Effects of Polyglycerine Fatty Acid Esters Having Different Fatty Acid

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Effects of Polyglycerine Fatty Acid Esters Having Different Fatty Acid Moieties on Crystallization of Palm Stearin Keisuke Shimamura,*,† Satoru Ueno,‡ Yoshiro Miyamoto,† and Kiyotaka Sato‡ †

Research Laboratory, Sakamoto Yakuhin Kogyo Co., Ltd., 1325-93 Kizu, Ako City, Hyogo 678-0165, Japan Laboratory of Food Biophysics, Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima, 739-8528, Japan

‡

ABSTRACT: We studied the effects of adding emulsifiers, specifically polyglycerine fatty acid esters (PGFEs), on the crystallization of palm stearin (PS), using synchrotron radiation X-ray diffraction and DSC methods. Our main aim was to examine the effects of the molecular shapes of PGFEs containing palmitic and oleic acid moieties at different ratios, the concentration of PGFE additives, and the cooling rate on the crystallization kinetics of PS in combined ways. We found that all PGFE additives retarded the crystallization of PS when the concentration of PGFEs was low and the cooling rate was low. However, crystallization was promoted when the concentration of PGFEs containing a high amount of palmitic acid moiety was increased and the cooling rate was increased. It was evident that the additive effects due to the PGFEs became less remarkable with increasing concentrations of oleic acid moiety. By contrast, emulsifiers containing palmitic acid moiety strongly affected the crystallization. We discuss the fact that crystallization was promoted by the template effects of PGFEs but was retarded by the disturbance of nucleation and subsequent crystal growth processes caused by the PGFE molecules.

1. INTRODUCTION Emulsifiers are widely employed in food, cosmetic, and pharmaceutical technologies as major reagents for emulsification, dispersion, wetting, foaming, defoaming, and solubilization.1 In addition, emulsifiers can modify nucleation, crystal growth, and polymorphic transformation processes of fats,2,3 as revealed by many studies in bulk4−14 and emulsion states.15−20 Despite these studies, however, the mechanisms of the effects of adding emulsifiers on the crystallization of fat crystals are not fully understood, most particularly with respect to the nucleation process. For example, certain kinds of emulsifiers promote nucleation, whereas others retard it. Further work is needed to successfully explain such complicated effects. Many factors are involved in the effects of emulsifiers on crystallization, such as the types of head (polar) groups, the types of fatty acid moiety, the degree of hydrophobicity or hydrophilicity, the solubility of fats in supercooled liquid, the similarity or dissimilarity in fatty acid moieties between the emulsifiers and the fats, the concentration of the emulsifiers to be added, the rate of cooling, and the polymorphism of the fats.2,3 In addition, the physical states of the crystallization system, such as neat liquid, oil-in-water emulsion, or water-inoil emulsion, are also important, since the interfacial properties can affect the fat−emulsifier interaction when heterogeneous nucleation occurs at the oil and water interfaces in the emulsion states.18−20 For example, let us consider the fat crystallization in neat liquid using an emulsifier of one type of polar group such as sucrose, or polyglycerines, where different types of fatty acid chains can be esterified. The effects of the addition of the emulsifier on the fat crystallization may be considered in two © XXXX American Chemical Society

ways: solubility of the emulsifier in neat liquid of fat and molecular interactions between the emulsifier and the fat. Changing the types of fatty acid moieties of the emulsifier causes the changes in both the solubility and the molecular interactions with the fats simultaneously as explained in the following. There may be a critical concentration of the emulsifier, as determined by its solubility in supercooled liquid of the fat. When the solubility of the emulsifier is high or the concentration of the emulsifier is lower than the solubility limit, the emulsifier may not crystallize prior to the fat during cooling, but prevent the formation of crystal nuclei of the fat through attractive molecular interactions between the emulsifier and the fat molecules. In contrast, the emulsifier having low solubility may crystallize prior to the fat when the concentration of the emulsifier exceeds its solubility limit. In this case, the emulsifier may act as a “template” which promotes the crystallization. We may decrease the solubility of the emulsifiers in supercooled liquid of the fats by putting on a long-chain saturated fatty acid moiety. In contrast, the emulsifiers become more soluble when the concentration of unsaturated fatty acid moiety increases. In both cases of promotion and retardation of crystallization, it is reasonable to assume that similarity in molecular shape between the emulsifier and the fat, especially the fatty acid moiety, is important as Smith et al. indicated.3 For example, when the fat contains a long-chain saturated fatty acid moiety, Received: June 17, 2013 Revised: September 28, 2013

