K. Sato’s research while affiliated with Hiroshima University and other places

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Differential scanning calorimetry (DSC), optical microscopy, and X-ray diffraction (XRD) were used to examine the thermal behavior, crystal structure, and crystal morphology of rice bran wax (RBX) in bulk and oil–wax mixtures, and to compare them with those of carnauba wax (CRX) and candellila wax (CLX). The RBX employed in the present study was separated from rice bran oil by winterization, filtration, refinement, bleaching, and deodorization. The RBX crystals melted in the bulk state at 77–79°C with ΔH melting=190.5J/g, which is quite large compared with CLX (129J/g) and CRX (137.6J/g). XRD data of the RBX crystals revealed O⊥ subcell packing and a long spacing value of 6.9nm. Thin long needle-shaped crystals were observed in the mixtures of RBX and liquid oils [olive oil and salad oil (canola:soy bean oil=50:50)]; therefore, the dispersion of RBX crystals in these liquid oils was much finer than that of CRX and CLX crystals. Organogels formed when the mixture of every plant wax and liquid oil was melted at elevated temperature and cooled to ambient temperature. However, the mixture of RBX and olive oil at a concentration ratio of 1:99wt.% formed an organogel at 20°C, whereas the lowest concentration necessary for CRX to form an organogel in olive oil was 4wt.% and that for CLX was 2wt.%. Observation of the rate of gel formation using DSC and viscosity measurements indicated that the gel structure formed soon after RBX crystallized, whereas a time delay was observed between the organogel formation and wax crystallization of CRX and CLX. These results demonstrate RBX’s good organogel-forming properties, mostly because of its fine dispersion of long needle like crystals in liquid oil phases.

We have conducted thermal and X-ray diffraction experiments on binary mixtures of symmetric stearic-oleic mixed-acid triacylglycerol (TAG) (1,3-distearoyl-2-oleoyl-glycerol: SOS) and asymmetric stearic-oleic mixed-acid TAG (1,2-dioleoyl-3-stearoyl-sn-glycerol: OOS), in which optically active sn-OOS was employed. We found that SOS–OOS mixtures exhibited immiscible monotectic or peritectic mixing behavior. This result was consistent with previous study on binary mixtures of 1,3-dipalmitoyl-2-oleoyl-glycerol (POP) and 1,2-dioleoyl-3-palmitoyl-rac-glycerol (OOP), in which racemic rac-OOP molecules were employed. The differences between the SOS–OOS and POP–OOP mixtures were in the polymorphic behavior of the fractions of POP and SOS. No effect was found from using an optically active (sn-OOS) or racemic mixture (rac-OOP) as an asymmetric oleic–oleic-saturated acid TAG. From the two results, we may conclude that an immiscible phase was formed in the binary mixtures of symmetric saturated-oleic-saturated TAGs and asymmetric oleic–oleic-saturated TAGs, of both racemic and optically active types. This result stands in contrast to mixtures of SOS–OSO (1,3-dioleoyl-2-stearoyl-glycerol), SOS–SSO (1,2-distearoyl-3-oleoyl-rac-glycerol), POP–OPO (1,3-dioleoyl-2-palmitoyl-glycerol), and POP–PPO (1,2-dipalmitoyl-3-oleoyl-rac-glycerol), all of which exhibited molecular-compound-forming behavior with molecular compound crystals at an equal ratio of the binary mixtures. Molecular-level mechanisms to explain this difference are discussed, based on possible roles of glycerol groups acting during the mixing processes of saturated–unsaturated mixed-acid TAGs.

