Principles of palm olein fractionation: a bit of science behind the technology

Dry fractionation is a well‐established and versatile fat modification technology that can produce a broad spectrum of edible oils and fats. Due to its specific chemical composition, especially palm oil can be processed by this technology into fractions that serve as salad oils, frying oils, margarine fats. Whereas the first step of this multi‐stage production process is well understood, the edible oil industry all over the world is much more often confronted with problems in the second stage of the process, when the liquid palm olein is further fractionated. The process of palm olein crystallization is indeed a lot more difficult to control. This article therefore elaborately explains the main, fundamental causes for this sensitivity of palm olein during the fractional crystallization. It further discusses which and how components present in refined palm olein can be responsible for process instability and how they affect the quality of the end products on an industrial scale. The article also highlights which novelties and innovation in dry fractionation technology are currently under investigation.     

Palm oil and palm kernel oil in food products

Cocoa butter equivalent (CBE) formulation, especially the compatibility of palm oil based CBE with cocoa butter, is of special interest to chocolate manufacturers. Traditionally palm oil is fractionated to obtain high-melting stearin and olein with a clear point of around 25 C, the latter serving as cooking oil. Recently, palm oil has been fractionated to recover an intermediate fraction known as palm mid-fraction (PMF), which is suitable for CBE formulations. Generally, production of PMF is based on a three-step procedure. However, a dry fractionation system, which includes selective crystallization and removal of liquid olein by means of a hydraulic press, has been developed. Iodine value, solid content (SFI) at different temperatures, cooling curves (Shukoff 0°) and triglyceride/fatty acid composition determination confirmed effectiveness of the procedure followed. A direct relationship between yield, quality of PMF and crystallization temperature during fractionation has been achieved. Yield of 60% for olein of IV 64–67 has been achieved. Yield of 30% for PMF of IV 36–38 and 10% for high melting stearin of IV of 20–22 are also being achieved. High-melting stearin may be used in oleochemical applications, soaps, food emulsifiers and other industrial applications such as lubricating oil. Olein fraction, especially after flash hydrogenation thereby reducing the IV to 62/64, has excellent frying and cooking oil characteristics. Palm olein is also suitable as dietary fat and in infant formulation. Studies on interesterification of high-melting stearin with olein showed possibilities to formulate hardstocks for margarine and spread formulations, even without using hydrogenated fat components. Palm kernel and coconut fats or fractions or derived products are used for confectionery products as partial CB replacers and as ice cream fats and coatings. Coconut oil also serves as a starting material for the production of medium-chain triglycerides.     

Fractionation of lauric oils

The different methods of edible oil fractionation are reviewed, and the applicability of these to the fractionation of palm kernel and coconut oils is discussed. Crystallization from solvents such as acetone, hexane or 2-nitro-propane, is the most easily understood and most convenient for small-scale laboratory trials, but the cost of solvents and the need to flameproof plants makes it uneconomical for an industrial undertaking. Dry crystallization is commonly employed, and there are several methods, described here, for subsequent separation of solid stearin from liquid olein. Chemical and physical properties of the separated stearins and oleins depend on fractionation conditions and on the yields sought. These are reviewed. The properties of the fractions may be further modified by hydrogenation, interesterification, blending or combinations of these techniques. Many sophisticated confectionery fats are manufactured from lauric stearins and their methods of manufacture and product applications are reviewed. A commercial operation must take care to find a good outlet for the secondary fractionation products (or byproducts) however, and useful outlets for these secondary fractions are therefore considered in addition to those of the main product.     

Production of cocoa butter substitutes <Emphasis Type="Italic">via</Emphasis> two-stage static fractionation of palm kernel oil

