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338 M. Kellens et al. Eur. J. Lipid Sci. Technol. 109 (2007) 336–349

are therefore used, with the advantage of producinga wide range of fractions suitable for different ap-plications.

The process consists in two steps: the crystallizationstage which produces solid crystals in a liquid matrix andthe separation stage where the liquid phase (olein) isseparated from the crystals (stearin).

4.1 Crystallization

4.1.1 Polymorphism

In the solid state, TAG are packed together side by side inseparate layers in a head-to-tail arrangement [5]. Differentpacking modes are possible, in pairs of two, three or morefatty acid chain lengths. There are generally three crys-

talline states in which an oil or fat can solidify. Accordingto the thermal conditions, TAG tend to pack together in ahexagonal structure ( a crystal form), an orthorhombic( b’ form), or a triclinic form ( b form). The stability increasesfrom a to b’ to the b crystal form. Furthermore, the rate ofcrystallization of the a form is greater than that of theb’ form, which in turn is greater than that of the b poly-morph. Different thermal conditions are needed to inducecrystallization of the different polymorphic forms. In highsupercooling conditions, a crystallization occurs, yieldinga dense mass of very small crystals. The b form, on theother hand, is more difficult to obtain and only appears insome exceptional cases, as for example in CB. Theb crystal texture does not always allow easy separationand should therefore be avoided in most cases.

In order to achieve good separation, the crystals shouldbe firm and of uniform spherical size, which is a conditionfound when they are mainly in the b’ form.

The physical properties of the polymorphic forms are asfollows, with b ’ always in midrange: melting point, melting

enthalpy, stability, selectivity, and activation energy are alllow for a and high for b; the nucleation rate, crystallizationrate, miscibility in solid state, and TAG compatibility are allhigh for a and low for b.

4.1.2 Intersolubility

Depending on the chemical composition and the crystalstructure (polymorph), TAG may form different kinds ofsolid solutions. In consequence, the efficiency of frac-tional crystallization is not only dependent on the effi-ciency of separation but is limited by the phase behavior

of TAG in the solid state.

Because of closely linked structural properties, TAG canproduce co-crystals by intersolubility; theymostfrequentlyshow solid solutions, monotectic interactions, eutecticsystems, molecular compounds, etc. Binary systems ofpure components have been extensively studied [6–11].The case of edible oils and fats is more complex: they aremade of numerous TAG that have very similar chemicalstructuresbut variable chain lengths, degrees of unsatura-tion and positional isomers. Depending on their chemicalstructure or polymorphic form, some TAG will be very sol-uble when mixed andform solid solutions; others will crys-tallize separately, being immiscible in the solid state andgiving rise to eutectic or monotectic interactions (Fig. 1) [4].

Fig. 1. Binary phase diagrams (temperature-composition) of PPP/PStP and PPP/POO(established based on powder X-ray diffractionand DSC data) [4]. Binary phase diagrams havebeen established by mixing and melting pureTAG. The samples were afterwards quenched at–40 7 C and heated at a constant rate (5 7 C/min):transition (squares) and melting (circles) peaksare detected by DSC. Powder X-ray diffractionis used under similar thermal conditions todetermine the polymorphic behavior.P: palmitic fatty acid. O: oleic fatty acid. St:stearic fatty acid.

2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ejlst.com

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Eur. J. Lipid Sci. Technol. 109 (2007) 336–349 Dry fractionation of palm oil 339

In practice, the phase behavior of a fat blend in the solidstate is considerably influenced by the crystallizationconditions. Crystallization does not start once the “liqui-dus” temperature is reached as a consequence of theexisting activation free energy of crystallization. Crystal-lization normally takes place at a lower temperature, i.e.the process needs a certain degree of supercooling tostart. This activation free energy of crystallization is thehighest for b, intermediate for b’ and the lowest for a. Thecrystallization behavior is a function of the intersolubilityof the different components in the solid state under thegiven conditions of cooling. These conditions normally donot conform to the rule of true thermodynamic equilibriumbut are mainly kinetically dependent.