We performed scanning microbeam small-angle X-ray diffraction (micro-SAXD) experiments, differential scanning calorimetry (DSC) analysis, and optical microscopic observation of palm mid fraction (PMF) crystals in oil-in-water emulsion droplets. The scanning micro-SAXD experiment was performed by irradiating a synchrotron radiation X-ray microbeam having an area of 5 x 5 microm(2) onto different positions on a 50 microm diameter emulsion droplet after the crystallization of PMF by chilling the emulsion at 5 degrees C. The micro-SAXD patterns were recorded with a two-dimensional (2D) detector, which enabled spatial analysis of polymorphic structures and the orientation of lamella planes of PMF crystals at different positions inside the emulsion droplet. Particular attention was paid to compare the crystallization of PMF in two types of emulsion droplets, hydrophilic polyoxyethylene sorbitan mono-oleate (Tween 80) alone (Tween 80 emulsion) and Tween 80 and hydrophobic sucrose palmitic acid oligoester (P-170) (Tween 80+P-170 emulsion). The DSC study revealed that the PMF crystallization temperature in the Tween 80+P-170 emulsion droplets increased by 3 degrees C compared to that of the Tween 80 emulsion because of the effects of the P-170 additive in promoting PMF crystallization. The micro-SAXD studies revealed the following results. (1) The lamella planes of PMF crystals near the outer edges of the droplet in the Tween 80+P-170 emulsion were mostly parallel to an oil-water interface, whereas the lamella planes of PMF crystals were not always aligned with the oil-water interface in the Tween 80 emulsion droplet. (2) The degree of orientation of the lamellar planes of PMF crystals, which was evaluated from the values of full width at half-maximum of 2D micro-SAXD patterns with respect to azimuthal angle extension, was remarkably higher in the Tween 80+P-170 emulsion than in the Tween 80 emulsion. (3) Polymorphic transformation of PMF from alpha to beta' in the Tween 80+P-170 emulsion was retarded compared to that in the Tween 80 emulsion. These results confirmed that the P-170 additive caused interfacial heterogeneous nucleation through hydrophobic interactions at the oil-water interfaces in the emulsion, which subsequently influenced the arrangements of fat crystals so that the lamellar planes of fat crystals were parallel to the oil-water interface.

We measured the rate and extent of solubilization and crystallization of lipids (long-chain fatty acids; lauric, myristic and palmitic acids) in oil-in-water emulsion with average oil droplet diameters of 120 nm, 170 nm, and 220 nm. We found that the rate and extent of the solubilized materials increased as the oil concentration in emulsion increased, the diameter of emulsion droplets decreased, the temperature of solubilization increased, and the melting points of the fatty acids decreased. The crystallization temperatures of the fatty acids in emulsion remarkably decreased compared with bulk oil. The present study demonstrated that the fatty acids became more solubilized and less crystallized when they are embedded in the nm-size emulsion droplets.

1,3-Dipalmitoyl-2-oleoyl-sn-glycerol (POP) and 1,2-dioleoyl-3-palmitoyl-rac-glycerol (OOP) are two major molecular species that account for roughly half of the total triacylglycerols in palm oil. The binary phase behavior of a POP/OOP mixture plays an important role in the crystallization of palm oil. We conducted thermodynamic and kinetic studies of OOP and its mixtures with POP using differential scanning calorimetry and X-ray diffraction with a conventional generator and synchrotron radiation. We found that OOP has two polymorphs, α as a metastable form and β′ as the most stable form, and that the two forms are stacked in a triple-chain-length structure. The POP/OOP mixtures exhibited immiscible eutectic natures in both their metastable and their most stable states, in contrast to POP/1,2-dipalmitoyl-3-oleoyl-rac-glycerol and POP/1,3-dioleoyl-2-palmitoyl-sn-glycerol mixtures, in which molecular compounds of a double-chain-length structure were formed. A time-resolved synchrotron radiation X-ray diffraction study undertaken during the cooling and heating processes indicated that the α and β′ forms of the POP and OOP fractions crystallized and melted in separate manners, and that crystallization of the β′ form and the polymorphic transformation from α to β′ of POP and OOP are promoted in the presence of another component. The absence of molecular compound crystals in the binary mixtures of POP/OOP is explained by taking into account the molecular interactions of acyl chain packing, glycerol conformation, and methyl end stacking, among which glycerol conformation appeared to be most influential.