As are traditional fractionation technologies, static dry fractionation is a highly reliable technology for the consistent production of good-quality palm kernel stearin (PKS) for use as cocoa butter substitute (CBS) after total hydrogenation. A new process route now permits the production of unhardened yet high-quality CBS. Also an increase in total stearin yield can be achieved, via a successful refractionation of palm kernel olein. DSC analysis together with pilot static fractionation trials on the palm kernel olein indicates that a cooling water temperature that is too low (e.g., 17°C) may result in the quick formation of unstable crystals that are possibly later converted to a more stable form. The resulting mixture of crystals with a possibly different polymorphic structure is easily squeezed through the filter cloth during filtration, whereas a slower, but more homogeneous co-crystallization occurs at higher temperature (18°C or higher) and results in a much more stress-resistant slurry. Polarized light microscopy analysis confirmed that crystal size is not the only determining factor for a successful filtration. The total two-stage static fractionation of palm kernel oil (PKO) [iodine value (IV) 18] on a pilot scale results in the following three end products: PKS IV 5 (yield: 29%, for direct use as CBS), PK olein IV 27 (yield: 58%), and PKS IV 7 (yield: 13% for use as CBS after full hydrogenation). The unhardened PKS IV 5 has outstanding melting and crystallization properties, comparable to traditional hydrogenated stearin fractions. Therefore, rather than the higher stearin yield, the reduced hydrogenation capacity is most probably the most important benefit of the two-stage static fractionation process.     

Processing of Palm Kernel Oil

By the end of the century palm kernel (PK) oil is forecast to rank fifth in volume of world trade in oils, only slightly behind coconut oil. PK oil received by Malaysian refiners is of good quality, typically below 2% in free fatty acids, and is easily refined. PK oil is fractionated to give a higher melting fraction (stearin) and a lower melting fraction (olein). The various processes used are described. PK oil, olein or stearin are also hydrogenated to give a range of products ranging in melting point from about 24 to 44°C. PK oil and olein are interesterified alone or in blends with a non-lauric oil to give products with improved melting properties and utility. The utilisation of PK oil in the main application areas of confectionery fats, non-dairy/imitation dairy products, biscuit creams, industrial margarines, nut roasting and spray oils are discussed.     

Preparation of plastic fats with zero <Emphasis Type="Italic">trans</Emphasis> FA from palm oil

A series of plastic fats containing no trans FA and having varying melting or plastic ranges, suitable for use in bakery, margarines, and for cooking purposes as vanaspati, were prepared from palm oil. The process of fractionating palm oil under different conditions by dry and solvent fractionation processes produced stearins of different yields. Melting characteristics of stearin fractions varied depending on the yield and the process. The lower-yield stearins were harder and had a wider plastic range than those of higher yields. The fractions with yields of about 35% had melting profiles similar to those of commercial vanaspati. The plastic range of palm stearins was further improved by blending them with corresponding oleins and with other vegetable oils. The plasticity or solid fat content varied depending on the proportion of stearin. Blends with higher proportions of stearins were harder than those with lower proportions. the melting profiles of some blends, especially those containing 40–60% stearin of about 25% yield and 40–60% corresponding oleins or mahua or rice bran oils, were similar to those of commercial vanaspati and bakery shortenings. These formulations did not contain any trans FA, unlike those of commercial hydrogenated fats. Thus, by fractionation and blending, plastic fats with no trans acids could be prepared for different purposes to replace hydrogenated fats, and palm oil could be utilized to the maximum extent.     

Monitoring milk fat fractionation: Filtration properties and crystallization kinetics

The effect of fractionation temperature, residence time, and agitation rate on the chemical composition of the stearin and olein milk fat fractions was studied. During fractionation, filtration properties of the crystal suspension were monitored; crystallization kinetics was determined by ¹H NMR. Higher fractionation temperatures result in a lower stearin yield, more oil entrapment, and a lower final solid fat content of the crystal suspension. On the other hand, the chemical composition of the resulting fractions is not influenced. Longer residence times lead to longer filtration times and lower oil entrapment, whereas the yield is not affected. Longer residence times induced lower growth rates, but chemical composition is not influenced. Agitation rates varying from 10 to 15 rpm have no influence on the chemical composition of stearin and olein milk fat fractions. Higher agitation rates decrease the filtration quality and increase stearin yield, causing a softer stearin. In designing and monitoring milk fat fractionation, filtration experiments and the assessment of crystallization kinetics are valuable techniques, but compositional chemical analysis is not favorable.     