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hexaoleate) containing palmitic and oleic acid moieties at a ratio of 1:1. The peak-top melting temperatures examined with DSC were 46.8 °C (10G12P), 33.7 °C (10G8P6O) and 23.7 °C (10G6P6O). The PGFEs were added to the mixtures of PS and SO at 0.05, 0.5 and 1 wt % with respect to the total mixture. Differential Scanning Calorimetry. DSC measurements were carried out using a Thermo Plus 8240 (Rigaku Co., Tokyo, Japan). The sample (10 mg) was sealed in an aluminum pan and set in the DSC furnace. The PS/SO = 50/50 samples were heated to 60 °C and held for 5 min at this temperature. The samples were then cooled to 20 °C at rates of 0.1 °C/min and 2 °C/min, held for five minutes at this temperature, and then heated to 60 °C at a rate of 2 °C/min. The PS/SO = 10/90 samples were heated to 50 °C, held for five minutes at this temperature, and then cooled to 0 °C at rates of 0.1 °C/min and 2 °C/min, held for five minutes at this temperature, and heated to 50 °C/min at a rate of 2 °C/min. We chose different scanning temperature ranges of the DSC studies for the two mixtures, because of the differences in crystallization temperature of high melting TAGs present in the mixture due to changes in the SO concentration. Duplicated measurements were done and the values of crystallization temperature (Tc), were the average of them under every condition. Synchrotron Radiation X-ray Diffraction Measurement. The crystallization of the PS/SO = 50/50 mixture and PS/SO = 10/90 mixture was measured by synchrotron radiation X-ray diffraction (SRXRD, wavelength λ = 0.150 nm) using two beamlines, BL-9C and BL15A, in the synchrotron radiation facility (Photon Factory, PF) at the High Energy Accelerator Research Organization in Tsukuba, Japan. The camera lengths of the small-angle and wide-angle X-ray diffraction (SAXD and WAXD) in BL-9C were 1100 mm and 550 mm, and in BL-15A they were 113 mm and 280 mm, respectively. A positionsensitive proportional counter (PSPC) was used for SAXD and WAXD data acquisition in BL-9C, and an X-ray CCD detector and a PSPC were used for SAXD and WAXD data acquisition in BL-15A. One-dimensional SAXD patterns obtained with the X-ray CCD detector were circularly averaged using software (23). A 6 mm square sample cell with a 1 mm thick, 2 mm square window was sealed with polyimide film (thickness 25 μm) and put on each sample holder. The temperature of the samples was changed with a Mettler DSC-FP84 (Metter Instrument Corp., N.J. USA) using FP 99 system software. The polymorphic forms of the fat crystals were determined by SAXD and WAXD measurements. We found that α, β′ and β forms showed the following values, which did not change among the samples employed in the present study: α, long spacing value of 4.6 nm, short spacing value of 0.41 nm; β′, long spacing value of 4.2 nm, short spacing values of 0.43, 0.42, and 0.38 nm; and β, long spacing value of 4.0 nm, short spacing values of 0.46, 0.42, and 0.38 nm.

the emulsifier having the same or a similar acyl chain structure may affect crystallization more than those containing shortchain or unsaturated fatty acid moieties. Such effects were observed for polyglycerine fatty acid esters having different fatty acid moieties.15 In the present paper, we report on experiments on the effects of adding emulsifiers, specifically polyglycerine fatty acid esters (PGFEs), on promoting or retarding the nucleation kinetics of semisolid fats in a bulk system (Figure 1). Particular interest

Figure 1. A molecular model of polyglycerine fatty acid ester (PGFE) with polymerized 10 glycerines. Solid lines represent hydrocarbon chains.

was focused on the effects of the fatty acid moiety of emulsifiers having the same polar group on the kinetics of crystallization of palm stearin (PS), a high-melting fraction of palm oil.21 We employed three types of PGFEs differing in their relative concentrations of palmitic and oleic acid moieties in order to observe the effects of the fatty acid composition of the PGFEs. In the first type of PGFE, the fatty acid moiety was solely palmitic acid. In the second type, palmitic acid and oleic acid were esterified at the same concentration ratio, whereas the concentration of palmitic acid was twice that of the oleic acid moiety in the third type. We examined the effects of cooling rate on the crystallization kinetics of the fat with the emulsifier additives, at 2 °C/min (rapid cooling) and 0.1 °C/min (slow cooling). The effects of the additives on the crystallization kinetics were measured as variations in crystallization temperature (Tc) were observed both by cooling scan of differential scanning calorimetry (DSC) and synchrotron radiation X-ray diffraction (SR-XRD).

