Malaysian Journal of Analytical Sciences, Vol. 7, No. 1 (2001) 93-96
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Effects of Monoglycerol Stearate and Free Fatty Acids on the Melting Point of Palm Olein
Dzulkefly K1, Wan Rusmawati W M3, Wen-Huei Lim2, Anuar K1, and Hamdan S4
1Department of Chemistry, Universiti Putra Malaysia (UPM)
2Advanced Oleochemical Technology Centre (AOTC), Malaysia Palm Oil Board (MPOB)
3Department of Chemistry, Universiti Pendidikan Sultan Idris (UPSI)
4Faculty of Science and Technology, University College Terengganu
(Received 6 September 2000)
Abstract. RBD palm oil was fractionated by using various solvents to obtain olein and stearin fractions. The triglyceride (TG) content of palm olein was then determined to find the best solvent for the fractionation process. The effects of monoglyceride (MG) and free fatty acid on the melting point of the fractionated palm olein were also studied. The result showed that acetone was probably the best solvent for fractionation of palm oil in respect to olein yield and triglyceride content. On the other hand, the presence of MS (monoglycerol stearate) increased the melting point of palm olein, whereas the free fatty acid reduced it.
Abstrak. Pemeringkatan minyak sawit RBD untuk mendapatkan pecahan olein dan stearin telah dilakukan dalam berbagai pelarut. Kandungan trigliserida (TG) dalam pecahan olein kemudian ditentukan untuk mendapatkan pelarut terbaik bagi proses pemeringkatan tersebut. Kesan kehadiran monogliserida (MG) dan asid lemak bebas ke atas suhu lebur pecahan olein juga dikaji. Keputusan menunjukan bahawa pelarut asiton mungkin yang terbaik bagi pemeringkatan minyak sawit tersebut jika dilihat dari peratus olein dan kandungan TG. Kehadiran MS (monoglisirol stearat) telah didapati menaikan suhu lebur olein sawit tersebut, tetapi sebaliknya, asid lemak bebas didapati menurunkan suhu lebur.
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Key words: palm oil, palm olein, melting point, free fatty acid, fractionation
Introduction
Palm oil is a complex mixture of glycerides but can be fractionated into liquid olein and solid stearin fractions [1]. Olein fraction is used for frying purposes (both in the industry and at home) mainly in tropical climates [2]. However, high-quality cooking oil is expected to remain liquid during storage.
Malaysian palm olein has relatively higher melting point of 22-24oC [3], thus its tends to crystallize during storage at temperature below 20oC. The crystallization of palm olein is caused by the presence of high-melting triglycerides and non-oil components, namely monoglyceride (MG), diglyceride (DG) and free fatty acids. These components have significant effect on the melting point of palm olein and their existence in palm olein depend greatly on the efficiently of the fractionation process. In a solvent- fractionation process, the polarity of solvent used will determine the amount of MG and DG retained in the fractionated palm olein, thus its melting point. The DG contents of palm olein were found between 4-6 % and 5.1-5.8 % when fractionated using acetone and hexane solvents respectively at temperatures between 19-20oC [4]. It was also reported that during fractionation, diglycerides inhibited crystallization that led to loss of olein (low yield of olein). On the other hand, saturated MG increases the melting point
of vegetable oil, whereas with unsaturated MG, the melting point decreases. Recently, monounsaturated triglycerides (POP, POS) are known causing early crystallization of palm olein during storage at temperature below 25oC [5].
There is limited report on the effect of various solvents on the properties of fractionated palm olein. In this paper, we report the effects of solvent on the yield and triglyceride content of fractionated palm olein. The effects of adding monoglyceride and free fatty acids on the melting point of fractionated palm olein were also studied.
Materials and Method Materials
Malaysian RBD palm oil and palm olein were obtained from a local oleochemical company.
Standard triolein was purchased from Sigma Chemical Co. and monoglycerol stearate was obtained from Palm Oleo, Rawang. The organic solvents used were: diethyl ether, ethylacetate, n-pentene, petroleum ether, n-hexane, acetone, cyclohexane, toluene, isooctane and propanol. All solvents were of high purity and used as received.
DZULKEFLY K et al.: EFFECTS OF MONOGLYCEROL STEARATE AND FREE FATTY ACIDS
Methods
Fractionation of RBD palm oil into palm olein and palm stearin
RBD palm oil was heated at 70oC until it melt completely before used. Samples of heated palm oil (about 10 g) were then dissolved in various organic solvents, at ratio of palm oil to solvent of 1:10 (v/v) and held at the fractionation temperature (4oC) for 24 hours to allow complete crystallization.
The liquid fraction (palm olein) was then separated from the solid stearin by vacuum filtration.
The solid stearin was washed with 3x20ml cold solvent to remove traces of olein. Palm olein, obtained after removal of solvent by rotary evaporation, and the solid stearin were then dried under vacuum to remove traces of solvent and weighted.
The triglyceride content of the fractionated palm olein was determined by using HPLC (JASCO UV-970) equipped with a normal phase silica column (ECONOSIL SI10U, 250 mm length and 4.6mm i.d).
A mixture of isooctane:propanol (94:6, v/v) was used as mobile phase with a flow rate of 1ml/min.
The triglyceride was detected using UV detector set at 213 nm. The external standard method was used to determine the concentration of triglyceride in olein fraction and a calibration curve was prepared using pure triolein.
Effect Of Monoglycerol Stearate And Free Fatty Acids On The Melting Point Of Palm Olein.
