Influence of Monoglycerides on the Crystallization Behavior of Palm

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Influence of Monoglycerides on the Crystallization Behavior of Palm Oil Eveline Fredrick,*,† Imogen Foubert,† John Van De Sype,‡ and Koen Dewettinck† Laboratory of Food Technology and Engineering, Ghent UniVersity, Coupure Links 653, 9000 Ghent, Belgium, and Cargill R&D Centre, HaVenstraat 84, 1800 VilVoorde, Belgium

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 6 1833–1839

ReceiVed January 9, 2007; ReVised Manuscript ReceiVed January 9, 2008

ABSTRACT: Differential scanning calorimetry (DSC) and time-resolved X-ray diffraction (XRD) were used to elucidate the mechanism of isothermal palm oil crystallization at 25 °C and gain more insight into the effect of monoglycerides (MGs) on the palm oil crystallization behavior. At this temperature, it was confirmed that palm oil first partially crystallizes from the melt in R crystals before β′ crystals are formed by a polymorphic transition. Subsequently, additional β′ crystals are formed directly from the melt. MGs of sunflower oil (MGSFO) only accelerate the polymorphic transition whereas MGs of hydrogenated palm oil (MGHPO) additionally promote the nucleation of the R crystals. The homogeneity between the fatty acids of MGHPO of palm oil and the higher degree of saturation of MGHPO are suggested as the main causes for the larger effectiveness in accelerating palm oil crystallization. Introduction Palm oil and its fractions have frequently been used in transfat-free products as an alternative source of hard stock in margarines and shortenings. The frequent use of palm oil results from its tendency to form β′ crystals, which are required for the smooth texture in those products.1,2 This beneficial effect is most likely related to the unique chemical composition of palm oil. It differs from all other vegetable oils in its high level of palmitic acid,3 its significant amount of saturated fatty acids at the sn2-position of the triglycerides (TGs)4,5 and its liability to develop a higher free fatty acid and diglyceride content during harvesting.6 In addition, palm oil has a lot of practical advantages because of its high productivity, low price, high thermal and oxidative stability, plasticity at room temperature, and accessibility to fractionation.4,5,7–9 The disadvantages of using palm oil at the industrial level, on the other hand, is its considerably slower crystallization rate5,10,11 and the unusually long R-lifetime6,12 compared to other natural fats. Adding emulsifiers could possibly be an answer to those weaknesses since earlier studies evidence their effect on the crystallization behavior of palm oil and its fractions.11,13–15 Despite the complex mixture of TGs in palm oil,4 only two groups of crystallization peaks have been distinguished during cooling using differential scanning calorimetry (DSC) corresponding with a high melting (“hard”) fraction and a low melting (“soft”) fraction.10 The crystals of the high melting fraction are generally rich in disaturated and trisaturated TGs, whereas the low melting fraction is mainly rich in monosaturated TGs.16 The isothermal crystallization behavior of palm oil is, on the contrary, more complex. From various studies,17–19 it appears that the crystallization behavior changes depending on the crystallization temperature (degree of supercooling). Several authors describe a cutoff temperature. At temperatures lower than the cutoff temperature, a two-step crystallization occurs and above the cutoff point the crystallization takes place in only one step. However, there is disagreement in literature on the exact cutoff temperature and the details of the two crystallization * Corresponding author. E-mail: [email protected]. Tel: 0032/ 9.264.61.62. Fax: 0032/9.264.62.18. † Ghent University. ‡ Cargill R&D Centre.