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EXPERIMENTAL SECTION

Materials and Methods. PS was supplied by Fuji Oil Co., Ltd. (Osaka, Japan), and soybean oil (SO) was purchased from Showa Sangyo Co., Ltd. (Tokyo, Japan). PS was mixed with SO at different concentration ratios. PS and SO were mixed with 50 wt %, and the mixture was called PS/SO = 50/50 as a model of shortening. PS of 10 wt % was mixed with 90 wt % of SO (PS/SO = 10/90 mixture) as a model of a low-saturated fat blend. According to the data provided by Fuji Oil Co., Ltd., the major high-melting triacylglycerols (TAGs) present in the PS were PPP (19.8%), POP (31.2%) and PPO (10.8%), in which P and O refer to palmitic acid and oleic acid. In SO, a total of TAGs (97.9%) contained the major TAGs of LLL (17.4%), OLL (16.4%), PLL (12.5%) and POL (11.4%), in which L refers to linoleic acid,22 and minor components are diacylglycerols (1%) and others (1.1%).23 The PGFEs employed in the present study were produced by Sakamoto Yakuhin Kogyo Co., Ltd. (Osaka, Japan). The polyglycerin employed was POLYGLYCERIN #750 (polyglycerin-10: hydroxyl value 890 mg of KOH/g) produced by Sakamoto Yakuhin Kogyo Co., Ltd. (Osaka, Japan). The polyglycerin and commercially available palmitic acid (98% pure, Miyoshi Yushi Co.,Ltd., Tokyo, Japan) and oleic acid (98% pure, Miyoshi Yushi Co.,Ltd.) were mixed and reacted with 0.05% sodium hydroxide for 4 to 14 h at 245 °C under nitrogen flow at atmospheric pressure. In order to examine the effects of the fatty acid composition, three types of PGFEs were prepared: 10G12P (decaglycerin dodecapalmitate) containing palmitic acid alone, 10G8P4O (decaglycerin octapalmitoyl tetraoleate) containing palmitic and oleic acid moieties at a ratio of 2:1, and 10G6P6O (decaglycerin hexapalmitoyl

3. RESULTS AND DISCUSSION PS/SO = 50/50 Mixture. We observed the crystallization of the PS/SO = 50/50 mixture at different rates of cooling to compare the effects of emulsifiers on crystallization at different cooling rates. Figure 2 presents the DSC cooling and heating thermopeaks of the PS/SO = 50/50 mixture without PGFE additives, examined at a rate of cooling of 2 °C/min. A sharp, large exothermic peak was observed at 25.6 °C in the cooling pattern, whereas broad endothermic peaks appeared in a temperature range of 23 to 40 °C, and a large endothermic peak appeared at around 47 °C in the heating pattern. PS is a mixture of different TAGs, so the broad DSC heating peaks must be due to different TAG fractions in the PS. In the present study, we paid particular attention to the effects of PGFE additives on the crystallization expressed in the sharp exothermic peak at 25.6 °C in Figure 2. As with Figure 2, we examined the DSC cooling thermopeaks of PS/SO = 50/50 at a cooling rate of 0.1 °C/ min, and those of PS/SO = 10/90 at cooling rates of 2 °C/min B