4.1.3 Crystallization

Crystallization consists of successive stages: super-cooling of the melt, nucleation, and crystal growth; crys-tals normally do not remain single but tend to agglomer-ate. The key point in fractionation consists in controllingthe selectivity of crystallization and separation. Thisselectivity is limited by the degree of compatibility of thedifferent TAG in the solid state, which in turn is a functionof the crystal form and of the composition. How far thisselectivity is affected by the cooling conditions dependsnot only on the overall cooling rate but also on the prac-tical limitations imposed by the unit process itself. Theheat transfer characteristics of the crystallizer vessel aswell as the efficiency of the separation technique largelydetermine the quality of the obtained fractions. Severalcooling modes are possible (Fig. 2) [12]. The oil coolingcurve can be based on a fixed water cooling profile; in thiscase, the temperature of the oil will depend on the tem-perature of the water. Another design is based on the DTprinciple; here, the temperature of the water is regulatedby the temperature of the oil itself. The differences in thecooling schemes result in technological variations essen-tially expressed in the geometry of the cooling surfacesand in the agitation design (Fig. 3) [4].

Normally, prior to crystallization, the oil is fully melted inorder to destroy the crystals present in the oil phase (toerase the thermal memory). Thereafter, the oil is cooledin a controlled manner according to a given coolingprofile that is a function of the feedstock and of therequired fractions. Nucleation occurs when the temper-ature of the melt is much lower than the thermodynamicequilibrium temperature, i.e. when the melt becomessupercooled.

Three types of nucleation phenomena can occur. Homo-geneous nucleation takes place in the bulk of the motherphase. Heterogeneous nucleation refers to formation ofnuclei on foreign substances. Secondary nucleationappears when tiny crystallites are removed from the sur-face of existing crystals, which in turn act as new nuclei.In many real systems, heterogeneous nucleation occursbefore homogeneous nucleation. It takes place at solid

particles for which the new phase has some chemical orphysical affinity, such as dust particles, walls of crystal-lizers, or foreign material.

Once the nuclei are formed, they grow further. The rate atwhich they further develop depends not only on externalfactors (degree of supercooling, presence of inhibitors,etc. ) but also on internal factors (polymorphic form, crys-tal morphology, crystal defects, etc. ). The growth rate isproportional to supercooling and inversely proportional toviscosity. The higher the viscosity, the more difficult is theexchange of material between the bulk phase and thecrystal surface and the slower will be the crystal growth.Therefore, in order to allow a continuous and uniformcrystallization, the fatty matter needs to be kept homo-geneous. This requires an intense but non-destructiveagitation.

During cooling and subsequent crystallization, the vis-cosity increases (Fig. 4). Viscosity is not only a result ofincreasing amounts of solids present in the liquid, but isalso influenced by the crystal size distribution as well asthe interactions between the different crystals.

Fig. 2. Schematic representation of (a) an oiltemperature-related profile and (b) an independ-ent water cooling profile, during the crystallizationstep of a dry fractionation operation [12].


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Fig. 3. Different crystallization equipments: (a) propelleragitation, (b) concentric jackets and sweeping surfaceagitation, and (c) stirring cooling surfaces.

Due to attractive interactions between the crystals, theytend to form agglomerates. The very large crystals thatcan be observed during crystallization are often com-posed of different small crystals held together by weakbonds. Agglomeration, however, can lead to lowerseparation efficiency due to a higher entrainment of liquidinside the clusters.

The crystal morphology is determined by internal as wellas external conditions. The overall crystallization kineticsdepends on the rate of formation of nuclei as well as onthe rate at which the nuclei will grow. The size and shapeof the ultimate crystals depend on the relationship be-tween these two factors. Normally, slow cooling results in

large crystals, whereas fast cooling gives smaller crystals.The optimal crystal size is largely determined by theseparation technique, more specially the fineness of thefilter belt or filter cloth. More important than the size,however, is the uniformity in crystal size and shape as wellas the resistance against mechanical stress. To reach thisstate, controlled nucleation and selective crystallizationare required.

