Full text - Palm oil consists of by fjwuxn


									Journal of Oil Palm Research Vol. 12 No. 1, June 2000, p. l-7

     DIFFERENTIAL                                                         he melting and crystallization beha-
                                                                          viour of palm oil triglycerides was
       SCANNING                                                           characterized by differential scan-
                                                               ning calorimetry (DSC).      The influence that
    THERMOGRAMS                                                diglycerides have on the melting and cooling

      OF PALM OIL                                              behaviour of palm oil is dependent on the
                                                               concentration and nature of dig-lye&-ides. All the
    TRIGLYCERIDES                                              diglycerides (PP, PO and 00) and the palm

         IN THE                                                diglyceride mixture (PDG) depressed the melting
                                                               point of the oil. The 1,2 isomer was more effective
     PRESENCEOF                                                than the 1,3 isomer. An excess of 1,3 PP elevated
                                                               the meltingpoint. The crystallization behaviour
     DIGLYCERIDES                                              is complicated if 1,3 PP is present in high
                                                               concentrations. All the diglycerides studied can
                                                               undergo co-crystallization with palm triglyce-
                                                               rides, except for 1,3 PP at high concentration.


        Keywords: Diglycerides, palm oil, melting,
      crystallization differential scanning calorimetry.
                                                               P      alm oil consists of four main triglycerides
                                                                     - POP and POO, together forming about
                                                               53%-57% of total triglycerides, POL 8%-11%,
                                                               and PPP 5%-6% (Sassano and Jeffrey, 1993;
                                                               Hamirin, 1981). In addition, it also contains
                                                               about 4%-7.5% diglycerides (Jacobsberg, 1976;
                                                               Goh and Timms, 1985; Siew and Ng, 1995).
                                                               Under ambient conditions, palm oil appears as
                                                               a heterogeneous slurry of crystals in a liquid oil.
                                                               The two phases are distinct and can be clearly
                                                               observed in their DSC crystallization and melt-
             SIEW, W L* and NG, W L**                          ing curves. The cooling therm,ogram of palm oil
                                                               shows two exotherms which correspond to hard
                                                               and soft components as discussed by Ng and Oh
                                                               (1994). Fractionation of the oil gives a liquid
                                                               fraction with one sharp exotherm and a solid
                                                               fraction, stearin, with a complex crystallization

                                                                   The high diglyceride content in palm oil
    Malaysian Palm Oil Board,
                                                               affects its crystallization properties (Okiy et al.,

    P.O. Box 10620, 50720 Kuala Lumpur. Malaysia.
*+ 15. Hujan Bubuk, Overseas Union Garden,                     1978; Okiy, 1978). In other oils, diglycerides
   58200 Kuala Lumpur, Malaysia.                               retard their polymorphic change from one crys-


tal form to another (Hernquist et al., 1981;           PP from Sigma Chemicals, USA and rat 1,3 PO,
Hernquist and Anjou, 1983) with the saturated          1,2 PO, 1,3 00 and 1,2 00 from Larodan Fine
1,2 isomer the more effective in doing so (Van         Chemicals, Sweden were mixed separately with
Sabine Walnett et al., 1991). In this study, the       the PTG oil. The concentrations prepared were
melting and crystallization behaviour of palm          1% and 5% for 1,2 00 and 1,3 PP and 2.5%,
oil under dynamic conditions was evaluated in          5% and 10% for the other standard diglycerides.
the presence of different diglycerides.                For 1,2 00, only a 2.5% mixture was prepared.
                                                       At the 5% and 10% levels, cloudy solutions
                                                       Thermal Analysis by DSC
Sample Preparation
                                                           DSC analysis of the samples was performed
    Palm oil was purified using column chroma-         with a Perkin Elmer DSC-7 instrument
tography with a 2.5 cm diameter and 40 cm long         (Norwalk, CN, USA). The sample was com-
column. About 30 g of oil were dissolved in            pletely melted at 80°C before being weighed
120 ml petroleum ether. Silica gel (30 g) was          (10 mg) into a aluminium pan which was -then
mixed with sufficient petroleum ether and the          sealed using a sample pan crimper. The thermal
mixture transferred to the chromatographic             history of the sample was erased by heating it
column. The sample was eluted through the              to 80°C in the DSC instrument and holding it
column with 1 litre petroleum ether-diethyl            for 10 min. The sample was then cooled to -30°C
ether (95:5 v/v). The solvent was removed from         at a rate of 40°C min.‘. To obtain the heating
the eluate under vacuum in a rotary evaporator.        thermogram, the sample was held at -30°C for
The purity of the triglycerides (PTG oil) was          10 min and then heated to 80°C at the rate of
checked by HPLC using AOCS official Ce 5b-             10°C min.‘. The reference cell used in the
89 method.                                             analysis was an empty aluminum pan. The
    Diglyceride from palm oil (PDG) was                temperature scale was calibrated with gallium
obtained from the same column after all the            and indium. For the cooling thermogram, the
triglycerides had been eluted. The eluant              sample was held at 80°C for 10 min and then
(900 ml) consisting of petroleum ether-                cooled to -30°C at the rate of 10°C mini’.
diethylether (85:15 v/v) was added to the co-
lumn The solvent was removed by rotary
evaporator from the PDG extract. The PDG                           RESULTS AND DISCUSSION
consisted of62.3% palmitoyloleoyl-glycerol (PO),
23.2% dioleolylglycerol (00) and 14.4%                 The melting and crystallization peaks of the
dipalmitoylglycerol (PP).                              pure diglycerides and PDG are given in Table
    PTG oil was mixed with PDG in the propor-          1. While the pure diglycerides had only one
tions of 2.5%, 5% and 10% by weight. Individual        endotherm and exotherm, indicative of their
diglycerides (> 99% pure) of 1,3 sn - PP and 1,2       purity, PDG showed three fusion and crystal-