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disappear around 36 °C, and the peaks at 0.46 nm, 0.42 nm and 0.38 nm to disappear at 47.5 °C. Combining the results of the SAXD and WAXD patterns, we may conclude that the α form was crystallized during cooling at 24 °C, as revealed in the typical WAXD peak of 0.41 nm, and the long spacing value of 4.6 nm. This form was then transformed to β′ and β by a solid-state transformation during storage at 20 °C. Further heating caused the melting of β′ at around 36 °C and β at around 47 °C. These results correspond well to the DSC study in Figure 2, in which a large exothermic peak occurred at 25.6 °C during cooling. This temperature is a bit higher than that of the crystallization of the α form observed in the SR-XRD study, but we can conclude that both temperatures are due to the crystallization of α in the highmelting TAG fraction of PS. The broad endothermic peaks observed in the DSC heating pattern correspond to the melting of β′ and β of TAG fractions that are present in PS. In particular, the intensity of the SAXD peak at 4.2 nm decreased below 30 °C, corresponding to the small endothermic peak indicated by an arrow in Figure 2b. Figure 4 presents the SR-XRD and DSC thermopeaks of the PS/SO = 50/50 mixture without additives, taken during cooling at a rate of 0.1 °C/min. The β′ form of PS showing the SAXD peak at 4.2 nm and WAXD peaks at 0.43 nm, 0.42 nm, and 0.38 nm occurred at 30.1 to 29.8 °C. The DSC cooling thermopeak also had an exothermic peak at 30.2 °C. The results in Figures 3 and 4 mean that polymorphic crystallization in the PS/SO = 50/50 mixture is largely affected by the cooling rate in such a way that rapid cooling crystallized α and slow cooling crystallized β′. Having the controlled data in Figures 2−4 without the additives, we examined the effects of adding the three PGFEs on the crystallization of PS in the PS/SO = 50/50 mixture by cooling the mixture from 60 to 20 °C at rates of 2 °C/min and 0.1 °C/min, and also by changing the concentration of the PGFE additives. We summarize the results of the crystallization temperature (Tc) of PS at the different concentrations of the additives examined by DSC in Figure 5 and Table 1a. For the cooling 2 °C/min rate, adding 10G6P6O did not affect Tc even at a concentration of 1%. However, adding 10G8P4O gradually decreased Tc from 25.4 to 22.5 °C at a concentration of 1%. When 10G12P was added, Tc first decreased to 24.4 °C at a concentration of 0.05% and then increased to 26.6 °C at an additive concentration of 1%. This means that adding 10G12P promoted the crystallization of PS at high additive concentrations for crystallization at a cooling rate of 2 °C/min. However, a large difference was observed when the crystallization was carried out at a rate of 0.1 °C/min, although the effects of adding 10G8P4O and 10G6P6O were the same (Figure 5b). Tc did not change with the addition of 10G6P6O but decreased with increasing concentration of 10G8P4O. Similarly, adding 10G12P always decreased Tc with the concentration increasing to 1%, although the degree of decrease was minimized at high concentrations. The results in Figure 5 indicate that adding 10G12P had opposing effects: the first effect was retardation at slow cooling rates and lower additive concentrations; the second effect was the promotion of crystallization at higher cooling rates and higher additive concentrations. We performed in situ SR-XRD measurements taken at different rates of cooling to confirm whether adding 10G8P4O and 10G12P retards or promotes PS crystallization, since these two additives presented the most typical effects observed by

Figure 2. DSC thermopeaks of PS/SO = 50/50 mixture taken during (a) cooling and (b) heating between 60 and 20 °C at the rate of 2 °C/ min.

and 0.1 °C/min. The crystallization temperatures observed in these conditions are summarized in Table 1. Table 1. Effects of Emulsifier Additives on Tc of Palm Stearin (PS) in Soybean Oil (SO) Examined by DSC at Different Emulsifier Concentrations and Cooling Rates concentrations of additives emulsifiers

0.05 wt %

0.5 wt %

1.0 wt %

(a) PS/SO = 50/50 Mixture Cooling Rate of 2 °C/min: Tc = 25.6 °C (without Additive) 10G12P 24.4 26.0 26.6 10G8P4O 25.4 24.0 22.5 10G6P6O 25.6 25.3 25.3 Cooling Rate of 0.1 °C/min: Tc = 30.2 °C (without Additive) 10G12P 26.0 26.7 27.0 10G8P4O 29.5 28.7 28.1 10G6P6O 30.2 29.9 29.8 (b) PS/SO = 10/90 Cooling Rate of 2 °C/min: Tc = 13.3 °C (without Additive) 10G12P 14.6 16.5 17.2 10G8P4O 8.6 7.1 6.6 10G6P6O 11.3 9.1 7.7 Cooling Rate of 0.1 °C/min: Tc = 19.4 °C (without Additive) 10G12P 16.3 17.1 17.2 10G8P4O 11.7 9.3 8.7 10G6P6O 17.5 15.8 14.8

