Effect of fatty acid structure on the morphology of spherulites formed from jet cooked mixtures of fatty acids and defatted cornstarch

Abstract

Aqueous mixtures of defatted cornstarch and fatty acids were jet cooked and slowly cooled in order to examine the influence of specific fatty acids on spherulite morphology. Addition of 0.7% palmitic acid yielded a spherulite mixture containing 97% of the torus/disc species, having the expected 6 1 V X-ray diffraction pattern. Linoleic acid, as either the free acid or the partially neutralized sodium salt, yielded spherical/lobed spherulites; and diffraction patterns indicated the 7 1 V conformation. Oleic acid also yielded largely the spherical/lobed species, consistent with its mono-unsaturated structure. The spherulite mixture with palmitic acid varied with both the pH and the amount of palmitic acid used relative to starch. When palmitic acid was partially neutralized with NaOH, significant quantities of spherical/lobed spherulites were observed in addition to the expected torus/disc species. Increasing the amount of free palmitic acid from 0.7% to 2.5%, based on starch, yielded a spherulite mixture containing 77% spherical/lobed particles. Stearic acid yielded a spherulite mixture that contained only 20% of the torus/disc species and 80% of a lobed species that unexpectedly exhibited a 6 1 V diffraction pattern. Myristic acid yielded both torus/disc spherulites and a small, narrow-lobed spherulite species that was almost rod-like in appearance. The fact that different spherulite mixtures were observed with fatty acids having similar chemical structures (and also with the same fatty acid under different experimental conditions) indicates that the influence of fatty acid structure on the conformation of the amylose helix is not the only factor controlling spherulite morphology. The variety of different morphologies observed for these particles under different experimental conditions can be explained by the well-known fact that spherulites are formed from chain-folded, crystalline lamellae that have individual chains of complexed amylose oriented perpendicular to the plane of each lamella. Variables that can affect the formation and growth of crystalline lamellae are considered.