Crystallization of Polymorphs - Jingfei Guo

. .

Résumé :
This thesis describes the polymorphism of two carotenoid compound, astaxanthin and apocarotenoic ester, on their crystal structures and properties. 3,3'-dihydroxy-ß,ß-carotene-4,4'-dione (astaxanthin) is a carotenoid used for the pink coloration in animal and fish foods. The screening methods for obtaining different polymorphic forms are described in details. Two nonsolvated and one solvated forms of this compound have been discovered and characterized using different analytical techniques such as XRPD, DSC, TG, Raman Spectroscopy, microscopy, etc. The variations in morphology, color and solubility of different polymorphic forms are evaluated and compared. The thermodynamic stability of the relevant polymorphs is revealed. The influences of process conditions on the phase transformation are discussed. Factors such as temperature, solvent and retention time have been considered. The transformation rate depends strongly on the selection of solvent medium. Therefore, the right process conditions can be developped according to the requirement of the final crystal form. Moreover, different chemical stability refers to the shelf life is studied correlating to the different polymorphs. The results show the possibilities to improve the pigmentation efficiency and chemical stability by a changing in the crystal polymorphs. The second compound, apocarotenoic ester shows the limitation of application of conventional crystallization methods of polymorphism studying due to the very less chemical stability.