Figure 3 presents SR-XRD small-angle diffraction (SAXD) and wide-angle diffraction (SAXD) patterns of the PS/SO = 50/50 mixture taken during the cooling and heating processes at a rate of 2 °C/min. In the SAXD patterns, a strong peak was observed at 4.60 nm during cooling at 24 °C, and this peak increased in intensity during further cooling to 20 °C; no other peak was observed. However, this peak shifted to 4.20 nm and its intensity increased when the mixture was kept at 20 °C for five minutes. On heating from 20 to 60 °C, the SAXD patterns shifted further from 4.20 nm to 4.00 nm at 36 °C, and this peak disappeared at 47.4 °C. The temperature variation of the WAXD patterns reflects the polymorphic crystallization and transformation from α to β′ and β forms during the cooling and heating processes. The WAXD peaks are not very sharp compared to those of SAXD, because of the presence of a wide diffraction peak of the liquid oil around 2θ = 18°. A weak peak of 0.41 nm was observed during cooling at 24 °C, and this peak disappeared and new peaks of 0.46 nm, 0.42 nm, and 0.38 nm appeared during storage at 20 °C. Further heating caused the peak of 0.42 nm to C

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Figure 3. SR-XRD patterns of PS/SO = 50/50 mixture taken during cooling (2 °C/min) from 40 to 20 °C, isothermal treatment at 20 °C for 5 min and heating (2 °C/min) from 20 to 60 °C. Unit, nm.

Figure 4. SR-XRD and DSC cooling patterns of PS/SO = 50/50 mixture during cooling at 0.1 °C/min. Unit, nm.

DSC. We focused on the polymorphic forms and Tc of the PS in the SR-XRD measurements. Figure 6 depicts SR-XRD patterns of PS/SO = 50/50 mixtures taken at a cooling rate of 2 °C/min with additive 10G12P at concentrations of 0.05% and 1%. When 0.05% was added, the SAXD and WAXD patterns indicated α at 22.5 °C, which is 2 °C lower than Tc without the additive (Figures 3 and 6a). The Tc value, however, of PS with additive at 1% increased to 25.0 °C as revealed in the SAXD and WAXD patterns in Figure 6b. In these two cases, the polymorphic form was α. Therefore, the results observed by SR-XRD mean that adding 10G12P at low concentrations retarded crystallization, whereas

adding 10G12P at high concentrations promoted crystallization, when the crystallization was performed at a cooling rate of 2 °C/min. These results are exactly the same as those observed by the DSC experiments summarized in Figure 5 and Table 1a. For a slow cooling rate of 0.1 °C/min, adding the three PGFEs always retarded crystallization. Figure 7 presents in situ SR-XRD SAXD patterns of the crystallization processes of PS in the PS/SO = 50/50 mixture acquired during cooling. The most typical retardation effects are shown for additives 10G12P (0.05 wt %; Figure 7b), 10G8P4O (1 wt %; Figure 7c), and 10G6P6O (1 wt %; Figure 7d) in comparison to the pure mixture (Figure 7a). Tc without the additive was 29.2 °C D

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Figure 5. Crystallization temperatures of PS in PS/SO = 50/50 mixture with PGFE additives measured by DSC. Cooling rate at (a) 2 °C/min and (b) 0.1 °C/min.

Figure 6. SR-XRD cooling patterns of PS/SO = 50/50 mixture with (a) 0.05% addition and (b) 1% addition of 10G12P at a cooling rate of 2 °C/ min.

