DSC and synchrotron‐radiation X‐ray diffraction studies on crystallization and polymorphic behavior of palm stearin in bulk and oil‐in‐water emulsion states

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.