steps. Kawamura17 observed a two-step crystallization associated with a polymorphic transition below 24 °C using DSC. Ng and Oh18 located the cutoff point at 25 °C using nuclear magnetic resonance (NMR) spectrometry and DSC. According to Chen et al.,19 using viscometry and polarized light microscopy, the cutoff point is 22 °C. In the latter study, the two-step crystallization was attributed to crystallization of first the R crystals and second the β′ crystals directly from the melt resulting in a coexistence of the R and β′ polymorph. Only direct crystallization in the final β′ polymorph occurs at temperatures where a one-step crystallization was observed. The differences in the cutoff temperature could possibly be ascribed to the differences in sensitivity of the monitoring systems used19 or by slight variability in the composition (major and minor components) of the palm oil depending on its origin (region and climate) and extraction and refining conditions applied.6,16 Mazzanti et al.20 studied the isothermal crystallization of palm oil under static conditions and under shear at 17 and 22 °C. They concluded that at both temperatures R nucleates from the melt and after some time the R crystals act as nucleation sites for the formation of β′ crystals, which grow at the expense of the R crystallites as well as from the melt. Apart from the TG composition, the physical properties of fats can be influenced by minor components present by nature or added on purpose such as emulsifiers. Emulsifiers are amphiphilic molecules containing a hydrophobic and a hydrophilic moiety. The hydrophobic moiety, mostly fatty acids, is attracted by the TGs in the fat system depending on the chain length and the degree of saturation homogeneity between the TGs and the emulsifiers while the hydrophilic moiety performs a repulsive power. Depending on the miscibility of the emulsifiers and the fat, the emulsifiers can interact (retard or accelerate) with the nucleation, crystal growth, and/or polymorphic transitions.21 The emulsifiers can act as impurities and, thereby, perform a catalytic action resulting in an acceleration of the heterogeneous nucleation. The rate of crystal growth alters as a consequence of adsorption of the emulsifiers on the fat crystal surface or inclusion in the fat crystals. Considering the interaction of emulsifiers with the polymorphic transition, emulsifiers were initially used to retard polymorphic transitions. Studies with tristearin later showed that emulsifiers can accelerate the polymorphic transition as well.22 Distinction is made between

10.1021/cg070025a CCC: $40.75  2008 American Chemical Society Published on Web 04/29/2008

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liquid and solid emulsifiers. The liquid emulsifiers always accelerated the polymorphic transition in a tristearin system. They provide a higher mobility in the tristearin crystals, thus promoting the polymorphic transition. Solid emulsifiers performed an ambiguous effect. They can both retard or accelerate polymorphic transitions. The different effects are attributed to the capacity of emulsifiers to create hydrogen bridges between the hydroxyl group of the hydrophilic head of the emulsifier and the carbonyl group of the neighboring TGs. Emulsifiers that are capable to form hydrogen bounds retard and the others accelerate the polymorphic transition because of the increasing mobility of the TGs caused by the absent fatty acids in the emulsifiers compared with TGs.22 In this study, the effect of monoglycerides (MGs) is investigated. In general, it is known that MGs can autoassociate to inverse micelles and these are suspected to act as templates for crystallization (heterogeneous nucleation).24 Niiya et al.25 found that saturated MGs increased the melting point and accelerated the crystal growth slightly in hydrogenated soybean and palm kernel oil. In the presence of unsaturated MGs, the melting point decreased and crystal growth was seemingly accelerated. Sambuc et al.26 concluded that the addition of a mixture of monopalmitin and monostearin decreased the induction time of vegetable fats. Smith et al.27 and Smith and Povey28 discussed a trilaurin model system. The crystal growth rate increased in the presence of monolaurin, while the crystal growth rate was hardly affected by the MGs with chain length deviating from lauric acids. Foubert et al.29 investigated the temperature and concentration dependent effect of monoolein and monostearin on milk fat crystallization. Monostearin interacts with the nucleation and crystal growth rate dependent on the degree of supersaturation, which is determined both by temperature and concentration. For monoolein, a more straightforward relation was detected. The crystal growth rate is enhanced, whereas the induction time is less affected. Research papers regarding the role of the MGs on the polymorphic transition of fats (in general) and the effect on palm oil crystallization behavior are quite limited. Only Miura et al.30 concluded that pure monoolein and monopalmitin did not affect the solid fat content (SFC) curve at the considered temperature (5 °C) and concentration (1%). This study applies a combination of techniques to elucidate the disagreement on the mechanism behind the two-step crystallization of palm oil and to investigate the effect of two different commercial types of MGs (2%) on the nucleation, crystal growth, and polymorphic transitions during the isothermal crystallization of palm oil. Experimental Section Refined Bleached and Deodorized Palm Oil. The palm oil used in the experiments was supplied by Cargill (Vilvoorde, Belgium). Monoglycerides. Dimodan distilled monoglycerides (MGs) produced from a glycerolysis reaction with sunflower oil (MGSFO) or hydrogenated palm oil (MGHPO) were used. After destillation, the products contain a minimum of 90% MGs. Preparation of the Palm Oil-Monoglyceride Blends. MGs (2% w/w) were dispersed in the melted palm oil and stirred with a magnetic stirrer at 70 °C until a homogeneous sample was obtained. When the blend was visibly free of dispersed material, an additional mixing was done for at least 2 h. Fatty Acid Composition. Fatty acid methyl esters (FAME) were produced according to the AOCS official method Ce 2–66 and, subsequently, analyzed on a Varian GC (Varian, Sint-Katelijne Waver, Belgium) with a WCOT CP-sil88 column, split injector, and FID according to the AOCS official method Ce 1–62. Each analysis was executed in triplicate. Table 1 gives the fatty acid composition of palm oil, MGHPO, and MGSFO.

