Polymorphism and crystallization behaviour of triacylglycerols. From pure components to mixtures rich in oleic acid

Abstract

Quality attributes of food products such as visual appearance, melting behavior, or shelf-life are strongly determined by the crystallization behavior of natural fats and oils within. In turn, this behavior depends on the polymorphism and the sort of mixing states (eutectic phase, solid solution phase, molecular compound formation) displayed by their major triacylglycerol components. Therefore, it becomes essential that studies regarding lipid crystallization cover from pure molecular components and their mixtures to real fats and oils.

In connection with the above, this thesis focuses on gaining further fundamental knowledge about the polymorphism and crystalline behavior of lipid systems with varying degrees of complexity. Thus, the first stage of the research involved a study of pure triacylglycerols based on palmitic, lauric, and oleic acids (P, L, and O, respectively), which are typical of natural fats and oils such as lard and palm, olive, and coconut oils. During the second stage, binary and ternary mixtures including these triacylglycerols were examined to obtain an insight into their solid-state miscibility properties. Concretely, the mixtures were composed of a fully saturated triacylglycerol and one or two saturated-unsaturated triacylglycerols able to form a molecular compound. Despite the presumable crucial role of interactions including triacylglycerols forming molecular compound in the physical behavior of natural lipids and their blends, the phase behavior of triacylglycerol mixtures with such configuration is barely known. Finally, the third stage consisted of a more applied investigation of the crystalline and thermal properties of blends including natural fats. More specifically, binary blends of cocoa butter and shea butter stearin. Regarding the investigation of pure TAGs, calorimetric and X-ray diffraction techniques were used to examine the polymorphic occurrence and transformation behavior of PPP, LLL, OPO, and PPO during the application of dynamic thermal treatments at varying cooling and heating rates. The results showed that the polymorphic occurrence on cooling, as well as the sequence and type of transformations during heating, were influenced by the rate of the thermal treatments applied. However, it was also found that the extent of the kinetic influence depended on the intrinsic properties of triacylglycerols such as the length of the fatty acid chains or the characteristic polymorphism. The mixtures under examination during the phase behavior studies included the binary systems PPP/POP, PPP/PPO, PPP/OPO, LLL/POP, and LLL/PPO, and the ternary systems PPP/POP/OPO, PPP/POP/PPO, and LLL/POP/PPO (keeping an equimolecular ratio of POP/OPO or POP/PPO to favor molecular compound formation). Eutectic behavior with partial miscibility was observed under metastable and stable conditions. Furthermore, the results obtained for stable binary mixtures showed that the degree of miscibility was primarily influenced by the oleic acid content of the unsaturated component, as well as the differences in the size of the triacylglycerols within the mixture. From the solid-state miscibility observed in ternary systems containing LLL or PPP, it was concluded that third components may affect the mixing interactions associated with molecular compound formation. In addition, the results suggested that the ternary interactions studied may help to explain the crystallization behavior of edible lipids like palm oil and be useful in fat structuring applications. As a case study of real fat blends, combinations of cocoa butter and shea butter stearin at varying proportions were examined through calorimetric, X-ray diffraction, spectroscopic, and microscopic techniques. These made possible to characterize the effect of composition on the polymorphic crystallization and transformation properties, the melting profile, and the microstructural development. The results obtained for some of the parameters under study suggested that specific combinations might be considered for its application in chocolate-type confectionery products.

Atributos de calidad de productos alimentarios con base lipídica, como la apariencia visual, el perfil de fusión o la vida útil, están determinados por el comportamiento cristalino de las grasas, polimorfismo y del estado de mezcla (fases eutécticas, fase de solución sólida, formación de compuesto molecular) de sus triacilgliceroles mayoritarios. Por tanto, los estudios en cristalografía de lípidos deben abarcar desde componentes moleculares puros y sus mezclas hasta grasas y aceites reales.

En línea con esto, la presente tesis se centra en el estudio del polimorfismo y comportamiento cristalino de sistemas lipídicos atendiendo a los diferentes grados de complejidad. Por un lado, se ha investigado la cristalización y transformación polimórfica bajo condiciones dinámicas de temperatura de triacilgliceroles basados en ácidos palmítico, láurico y oleico (P, L y O, respectivamente), comunes en la manteca y los aceites de palma, oliva y coco. Además, se han estudiado mezclas binarias y ternarias incluyendo estos triacilgliceroles para determinar sus propiedades de miscibilidad en estado sólido. Específicamente, las mezclas han consistido en un triacilglicerol trisaturado y uno o dos triacilgliceroles con cierto grado de insaturación capaces de formar un compuesto molecular. A pesar del rol fundamental de las interacciones entre triacilgliceroles capaces de formar un compuesto molecular y otros componentes en las propiedades físicas de lípidos naturales y sus combinaciones, el comportamiento de fase de mezclas de triacilgliceroles con esta configuración apenas se ha investigado con anterioridad. Por último, esta tesis también recoge un estudio de carácter más aplicado sobre las propiedades térmicas y cristalinas de mezclas de grasas naturales. Más concretamente, de mezclas binarias de manteca de cacao y estearina de karité. El interés de este estudio reside en la importancia a nivel económico de los productos relacionados con el chocolate y en la necesidad de encontrar nuevas mezclas lipídicas que permitan satisfacer la creciente demanda global de manteca de cacao u obtener nuevas funcionalidades en los productos finales.