stearin
HPKS35 – Hydrogenated Palm Kernel Stearin

Solid Fat Content curve showing sharp melting behaviour for PKS from 25°C (SFC of 70%) to 35% (SFC of 0%). In contrast PO has gradual melting characteristics.

Triacylglycerol (TAG) Composition of Palm Kernel Oil

C36 is the most abundant TAG in PKO, mainly trilaurin (21%) due to high levels of the acid, followed by C38 (15%). Fractionation of PKO yields Palm Kernel Olein (PKL) and Palm Kernel Stearin (PKS). Oleic and linoleic acids from PKO goes into the liquid fraction (PKL) during fractionation while the more saturated fatty acids (lauric and myristic acids) go to the solid fraction (PKS).

Reference:

1. The Chemistry of Oils and Fats, CRC Press, 2004
2. Bailey’s Industrial Oil and Fat Products, Wiley, 2005
3. Palm Oil : Production, Processing, Characterization and Uses, AOCS Press, 2012
4. Vegetable Oil in Food Technology, Blackwell Publishing, 2012
5. Characteristics of Malaysian Palm Kernel and Its Products, Journal of Oil Palm Research Vol 25, 2013