    Diglyceride     Fusion peak       Melting point       Crystallization     onset of
                       CT)                 (“C)           peak PC)            crystallization (“C)
      1,2 PP          50.7                 52.5             47.4                  50.0
      1,3 PP          71.5                 75.0             62.5                  65.0
      1,2   0 0       22.7                 26.0              3.2                  10.0
      1,3   0 0       24.6                 28.3             10.9                  15.7
      1,2             51.5                 54.0             47.5                  49.8
      1,3             41.7                 45.3             28.0                  30.6
      PDG             22.8, 30. 4 3        47.0             -15, 9. 2 6 . 7       32.0


lization peaks. This was expected since the              triglycerides. The bulk of the triglycerides crys-
mixture contained 62.3% PO, 23.2% 00 and                 tallizes out in the second exothermic peak. In
14.4% PP. The onset ofcrystallization was 32°C           the presence of PDG, the first peak becomes
close to that of 1,3 PO. Similarly, the first            broader and shifts slightly to a lower tempera-
crystallization peakwas at 26.7”C, being so close        ture (Figure I). In the case of added PO or 00,
to that of 1,3 PO as to suggest that the exotherm        this exothermic peak shifts to 12.3”C and 11.9”C
represents the crystallization of the 1,3 PO, the        respectively Wgures 2 to 4). This shift to a lower
main diglyceride in the mixture. The fusion              temperature is in compliance with the melting
peak also reflects that of the 1,3 PO. The various       point depression effected by the diglycerides.
diglycerides showed a large range in melting             On the other hand, the broadening of this peak
and crystallization behaviour. The 1,2 and I,3           probably indicates that the high-melting com-
positioning of the fatty acids in the diglycerides       ponent triglycerides which crystallize out to-
gave differences in the melting behaviour,               gether, had been affected to a different extent
indicative of polymorphism.                              by the diglycerides. In the case of added PP, the
    The DSC cooling thermograms showed                   effect is rather abnormal (Figure 5). For in-
differences caused by the diglycerides. In the           stance, in the presence of a low level (1%) of
thermogram ofPTG oil, there are two exothermic           the 1,3 isomer, the highest exothermic peak is
peaks, the first at 14°C and the second at -0.3”C        considerably reduced. At 5%, this peak dimin-
(Figure I). The first peak is sharp and repre-           ishes in intensity and an additional peak is
sents the crystallization of the higher-melting          observed at a higher temperature of 27°C. This

                                                                         Temperature (“C)

  Figure 1. DSC thermograms of PTG oil with                Figure 2. DSC thermograms of PTG oil with
                  added PDG.                                              added 1,3 PO.


