Many physical properties (e.g., hardness, texture, rheology, and spreadability) of lipid-based products largely depend on the extent of crystallization and transformation of lipids, and their network formation. Therefore, many studies have focused on controlling the crystallization of lipids in order to determine the functionality of lipid crystals. Both internal and external factors greatly affect the processes of lipid crystallization. The most important internal factors are polymorphism, which depends on variation in fatty acid moieties, and the composition or blending of different lipid materials. Important external factors are thermal treatment, additives, application of shear, sonication, and pressure. This paper briefly reviews recent advances in research on these external factors. We discuss the results by considering the relationships between external factors and thermodynamics, as well as kinetic properties of the crystallization and transformation of polymorphic forms of lipid crystals.

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In situ observation of transformation pathways of polymorphic forms of 1,3-dipalmitoyl-2-oleoyl glycerol (POP) examined with synchrotron radiation X-ray diffraction and DSC3

We systematically examined the phase behavior of binary mixtures of mixed-acid triacylglycerols (TAGs) containing palmitic and oleic acid moieties 1,3-dioleoyl-2-palmitoyl-glycerol (OPO), 1,2-dipalmitoyl-3-oleoyl-rac-glycerol (PPO), and 1,2-dioleoyl-3-palmitoyl-rac-glycerol (OOP), which are widely present in natural fats and are employed in the food, pharmaceutical, and cosmetic industries. Differential scanning calorimetry and X-ray diffraction methods were applied to observe the mixing behavior of PPO/OPO, OOP/OPO, and PPO/OOP under metastable and stable conditions. The results led to three conclusions: (1) Eutectic behavior was observed in PPO/OPO. (2) Molecular compound (MC) crystals were formed in the mixtures of OOP/OPO and PPO/OOP. (3) However, the MC crystals occurred only under metastable conditions and tended to separate into component TAGs to form eutectic mixture systems after 17 months of incubation. These results were contrary to those of previous studies on 1,3-dipalmitoyl-2-oleoyl glycerol (POP)/OPO and POP/PPO in which the MC crystals were thermodynamically stable. We determined that specific molecular interactions may cause this different phase behavior (stability of POP/OPO and POP/PPO MC crystals and metastability of OOP/OPO and PPO/OOP MC crystals). All results confirm the significant effects of molecular structures of glycerol groups, interactions of fatty acid chains, and polymorphism of the component TAGs on the mixing behavior of mixed-acid TAGs.

We examined the influence of kinetics on the polymorphic behavior of 1,3-dioleoyl-2-palmitoyl glycerol (OPO), a triacylglycerol (TAG) present in natural oils such as olive oil and human breast milk. Pure OPO was heated at 15ºC• min-1 in all cases. The polymorphic crystallization of OPO was studied at different cooling rates (15º, 2º, 1º, and 0.5ºC• min-1), and the dynamic polymorphic transformations were characterized on heating by simultaneously using Differential Scanning Calorimetry (DSC) and Synchrotron Radiation X-Ray Diffraction (SR-XRD) with smallangle X-ray diffraction (SAXD) and wide-angle X-ray diffraction (WAXD). Thermo-optical Microscopy (TOM) was also used in order to observe the polymorphic transitions. Polymorphic forms of OPO were identified and characterized. As the cooling rate decreased, more stable forms crystallized, not following the Ostwald step rule, although in most cases concurrent crystallization occurred and the whole process was highly complex.