Structure-modification by moderate oxidation in hydroxyl radical-generating systems promote the emulsifying properties of soy protein isolate

• The emulsifying property of SPI was improved through moderate oxidation.
• Oxidation by hydroxyl radicals affected the CD spectrum of SPI significantly.
• Moderate oxidization generates soluble protein aggregates with a more flexible structure.

Oxidative modification of soy protein isolate (SPI) by hydroxyl radicals derived from FeCl3/H2O2/ascorbic acid hydroxyl radical-generating systems (HRGS) at room temperature (20 °C) for 5 h was investigated. Increasing the H2O2 concentration resulted in the increase of protein carbonyl groups and dityrosine contents (P < 0.05), and the decrease of free sulfhydryl groups (P < 0.05). Circular dichroism spectra confirmed that oxidation resulted in a gradual loss of α-helical structure with a concomitant increase of β-sheet structure. Fluorescence spectroscopy revealed that oxidation treatment greatly increases the extent of exposed hydrophobic domains. The emulsifying activities of SPI were significantly improved at H2O2 concentrations up to 1.0 mM, and then declined at higher H2O2 concentration (P < 0.05). These results suggest that moderate oxidization could significantly enhance the emulsifying properties of SPI, which partly caused by the expose of surface hydrophobic groups and larger soluble aggregates, as well as mainly due to the enhancement of the electrostatic repulsive force between emulsion droplets.

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