We observed the crystallization and polymorphic transformation processes of palm stearin (PS) dispersed in nanometer-size oil-in-water emulsion droplets (nanometer-size droplets, average diameter 120 ± 30 nm) using DSC and in-situ synchrotron radiation X-ray diffraction (SR-XRD) techniques. The nanometer-size emulsion droplets were prepared by using four types of highly hydrophilic polyglycerine fatty-acid mono-esters, in which a polar moiety was made of 10 polymerized glycerins (decaglycerine, 10G) and nonpolar moieties were lauric, myristic, palmitic, and stearic fatty acids. When decaglycerin monolaurate (10G1L) was employed, crystallization temperatures (Tc's) of high-melting and low-melting fractions of PS were 8 and 2 °C, which were decreased from 31 and 3 °C for the bulk PS. However, the Tc of the high-melting fraction of PS was increased as the fatty-acid moiety was changed to myristic (10G1M), palmitic (10G1P), and stearic (10G1S) acids; in particular, the Tc became 38 °C with 10G1S. Furthermore, the crystallization of the high-melting fraction with 10G1M, 10G1P, and 10G1S exhibited only wide-angle SR-XRD patterns without small-angle SR-XRD patterns, indicating that these crystals formed thin films in the nanometer-size emulsion droplets that did not diffract small-angle SR-XRD patterns. We did not observe these features in our previous work on PS crystallization in bulk and micrometer-size emulsion droplets (Sonoda, et al. J. Am. Oil Chem. Soc. 2004, 81, 365). It was assumed that the freezing of a high-melting emulsifier may act as a template for nucleation of a high-melting fraction of PS, whose effects are more remarkable in nanometer-size emulsion than in micrometer-size emulsion, due to the tight packing of the hydrophobic region of the interfacial emulsifier membrane of the nanometer-size emulsion.

Rheological analysis was conducted to study the physical properties of the mixtures of high-melting (HM) and low-melting (LM) fats. The HM and LM fats examined in the present work were fully hydrogenated rapeseed oil with a high amount of behenic acid (FHR-B), and sal fat olein (SFO). Gel-like behavior of the FHR-B/SFO fat mixture with FHR-B concentration of 1.5–4.0 wt% indicated the existence of many small FHR-B crystals of β polymorph that were formed by the following thermal treatment: rapid cooling (Vc=10 °C/min) from 70 °C to crystallization temperature (Tc=20 °C), and heating from Tc to final temperature (Tf=38 °C) where the fat mixture was held for 60 min. These results were obtained: (1) FHR-B/SFO fat mixture exhibiting gel-like behavior formed by Vc=10 °C/min exhibited pseudo-plastic flow properties with the apparent yield value at Tf=38 °C. Storage moduli G′ showed approximately the same values during the frequency scanned. Loss moduli G″, which were smaller than G′, decreased with increasing frequency in the measured range, whereas the gel-like fat was not formed at Vc=1.5 °C/min. In this mixture, G′ and G″ were increased with increasing frequency, resulting in smaller values than those of the rapid cooling. Additionally, G″ dominated the viscoelasticity, which corresponds to liquid-like behavior. (2) At the FHR-B concentration of 1.5 wt% or higher, the FHR-B/SFO fat mixtures exhibited gel-like behavior with proper thermal treatment, whereas G′ and G″ showed liquid-like behavior when FHR-B concentration was less than 1.5 wt%. (3) G′ and G″ of gel-like behavior of FHR-B (2.0 wt%)/SFO (98.0 wt%) decreased drastically around Tf=38 °C, and rheological behavior changed from gel-like to liquid-like at 43 °C. (4) The many small β crystals of FHR-B crystallized by proper thermal treatment may play decisive roles in viscoelastic behavior of β-fat gel.

The crystallization and polymorphic behavior of palm stearin (PS) in a bulk state and in oil-in-water (O/W) emulsion droplets (average diameter, 1.7±0.3 μm) was observed by using DSC, optical microscopy, and in situ X-ray diffraction with synchrotron radiation (SR-XRD). For the bulk sample the DSC measurements revealed three main exothermic peaks at approximately 31 (large), 21 (small) and 3°C (medium) on cooling, and broad endothermic peaks at approximate −3 (small), 8, 15 to 25 (medium), and 37 and 53°C upon heating. The SR-XRD patterns taken during cooling from 60 to −5°C clarified that the DSC exothermic peaks around 31 and 3°C corresponded to crystallization of the α form of high-melting and low-melting fractions, respectively, and that the occurrence of β′ corresponded to the small exothermic peak around 21°C. The XR-XRD patterns taken during heating from −5 to 60°C demonstrated that the DSC endothermic peaks corresponded to the following transformation processes: melting of α of the low-melting fraction (−3°C), melt-mediated transformation from α to ∇′ (15–25°C), melting of β′ (36°C), and melting of β (53°C) of the high-melting fraction. As for the O/W emulsion sample, the DSC and SR-XRD measurements during the cooling and heating processes exhibited basically the same behavior as that of PS in the bulk state, except that β′ did not crystallize during the cooling process, and the temperatures of crystallization of α, melt-mediated α→β′→β transformation, and melting of β were lower in the emulsion droplets than in the bulk state.