new peak can be attributed to the crystalliza-
tion of excess 1,3 PP. Also, 1,3 PP has a much
higher crystallization temperature than 1,2 PP
(Table 1). In addition, it is probable that some
of the high-melting triglycerides of palm oil may
co-crystallize with the 1,3 PP. In contrast, the
1,2 isomer does not exhibit similar effects to
those of the 1,3 isomer. It behaves just like the
rest of the diglycerides studied. The above
results show that all the diglycerides studied,
with the exception of 1,3 PP at high concentra-
tion, can easily undergo co-crystallization with
the PTG oil.
    In the melting thermogram of PTG oil, there
are two broad endotherms which are well se-
parated from each other. The low-melting
endotherm shows a sharp-peak at 4.9”C (Figure
1). The melting characteristics of PTG oil are
affected to a different extent by the different
types of diglycerides, and the effect is more
significant at higher concentrations. Because of
the rapid cooling of the fat, it probably crystal-
lized in the alpha form. During melting, poly-
morphic transition takes place as can be seen
in the thermograms.
    In the case of added PDG, the high-melting
endotherm diminishes in intensity and shifts to                          Temperature (“C)
a lower temperature, thus showing a distinct
effect of melting point depression. In the low-            Figure 3. DSC thermograms of PTG oil with
melting endotherm, the sub-peak at 4.9”C di-                             added I,2 PO.
minishes in intensity and eventually disap-
pears, causing the endotherm to be much sharper
with the mean peak around 3°C (Figure I). Since          nevertheless distinct, with an intensity propor-
PDG contains 62% PO, it is not surprising that           tional to the concentration of 1,2 PO. This
the crystallization and melting thermograms              suggests that the endotherm is due to the
are quite similar to those of 1,3 PO.                    melting of the diglyceride or its mixture with
    In the case of added palmitoyloleoyl                 some palm triglycerides.
diglyceride, there was a difference in the effect            In the case of 1,3 00 (Figure 41, the effect
of 1,2 and 1,3 isomers. For 1,3 PO (Figure 2),           is rather complex. The high-melting endotherm
the broad high-melting endotherm narmws                  shifts to a lower temperature with a concurrent
down, with diminishing intensity, showing the            split into two peaks. The intensity of the low-
effect of melting point depression. The low-             temperature peak continues to increase while
melting endotherm also narrows down, with a              that of the high temperature peak diminishes,
concurrent increase in intensity resulting in the        There is hardly any effect on the low-melting
peak becoming very sharp. In the presence of             endotherm except that the peak at 4.9”C is
the 1,2 isomer (Figure 31, the high-melting              shifted slightly higher and eventually merges
endotherm shifts to a lower temperature which            with the low temperature peak of the high-
is indicative of the melting point depression.           melting endotherm. The 1,2 00 is unusual
This peak also becomes narrower. The low-                because of its low solubility in PTG. The pres-
melting endotherm becomes narrower and                   ence of a small amount of dissolved 1,2 00 does
sharper while a new peak emerges between the             not seem to have any significant effect except
two endotherms. This peak, although small, is            for the melting point depression.


                                                                30   -20   ~10     0   10   20   30     40   50   60   70   R”

                Temperature (“C)                                                 Temperature     (“C)

  Figure 4. DSC thermograms of PTG oil with                 Figure 5. DSC thermograms of PTG oil with
                  added 00.                                                  added PP.

     The 1,2 and 1,3 isomers of PP exert different        unequal intensity. Thus, while 1,2 PP forms an
effects on the melting behaviour of PTG oil               eutectic mixture with the PTG oil, this is not
(Figure 5). As shown in Table 1, the 1,3 isomer           so for the 1,3 isomer of PP as it melts separately.
melts and crystallizes at a much higher tem-                  In conclusion, the melting and crystalliza-
perature than the 1,2 isomer. With added 1,3             tion behaviour of palm oil is affected greatly
PP, the high-melting endotherm narrows down              by its diglyceride content. As the effect is
with its higher end shifting to a lower tempera-         concentration dependent, palm oil with more
ture. However, a new peak then appears at the            consistent properties can be produced if the
temperature just above the high-melting                  variation in diglyceride composition can be
endotherm. This peak not only increases in               minimized or better controlled.
intensity but also shifts to a high temperature               The melting point of an oil ,is an important
as the concentration of 1,3 PP increases. This           parameter as it determines its physical state at
new peak is undoubtedly due to the diglyceride,          any temperature. It is one of the specifications
and the fact that the peak position depends on           normally required in the vegetable oil industry.
the composition suggests intermolecular inter-           In this study, the melting points of PTG oil and
actions between 1,3 PP and the saturated                 its fractions were determined from the DSC
triglycerides in the PTG oil. In the case of the         melting thermograms (Table 21. The melting
1,2 isomer, the effect is mainly on the high-            point of PTG oil was 44.5”C, compared to the
melting endotherm, which becomes much nar-               average slip melting point of palm oil at 36.0
rower and splits into two overlapping peaks of           (Tan and Oh, 1981). In general, all the indi-


