Abstract: The freeze-thaw stability of composite emulsion systems prepared with ultrasonic-modified soy protein isolate (SPI) and soluble soy polysaccharide (SSP) was studied and correlated with their structural properties. Confocal laser scanning microscopic (CLSM) analysis of the emulsions before and after two freeze-thaw cycles was conducted. Simultaneously, changes in the percentage of solid fat content (SFC) during isothermal crystallization, the amount of emulsified oil and the secondary structure of the emulsifier SPI under different ultrasonic treatments (0, 200, 300, 400, and 500 W) were investigated as well as their relationships with the freeze-thaw stability of the emulsion system. The results showed that the coalescence of droplets in the emulsion system after being subjected to two freeze-thaw cycles decreased with the increase of ultrasonic power. The emulsion with SPI subjected to ultrasonic treatment at 400 W showed the highest stability. The rate of increase in SFC during isothermal crystallization varied among different emulsion systems, but the total SFC was identical in the final balanced emulsions. The amount of emulsified oil also changed. Similarly, different ultrasonic treatments changed the secondary structure of soy protein isolate. The highest random coil structure content in soy protein isolate subjected to 400 W ultrasonic treatment was observed. SPI treated at different ultrasonic powers formed complexes having different structures with SSP, affecting the freeze-thaw stability of emulsions. Therefore, this study indicated that appropriate ultrasonic treatment could improve the conformation and spatial structure of soy protein isolate and promote its binding with soluble polysaccharide molecules, consequently affecting the interfacial structural properties between soy protein isolate and soluble polysaccharide and the freeze-thaw stability of the emulsion system.

Key words: ultrasonic treatment, soy protein isolate, soy soluble polysaccharide, freeze-thaw stability