Effect of dietary palm oil and its fractions on rat plasma and high density lipoprotein lipids

Male Sprague Dawley rats were fed semipurified diets containing 20% fat for 15 weeks. The dietary fats were corn oil, soybean oil, palm oil, palm olein and palm stearin. No differences in the body and organ weights of rats fed the various diets were evident. Plasma cholesterol levels of rats fed soybean oil were significantly lower than those of rats fed corn oil, palm oil, palm olein or palm stearin. Significant differences between the plasma cholesterol content of rats fed corn oil and rats fed the three palm oils were not evident. HDL cholesterol was raised in rats fed the three palm oil diets compared to the rats fed either corn oil or soybean oil. The cholesterol-phospholipid molar ratio of rat platelets was not influenced by the dietary fat type. The formation of 6-keto-PGF_1α was significantly enhanced in palm oil-fed rats compared to all other dietary treatments. Fatty acid compositional changes in the plasma cholesterol esters and plasma triglycerides were diet regulated with significant differences between rats fed the polyunsaturated corn and soybean oil compared to the three palm oils.     

Fractionation studies of palm oil by density gradient

The paper describes a method of fractionating vegetable, animal and fish oils, and in particular palm oil. The method involves addition of a medium comprising two common solvents to the semisolid oils. On centrifugation, the olein and stearin are separated by the medium in the middle. Thirteen media made up from binary combinations of nine solvents, viz. water, propylene glycol, glycerine, methanol, ethanol,n-propanol, isopropanol (IPA), acetone and butanone, are found to be effective in olein-stearin separation. However, only the water/IPA and water/methanol systems have been studied in detail. The aqueous IPA provides a higher yield of olein than water/ methanol but intersolubility between oil and medium is also greater. The fractionation process can be carried out at any suitable temperature. Fractionation of the special prime bleached (SPB) palm oil at 16 C yields an olein with a cloud point of 4.8 C. Some hybrid palm oils produce a large quantity of low cloud point olein which can be bleached readily. The process can be extended to include degumming and neutralization by using an alkaline medium for centrifugation. The olein fractions obtained have been found to be free of phosphatides and the free fatty acids reduced to as low as 0.02%. Metal-scavenging agents have also been added to the medium in an attempt to remove copper and iron. The development of this process into a continuous one has been demonstrated on the AlfaLaval LAPX 202 Separator. Fractionation of crude palm oil using a density gradient provides seven fractions of different characteristics. The iodine values vary from 37.5 to 57.4 and the unsaturated fatty acids range from 32.7% to 51.2%. Triglyceride analysis by carbon numbers shows great differences in the C₄₈ and C₅₂ constituents of the fractions. aThe volume ratio of oil to medium in each case was 1:1. The separation involved the oil and wax.     

Crystallization Rates of Palm Oil and Modified Palm Oils

Differential scanning calorimetry and pulsed nuclear magnetic resonance were used in the estimation of crystallization kinetics of palm oil and modified palm oils. Differential scanning calorimetry was found to be more sensitive and could differentiate between crystallization during cooling and crystallization during isothermal conditions. Hydrogenated palm oils crystallized quickly and completely when cooled from 60° to 20°C, while palm oil and fractionated palm stearin continued to crystallize when held isothermally at 20°C.     

Determination of Metastable Zone Width and Nucleation Induction Period of Palm Oil and its Olein/Stearin in Melting Layer Crystallization

Measurement methods of metastable zone width (MZW) and nucleation induction time for melting layer crystallization of palm oil (PO) and its olein/stearin (POL/PST) were established, and the effects of cooling rate (corresponding to various supercoolings) on MZWs and induction time for melting layer crystallization of PO, POL, and PST were determined. The results indicated that the MZW coherently rose with increasing cooling rates with respect to PO and POL, while it declined with higher cooling rates for PST. The induction period results demonstrated that the nucleation induction periods of PO, PST, and POL decreased with increasing supercoolings, and the lag time for nucleation negatively correlated to the melting point of oils at the same supercooling. These data could offer significant instruction in designing and controlling the melting layer crystallization process for palm oil.     

