ion and Melting Behaviour of Palm

8/7/2019 ion and Melting Behaviour of Palm

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WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001 0931-5985/2001/1111-0729 $17.50+.50/0

Eur. J. Lipid Sci. Technol. 103 (2001) 729734 729

1 Introduction

Lauric oils such as coconut, palm kernel, babassu areused extensively both in the food industry and in the oleo-chemical industry. The high demand for lauric oils andtheir price variation has caused considerable interest andhence research is carried out to look for alternativesources via gene manipulation. Nature has endowed thecoconut and the oil palm to produce oils with a high con-tent of medium-chain triacylglycerols. In the case of the oilpalm, two types of oil are extracted from the fruits. Theouter mesocarp gives palm oil while the inner kernel pro-duces the kernel oil. Palm kernel oil (PKO) can be frac-tionated to produce a harder fraction, palm kernel stearin(PKS), used as cocoa butter substitute and a liquid frac-tion, palm kernel olein (PKOO). These products and thekernel oil are hydrogenated to various degrees to furtherreduce the iodine value of the oil. The chemical composi-tion and physical properties of some of these productshave been documented [1, 2]. Earlier works by Bezard [3], Bezard et al. [4] and Cornelius [5] have documentedthe composition of palm kernel oil only. In the trade, theoils are characterised by their respective iodine value,which is an important parameter to specify the oil compo-

sition and quality. The iodine value of PKO lies for exam-ple between 16 and 19 and the end users often scrutinisesamples having higher iodine values for probable conta-mination. On the other hand, PKS specifications arebased on a maximum iodine value of 8. While there isenough information on the composition of the oils, there is

however, scarce information in the literature on the crys-tallisation and melting behaviour of these oils. This paperexamines the crystallisation and melting behaviour ofsome of these products using differential scanningcalorimetry (DSC) and nuclear magnetic resonance(NMR).

2 Materials and methods

2.1 Materials

PKO and its fractions were obtained from local refineries Southern Edible Oil Sdn Bhd and Soctek Sdn Bhd,PKOO and PKS from dry fractionation processes. Thehydrogenated oils (HPKO, HPKOO and HPKS) were ob-tained by hydrogenation of the respective oils. All chemi-cals used were either of analytical or high-performanceliquid chromatography grade.

2.2 DSC analysis

DSC analyses of the oils were performed with a Perkin

Elmer 7 DSC ( Perkin Elmer , Norwalk, CN, USA). Prior toweighing (10 mg) each sample was completely melted at80 C into an aluminium pan, which was then sealed us-ing a sample pan crimper. Heating the samples to 80 Cin the DSC instrument and keeping it for 10 min at thistemperature erased the previous history of the sample.The samples were cooled to 30 C at a rate of

40 C/min. At the end of the period, the sample washeated in steps of 5 C/min to 80 C for 10 min and thencooled again in steps of 5 C/min to 30 C. The meltingand cooling thermograms were recorded.

Wai Lin Siew

Malaysian Palm Oil board(MPOB)

Crystallisation and melting behaviour of palmkernel oil and related products by differentialscanning calorimetryLauric oils are valuable sources for oils suitable for various food applications. They are

particularly useful as cocoa butter substitutes for which steep solid fat content profilesare required. Palm kernel oil is one such fat, which upon fractionation and/or hydro-genation provides a variety of oil fractions with different oil composition and properties.The stearins have excellent properties for confectionery fats, while the oleins can befurther hydrogenated to improve their properties. This paper gives an overview of theproperties of products of palm kernel oil, produced from fractionation and hydrogena-tion. The melting and crystallisation properties from differential scanning calorimetrystudies are discussed in relation to the triacylglycerols of the oils.

Keywords: Crystallisation, melting, differential scanning calorimetry, palm kernel oilproducts.

Correspondence: Wai Lin Siew , Malaysian Palm Oil board(MPOB), P.O.Box 10620, 50720 Kuala Lumpur, Malaysia.Phone: +603-8928-2430, Fax: +603-8925-9446; e-mail:[email protected]


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2.3 Fatty acid composition

The methyl esters were prepared by transesterificationof the sample with 1 M sodium methoxide [6]. 0.95 mln-hexane were added to the oil sample (6 drops or

50 mg) in a 2 ml vial using a graduated pipette. The mix-ture was well shaken and 0.5 ml sodium methoxide wereadded. The vial was shaken vigorously for 5 s with thehelp of a vortex mixer. At first the mixture appeared clear,but it turned turbid when sodium glyceroxide precipitated.After 5 min the clear upper layer of methyl ester waspipetted off for GC analysis.

