Evaluation of ohmic heating modified soybean protein isolate structure and antioxidant film under different catechin concentrations

To produce edible films with excellent antioxidant properties, soy protein isolate (SPI) was pretreated using ohmic heating (OH) to create more catechin binding sites. Compared with untreated SPI, ohmic heating-pretreated SPI (OH-SPI) had a looser secondary structure, higher fluorescence intensity, more free sulfhydryl groups, higher hydrophobicity (H0), and lower protein particles, all of which facilitated covalent binding between SPI and catechin. The binding equivalent of OH-SPI and catechin increased from 128.90 to 237.43 nmol/mg protein. The above results proved that ohmic heating pretreatment benefited the covalent binding between SPI and catechin and the development of the antioxidant film. Scanning electron microscopy (SEM) showed that catechin-modified OH-SPI (OH-SPI-CC) films had a rough and aggregated structure. As the concentration of catechin increased, the mechanical properties of the film improved while the barrier properties slightly decreased. Fourier transform infrared (FTIR) and X-ray diffraction (XRD) confirmed that the cross-linking between catechin and OH-SPI was covalent. In application testing of OH-SPI-CC film, OH-SPI-CC film at a catechin concentration of 4.0 mg/mL (OH-SPI-CC-4) showed the best DPPH and ABTS free radical scavenging rate and the highest cumulative release in food simulators. Therefore, cross-linking catechin with OH-SPI was an effective method for preparing antioxidant films.