Thermal properties, powder X-ray diffraction patterns and FT-IR absorption spectra of crystals of two isomers of conjugated linoleic acid (CLA), 9-cis, 11-trans-CLA (c9t11), 10-trans, 12-cis-CLA (t10c12) were examined. To search for polymorphic modifications, we carefully performed crystallization from melt and solution phases, and isolated one type of crystalline form in c9t11 and t10c12. The melting temperature (T m ) was 14.9°C, enthalpy of fusion (DH) was 38.7 kJ/mol, and entropy of fusion (DS) was 134 J/mol K for c9t11, and T m = 19.8°C, DH = 35.6 kJ/mol and DS = 122 J/mol K for t10c12. The X-ray diffraction and FT-IR measurements indicated O \ subcell packing in the crystals of c9t11 and t10c12, and long spacing values of 4.22 nm for c9t11 and 3.88 nm for t10c12. The unique molecular structures of the two isomers of CLA are discussed in comparison to the polymorphism of oleic acid, petroselinic acid, elaidic acid and linoleic acid, all of which are unsaturated fatty acids having the same carbon number of 18 as that of the two CLA isomers.
We have investigated the polymorphism of triacylglycerol (TAG) crystals as they affect the qualities such as shelf life, mouth feel, and texture of chocolate and other products. Saturated monoacid TAGs, like trilaurin, are considered as models for TAG crystallization; however, there is still debate about the number of their polymorphs that exist. In this study, we characterized a set of novel polymorphs, β forms of saturated monoacid TAGs, which were obtained via different pathways depending on the crystallization history, by polarized light microscopy, X-ray diffraction, and differential scanning calorimetry. Saturated monoacid TAGs were crystallized as the unstable polymorphs, the a or β’ forms first, and then they were transformed into β forms by solid–solid transformations. The βform that had transformed from β’ changed its morphology by a polymorphic transformation, while the β form made from the a form kept its spherulite morphology. The β forms obtained showed different melting points. Additional heat treatment promoted further polymorphic transformation. Four novel β forms were found for each of the saturated monoacid TAGs, trilaurin, trimyristin, tripalmitin, and tristearin. They showed similar polymorphism with the same subcell packing.
Conjugated linoleic acid (CLA) is commercially available as a mixture consisting of almost equal amounts of the cis-9,trans-11-CLA (c9,t11) and trans-10,cis-12-CLA (t10,c12) isomers. Separation of the two isomers is highly significant since each exhibits different biochemical properties. Highly efficient separation could be accomplished by crystallization in acetone (solvent) of the two CLA isomers (solutes) in the presence of medium-chain fatty-acid (MCFA) additives. The relative concentration ratios of the two CLA isomers in the solvent-crystallized materials varied depending on which MCFA were added. Addition of lauric and decanoic acids resulted in the crystals predominantly containing t10,c12, whereas octanoic acid yielded those predominantly containing c9,t11. We have confirmed that onetime solvent crystallization using decanoic acid and octanoic acid additives increased the t10,c12 and c9,t11 concentrations, and that repeated solvent crystallization resulted in the ratio of c9,t11 to t10,c12 of at least 4:96 or 98:2.
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