4.2 Separation

The goal of the separation is to produce a solid (stearin)and a liquid (olein) phase, each having its proper physi-cochemical characteristics and its particular applications. At the end of the crystallization process, TAG are distrib-uted in three locations: (1) as solids in the form of co-crystals, (2) as liquids (non-crystallized oil), and (3) as

liquids physically trapped on the surface of the crystals.Based on this, different separation equipments are avail-able, depending on the efficiency of the separationrequired: (1) vacuum filters (rotary drum or belt filters),(2) press filters (membrane or hydraulic), and (3) cen-trifuges (nozzle centrifuge [13], centrifugal filter [14] andcentrifugal decanter [15]) (Fig. 5).

Two types of vacuum filters are in use: the rotary drumsand the belt filters, which both operate in two stages.The first stage consists in the separation of the crystalsfrom the mother oil, and the second stage permits adrying of the cake by sucking under vacuum in order toreduce the entrained liquid oil. Such filters are still inoperation in fractionation plants when the market favorssoft stearins.

Fig. 4. Effect of cooling on SFC formation(followed by pulsed NMR) and viscosityincrease during fractional crystallization ofpalm oil (fixed water cooling profile).


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Fig. 5. Separation technolo-gies: (a) rotary drum, (b) Flor-entine belt filter, (c) membranepress filter, (d) nozzle centrifuge,(e) centrifugal filter, and (f) cen-trifuge decanter.

Nowadays, many users of fractionation plants favor theautomatic press filter, fitted with airtight membranes. Al-though it does not have the benefit of the continuousoperation of vacuum filters, its advantage lies mainly in

the higher percentage of liquid it yields, by applying apressure to the cake during each filtration cycle. Theresult is to expel more of the liquid physically trapped onthe crystals.


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A membrane press filter consists of a series of filterchamber plates that are kept together by means of ahydraulic cylinder. The available filter surface is muchlarger compared to a vacuum filter, allowing a much fasterand more uniform filtration. The stearin crystals, afterbeing concentrated upon filling the filter chambers, areadditionally squeezed together by means of the inflatablemembrane, which results in better removal of theentrained liquid phase and a higher yield of olein. Due tothe higher differential pressures applied, separation is lesssensitive to changes in crystal structure. Membrane pressfiltration is a semi-continuous process that can be dividedinto two sequences: filtration and squeezing (Fig. 6). Onfilling the filter, the slurry is pressed into the filter cham-bers, which allows a large part of the free olein to beseparated from the slurry. In the second step, the con-centrated crystals are mechanically squeezed betweenthe filter cloths by inflating the membranes, in order to

squeeze out part of the olein that is entrapped inside thesolid mass. Thereafter, the filter is opened and the cakesare discharged by gravitation.

The use of centrifuges to separate the different fractionsis based on the difference in density between the liquidand the solid phases. The density of the solid fraction isdependent on both the crystal size as well as on thecrystal habit and increases from a to b’ and further to theb form, as a result of the closer packing mode of themolecules in the crystal lattices. The density differencebetween stearin and olein fractions (about 100 kg/m 3 ) isfurthermore determined by the amount of liquid entrainedinside the crystal mass. A new type of separator (nozzlecentrifuge) has been developed and launched for theseparation of fat crystals in a centrifugal field by densitydifference between olein and stearin without additives[13]. Another type of centrifuge, a conical sieve centrifuge

fitted with a tightly fitting, co-rotating worm, has also beeninvestigated [14]. Recent development work in centrifugedecantation [15] has shown that it is possible to easilyseparate a stearin from an olein as the sedimentation ratedepends on the crystal size, the crystal shape, the densitydifference between olein and stearin and the viscosity ofthe olein.

In Fig. 7, a typical diagram of a dry fractionation systemincluding separation on a membrane press filter is pre-sented.

