Effects of Emulsifiers on Crystallization Behavior of Lipid Crystals in Nanometer-Size Oil-in-Water Emulsion Droplets

Abstract

We observed the crystallization and polymorphic transformation processes of palm stearin (PS) dispersed in nanometer-size oil-in-water emulsion droplets (nanometer-size droplets, average diameter 120 ± 30 nm) using DSC and in-situ synchrotron radiation X-ray diffraction (SR-XRD) techniques. The nanometer-size emulsion droplets were prepared by using four types of highly hydrophilic polyglycerine fatty-acid mono-esters, in which a polar moiety was made of 10 polymerized glycerins (decaglycerine, 10G) and nonpolar moieties were lauric, myristic, palmitic, and stearic fatty acids. When decaglycerin monolaurate (10G1L) was employed, crystallization temperatures (Tc's) of high-melting and low-melting fractions of PS were 8 and 2 °C, which were decreased from 31 and 3 °C for the bulk PS. However, the Tc of the high-melting fraction of PS was increased as the fatty-acid moiety was changed to myristic (10G1M), palmitic (10G1P), and stearic (10G1S) acids; in particular, the Tc became 38 °C with 10G1S. Furthermore, the crystallization of the high-melting fraction with 10G1M, 10G1P, and 10G1S exhibited only wide-angle SR-XRD patterns without small-angle SR-XRD patterns, indicating that these crystals formed thin films in the nanometer-size emulsion droplets that did not diffract small-angle SR-XRD patterns. We did not observe these features in our previous work on PS crystallization in bulk and micrometer-size emulsion droplets (Sonoda, et al. J. Am. Oil Chem. Soc. 2004, 81, 365). It was assumed that the freezing of a high-melting emulsifier may act as a template for nucleation of a high-melting fraction of PS, whose effects are more remarkable in nanometer-size emulsion than in micrometer-size emulsion, due to the tight packing of the hydrophobic region of the interfacial emulsifier membrane of the nanometer-size emulsion.