Determination of mono- and diglycerides in palm oil,olein and stearin

Partial glycerides are important constituents of palm oil and can have significant effects on the physical properties of products containing palm oil or on the fractionation of palm oil. A method is described for their routine determination in palm oil. By analysis of 28 weekly composite samples of crude palm oil the following results were obtained: free fatty acids, mean=3.76%, range 2.4 to 4.5%; monoglycerides, mean=0.28%, range 0.21 to 0.34%; diglycerides, mean=6.30%, range 5.3 to 7.7%. During detergent fractionation of palm oil, diglycerides concentrate in the palm olein, but monoglycerides concentrate in the palm stearin. Palm fatty acid distillate was found to contain approximately 3% each of mono- and diglycerides. Because the refining and fractionation processes are continuous in the refinery, it is not possible to follow a single identifiable batch of crude palm oil through the refinery. To circumvent this problem, crude palm oil, stearin and olein from the refinery were bleached and steam refined in the laboratory and the partial glyceride contents determined at each stage of processing. Except for fractionation, the content of glycerides did not change during processing. For oil, olein and stearin, monoglycerides were reduced significantly both after bleaching and after steam refining.     

Insight into thermal,crystallization kinetics,and phase behaviors of palm kernel oil and its olein/stearin by differential scanning calorimetry

The fractionation of oils has received considerable research attention, aiming at the modification of the properties of oils. The thermal and crystallization properties of palm kernel oil (PKO) and its olein/stearin (PKOL/PKST) were systematically investigated using differential scanning calorimetry (DSC). Based on the isothermal DSC results, crystallization temperature exerted critical effect on co-crystallization of PKOL and PKST. Moreover, it is reported that PKOL could be further separated by controlling the crystallization temperature, while PKST is hard to be further separated. The PKOL/PKST mixtures with different compositions were prepared and subjected to dynamic DSC analysis. The results illustrated that PKO meets the phase equilibrium of quasi-solid solutions.     

Isolation and Evaluation of Stearin and Olein Fractions from Rice Bran Oil Fatty Acid Distillate by Detergent Fractionation and Conversion into Neutral Glycerides by Autocatalytic Esterification Reaction

Detergent fractionation (Lanza process) offers a valuable separation process for edible oils that contain varying amounts of saturated and unsaturated fatty acids. The rice bran oil fatty acid distillate (RBOFAD), obtained as a major byproduct of rice bran oil deacidification refining process, was fractionated by detergent solution into a fatty acid mixture as follows: low-melting (19.00 °C) fraction of fatty acids as olein fraction (44.50 g/100 g) and high-melting (49.00 °C) fatty acids as stearin fraction (37.15 g/100 g). A high amount of palmitic acid (42.75 wt%) is present in stearin fraction, while oleic acid is higher (48.21 wt%) in the olein fraction. The stearin and olein fractions of RBOFAD with very high content of free fatty acids are converted into neutral glycerides by autocatalytic esterification reaction with a theoretical amount of glycerol at high temperatures (130–230 °C) and at a reduced pressure (30 mmHg). Acid value, peroxide value, saponification value, and unsaponifiable matters are important analytical parameters to identity for quality assurance. These neutral glyceride-rich stearin and olein fractions, along with unsaponifiable matters, can be used as nutritionally and functionally superior quality food ingredients in margarine and in baked goods as shortenings.     

Physical properties of oils and mixtures of oils

The physical properties of palm, palm kernel and coconut oils are reviewed and compared and contrasted with the properties of other oils and fats. More information is available for palm oil than for the other two. The properties of mixtures of the oils also are considered, especially mixtures of palm and palm kernel oils in which a eutectic interaction occurs. Basic physical properties considered are density, specific heat, heat of fusion and viscosity. Where appropriate, data is tabulated in SI and Imperial units. Experimental methods used for determining melting points and solid fat contents are discussed and the empirical nature of the results emphasized. Wiley melting points and Slip melting points, and Solid Fat Content by NMR and Solid Fat Index by dilatometry, are compared and comparative data given. For palm oil, detailed olein and stearin information is presented. The phase behavior and polymorphism of the three oils is reviewed. Special attention is given to the post-hardening phenomenon in palm oil and the effects of diglycerides and storage time on phase behavior.     

Fractionation of palm oil

Because of its fatty acid composition, which includes 50% saturated and 50% unsaturated fatty acids, palm oil can readily be fractionated, i.e. partially crystallized and separated into a high melting fraction or stearin and a low melting fraction or olein. Three main commercial processes for fractionating palm oil are in use: the fast dry process, the slow dry process and the detergent process. All these processes lead to specific products of different quality with different yield and operating costs. The physical and chemical characteristics as well as the triglyceride compositions by high performance liquid chromatography (HPLC) of palm oil fractions from these industrial fractionation processes are given. Other varieties of products produced by specific fractionation are presented with analytical data: the superoleins, palm-mid-fractions and cocoa butter substitutes.     

