POP (1,3-dipalmitoyl-2-oleoyl glycerol), a major triacylglycerol component of vegetable and animal fats found in chocolate, fat spread, and other edible fats, crystallizes in seven polymorphic forms: α, γ, δ, β′₂, β′₁, β₂, and β₁. The crystallization and transformation pathways of the polymorphs of POP are influenced by many factors. In the present work, we studied the influence of the rates of cooling and heating on the polymorphic crystallization and transformation of POP. The rate of cooling (heating) was changed from 15 to 2, 1, and 0.5 °C min⁻¹ (15 to 2, 1, 0.5, and 0.1 °C min⁻¹). The crystallization and transformation processes were examined with differential scanning calorimetry (DSC) and synchrotron radiation X-ray diffraction (SR-XRD). The results indicated that more stable forms were formed in a higher quantity when POP was slowly cooled and heated, whereas less stable forms occurred at higher rates of cooling and heating. Specifically, less stable α and γ forms were directly obtained at cooling rates of 15 to 0.5 °C min⁻¹. The more stable forms of β′ or β did not occur even at a rate of cooling of 0.5 °C min⁻¹, whereas α or γ transformed to β′ or β at heating rates of 0.5 and 0.1 °C min⁻¹. The polymorphic transformations occurred either in solid-state or melt-mediation and were largely influenced by the heating rates. We discuss the present results by considering the differences in activation free energies for the transformations from α or γ to more stable forms, which may determine the heating-rate-dependent transformation pathways.

The crystallization properties of triacylglycerols (TAGs) strongly determine the functional properties of natural lipids. The polymorphic and mixing phase behavior of TAG molecules have long been, and still are, a hot topic of research with special relevance for the cosmetic, pharmaceutical, and food industry. To avoid the difficulties arising from the study of whole real fats, studies at the molecular level on mixtures of a limited number of TAGs has become an indispensable tool to identify the underlying causes of the physical properties in lipid systems. In particular, phase diagrams of binary mixtures of TAGs exhibiting a different degree of heterogeneity (monoacid or mixed fatty acids; molecular symmetry; the presence of cis or trans double bonds) have resulted in a significant breakthrough in our knowledge about structure–interaction–function relationships. The present work aims to provide an overview of the main reports regarding binary and ternary TAG systems, from the early studies to the most recent developments.