     TABLE 2. MELTING POINT OF PALM                        The variable diglyceride contents of crude
    TRIGLYCERIDES IN THE PRESENCE OF                   and refined palm oil and their fractions obvi-
              DIGLYCERIDES                             ously have an effect on their melting points.
Diglyceride content             Melting point          This was observed in the slip melting point of
                                                       palm oil which varied from 32°C to 39°C (Siew
       Nil                           44.5              et al., 1993). There was no significant correlation
                                                       between melting point and iodine value (Timms,
       1,3 PP                                          1985) for crude palm oil. This somewhat sur-
       1%                            45.0
                                                       prising observation can be attributed to the free
                                                       fatty acids and diglyceride contents.
       1.2 PP
       1%                            40.0
       5%                            38.0                           ACKNOWLEDGEMENTS

       1,3 00                                          The authors thank the Director-General of
       2.5%                          44.0              MPOB for permission to publish this paper.
       5%                            43.0              Thanks are also due to the staff of the Analyt-
       10%                           42.0              cal, Physics and Chemistry Groups for their
                                                       technical support.
       2.5%                          44.0

       1,3 PO
       2.5%                          41.0
       5%                            40.0              BERGER, K G and WRIGHT, W B (1976).
       10%                           40.0              Crystallization behaviour of palm oil. Paper
                                                       presented at ISF Conference, Marseilles.
       1,2 PO
       2.5%                          39.5              GOH, E M and TIMMS, R E (1985). Determi-
       5%                            39.0              nation of mono- and diglycerides in palm oil,
       10%                           38.5              olein and stearin, J. Am. Oil Chem. Sm., 62: 730.
       2.5%                          39.5
       5%                            39.0
                                                       HERNQUIST, L and ANJOU, K (1983).
       10%                           39.0              Diglycerides as stabilisers of the 8’ crystal form
                                                       in margarines and fats. F&e Seifen Anstrichm,
                                                       85: 64-66.
vidual diglycerides, as well as PDG, depress the
melting point of PTG oil. The 1,2 isomer is more       HERNQUIST, L; HERSLOF, B; LARSSON, K
effective than the 1,3 isomer in melting point         and PODLAHA, 0 (1981). Polymorphism of
depression. This effect of melting point depres-       rapeseed oil with low content of erucic acid and
sion has been attributed by other workers (Okiy        possibilities to stabilise the p’ crystal form in
et al., 1978; Okiy, 1978; Berger and Wright,           fats. J. Sci. Food Agric, 32: 1197.1202.
1976) to the formation of eutectic mixtures
between diglycerides and triglycerides. How-           HAMIRIN, K (19811. Studies on palm oil with
ever, 1,3 PP is an exception as it elevates the        special reference to interesterification. Ph.D
melting point of PTG oil. From the DSC melting         thesis, University of St. Andrews, U.K. p. 26.
thermograms, an additional peak at the highest
temperature was observed. This peak is well            JACOBSBERG, B and OH, C H (1976). Studies
separated from the endotherm of the PTG oil,           in palm oil crystallisation. J. Am. Oil Chem SOL,
implying that no eutectic mixture was formed           53: 609-617.
between 1,3 PP and the component triglycerides
in PTG oil.                                            NG, W L and OH, C H (1994). A kinetic study


on isothermal crystallisation by solid fat content       SIEW, W L; CHONG, C L; TAN, Y A; TANG,
measurements. J.Am. Oil Chem. Sot., 71: 1135-            T S and OH, C H (1993). Identity characteristics
1139.                                                    of Malaysian palm oil products: fatty acid and
                                                         triglyceride composition and solid fat content.
OKIY, D A; WRIGHT, W B; BERGER, K G and                  Elaeis, 5: 38-46.
MORTON, I D (1978). The physical properties
of modified palm oil. J. Sci. Food Agric., 29:           TIMMS, R E (1985). Physical properties of oils
1061-1068.                                               and mixtures of oils. J. Am. Oil Chem. Sot. 62:
OKIY, D A (1978). Interaction of triglycerides
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                                                         TAN, B K and FLINGOH OH, C H (1981).
                                                         Malaysian palm oil, chemical and physical
SASSANO, G H and JEFFREY, B S J (1993).                  characteristics. PORZM Technology No. 3, Palm
Gas chromatography of triacylglycerols in palm           Oil Research Insitute of Malaysia, Bangi.
oil fractions with medium polarity wide-bore
columns. J. Am. Oil Chem. Sot., 70; 1111-1114.           VON SABINE WAHNELT; MANSEL, D and
                                                         TULSNER, M (1991). Influence of isomeric
SIEW, W L and NG, W L (1995). Diglyceride                diglycerides on phase transitions of cocoabutter
content and composition as indicators of palm            Investigations by isothermal DSC. Fat Sci.
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FATS and OILS. AOCS Official Method Ce 5b-
89 Triglycerides in Vegetable Oils by HPLC.


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