5 Dry fractionation of palm oil

Palm oil is by far the most important fractionated oil.There are industrial fractionation installations in operation

that process up to 2000 tons of palm oil per day. Bothcrude as well as refined palm oil are fractionated in multi-stage giving rise to several applications (Figs. 8, 9,Tab. 1). The early reasons of fractionating palm oil haveless to do with product quality than with a simple politicalchoice of the Malaysian government aiming at rapidlydeveloping its local industry [16]. Since the early 1970’s,the export tax on palm oil being shipped from Malaysiahas been decreased, provided the oil had undergone fur-ther processing. This policy naturally triggered a boom inthe production of processed palm oil. For a long time,Malaysia has fractionated palm oil, mainly taking advan-tage of such duty structure. The result has been thecreation of new commodities: palm olein and palmstearin, both having broad specifications (olein not liquidenough to resist low temperatures and stearin not suffi-ciently tailored for direct use). Both had, however, the bigadvantage of being cheap.

Fig. 6. Filtration sequences in a membrane press filter: (a) filling, (b) squeezing, and (c) discharge.


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Fig.7. Flow sheetof a typicaldry fractionation plant oper-ating with a membrane pressfilter.

Fig. 8. Multistage dry fractionation process ofpalm oil (PMF: palm mid fraction; IV: iodinevalue).

Fig. 9. SFC profiles (SFC by pulsed NMR) of dif-ferent palm oil fractions (Ol: olein, SOl: super olein,MOl: mid olein, PO: palm oil, MSt: mid stearin,SPMF: soft palm mid fraction, HPMF: hard palmmid fraction, St: stearin, SSt: super stearin).


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Tab. 1. Some applications of palm oil fractions. §

Product Palm oil Olein Stearin Superolein

Middlestearin

Palm midfraction

Shortenings 111 111 11 – 111 1

Margarines 11 111 1

– 111 1

Frying fats 111 111 – 111 11 1Cooking oils – 11 – 111 – –Salad oils – 1 – 111 – –Specialty fats for coatings – – – – 1 11Cocoa butter extenders – – – – 1 111Ice cream 111 – – – – –Icings 11 – – – 1 11Biscuits 111 1 1 – 11 –Cakes 111 – 1 – 11 –Cookies 111 – 1 – 11 –Crackers 111 1 1 – 11 –Noodles 111 111 – – 11 –Fatty acids source 1 – 111 – – –

Hard coatings – – 11

– – –§ 111 , highly suitable; 11 , suitable; 1 , limited application; –, not suitable

Elsewhere, for example in Colombia, the reasons forfractionating have been very different. Unlike Malaysia,Colombia is an importer of liquid oils (soybean and sun-flower). Its palm production has increased continuously,but most of the palm oil is being used locally. In order tobottle liquid oil for the Bogota market without running therisk of having sediment formation, the Colombian refinerhas to blend its palm olein with imported and taxed soy-bean oil. The higher the quality of the olein (or superolein),the more will be allowed in blends and the higher is theprofit margin. According to olein (or superolein) quality,this percentage can vary from 30 to 10% only (Tab. 2).

Oils and fats are complex mixtures of acylglycerols thatare composed of a whole variety of different fatty acids:palmitic acid (P, C16:0), stearic acid (St, C18:0), oleic acid(O, C18:1), linoleic acid (L, C18:2), and linolenic acid (Ln,C18:3) are usually predominant. These fatty acids can bebroadly divided into saturated and unsaturated ones; inconsequence, TAG are generally divided into four classes:

the trisaturated (SSS), the disaturated (SSU-SUS), thediunsaturated (SUU-USU), and the fully unsaturated(UUU) ones. These TAG exhibit different physical andchemical properties and specific potential for end-useapplications (Tab. 3) [12]. Palm oil is principally made upof 6–10% SSS (mainly PPP, tripalmitoylglycerol), 44–50%SSU-SUS (mainly POP, dipalmitoyl-oleylglycerol, andPLP, dipalmitoyl-linoleylglycerol), 38–42% SUU-USU(mainly POO, dioleyl-palmitoylglycerol, and PLO, palmi-toyl-linoleyl-oleylglycerol) and 5–8% UUU (mainly OOO,trioleylglycerol, and OOL, dioleyl-linoleylglycerol). Basi-

Tab. 2. Percentages of palm liquid fraction allowed to beblended with soybean oil keeping cold test at 0 7 C lowerthan 5.5 h.

