Effects and mechanisms of ultrasonic-assisted emulsification on the properties of rennet casein emulsions under low salt conditions

The physical instability of rennet casein emulsions in natural systems limits their applications in low-salt products. This study was designed to investigate the effects and mechanisms of ultrasonic-assisted emulsification on the properties of rennet casein emulsion under simulated milk ultrafiltrate conditions. Results showed that ultrasonic treatment at 500 W for 7 min can significantly improve the stability of a rennet casein-corn oil emulsion. Compared with rennet casein emulsions prepared by high-speed homogenization, the emulsions prepared by ultrasonics had a higher emulsifying activity index (0.17-1.00 m²/g) and better storage stability. The apparent viscosity of rennet casein emulsions was also increased after ultrasonic treatment. Ultrasonic treatment significantly reduced the droplet size (D_3,2 decreased from 8.10 to 1.66 μm) and protein aggregates in the rennet casein emulsions. The concentration of rennet casein at the water-oil interface and the absolute zeta potential of emulsions were increased after ultrasonic treatment. The oil contact angle and surface hydrophobicity of rennet casein after ultrasonic treatment were also higher than those of the high-speed shear homogenization-treated samples. In summary, ultrasonic-assisted emulsification treatment can significantly improve the stability of rennet casein emulsions under natural conditions.