Fredrick et al. Table 1. Fatty Acid Composition of Palm Oil, MGHPO, and MGSFO C C 12:0 14:0 palm oil std. dev. MGHPO (%) std. dev. MGSFO (%) std. dev.

0.18 0.01 0.39 0.01 0.00 0.00

C 16:0

C 18:0

C 18:1

C 18:2

C 18:3 others

1.12 46.64 4.18 37.79 7.56 0.16 0.01 0.13 0.02 0.21 0.10 0.02 1.26 46.72 50.89 0.16 0.02 0.00 0.01 0.04 0.07 0.01 0.02 0.00 0.09 7.68 5.37 21.31 64.01 0.11 0.01 0.04 0.04 0.19 0.32 0.11

0.33 0.20 0.55 0.07 0.31 0.24

Differential Scanning Calorimetry. The differential scanning calorimetry (DSC) experiments were performed with a Q1000 DSC (TA instruments, New Castle, DE). The DSC was calibrated with indium (TA Instruments, New Castle, DE), azobenzene (Sigma-Aldrich, Bornem, Belgium) and undecane (Acros organics, Geel, Belgium) prior to analysis. Nitrogen was used to purge the system. Palm oil and the palm oil-MGs blends were sealed in hermetic pans and an empty pan was used as a reference. For the isothermal crystallization, the following time–temperature (t-T) program was applied: holding at 70 °C for 15 min to ensure a complete liquid state erasing the crystal memory, cooling at 8 °C min-1 to the isothermal crystallization temperature (25 °C), and keeping at this temperature until crystallization was finished. For the stop-and-return method, the same pan as for the isothermal crystallization experiment was used. The procedure is basically identical to the isothermal crystallization experiment except that the sample is heated after a given isothermal period, prior to completion of the crystallization, with a heating rate of 20 °C min-1 until 70 °C. This heating rate was chosen to prevent the occurrence of polymorphic transitions as much as possible and slow enough to prevent thermal lag. All DSC analyses were performed in triplicate. DSC profiles were analyzed with the Universal Analysis software version 4.1 (TA instruments, New Castle, DE). Time-Resolved Synchrotron X-ray Diffraction. Time-resolved synchrotron X-ray diffraction (XRD) measurements were performed on the Dutch-Belgian (DUBBLE) beamline BM26B at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). The experiments were performed at a fixed wavelength λ of 1.24 Å. A curved 1D microstrip gas chamber detector was used for wide-angle X-ray diffraction (WAXD), whereas a 2D multiwire gas-filled detector was used for small-angle X-ray scattering (SAXS). The samples were enclosed into a perforated aluminum DSC cup and the gap was covered by thin mica. The temperature was controlled by a Linkam hot stage. The following t-T program was applied: heating to 70 °C, holding at that temperature for 15 min, cooling at 8 °C min-1 to the crystallization temperature (25 °C), and keeping at that temperature until crystallization was completed. Scattering patterns were taken every 30 s from the start of the isothermal period. The known reflections of silverbehenate and silicon powder were respectively used to calibrate the SAXS and WAXD scattering angles, 2θ. In the case of SAXS, intensities are presented as a function of s, with s ) 2 sin(θ/λ). All scattering patterns were corrected for the detector response and normalized to the intensity of the primary beam which is measured by an ionization chamber placed after the sample. The 2D SAXS powder patterns were radial averaged to yield 1D patterns and, finally, a melt pattern was subtracted as a background. For WAXD, only the melt scattering pattern was subtracted.

