Abstract: Compared to the non-covalent complex, the covalent complex between black soybean protein isolate (BSPI) and chlorogenic acid (CA) was found to have higher polyphenol-binding capacity. Fluorescence spectroscopy and circular dichroism (CD) spectroscopy were used to analyze the effect of CA cross-linking on BSPI structure. It was found that after the addition of CA, the secondary and tertiary structures of BSPI were changed. The proportion of α-helix decreased significantly and the proportion of random coil increased significantly. The protein structure became more loose and disordered, and this effect was more obvious for the covalent complex than the non-covalent one. Crosslinking with CA greatly improved the emulsifying capacity and antioxidant activity of BSPI. Nanoemulsions prepared with the BSPI-CA conjugates had smaller mean particle sizes and higher absolute values of zeta-potential than those prepared with native BSPI. The storage stability and oxidative stability of nanoemulsions with the BSPI-CA conjugates were enhanced significantly, and the covalent conjugate was more beneficial to the basic properties and stability of nanoemulsions.

Key words: black soybean protein isolate; chlorogenic acid; covalent interaction; non-covalent interaction; emulsifying capacity; antioxidant activity