Impact of polyelectrolyte complex layer on the stability of palm oil multiple emulsions encapsulating a water-soluble compound during heating, cooling, and storage processes

The effect of an interfacial polyelectrolyte complex layer on the stability of multiple water-in-oil-in-water (W/O/W) emulsions containing palm oil (oil phase) was investigated. Monodisperse W/O/W emulsions prepared by primary ultrasonic emulsification using polyglycerin polycondensed ricinoleic acid ester as an oil-soluble emulsifier followed by secondary microchannel emulsification contained multiple sodium caseinate-stabilized droplets with a mean diameter of 44 μm. The multiple droplets containing palm oil maintained their diameter at 60 °C for a minimum of 24 h, whereas significant aggregation of the multiple droplets, with a partially crystallized palm oil phase, was observed below its crystallization temperature (∼27 °C). The formation of a chitosan-sodium caseinate complex layer by the electrostatic interaction of positively charged chitosan with negatively charged sodium caseinate at the oil–water interface could suppress the aggregation of the multiple droplets, even below the crystallization temperature of palm oil. Calcein, a water-soluble fluorescent dye, was stably encapsulated for a month with a high entrapment yield at 25 °C with a chitosan-sodium caseinate complex layer, although without the complex layer, it leaked to the external water phase within a day. The W/O/W emulsion with the polyelectrolyte complex layer improved both the droplet dispersibility and encapsulation efficiency of the multiple droplets during heating at 80 °C, followed by cooling at 25 °C. Polarized microscopic observations indicated that the chitosan-sodium caseinate complex layer affected the crystallization of palm oil in multiple droplets: partial crystallization of palm oil in the multiple droplets may be delayed by the presence of the polyelectrolyte complex layer at the oil–water interface.