Results and Discussion The isothermal crystallization of palm oil was followed at 25 °C using DSC. To gain more insight in the isothermal crystallization mechanism, the crystallization process was interrupted at different moments and the samples were heated (stopand-return experiments). The obtained melting profiles give an indication of the amount of crystallized fat (peak area) and the occurrence of polymorphic transitions (by shifts in peak maximum). Time-resolved synchrotron XRD experiments were performed, allowing unambiguous identification of the DSC crystallization and melting peaks. By merging the information obtained from the different experimental techniques a potential isothermal crystallization mechanism of palm oil at 25 °C was derived. The three different techniques were further used to

Influence of MGs on Behavior of Palm Oil

Figure 1. Isothermal crystallization of palm oil at 25 °C as measured by DSC.

establish the effect of addition of 2% commercial monoglycerides (MGs) on the nucleation, crystal growth and polymorphic transition. Isothermal Crystallization Behavior of Palm Oil as Measured by DSC. Figure 1 shows the isothermal crystallization curve of pure palm oil at 25 °C as measured by DSC. The DSC curve shows one exothermal peak (between 1.5 and 55 min isothermal time) with a clear shoulder before the peak maximum. The shoulder can possibly indicate a two-step crystallization where the peaks are not completely separated which can be attributed to an overlap of two crystallization steps. Such multiple-step crystallization curves can result from either the formation of different polymorphic forms, the crystallization of different fractions or the combination of both.31 In literature disagreement exists about the cause of the two-step crystallization of palm oil. Kawamura17 and Mazzanti et al.20 attributed the two-step crystallization to a polymorphic transition during the isothermal crystallization but Chen et al.19 maintained a separated crystallization of two different polymorphs, which finally coexist. Stop-and-Return Experiments of Palm Oil as Measured by DSC. The principle of the stop-and-return technique is to interrupt the isothermal crystallization at different moments and, subsequently, heat the sample to obtain the melting profile of the crystals formed during the considered isothermal period. Figure 2 gives the melting profiles in which the isothermal crystallization at 25 °C is interrupted every 5 min. Remarkable is the bending at lower temperatures (30-35 °C) of the melting curves after 5 and 10 min isothermal crystallization (shown by the encircling in the figure). Because palm oil is already partly crystallized after 5 min isothermal period (Figure 1), the bend can possibly originate from the melting of crystals with a lower melting point than those present after 15 min isothermal crystallization. The absence of the bending after 15 min isothermal crystallization can be associated with the disappearance of the crystals with a lower melting point. It is thus very plausible that a polymorphic transition takes place from a metastable to a more stable polymorph causing the two-step crystallization. After 15 min of isothermal crystallization, no shifts of the peak maxima and a steady growth of the peak can be observed as the isothermal period rises. The increasing peak area suggests that after the polymorphic transition is complete, extra crystals are formed from the melt. Isothermal Crystallization Behavior of Palm Oil as Measured by Time-Resolved Synchrotron XRD. To identify the polymorphs formed during the isothermal crystallization of palm oil at 25 °C, we subjected palm oil to time-resolved synchrotron XRD. SAXS and WAXD diffraction patterns were simultaneously recorded during the isothermal period. SAXS

