Submitted to: Annual Meeting and Expo of the American Oil Chemists' Society
Publication Type: Abstract Only
Publication Acceptance Date: 3/15/2005
Publication Date: 5/1/2005
Citation: Zhang, L., Ueno, S., Sato, K., Adlof, R.O., List, G.R. 2005. Polymorphism and crystallization kinetics of 1,2-dioleyol-3-stearoyl-sn-glycerol [abstract]. 96th Annual Meeting and Expo of the American Oil Chemists' Society. p.13. Interpretive Summary:
Technical Abstract: Polymorphism of triacylglycerols (TAGs) is an important phenomenon which influences the physical and chemical properties of fats employed in foods, pharmaceuticals, cosmetics, etc. In particular, precise analysis of kinetic properties of fat crystallization is closely related to the technical control of fat crystallization in confectionery and food industry. The structural properties of TAG crystals are sensibly influenced by molecular properties of TAGs, such as saturated/unsaturated fatty acid moieties, glycerol conformations, symmetry/asymmetry of the fatty acid composition connected to the glycerol group, etc. Particular attention should be paid to saturated-unsaturated (Sat-Unsat) mixed acid TAGs, since these TAGs are most abundantly present in vegetable oils and fats for edible applications. For these Sat-Unsat TAGs, enough basic data is not yet available. Furthermore, most naturally occurring TAGs show optical activity, indicating that the specific distributions of fatty acids at the sn-1 and sn-3 positions are different. Most of the previous studies have been done by using racemic type TAGs, however, properties of the racemic mixtures are not necessarily the same as those of the specific isomers. Stereospecifically-numbered (sn) 1,2-dioleoyl-3-saturated- acyl-sn-glycerols (O-O-Sat) were studied by Fahey et al (Fahey et al, Biochemistry 1985). In O-O-Sat, the carbon number of the sn-3 even-numbered saturated fatty acid chain was varied from 14 to 24. They pointed out that the beta subcell reflections intensified when the compounds were heated to within 5 deg C of the melting temperature of the highest melting polymorph and the stable form of all compounds exhibits a triple layer packing mode. But the details of polymorphic transformation were missing and the evidence of the alpha phase was not seen. Since, in some cases, the solvent crystallization was the single way to grow the most stable form of crystal, it is necessary to examine the crystalline phases grown via solution-mediated transformation. In the present work, we report the packing arrangements of the most stable crystal form of 1,2-dioleyol-3-stearoyl-sn-glycerol (sn-SOO) obtained from the solvent crystallization and melt-mediated transformation, which were studied by using DSC and X-ray diffraction. In the solvent crystallization, acetonitrile and octane were chosen, because of the remarkable difference of solubility. The thermal and structural properties of solvent-grown crystals showed basically the same as those of melt-grown crystals incubated at 20 deg C over one month. These results indicate that the most stable crystal structure of sn-SOO is beta-prime form with triple chain length structure. Furthermore, kinetic properties in polymorphic crystallization of sn-SOO in alpha-melt-mediated transformation have been unveiled by in-situ time-resolved synchrotron X-ray diffraction.