Crystallization of the Coconut Oil at the Oil/Water Interface: Impact of Monoglycerides and Sodium Caseinate

Abstract

Fats and oils are ubiquitous in various research fields, such as food, cosmetic, and pharmaceutical industries, and their crystallization process at the water interface is critical for determining product texture, stability, and functionality. However, the molecular mechanisms governing crystallization at the oil/water interface remain poorly understood. This study investigates in situ interfacial solidification and crystallization of coconut oil using interface-specific vibrational sum-frequency generation spectroscopy, focusing on the effects of adding monoglycerides and proteins. For pure coconut oil, cooling induces a gradual ordering of triacylglycerol molecules at the aqueous interface, preceding bulk solidification of coconut oil. By contrast, the addition of monoglycerides significantly promotes the interfacial crystallization upon cooling. Notably, saturated monoglycerides induced molecular ordering at higher temperatures than unsaturated ones, acting as effective interfacial templates that facilitate heterogeneous nucleation of microdomains. Conversely, sodium caseinate was strongly adsorbed at the aqueous interface regardless of temperature or the presence of monoglycerides. This protein layer significantly suppressed the crystallization of triacylglycerols and modified the interfacial water environment, as reflected in the weak temperature dependence of the OH/NH region SFG response. These findings elucidate the opposing molecular mechanisms of monoglycerides as crystallization promoters and sodium caseinate as an inhibitor. This molecular-level insight into competitive crystallization phenomena of oil/water interface provides a critical foundation for the rational design of stable, high-quality emulsion systems. More broadly, these results identify the buried oil/water interface as an active molecular environment in which competitive adsorption and interfacial organization determine the crystallization pathways in multicomponent soft-matter systems.