Iodinevalue

Mettler cloudpoint [ 7 C]

Palm liquidfraction [%]

Olein 57 7.2 –Super olein 63 2.5 10Super olein 65 1.3 30Top olein 71 2 4.5 100 (24 h min.

at 0 7 C)

Tab. 3. Correlation between chemical composition (interms of TAG), physical state of the product and finalapplication (S: saturated fatty acid; U: unsaturated fattyacid).

Triacylglycerols Physical state of theproduct

Final application

SSS Solid Fatty acid productionHard coatings

SSU-SUS Solid? semi-solid ConfectionerySUU-USU Semi-solid? liquid MargarinesUUU Liquid Salad (dressing) oils

Liquid frying oils

cally, the goal of dry fractionation is to separate thetrisaturated fraction first and then the disaturated and thediunsaturated ones. Due to intersolubility (closely linked


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to polymorphism), it is clear that the trisaturated fractionwill inevitably contain also some SSU-SUS, SUU-USUand UUU components. The first fractionation step of palmoil will reduce the SSS content in the olein as much aspossible. When the crystallization is operated properly,this content falls to zero in favor of higher levels of SUU-USU and UUU, while SSU-SUS remains unchanged(Tab. 4). When SSS is totally removed in the first step,SSU-SUS begins to decrease selectively in the secondstep, leading to higher iodine value (IV) and lower cloudpoint (CP) super oleins. Further enrichments in POO andPLO are responsible for the high drop in CP observed forthe top oleins (Fig. 10) [17].

On the other hand, palm oilis a valuable source of SUS; athigh levels of POP, it becomes after fractionation suitablefor CB replacement fats [principally the hard palm midfraction (PMF)] (Tabs 5, 6). The physicochemical char-

acteristics of CB equivalents (CBE) are relatively close tothe ones of CB with regard to crystallization, texture andmelting properties; this is due to similar TAG composition(SSU-SUS components – POP, POSt and StOSt), whichmakes CBE fully compatible with CB and in consequenceusable in all types of applications to replace CB, partiallyor totally. Production of high-quality CBE specialty fats isdelicate and can be faced by several problems that causefailures in many confectionery products: (1) inadequateremoval of SSS with the effect of undesirable waxinessbetween 35 and 40 7 C, (2) high retention of SUU-USUandUUU components with deterioration influence on the solid

fat content (SFC) profile, and (3) bad removal of diac-ylglycerols (DAG) with detrimental effect on the crystal-lization performance of the specialty fat [18]. Optimiza-tions of the multistage dry fractionation route of palm oil,together with the new developments in viscosity-resistantcrystallizers and in high-pressure membrane filterpresses, have led to a technology capable of producingPMF as good as those obtained by solvent fractionation.Classically, two routes (Fig. 8) are proposed for producing

the PMF: the olein route (more commonly followed in Asia) and the stearin route, which is preferably used inSouth America, because of the need of high-IV olein in thefirst fractionation stage. Best CBE are obtained throughthe olein route where SSU-SUS concentrates moreselectively in the soft PMF at the second fractionationstep; refractionation of this soft PMF produces an excel-lent hard PMF particularly enriched in SSU-SUS with asteep SFC profile. Practically, in the dry fractionated softPMF, SSU-SUS counts for more than 73%, the SSU-SUS/ SUU-USU ratio is 3–4 and the SSS content is low.Refractionation of this soft PMF produces an excellenthard PMF made of 85–90%SSU-SUS, with an SSU-SUS/ SUU-USU ratio of 9–12 and less than a few percent ofSSS; the DAG content can be maintained sufficiently lowto avoid any adverse effect on the crystallization proper-ties of the fraction [4].

Another main characteristic of palm oil is its red color dueto its particularly high content in b-carotene. Chemicalneutralization followed by deodorizing in mild conditionsis able to produce a refined palm red oil rich in carotenes,tocopherols and tocotrienols. Fractionation is classicallyoperated on the fully refined oil, but the high vitamin con-tent of crude palm oil makes the dry fractionation processan attractive route for the specially refined oil; as a matterof fact, carotenes, tocopherols and tocotrienols con-centrate markedly in the liquid fractions.