Quantification of the peak areas was carried out with aHewlett-Packard HP5890 II gas chromatograph. Theanalysis conditions corresponded to those described bySiew and Ng [7]. A correction based on analysis of a ref-

erence mixture or methyl esters of known composition(20A from Nuchek Prep Inc., Nuchek , Elysian, MN, USA)was used for calculation of the weight percentage of thetotal mass.

2.4 Triacylglycerol (TAG) composition by highperformance liquid chromatography (HPLC)

TAG analyses were performed with Gilson 303 and 302pumps, a Waters 410 differential refractometer and aHewlett Packard 3396A integrator. The two columns were25 cm long, had an i.d. of 4 mm and were packed with5 m Lichrosphere RP18 ( Merck , Darmstadt, Germany).The columns were kept in an oven at 30 C. The mobilephase was acetonitrile : acetone (65:35, v/v) at a flow rateof 1.0 ml/min. Injection was achieved through a Rheo-dyne valve fitted with a 20 l loop. Samples were injectedas 10% (w/v) solutions in warm acetone. Identification ofthe TAGs was made by comparison with those of avail-able standards, purchased through Sigma Chemicals.The TAGs were expressed as equivalent carbon number(ECN) in weight percentage of the total TAGs.

2.5 Solid fat content by NMR

The samples were melted and filled into the sample tubes(10 mm o.d. x 75 mm length) up to a height of 3 cm. Priorto measurement the samples were melted at 70 C for30 min, chilled at 0 C for 90 min and then held at 10 Cfor 30 min. One tube was held at each measuring tem-perature for 30 min. The measuring temperatures were10, 15, 20 and 25 C. The instrument used was a BrukerMinispec PC 120 pulsed NMR. The percentage of solidsdetermined by pulsed NMR may be defined as the ratio ofthe response obtained from the hydrogen nuclei in thesolid phase and the response arising from all the hydro-gen nucleic in the sample. The direct display methodwas employed during the measurements. In the directmethod, magnetisation is sampled at two time intervals one shortly (11 s) after excitation of the magnetisationand another one after a longer interval (70 s). The mea-surement after the short interval provides the magnetisa-

tion of both liquid and solid phase. The repeatability ofmeasurements for the direct method is 1.0.

3 Results and discussion3.1 Fatty acid and triacylglycerols

The fatty acids and TAGs composition of the oils used inthis study are presented in Tabs. 1 and 2. The foundcompositions are consistent with that given by Siew andBerger [1] and Tang et al. [2]. Thus the oil used in thisstudy can be regarded as representative of the oils gen-erally obtained in Malaysia. The fractionated products,the kernel olein and stearin have compositions reflectingthe more liquid or solid nature of the oil. The main differ-ence between PKO and PKOO is the lower content of lau-ric and myristic acids and the higher content of oleic acidin the latter. In contrast the content of other acids, includ-ing palmitic acid, is roughly the same. Similarly, the maindifference between PKS and PKO is the content of thefatty acids, lauric, myristic and oleic acids, which is higherfor the two former acids and lower for the latter. Hydro-genated samples are easily distinguished by their highstearic acid content and by the absence of unsaturatedacids. The differences between hydrogenated and non-

730 Wai Lin Siew Eur. J. Lipid Sci. Technol. 103 (2001) 729734

Tab. 1. Fatty acid composition of palm kernel oil products.