Chicken fat dry fractionation: Effects of temperature and time on crystallization,filtration and fraction properties

A surface response experimental design was used to quantify the effects of the final cooling temperature and the residence time at this temperature on the filtration characteristics of the crystal suspension and the quality of the fractions during chicken fat dry fractionation. The crystal morphology was monitored to gain insight into the observed effects. Temperature affected crystallization and the characteristics of the olein and stearin fractions. The crystals became fragmented as the residence time increased, thus leading to a decrease in the filterability of the crystal suspension and an increase in the proportion of the entrained liquid phase. The residence time only affected the quality of the stearin fraction. This time effect was noted at high cooling temperatures in the investigated experimental domain. At low temperatures, increasing the residence time enhanced crystal growth and increased the extent of unsaturation of both fractions without modifying the filtration features.     

Composition and thermal profile of crude palm oil and its products

Gas-liquid chromatography and high-performance liquid chromatography (HPLC) were used to determine fatty acids and triglyceride (TG) compositions of crude palm oil (CPO), refined, bleached, and deodorized (RBD) palm oil, RBD palm olein, and RBD palm stearin, while their thermal profiles were analyzed by differential scanning calorimeter (DSC). The HPLC chromatograms showed that the TG composition of CPO and RBD palm oil were quite similar. The results showed that CPO, RBD palm oil, RBD olein, and superolein consist mainly of monosaturated and disaturated TG while RBD palm stearin consists mainly of disaturated and trisaturated TG. In DSC cooling thermograms the peaks of triunsaturated, monosaturated and disaturated TG were found at the range of −48.62 to −60.36, −25.89 to −29.19, and −11.22 to −1.69°C, respectively, while trisaturated TG were found between 13.72 and 27.64°C. The heating thermograms of CPO indicated the presence of polymorphs β₂′, α, β₂′, and β₁. The peak of CPO was found at 4.78°C. However, after refining, the peak shifted to 6.25°C and became smaller but more apparent as indicated by RBD palm oil thermograms. The heating and cooling thermograms of the RBD palm stearin were characterized by a sharp, high-melting point (high-T) peak temperature and a short and wide low-melting point (low-T) peak temperature, indicating the presence of occluded olein. However, for RBD palm olein, there was only an exothermic low-T peak temperature. The DSC thermograms expressed the thermal behavior of various palm oil and its products quite well, and the profiles can be used as guidelines for fractionation of CPO or RBD palm oil.     

Polymorphism of palm oil and palm oil products

Palm oil, palm stearin, hydrogenated palm oil (IV 27.5) and hydrogenated palm olein (IV 28) were crystallized at 5°C, temperature cycled between 5 and 20°C, and kept isothermally at 5°C for 36 days. The polymorphic state of the fats was monitored by X-ray diffraction analysis. Soft laser scanning of X-ray films was used to establish the increase inβ crystal content. Palm stearin was least stable in theβ′ form, followed by palm oil. The hydrogenated oils were very stable in theβ′ form. Differential scanning calorimeter (DSC) analysis was used to complement the X-ray data.     

Colorimetric determination of total tocopherols in palm oil,olein and stearin

By using a preliminary heat-bleach at 250 C the Emmerie-Engel method has been adapted for the determination of total tocopherols (including tocotrienols) in crude as well as refined palm oil, olein and stearin. Total tocopherol contents found were: Crude palm oil, 794 ppm (n=10); RBD palm oil, 563 ppm (n=13); RBD palm olein, 643 ppm (n=40); RBD palm stearin, 261 ppm (n=19), where n is the number of samples analyzed. During the detergent fractionation no tocopherols were lost, but the tocopherols were concentrated in the olein fraction. The fate of the tocopherols during degumming, bleaching and steam refining/deodorizing of Crude palm olein containing 978 ppm total tocopherol was studied. Over the whole refining process only 8% of the tocopherols were lost, 62% of the original tocopherols were retained in the RBD palm olein, while the remaining 30% were concentrated in the fatty acid distillate which contained 7,040 ppm tocopherol.