Such specially refined oil can be fractionated giving solid

and liquid red fractions [19]. The pigment concentration inthe liquid fractions is particularly important: b-carotene intop olein issued from the triple fractionation is nearly twicethe content in the neutralized oil, giving to this fractionvery high oxidative and cold stabilities and permittingpositive claims on the nutritional label. The red stearins aswell as red PMF find applications in the production ofmargarines, shortenings and CBE rich in vitamins for die-tetic use (Tab. 7).

Fig. 10. Correlation between IV and CP of liquid frac-tions in a multistage dry fractionation process of palmoil [17].


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Tab. 4. TAG composition (%) of palm oil, palm stearin, palm olein and palm super olein (M: myristic;P: palmitic; St: stearic; O: oleic; L: linoleic; S: saturated fatty acid; U: unsaturated fatty acid; IV: iodinevalue; nd: not detected).

Palm oilIV 52.3

Palm stearinIV 34.3

Palm oleinIV 56.7

Palm oleinIV 58.9

Palm super oleinIV 65.8

LLL 0.5 0.3 0.6 0.7 0.8PLL/MOL 2.7 1.5 0.6 0.6 0.3OOL 1.9 1.1 2.0 2.1 2.8PLO/SLL 10.7 5.9 12.0 12.9 16.6PLP/MOP 10.4 7.5 10.9 11.2 11.9MPP 0.4 2.0 4.0 4.4 5.4POO 22.7 12.9 24.5 26.3 33.4POP/PLSt 30.3 27.5 30.2 28.7 19.0PPP 6.1 26.5 1.7 nd ndStOO 2.5 1.5 3.1 3.3 3.8POSt 5.5 4.8 6.0 5.9 2.8PPSt 1.2 5.3 0.2 nd ndStOSt 0.7 0.5 0.4 0.3 nd

PStSt 0.1 0.6 nd nd ndUUU 6.0 3.4 6.7 7.2 9.0SUU-USU 38.6 21.8 42.9 46.1 57.1SSU-SUS 47.5 40.7 48.3 46.7 34.0SSS 7.9 34.1 2.1 nd nd

Tab. 5. Cocoa butter replacement fats (CBS: cocoa butter substitute; CBE: cocoa butter equivalent; CBR: cocoa butterreplacer) (La: lauric; M: myristic; P: palmitic; O: oleic; St: stearic; E: elaidic; S: saturated fatty acid; U: unsaturated fatty acid;CB: cocoa butter).

CBS: lauric oils rich in C36–C40 (LaLaLa, LaLaM, LaMM, etc. )Palm kernel stearin fraction (solvent or dry fractionated)?

not compatible with CB in the solid stateCBE: non lauric-based oils rich in SSU-SUS (POP, POSt, StOSt)

Cocoa butter, Illipe (natural fats)Palm oil fraction (hard PMF, solvent or dry fractionated)Sal fat, shea butter (stearin fraction, usually solvent fractionated or panned and pressed).? highly compatible with CB in the solid state

CBR: non lauric oils rich in ‘ trans ’ monounsaturated fatty acids (StEE, PEE, etc. )Partially hydrogenated oils (oils rich in SUU-USU triacylglycerols like soybean or sunflower).? partially compatible with CB in the solid state

Minor components like DAG, which cannot be

removed during the refining process of palm oil, haveseveral disadvantages during fractionation; TAG andDAG show eutectic interactions making separation dif-ficult and fractionation incomplete [20]. High-meltingDAG (mainly 1,3-dipalmitoyl glycerol) develop cloudi-ness upon storage of olein fractions at room tempera-ture [21]. As mentioned before, they have a detrimentaleffect on the crystallization performances of the speci-alty fats [18]. The effects of the DAG content of RBDolein on the IV and CP characteristics of super oleinsobtained after dry fractionation have been recently

studied [22]. It was shown that super olein obtained

from RBD olein with low DAG content (less than 1%)prevented a higher amount of SUU-USU in correlationwith better IV and CP, compared with non-treatedsuperolein (5% DAG).