C6:0 C8:0 C10:0 C12:0 C14:0 C16:0 C18:0 C18:1 C18:2

[wt-%]

PKO 0.3 3.9 3.4 49.9 16.5 8.1 2.8 12.9 2.1PKOO 0.1 4.0 3.6 45.6 14.4 8.2 2.3 18.6 3.1PKS 0.1 2.0 2.8 58.1 22.1 7.4 1.6 5.0 0.8HPKO 0.1 3.0 3.2 49.1 16.7 8.6 19.2 HPKOO 0.2 4.4 3.5 44.3 14.1 8.3 25.1 HPKS 0.1 1.9 2.8 56.6 22.0 7.9 8.6


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hydrogenated samples become apparent from the ab-sence of triacylglycerols containing double bonds. PKO,PKOO and PKS include only 4%, 2.8% and 6.7%, of ECN42, respectively, whereas the hydrogenated samples in-clude at least 9% of that TAG. Similarly the hydrogenatedsamples contain about 34% more ECN 44 TAG thannon-hydrogenated samples. The highest difference is

found for ECN 48, which is mainly due to contributions ofC18:0, C18:0, C12:0 and C14:0, C16:0, C18:0. Two newTAGs are present in the hydrogenated samples, ECN 50and ECN 52, likely originating from. C16:0, C16:0, C18:0and C16:0, C18:0, C18:0, respectively. These TAGs areonly present at levels of less than 3%.

3.2 Solid fat contents

PKO (Tab. 3) has a high content of solids at low tempera-tures but a low one at 25 C. A simple fractionation

process produces further a stearin where the content ofsolids is enhanced at low temperatures which shows asteep melting curve and complete melting at 35 C.

Thus this product is highly valued in the confectionery in-dustry. The low content of solids of PKOO can be furtherincreased by hydrogenation. Among the hydrogenatedsamples, the sharpest melting profile is shown by HPKS,followed by HPKO. HPKOO tends to be softer at lowertemperatures and also has some tailing of solids at highertemperatures. The observed lower tailing in the HPKS is areflection of the lower ECN 48 to ECN52 content in the oil.

3.3 DSC analyses

Tab. 4 shows the data of the DSC analyses of palm kerneloil products while the thermograms are illustrated inFigs. 1 and 2. When PKO is cooled at steps of 5 C/min tosub ambient temperatures, it crystallises rapidly at tem-

Eur. J. Lipid Sci. Technol. 103 (2001) 729734 Crystallisation and melting behaviour of palm kernel oil 731

Tab. 2. Triglyceride Composition of palm kernel products.

Triglycerides PKO PKOo PKS HPKO HPKOo HPKSECN : n

[wt-%]

C26 1.07 0.71 0.51 0.82 1.4 2.2C28 0.64 0.85 0.45 0.71 0.99 1.32C30:2 0.44 C30:1 0.91 C30 1.41 1.11 0.51 1.44 1.92 0.95C32 6.11 7.63 3.52 6.76 8.36 3.67C34:1 0.54 0.74 C34 8.36 9.32 6.85 8.62 9.69 6.56C36 21.35 18.61 28.21 21.68 18.63 26.04C38:2 0.62 1.03 C38:1 0.8 1.38 C38 15.23 10.57 24.42 16.39 12.33 22.93C40:2 0.88 C40:1 4.55 7.62 2.71 C40 8.64 5.41 14.32 9.62 7.16 13.7C42:2 0.84 1.21 C42:1 5.12 6.66 2.44 C42 4.11 2.75 6.7 9.61 9.68 9.11C44:1 3.44 4.5 C44 4.39 5.25 0.95 7.06 8.08 5.24C46:2 2.38 2.06 2.04 C46:1 1.78 2.3 2.8 C46 2.01 2.44 0.93 5.54 6.86 3.37C48:3 1.92 2.05 1.34 C48:2 1.78 2.19 0.49 C48:1 0.97 1.25 0.43 C48 0.24 0.59 0.38 6.46 8.51 2.85

C50:2 0.47 C50:1 0.34 0.41 C50 2.67 3.43 1.35C52 2.61 2.97 0.69


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heated in steps of 5 C/min the melting profile is broadand ill-defined, finally melting at 50 C. The slip meltingpoint for this oil is 41 C.

Hydrogenation of palm kernel products results in moreproducts available to the confectionery industry. This isestablished by Rossells work [8], which also shows thedifferences in thermograms of HPKO hydrogenated to dif-ferent degrees. In the present study the difference ob-served between hydrogenated and non-hydrogenated

sample is the crystallisation temperature, which is 5.6 Cfor PKO and 27.7 C for HPKO. In the thermogram ofHPKO the larger exotherm represents the lower meltingfraction. The fatty acid composition of PKOO and HPKOOdiffers considerably, whereby the hydrogenated samplecontains much more stearic acid. It appears that a highstearic acid content is not sufficient to allow sharp crys-tallisation. The thermogram of the sample shows a highmelting and a lower melting exotherm instead. Probablythe sample could be refractionated to gain the requiredproperties.

