Abstract: Soybean protein isolate (SPI) and lecithin can be self-assembled into a complex system at neutral condition (pH 7.0), while the unassembled compounds are still present in the solution. In an effort to achieve maximum complexation between SPI and lecithin and to elucidate the relationship between the functional properties of the complex system and the secondary structure of soybean proteins, we examined the structural variability of the system by Fourier transform infrared spectroscopy and measured its water-holding capacity, oil-holding capacity, gel texture characteristics and functional properties under ultrasound irradiation. The results showed that ultrasonic treatment significantly improved functional properties of the soybean protein-lecithin complex. Water-holding capacity and oil-holding capacity increased firstly then decreased with increasing ultrasonic power after a short irradiation time, but these functional properties continuously decreased with increasing ultrasonic power after a long irradiation time. Fourier transform infrared spectroscopic analysis revealed that the secondary structure of SPI contained a lower content of α-helix and a higher content of β-structure after ultrasonic irradiation at low and medium powers, indicating stronger protein-lecithin interactions. There was a certain relationship between the gel texture characteristics of the complex system and the secondary structure of SPI. The structure of soybean protein-lecithin system could be changed by proper ultrasonic treatment, thus improving its functional properties.

Key words: soybean protein, lecithin, functional properties, structural properties, Fourier transform infrared spectroscopy