Pea protein-p-coumaric acid conjugate-based antioxidant film: The relationship between protein structure and film properties after covalent bonding

In this study, pea protein isolates (PPI) have been used to make conjugates by covalent binding with p-coumaric acid (p-CA), and the effect of conjugation on PPI structural characteristics and the structural, optical, mechanical, and physicochemical properties of conjugate-based films were investigated. The conjugation with p-CA unfolds the tertiary structure of PPI and significantly reduces the surface charge. A tendency of aggregation with increasing p-CA addition was proposed. The conjugate-based films show stronger tensile strength (TS) and better water barrier properties than PPI-based films. However, no significant improvement of the elongation at break (EAB) and thermal stability was found for conjugate-based films. FTIR spectroscopic and zeta-potential analyses indicated that such improved film properties could be attributed to the enhancement of hydrogen bonds and weakening of electrostatic repulsion between conjugate-conjugate. Additionally, the conjugates impart the antioxidant activity to the films as confirmed by DPPH scavenging capacity. This study demonstrates the potential of using pea protein-phenolic acid conjugate to make PPI-based films with improved mechanical properties, water barrier properties, and antioxidant activity.