(Figure 7a), whereas it decreased to 25.8 °C with additive 10G12P at 0.05%, and to 28.3 °C with additive of 10G8P4O at 1.0%. Adding 10G6P6O did not change Tc, as seen in Figure 7d. The polymorphic form of the first crystals was β′ in all cases except for 10G12P, which caused the crystallization of α at the beginning and a transformation to β′ during further cooling. The polymorphic occurrence of PS in the PS/SO = 50/50 mixtures with the additives at 1.0 wt % examined by the SRXRD is summarized in Table 2a. The α form always occurred at a 2.0 °C/min cooling rate. However, β′ crystallized without additives and with additives 10G8P4O and 10G6P6O, whereas α crystallized with additive 10G12P at a 0.1 °C/min cooling rate. The reasons for this difference will be discussed later. PS/SO = 10/90 Mixture. We have conducted crystallization experiments with PS/SO = 10/90 mixtures using DSC with and without the three PGFEs at different cooling rates quite similarly to the experiments with PS/SO = 50/50 mixtures. The results are summarized in Figure 8 and Tables 1b and 2b. Figure 8a reveals that adding 10G6P6O and 10G8P4O decreased Tc from 13.3 °C (without additives) to 6.6 °C (10G8P4O) and 7.7 °C (10G6P6O) with an additive concentration of 1 wt % and a 2 °C/min cooling rate. For

10G12P, however, Tc gradually increased with increasing concentration, reaching 17.2 °C at a concentration of 1 wt %. This means that adding 10G12P promoted PS crystallization, whereas the other two additives retarded it, when the cooling rate was 2 °C/min. In contrast, adding the three PGFEs retarded crystallization at all concentrations when the cooling rate was 0.1 °C/min, as revealed by the decrease in Tc seen in Figure 8b. We performed in situ SR-XRD measurements to confirm whether adding the three types of PGFEs retards or promotes PS crystallization. Figure 9 presents SR-XRD patterns for the three PGFE additives at a concentration of 1% while cooling at 2 °C/min, together with the result without additives. The polymorphic form of the first crystals was α for the samples both with and without the additives. Tc without the additive was 12.0 °C, and it decreased to 4.0 and 7.0 °C with the addition of 10G8P4O and 10G6P6O at 1.0%, respectively. However, adding 10G12P caused a small peak in the SAXD pattern around 18 °C as noted by an arrow in Figure 9, and it increased at 14 °C during further cooling. This clearly demonstrated that adding 10G12P promoted PS crystallization. Although not shown here, the crystallization of the PS/SO = E

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Figure 7. SR-XRD small angle diffraction patterns of PS/SO = 50/50 mixture at cooling rate of 0.1 °C/min. (a) No additives, (b) with 10G12P (0.05 wt %) and (c) with 10G8P4O (1 wt %) (d) with 10G6P6O (1 wt %).

The experiment results obtained by the present study are

Table 2. Effects of Emulsifier Additives (1.0 wt %) on Polymorphic Crystallization of Palm Stearin (PS) in Soybean Oil (SO) examined by SR-XRD at Different Cooling Rates emulsifiers no additive 10G12P 10G8P4O 10G6P6O no additive 10G12P 10G8P4O 10G6P6O

2 °C/min (a) PS/SO = 50/50 Mixture α α α α (b) PS/SO = 10/90 α α α α

summarized in the following. (a) The effects of adding PGFEs on PS crystallization differed among three types of PGFEs, as summarized in Table 1. 10G8P4O and 10G6P6O always retarded PS crystallization in both the PS/SO = 50/50 mixture and the PS/SO = 10/90 mixture at slow and rapid cooling rates, as indicated by the decrease in Tc. 10G8P4O retarded crystallization more than 10G6P6O under all conditions examined. In contrast, adding 10G12P induced opposite effects as the additive concentration and the cooling rate varied. Crystallization was retarded for both the PS/SO = 50/50 and the PS/SO = 10/90 mixtures at all concentrations when the cooling rate was low (0.1 °C/min), as seen in Figures 5b and 8b. However, crystallization was promoted at a cooling rate of 2 °C/min when the additive concentrations were 0.5 wt % and 1 wt % (50/50 mixture) and 0.05 to 1 wt % (10/90 mixture).

0.1 °C/min β′ α > β′ β′ β′ β′ α > β′ α > β′ β′ > α

10/90 mixture with the three additives under different cooling conditions was confirmed by SR-XRD measurements, similar to the results for the PS/SO = 50/50 mixture, as summarized in Figure 8 and Tables 1 and 2.

Figure 8. Tc values of PS in PS/SO = 10/90 mixture with PGFE additives examined by DSC cooling at (a) 2 °C/min and (b) 0.1 °C/min with different concentrations of emulsifier additives. F

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Figure 9. SR-XRD small angle diffraction patterns of PS/SO = 10/90 mixtures with 1 wt % addition of emulsifiers at a cooling rate of 2 °C/min.

