Effect of phlorotannins modification on the physicochemical, structural and functional properties of soybean protein isolate and controlled hydrolysates: Covalent and non-covalent interactions

In this research, the physicochemical, structural and functional properties of soybean protein isolate (SPI) as well as controlled hydrolysates (HSPI) were investigated by treatment with different concentrations of phlorotannins (PT) (0, 0.2, 0.5, 1.0, 2.0, mg/mL) through covalent and non-covalent interactions. Native/SDS-PAGE confirmed the formation of covalent and non-covalent complexes between PT and SPI/HSPI, and the presence of covalent bonds other than disulfide bonds in the covalent complexes (C–N, C–S bonds). After binding to PT, the secondary and tertiary structures of SPI/HSPI were changed. The secondary structure of complexes was transformed from β-sheet and β-turn to α-helix and random coil, and higher rates of transformation for covalent complexes. With the increase of PT concentration, the fluorescence and UV intensity of covalent/non-covalent complexes gradually reduced and rose, respectively; covalent complexes had lower fluorescence and higher UV intensity at equal concentrations. The particle size and ζ-potential of covalent/non-covalent complexes rose with PT concentration, and the PDI decreased and then rose, reaching a minimum value of 0.21 (HSPI-PT covalent) at PT of 0.5 mg/mL. TEM showed PT addition induced the aggregation of the complexes to increase. Meanwhile, surface hydrophobicity reduced while the antioxidant activity rose. Solubility, foaming activity and emulsifying activity of HSPI-PT covalent complexes first rose and then declined, and were optimal at PT of 0.5 mg/mL, with 78%, 250% and 97 m2/g, respectively. These will provide references for the improvement of soy protein properties and the application of complexes in formulated foods.