The main TAGs involved in the crystallisation of PKS areECN 36, ECN 38 and ECN 40. Presumably ECN 40, melt-ing at a higher temperature than ECN 36 and ECN 38, isthe major TAG involved in the crystal nucleation. BesidesECN 42 and ECN 44 the above-mentioned TAGs are in-volved in the crystallisation of PKO. Next in line assisting

cristallisation are the TAGs with one and two doublebonds. The high-melting and low-melting peaks of PKOOreflect these TAGs and the composition of PKOO is nowmore evenly spread between the saturated and unsatu-rated TAGs. In the hydrogenated samples crystallisationis effected by the high-melting TAGs ECN 48, ECN 50and ECN 52 and is observed in the small crystallisationexotherms of HPKO and HPKOO and to a lesser extent inHPKS. Only about 4.9% of ECN 48, ECN 50 and ECN 52are present in HPKS as compared with 12-15% in HPKOand HPKOO. The DSC thermograms also revealedrelatively simple curves, characteristic of fats with littleforms of polymorphism. The X-ray data (Tab. 5) showonly polymorphs for all samples, indicating the strong tendencies of such fats. The polymorphs were obtainedupon cooling oil samples from melting temperatures totemperatures of 20, 30 and 40 C and maintaining themat these temperatures for 10 wk. The melting thermo-grams of PKS and HPKS show polymorphic transforma-tions at 20 and 25 C, respectively, from the unstable formto a more stable form. The polymorphic forms for the re-spective peaks shown in the DSC thermograms were notdetermined.

4 ConclusionsThe melting and crystallisation behaviour of PKO, PKOOand PKS and their hydrogenated samples can be evalu-ated from their TAG compositions. Two exotherms ob-served for PKO, PKOO and the hydrogenated samplesreflect the higher and lower melting TAGs. In the hydro-genated samples the higher melting TAGs are ECN 48 toECN 52 and for the natural PKO and its fractions theTAGs are ECN 40 to ECN 44. The strong polymorphsshown for all samples suggest that these TAGs or theircombinations are strongly in nature.

Acknowledgements

The authors thank the director general, MPOB for permis-sion to publish this paper and Dr Chong Chiew Let and hisstaff for kind assistance in NMR and DSC analyses.

References

[1] W. L. Siew, K. G. Berger : Malaysian palm kernel oil: Chemi-cal and physical characteristics. PORIM Technology 6(1986).

Eur. J. Lipid Sci. Technol. 103 (2001) 729734 Crystallisation and melting behaviour of palm kernel oil 733

Fig. 2. DSC melting thermograms of palm kernel oil prod-ucts (scan at 5 C/min).


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[2] T. S. Tang, C. L. Chong, M. S. A. Yusoff : Malaysian palm ker-nel stearin, palm olein and hydrogenated products. PORIMTechnology 16 (1995).

[3] J. A. Bezard : The component triacylglycerols of palm kerneloil. Lipids 6 (1971) 630634.

[4] J. A. Bezard, J. P. Moretain, M. Bugaut : The saturated sn-2-triacylglycerols of palm kernel oil. Fette Seifen Anstr. Mittel79 (1977) 399407.

[5] J. A. Cornelius : Palm kernel and palm kernel oil. Prog. ChemFats and other Lipids 15 (1977) 524.

[6] Malaysian Standard MS 252: Part 21: 1994 Animal and veg-etable fats and oils: Rapid method for the preparation ofmethyl esters of fatty acids.

[7] W. L. Siew, W. L. Ng : Characterisation of crystals in palmolein. J. Sci Food Agric. 70 (1996) 212216.

[8] J. B. Rossel : Fractionation of palm kernel oil. J Am. OilChem. Soc. 62 (1985) 385390.

[Received: February 20, 2001; accepted: July 24, 2001]

734 Wai Lin Siew Eur. J. Lipid Sci. Technol. 103 (2001) 729734

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ion and Melting Behaviour of Palm