Improved Stability of the Palm Oil-in-Water Emulsion via the Surface Modification of Droplets with a Chitosan-Casein Complex Layer

The stabilization effect of a biopolymer complex layer consisting of sodium caseinate (SC) and chitosan (CHI) formed via electrostatic interaction was investigated using an oil-in-water (O/ W) emulsion containing palm oil droplets as the dispersed phase. O/W emulsions containing palm oil droplets with a narrow droplet diameter distribution were successfully prepared using two microchannel emulsification devices using SC as an emulsifier at 60°C. Surface modification of the palm oil droplets was conducted using different CHI/SC ratios. The surface charge of the droplets was negative and positive at CHI/SC ratios of less than 0.025 and more than 2.5, respectively. At an intermediate CHI/SC ratio between 0.025 and 2.5, the droplets formed large aggregates and became polydisperse. The adsorption of CHI on the surface of palm oil droplets at a high CHI/SC ratio was confirmed by fluorescent microscopy and ζ-potential measurement. Improved stability of the O/ W emulsions with the CHI-SC complex layer was demonstrated during heating (60°C) and cooling (20°C) treatment, and long-term (7 months) storage at room temperature (20-25°C). Interestingly, separation of the large oil phase on the top of the emulsion samples and the formation of large crystals of palm oil were suppressed in the emulsion samples with the CHI-SC complex layer.