Physico-chemical properties improvement of soy protein isolate films through caffeic acid incorporation and tri-functional aziridine hybridization

In this study, tri-functional trimethylolpropane-tris-(2-methyl-1-aziridine) propionate (TMPTAP) and plant-derived caffeic acid (CA) were used as a cross-linking system to simultaneously enhance the strength and retain elongation at break (EB) of soy protein isolate (SPI) films. The cross-linking reactions in the film by CA/TMPTAP system were examined with attenuated total reflectance-Fourier transform infrared spectroscopy, 13C Nuclear Magnetic Resonance, UV–vis spectroscopy, X-ray diffraction, and thermogravimetric analysis. The film morphologies were observed with the scanning electron microscopy and atomic force microscopy. The results indicated that the catechol groups and oxidized quinone in CA readily reacted with the amine/sulfhydryl groups on SPI, while the tri-functional TMPTAP might react with carboxyl groups through aziridine ring-opening reactions. As expected, the modified SPI films by cross-linking with the CA/TMPTAP system simultaneously enhanced the strength and EB of SPI films. The tensile strength and EB of SPI/TMPTAP/CA film, in comparison to untreated SPI film, were improved by 103.0% and 17.1%, respectively, as a result of the relatively ordered three-dimensional cross-linked network among TMPTAP, CA, and SPI. The water absorption of SPI/TMPTAP/CA film also showed a 49.2% reduction compared to that of the unmodified SPI film.