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diffraction patterns provide information about the layer thickness (d) or the long spacings of the crystals. The long spacings allow the assignment of double (2L) or triple (3L) spacings of TGs and depend on the chain length of fatty acids and the angle of tilt formed between the chain axes and the basal plane formed by the end methyl groups.32 The latter thus allows to identify polymorphic transitions. WAXD diffraction patterns yield short spacings which are unequivocally related to the three basic polymorphic forms (R, β′, and β).32 The combination of both techniques has been shown to be important in recent evolutions in the field of phase behavior of pure TGs33–35 and commercial fats.36–41 Figure 3 gives the SAXS diffraction patterns of the isothermal crystallization of palm oil at 25 °C at different moments in time. To keep the figure clearly structured, only a few frames are selected and presented. The first peak, at s ) 0.0215 Å-1 (d ) 1/s )46.51 Å), appears after 1.5 min isothermal crystallization, indicating the start of the crystallization. This peak grows until 7.5 min isothermal crystallization. Simultaneously, WAXD diffraction patterns with a peak at 4.15 Å can be established (data not shown), which is unequivocally connected with an R polymorph.32 Therefore, the first peak in Figure 3 coincides with the long spacing of an R polymorph. After 7.5 min crystallization the first peak starts to vanish and simultaneously a second peak appears at 0.0237 Å-1 (42.19 Å). Concurrently, the WAXD diffraction patterns showed that the peak at 4.15 Å moves to 4.20 Å and, in addition, a second peak appears at d ) 3.8 Å (data not shown). The WAXD diffraction patterns typically conform to the β′ polymorph.32 The second peak of the SAXS diffraction pattern is thus connected to the β′ polymorph. The decrease in long spacings (or shift to a higher s-value) can be explained by the tilted fatty acid chains with repect to the methyl end group plane in the β′ polymorph. It is remarkable that the patterns of the frames between 7.5 and 15.5 min in Figure 3 pass through one single point, known as the isosbestic point. Such an isosbestic point indicates that the R crystal structure is transformed to the β′ crystal structure without a change in the total volume occupied by the R and β′ phase together. The existence of an isosbestic point during isothermal crystallization was previously observed in cocoa butter41 and milk fat.42 After 15.5 min isothermal crystallization the SAXS patterns do not pass through that point anymore. The isosbestic point can thus only be temporarily observed. The β′ crystallization proceeds even though all R crystals are already transformed. The extra β′ crystals can only originate directly from the melt. Proposed Mechanism. Based on the results from the different techniques, a potential mechanism of the isothermal crystallization of palm oil at 25 °C can be proposed. The crystallization process can be divided into different isothermal stages which can be justified by the different techniques used in this study: Stage I: Supercooled melt. No crystallization peak in DSC and no crystalline reflection patterns can be established in SAXS diffraction patterns, confirming that no crystals are formed or present in this first stage. Stage II: Direct crystallization of R crystals from the melt. An exothermic peak in the crystallization curve and a bending at low temperatures (endothermic) in the melting profile of the stop-and-return method by means of DSC can be observed. In the SAXS diffraction patterns a single peak associated with the R form is established during this period. Stage III: R-β′ polymorphic transition. The metastable R crystals vanish and the stable β′ crystals grow. The disappearance of the bending in the melting profile recorded via the stopand-return method by DSC and the vanishing of the R peak

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Figure 2. Melting curves of the stop-and-return method in which the isothermal crystallization of palm oil was interrupted at 5 (___), 10 (-----), 15 (___.), 20 (__--), 25 (__ __) and 30 (___ -) minutes as measured by DSC.

Figure 3. SAXS diffraction patterns of the isothermal crystallization of palm oil at 25 °C.

and the appearance of the β′ peak in the SAXS diffraction pattern confirms the R-β′ polymorphic transition. The occurrence of an isosbestic point in the SAXS diffraction patterns implies that the β′ crystals grow only at the expense of the R crystals and no β′ crystals are formed directly from the melt. Stage IV: Extra β′ crystals are formed directly from the melt. The continued steady growth of the melting profiles of the stopand-return method by the DSC and the absence of an isosbestic point until the end of the crystallization evidence the formation of extra β′ crystals that were not previously crystallized in the metastable R polymorph.