Figure 10. Schematic illustrations of the interactions between palm stearin (PS) and polyglycerine fatty acid esters (PGFEs) during (a) nucleation and (b) crystal growth processes.

(b) As to the polymorphic crystallization of the α or β′ forms of PS, slow crystallization formed β′ and rapid crystallization formed α when PGFEs were not added. This result did not change when the crystallization was carried out at a cooling rate of 2 °C/min (Table 2). In contrast, the crystallization of β′ became predominant at a cooling rate of 0.1 °C/min, except when adding 10G12P (50/50 mixture) and 10G12P, 10G8P4O, and 10G6P6O (10/90 mixture), in which α crystallized more than or the same as β′. It is worth noting that the degree of retardation expressed by the decrease in Tc was high under these conditions. (c) The differences in the effects of 10G12P between the PS/SO = 50/50 mixture and PS/SO = 10/90 mixture were observed at different ranges of crystallization temperature, when the cooling rate was 2 °C/min. In the PS/SO = 50/50 mixture, the addition of 10G12P of 0.05% retarded the crystallization, whereas the promotion effects were observed when the concentrations of 10G12p were 0.5% and 1%, as shown in Figure 5a. In the PS/SO = 10/90 mixture, however, the crystallization was promoted by the addition of 0.05%, 0.5% and 1% more remarkably than those in the PS/SO = 50/50 mixture, as shown in Figure 8a.

It is reasonable to assume that PPP, POP, and PPO crystallized in the PS/SO mixtures because of their high concentrations in PS and their high melting points. However, it is difficult to determine exactly which TAGs crystallized first in the present experiments. This is because the long spacing values for α and β′ of PPP (4.6 nm) and POP (4.2 nm) are almost the same,24 and POP and PPO form molecular compound crystals having a double-chain-length structure, whose long spacing values are similar to 4.6 nm (α) and 4.2 nm (β′).27 We may assume that PPP crystallizes first and then POP and PPO crystallize since PPP has a higher melting point than POP and PPO. Smith et al. pointed out in a recent review that similarity in the fatty acid compositions between the additives and fats is a necessary condition to determine the additive effects. The PGFEs we employed in this study satisfy this condition.3 Given this, the differences in the additive effects among the three PGFEs may be interpreted by the following consideration. 10G8P4O and 10G6P6O are more soluble than 10G12P in the supercooled liquid of the PS/SO mixtures, since the melting temperatures of the three PGFEs are 46.8 °C (10G12P), 33.7 °C (10G8P6O), and 23.7 °C (10G6P6O). Therefore, 10G8P4O and 10G6P6O may not crystallize prior to the TAGs in the PS while cooling, and this may prevent clusters of PS from forming crystal nuclei (declustering; Figure 10a). The interactions between 10G8P4O and PS may be much stronger than those between 10G6P6O and PS because of the higher concentration of palmitic acid moiety in 10G8P4O. For this, the retardation effects were always greater for 10G8P4O than for 10G6P6O.

The results summarized in point a can be discussed by considering the interactions between PS and PGFEs during nucleation and crystal growth in supercooled liquid of the PS/ SO mixtures (Figure 10). G