RBD palm olein sample (about 5.0 g) was mixed with various percentage of monoglycerol stearate (MS) before being melted at 70oC to homogenize. The mixture was then freezed at 0oC for 24 hours before subjected to differential scanning calorimetry (DSC) analysis. The weight percents of monoglycerol stearate used were: 0%, 2%, 4%, 6% and 8%. The melting point of the mixtures was determined from the DSC curve. The same procedure was repeated using lauric and oleic acids. The weight percents of acid added to palm olein were: 0%, 2%, 4%, 6% and 8%.
Results and Discussion
Palm oil consists of mainly triglycerides of different polarity. It can be fractionated into liquid fraction (olein) and solid fraction (stearin) using organic solvent. The polarity of solvent used is greatly influenced the yield of olein and stearin fractions.
Table 1 shows the yield of olein and stearin fractions after fractionated using various organic solvents.
Hexane gave the highest percentage of olein (88.4%), followed by cyclohexane (86.7%), petroleum ether (85.3%) and others. It was also observed that sedimentation occurred in the olein fractions obtained by using diethyl ether, toluene and cyclohexane after a few days storage at room temperature. The sediment was probably due to incomplete crystallization.
Table 1: Effect of solvent on the yield of palm olein and palm stearin fractions and the triglyceride (TG) content of palm olein
Solvent Yield of Olein (%)
Yield of Stearin (%)
TG Content of Olein Fraction (%)
Acetone 73.6 26.4 84.1
Cyclohexane 86.7 13.3 81.4
n-Hexane 88.4 11.6 78.1
Petroleum ether 85.3 14.7 65.5
n-pentane 80.0 20.0 65.5
Ethyl acetate 79.3 20.7 62.9
Diethly ether 77.0 23.0 82.9
Toluene 83.8 16.2 64.8
A good palm olein quality should contained high TG content, and low non-oil components.
Results from this study showed that fractionation using acetone gave the highest TG content (84.1%) in olein fraction, whereas toluene gave the lowest (64.8%). Other solvents - cyclohexane (81.4%) and diethyl ether (82.9%) obtained more than 80% TG.
Based on the high yield of olein (86.7%) and TG content (81.4%), cyclohexane should be the best solvent for fractionation of palm oil. However, cyclohexane was not able to remove completely from the product, therefore, acetone is more suitable solvent for fractionation of palm oil.
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DZULKEFLY K et al.: EFFECTS OF MONOGLYCEROL STEARATE AND FREE FATTY ACIDS
A B C
D A: palm olein + 0% MS
B: palm olein + 2% MS C: palm olein + 4% MS D: palm olein + 6% MS
Figure 1: DSC thermograms of fractionated palm olein with adding monoglycerol stearate (MS).
Effect Of Monoglycerol Stearate And Free Fatty Acid On The Melting Point Of Palm Olein
The effect of monoglycerol stearate (saturated MG) in fractinated palm olein was study by differential scanning calorimetry (DSC). The melting point of fractionated palm olein obtained in this study was very low, about 5.7oC, as compared to that of normal palm olein, which is about 22-24oC obtained from fractionation at 22oC. Figure 1 shows a typical DSC thermogram of fractionated palm olein in the presence of monoglycerol stearate (MS). It was observed that the melting curve of palm olein was shifted to the high temperature region with increasing amount of
Figure 2 : Effect of monoglycerol stearate (MS) on the melting point of palm olein
MS. Figure 2 shows a linear correlation between the melting point of palm olein and the percentage of MS.
In contrary, the melting point of palm olein decreases with free fatty acid. Figure 3 depicts a significant reduction of melting point with increasing the percentage of lauric (C12:0) and oleic (C18:1) acids respectively. Lauric acid caused greater reduction in melting point as compared to oleic acid.
The main reason was probably due to the shorter fatty chain of C12 as compared to C18:1. Therefore, the observed melting point of palm olein is actually a result of a combination of these two effects, i.e.
partial glycerides and free fatty acid contents.
0 2 4 6 8
0 2 4 6 8
% of lauric and oleic acids Temperature, oC
4.5 5 5.5 6 6.5 7
0 2 4 6 8
% MS Temperature, oC
10 10
Figure 3: Effect of lauric (♦) and oleic (•) acids on the melting point of palm olein.
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DZULKEFLY K et al.: EFFECTS OF MONOGLYCEROL STEARATE AND FREE FATTY ACIDS
Conclusion
Acetone is the best solvent for fractionation of palm oil because it gives the highest triglyceride (TG) content in olein fraction. The presence of unsaturated monoglyceride in palm olein resulted an increase of melting point, whereas free fatty acids reduced the melting point.
References
1. Deffense, E. (1985). Fractionation of palm oil, J.
Amer. Oil Chem. Soc., 62: 376-386
2. Pantzaris, T.P. (1988). Pocketbook of palm oil uses, PORIM, Malaysia
3. Selected Reading On Palm Oil And Its Uses, by Malaysia Palm Oil Board (MPOB), 1993
4. Goh, E.M and Timms, R.E. (1985). Determination of mono- and diglycerides in palm oil, olein and stearin, J. Amer. Oil Chem. Soc., 62: 776-781 5. Swe, P.Z., Che Man, Y.B., Ghazali, H.M and Wei,
C.S. (1994). Identification of major triglycerides causing the clauding of palm olein, J. Amer. Oil Chem. Soc., 71: 1141-1144
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