The fact that palm oil first crystallizes in the less stable R polymorph can be explained as follows. The R polymorph has a lower critical activation free energy for nucleation and a higher nucleation rate despite the larger difference in chemical potential and thus the larger driving force for crystallization for the more stable polymorphs. Therefore, the R polymorph is kinetically favored and starts to crystallize consuming part of the high melting fraction available for the R phase. Mazzanti et al.20 claimed that after some time the R crystals act as nucleation sites for the formation of the β′ crystals, which grow at the expense of the R crystals, and extra β′ crystals can be formed

Influence of MGs on Behavior of Palm Oil

Figure 4. Isothermal crystallization of palm oil, the MGSFO blend, and the MGHPO blend.

out of the melt. This study confirms Mazzanti’s findings and, in addition, elucidates the order of the different stages in the crystallization mechanism of palm oil. Isothermal Crystallization of Palm Oil with Monoglycerides Measured by DSC. In this study, commercially available MGs of hydrogenated palm oil (MGHPO) and of sunflower oil (MGSFO) were selected to study the influence of 2% (w/w) MGs on the crystallization behavior of palm oil. Figure 4 gives the isothermal crystallization profiles of the pure palm oil, the MGHPO blend and MGSFO blend. The crystallization curve of the MGSFO blend shows a shape comparable to the one of pure palm oil. Only a slightly earlier appearance of the peak maximum and a steeper slope of the second step can be observed. Addition of MGHPO changes the crystallization profile to a larger extent. Crystallization takes place from the beginning of the isothermal period. As a consequence, no induction time is measurable for the isothermal crystallization of the MGHPO blend at 25 °C. Besides, a second peak can be observed in the crystallization profile (between 5 and 30 min). Since no shoulders in the two peaks are present, it can be concluded that the isothermal crystallization occurs in two fairly separated steps, which is in contrast to two overlapping steps in pure palm oil and in the MGSFO blend. Stop-and-Return Experiments of Palm Oil with Monoglycerides as Measured by DSC. The melting profiles, after interruption of the isothermal crystallization at 25 °C, are recorded for both the MGSFO and the MGHPO blend. No large differences in the melting profiles of the pure palm oil and the MGSFO blend can be noticed. In both cases, a bending at low temperatures after 5 and 10 min crystallization appears (data not shown). In the melting profiles of the MGHPO blend (Figure 5), on the contrary, a remarkable difference after 5 and 7.5 min isothermal crystallization can be noticed compared to those of pure palm oil (Figure 2). In the melting profile after 5 min isothermal crystallization, a small peak at low temperature emerges (30-35 °C) instead of a bending. Its appearance demonstrates that a larger amount of TGs are crystallized at that moment compared with pure palm oil. After 10 min, a broader peak becomes visible at higher temperatures (35-50 °C), and no peak is observed at lower temperatures. The disappearance of the bending at low temperature in the MGSFO blend and the shift of the peak maximum to higher temperatures in the MGHPO blend is most probably caused by a polymorphic transition similar to palm oil. After 15 min of isothermal crystallization, no further shifts of the peak maximum and a steady growth of the peak in both the MGSFO and MGHPO blends can be observed as the isothermal period rises. The increasing peak area suggests that