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Taking cooling-rate-dependent polymorphic crystallization into account, we may discuss the results summarized in the above point (b) as follows. β′ crystals with an orthorhombic perpendicular (O⊥) subcell structure are more tightly packed than in α with hexagonal packing, so we assume that PGFEs may interact with β′ more than α, and that the retardation effects on the crystallization rate due to the PGFE additives may thus be more remarkable in β′ than in α. The nucleation of α may then become predominant at wider ranges of temperature compared to the pure system (Figure 11b). The more the retardation effects are revealed, the more the crystallization of α becomes predominant. Our experiments showed this tendency as summarized in Table 2. The results summarized in point c may be understood by taking into account both of the changes in crystallization temperature of the fat, and the solubility of 10G12P. The crystallization temperature decreases with increasing concentration of SO, e.g., 24−26 °C in the PS/SO = 50/50 mixture and 14−17 °C in the PS/SO = 10/90 mixture when the cooling rate was 2 °C/min. The solubility of 10G12P also increases with increasing SO concentration and decreasing temperature. Then, we assume that the template effect of 10G12P may occur more remarkably around 14−17 °C than around 24−26 °C because of the solubility effect. Due to lowered solubility of 10G12P at lower temperature range, the addition of 10G12P at the concentrations of 0.05−1% may exceed the solubility limit. Then the crystallization of 10G12P may occur at the cooling rate of 2 °C/min, causing the template effect. Such effects did not occur when the cooling rate was 0.1 °C/min, because the crystallization of 10G12P was delayed. We cannot simply compare, however, the solubility of 10G12P between the two mixtures, since the concentration of liquid oil (SO) in the PS/SO = 10/90 mixture is higher than that in the PS/SO = 50/50 mixture. So, precise measurements of the solubility of 10G12P and the observation of the competitive crystallization of the fat and 10G12P additive may clarify this consideration. To conclude, PS crystallization was promoted through template effects by adding a PGFE having a high-melting fatty acid moiety at a high concentration and with a high cooling rate. The template is formed when the PGFE crystallizes before the fat itself during cooling. However, crystallization was retarded by adding PGFEs that are dissolved in the supercooled liquid of the fat. Furthermore, adding the same PGFE can produce opposite effects (retardation or promotion) when the concentration and cooling rate vary. The key to tailoring the promotion or retardation effects is determining whether the additive crystallizes before the fat crystallizes or dissolves in the molten liquid. In all cases, a prerequisite to determining any additive effects is a similarity in fatty acid composition between the additive and the fat.

For 10G12P, however, the combined effect of the additive concentration and the cooling rate caused opposing effects. 10G12P promoted the crystallization in the following cases, as revealed in the increase in the Tc values: the concentrations of 0.5% and 1% in the PS-SO = 50/50 mixture at the cooling rate of 2 °C/min (Figure 5a), and the concentrations of 0.05%, 0.5% and 1% in the PS-SO = 10/90 mixture at the cooling rate of 2 °C/min (Figure.8a). By contrast, the crystallization was retarded in the other cases. We think that crystallization was promoted by template effects (Figure 10b). However, the template effect of 10G12P was only observed at high cooling rates (2 °C/min). When the cooling rate was decreased, adding 10G12P retarded crystallization more than adding the other two PGFEs (Figure 5b and Figure 8b) because 10G12P does not crystallize prior to PS, so the 10G12P molecules dissolving in the liquid may cause “de-clustering” similarly to 10G8P4O and 10G6P6O. The template effects of adding emulsifiers have recently been observed in W/O emulsion,19 O/W emulsion,18 and bulk systems.14 In all of these studies, the emulsifiers employed contained high-melting saturated fatty acid moieties, which may crystallize before the fats due to their low solubility in the supercooled liquid of the fats. Such a mechanism may occur in the present case of 10G12P at high concentrations and high cooling rates. For crystal growth, the retardation mechanisms may be understood in terms of the similarity/dissimilarity concept, with the incorporation of crystallizing materials at the kink sites on the growth steps at the crystal−liquid interface being hindered by the additives (Figure 10b). Specifically, emulsifiers having a fatty acid composition the same as or similar to that of PS can be adsorbed at the kink sites, while prohibiting the incorporation of the TAG molecules into the kink sites through large polar groups. As for polymorphic crystallization, the fact that the rapid (slow) crystallization caused α (β′) may be explained by the differing rates of nucleation of α and β′ (Figure 11a).26−28

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Figure 11. Schematic illustration of retardation mechanisms of crystallization rates of α and β′ forms of palm stearin (solid lines, without emulsifiers; dotted lines, with emulsifiers).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Since the rate of nucleation of α is higher than that of β′, the relative occurrence of α exceeds that of β′ as the crystallization temperature is lowered and the cooling rate is increased. This tendency was recently confirmed for OPO27 and POP.28 Therefore, rapid crystallization leads to the nucleation of α, whereas β′ is formed as the cooling rate is decreased. 26

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ACKNOWLEDGMENTS SR-XRD experiments were conducted with the approval of the Photon Factory Program Advisory Committee (proposals 2010G114 and 2010G656). The authors gratefully acknowlH

dx.doi.org/10.1021/cg400910g | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

edge the help of Prof. Masaharu Nomura, Station Manager of BL-9C at Photon Factory.

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REFERENCES

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dx.doi.org/10.1021/cg400910g | Cryst. Growth Des. XXXX, XXX, XXX−XXX