By removing minor components like free fatty acids andmonoacylglycerols during refining, modifications of thephysical properties of palm oil and fractions are observed.Increased values of SFC, especially at lower tempera-tures, are reported for refined palm oil compared to crudeoil [23]. Additionally, CP of refined palm oleins are usually


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Tab. 6. Different CBE qualities from palm oil mid fractions(HPMF: hard palm mid fraction). §

Iodinevalue

POPcontent [%]

SFC [% max.]at 35 7 C

HPMF 1 28–30 .

80 6HPMF 2 32–35 . 70 3–6HPMF 3 35–37 . 60 2–5HPMF 4 37–40 . 50 1–4

§ By pulsed NMR using tempering method.

Tab. 7. b-Carotene content of chemically refined redpalm oil and fractions [19].

b-Carotene [ppm]

Palm oil 382Palm olein 409Palm super olein 670Palm top olein 854Palm hard stearin 280Palm soft PMF 235Palm hard PMF 80

lower than CP of crude palm oleins. The influence of therefining conditions has been studied, leading to theobservation that a tailing effect in the SFC profile wasobserved in palm oleins subjected to severe refining

conditions. Although CP are generally reduced by deo-dorizing, this tailing effect could be attributed to intra-esterification of the fatty acids during refining, leading toearlier crystallization of palm oleins upon storage. Theeffect of time and temperature on the SUS/SSU ratio hasbeen deeply studied by Jeffrey [24]: long residence timesat high temperatures results in intra-esterification makingthe processed palm oil unacceptable for production ofPMF.

6 Influence of filtration conditions onqualities of palm fractions

6.1 Vacuum, centrifugal and membrane pressfiltration

Compared with vacuum and centrifugal filtration, mem-brane press filtration has some important advantages:higher separation efficiency, higher tolerance to crystalmorphology changes, better protection against oxidation,faster filtration, and much lower energy consumption. Dueto the improved separation, the stearin is characterized

Tab. 8. Characteristics of palm fractions issued fromvacuum filtration, nozzle centrifuge separation and mem-brane press filtration (IV: iodine value; SFC: solid fat con-tent by pulsed NMR).

Vacuum

filtration(rotarydrum)

Nozzle

centrifugeseparation

Membrane press

filtration(squeezingpressure: 16 bar)

IV palm oil 52 52 52IV palm olein 56–57 56–57 56–57IV palm stearin 40–42 36–38 30–32SFC of crystallized oil [%] 11 11 11SFC of stearin cake [%] 43 – 65Olein yield [%] 72 76 82

by a lower IV ( i.e. more saturated), a higher melting point

as well as a steeper SFC profile. The liquid phase, on theother hand, is of at least the same and in most cases of abetter quality than that achieved in a vacuum filter. In caseof palm fractionation (first stage), the yield of centrifugalseparation is reported to be in between the yieldsobtained by a membrane press filter and a vacuum filter(Tab. 8).

6.2 Squeezing pressure in membrane pressfiltration

As already said, it is not possible to remove all the liquidphase from the solid phase with any of the commercialseparation techniques, due to liquid entrainment be-tween and within the crystals. Removal of liquid fromthe solid phase is more efficient in a membrane pressfilter as compared to a vacuum filter. This is mainly dueto the larger differential pressure applied. In the case ofpalm oil, with a standard membrane press filter operat-ing at 6 bar pressure, the olein yield (calculated on theolein fraction) easily increases by 10% compared withvacuum filtration. However, the separation efficiency ofa filter press cannot be defined only in terms of the dif-

ferential pressure applied. The origin of the fatty matter,as well as the conditions of crystallization and filtration,also affect the residual olein content in a stearin cake.The positive effect of squeezing pressure on the qualityof the solid palm fraction is shown in Tab. 9. The higherthe squeezing pressure, the less residual olein remainsin the cake. However, not all crystals can withstand highsqueezing pressures, resulting in partial or even totalpassage of the stearin cake through the filter cloth. A solution can be found in applying a lower pressure on athinner stearin cake.