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after a polymorphic transition extra crystals are formed from the melt, also similar to palm oil. Isothermal Crystallization Behavior of Palm Oil with Monoglycerides as Measured by Time-Resolved XRD. Figure 6 shows the SAXS diffraction patterns of the isothermal crystallization of the MGHPO blend at 25 °C at different moments in time. When 25 °C is reached, only one peak is present at 0.019 Å-1 (52.59 Å) and remains unchanged during the isothermal period. This peak can probably be attributed to the crystallization of the MGs prior to formation of the palm oil crystals. After 0.5 min isothermal crystallization, a second peak appears at 0.215 Å-1 (46.51 Å) in the SAXS diffraction patterns simultaneous with an increasing intensity of the 4.15Å peak corresponding with the R polymorph in the WAXD diffraction pattern (data not shown). The second peak in Figure 6 corresponds thus to R crystals. This peak grows until 4.5 min isothermal crystallization and subsequently vanishes rapidly favoring a third peak at 0.237 Å-1 (42.19 Å). In the WAXD diffraction patterns, the peak at 4.15 Å shifts to 4.20 Å and a 3.8 Å peak appears. An R-β′ polymorphic transition takes place. Similar to palm oil, the SAXS diffraction patterns show a temporary appearance of an isosbestic point during polymorphic transition. Therefore, palm oil and MGHPO blends show a comparable crystallization mechanism. Influence of MGs on the Crystallization Kinetics. Although it has been shown above that the crystallization mechanism of palm oil is similar to that of the MGHPO and MGSFO blends, discrepancy can be made in terms of kinetics. No induction time for the crystallization of the MGHPO blend is observed, whereas the onset of crystallization is not affected by the addition of MGSFO. In general, it is expected that MGs can autoassociate as reverse micelles.24 These micellar structures can be regarded as liquid crystals having a certain order causing a decrease in the energy barrier for the nucleation of TGs. In addition, in the case of adding MGHPO not only micellar structures can be formed but also crystallization of the MGs occurs (Figure 6 at peak 0.019 Å-1) because of its higher melting point compared to MGSFO. These MG crystals may act as seeding material and are thus more effective catalytic impurities for heterogeneous nucleation then the micellar structures. The differences in melting point (solubility) and thus crystallization point of MGHPO and MGSFO explain therefore partly the differences in effect between the two types of MGs on the nucleation stage. The micellar structures of MGSFO were thus not sufficient to cause earlier nucleation, which is possibly due to the large difference in fatty acid composition of MGSFO and the palm oil. The larger the homogeneity between the fatty acids of the MGs and the palm oil, the larger the effect of the MGs. The higher crystallization rate of the first step in the MGHPO blend can be explained by the larger amount of nucleation sites, which implies the simultaneous formation of a larger quantity of smaller crystals. The difference in the size of the crystals is also reflected in the SAXS diffraction patterns. It is known from the Scherrer equation that the broader the diffraction peaks, the smaller the crystals are as described for milk fat in Lopez et al.43 Comparing Figures 3 and 6, it can be observed that the SAXS diffraction peaks of the MGHPO are broader then those of pure palm oil. The larger amount of nucleation sites could also result in a larger amount of crystallized TGs in the R polymorph, as can be concluded from the melting profiles (Figures 2 and 5). In both the MGSFO and MGHPO blends, the polymorphic transition is accelerated. The start of the polymorphic transition in the MGHPO blend is even earlier than in pure palm oil. This

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Figure 5. Melting curves of the stop-and-return method in which the isothermal crystallization of the MGHPO blend was interrupted at 5 (___), 7.5 (-----), 10 (___.), 15 (__--), 20 (__ __) and 30 (___ -) min as measured by DSC.

Figure 6. SAXS diffraction patterns of the isothermal crystallization of the MGHPO blend at 25 °C.

can probably be explained by an adsorption of MGs at the crystal surface or an incorporation of MGs in the crystal lattice. The presence of those liquid or solid MGs in the crystals results in defects in the crystal lattice increasing the mobility of neighboring TGs due to the absence of fatty acids in MGs compared to TGs.22 Conclusion Using DSC and time-resolved XRD, it was shown that when crystallizing palm oil at 25 °C R crystallization proceeds the formation of the β′ crystals. The β′ crystals were formed as a result of first a polymorphic transition and second an ongoing crystallization of β′ crystals from the melt. A similar mechanism was detected for the MGHPO and the MGSFO blends. Dis-

crepancy can be made between the two MGs. MGSFO only accelerates the polymorphic transition, whereas MGHPO also affects the crystallization on nucleation level. No induction time can be observed for the MGHPO blend. The higher degree of saturation of MGHPO and the match in chain length between the MGHPO and palm oil are suggested as the main factors causing the larger extent of the influence of MGHPO. Addition of the MGHPO is proposed as an answer for the problems of the longer R-lifetime and the slower crystallization rate of palm oil in the industrial applications. Acknowledgment. Imogen Foubert is a postdoctoral fellow of the Fund for Scientific Research-Flanders (F.W.O-Vlaanderen). Further, the authors acknowledge the Dutch-Belgian

Influence of MGs on Behavior of Palm Oil

Beamline (DUBBLE) research group at the ESRF (Grenoble, France) and the Dutch organization for scientific research (N.W.O.) as well as H. Reynaers (KULeuven) and B. Goderis (KULeuven, Belgium) for their help and continuous support of the DUBBLE project (ESRF, Grenoble, France). Cargill is acknowledged for financial support and for providing the samples.

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