Effects of Different Coagulants on Intermolecular Forces and Secondary Structure of Soybean Protein Isolate

To study the changes of intermolecular forces and secondary structure of soy protein induced by MgCl₂, CaSO₄, lactic acid, acetic acid, GDL and fermented soy whey, the effects of different coagulants on the pH value, surface hydrophobicity, free sulfhydryl group (SH), Zeta potential, α-helix, β-sheet, β-turn and random coil during SPI tofu processing were analyzed, while the SPI was used as the research object. Both salt coagulants and acid coagulants could reduce the pH values of SPI solution, and the final pH value was 6.08~6.14 and 5.25~5.58, respectively. Compared with heated SPI solution without coagulant, the addition of coagulants caused increases in the surface hydrophobicity and free SH content, and a decrease in Zeta potential in SPI solution. The surface hydrophobicity of SPI induced by coagulants increased first and then decreased, and free SH content decreased at 0~45 min. The Zeta potential of SPI solution added with fermented soy whey was significantly higher than that of other coagulants (P<0.05), ranging from 9.19 mV to 9.90 mV. In addition, salt coagulants induced a shift of α-helix and β-turn into β-sheet of soy proteins, and acid coagulants destroyed β-sheet of SPI. In particular, the SPI added with fermented soy whey had α-helix ratio between lactic acid and salt coagulants, and β-turn ratio between lactic acid and acetic acid.