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Tab. 9. Effect of squeezing pressure on separation effi-ciency and quality of palm oil fractions (SFC: solid fatcontent by pulsed NMR; IV: iodine value).

Squeezingpressure

SFC ofstearin cake

[%]

Yield [%] IV

Stearin Olein Stearin Olein

50 mm §

6 bar 55 23.6 76.4 39.7 57.115 bar 61 20.0 80.0 36.6 57.130 bar 65 18.3 81.7 34.7 57.125 mm §

6 bar 60 20.6 79.4 36.8 57.215 bar 66 18.8 81.2 34.9 57.230 bar 70 16.7 83.3 32.1 57.2

§ Chamber plate width.

7 Analytical methods used to determine thecharacteristics of palm fractions

A whole variety of analytical methods are used to deter-mine the characteristics of the fractions obtained as wellas to qualify the performance of the fractionation process[25]. Most of these methods can be found in the AOCSOfficial Methods and Recommended Practices handbook[26] or in the IUPAC Standard Methods for the Analysis of Oils, Fats and Derivatives [27].

The IV is a measure of the degree of unsaturation, usuallymade by titration (Wijs method). During crystallization, themore saturated and hence higher-melting TAG con-centrate in the solid phase, whereas the olein fractionbecomes enriched in more unsaturated TAG.

The change in IV is also a measure for the separationefficiency as it can be used to quantify the separation:

Yield stearin (%) = IVolein IVinitialIVolein IVstearin

6 100

The fatty acid and TAG composition is usually determinedby gas-liquid chromatography (GLC) or high-pressureliquid chromatography (HPLC). The fatty acid composi-tion also allows the calculation of the IV:

IV = 0.950 6 C16:1 1 0.860 6 C18:1 1 1.732 6 C18:2 12.616 6 C18:3 1 0.785 6 C20:1 1 0.723 6 C22:1

The CP and the cold test (CT) reveal certain aspects of thecrystallization behavior. They are predominantly used todefine the properties of the liquid fraction. The CP is anindication of the start of crystallization under the givenconditions of cooling. The CT, on the other hand, is a

measure of the resistance of the liquid fraction againstcrystal formation at a certain temperature for a certainperiod. When properly calibrated, the CP can be used topredict the cold stability.

The stearin fraction is more commonly characterized byits melting behavior ( e.g. slip melting point, clear meltingpoint, dropping point). These parameters, however, onlygive an indication of the end of melting. They are notrepresentative of the overall melting behavior. The deter-mination of the solid fat index (SFI) by dilatometry and theSFC by nuclear magnetic resonance (NMR) allows a morequantitative definition of the solid-phase behavior of thedifferent oil fractions. Today, the SFC method is morefavored than the SFI method, due to its higher accuracyand its ease of application. NMR can also be applied tofollow quantitatively the crystallization behavior of the oilor fat during the fractionation process (see Fig. 4). Also, it

permits to quickly determine the yield in stearin, simply bymeasuring the SFC of the slurry prior to filtration and ofthe stearin cake after filtration.

Yield stearin (%) = SFC slurrySFC cake

6 100

Differential scanning calorimetry (DSC) reveals by far themost direct information about the melting and crystal-lization behavior of an oil or fat. The interpretation of theDSC thermograms, however, is not easy, due to themultiple polymorphic transitions that may occur duringmelting and crystallization. Although the technique has

a large potential, standardization is necessary to use itas a routine technique in the oils and fats processingindustry.

8 Conclusions

The fractionation of oils, fats and their derivatives isbasically a rather simple technology, but it is based on acomplex crystallization process which, till today, is stillonly partially understood. Especially the dry fractionationprocess whereby the oils and fats are submitted to a

controlled cooling and hence crystallization, followed bya selective separation using filtration, has largelyimproved over the last years, enabling processors to dryfractionate nearly any type of oil, fat or derivative. Theongoing developments of more powerful dynamic crys-tallizers and efficient high-pressure filter presses and theimplementation of new principles of fractionation, suchas controlled static crystallization, continuous counter-current crystallization and centrifugal separation, are fur-ther expanding the applicability of the fractionationtechnology.


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