Improving the mechanical and water-resistance properties of pea protein-based edible film via wet-heating Maillard reaction: Insights into the simultaneous effect of heating and Maillard reaction

Wet-heating Maillard reaction (MR) is a novel method to modify protein structure for improved functionalities; yet information is often missing regarding comparisons between MR-treated samples with various heating times and controls treated for the same time. Here, wet-heating was applied to pea protein isolate (PPI) and pullulan (90 degrees C for 2-96 h), and the physicochemical, mechanical, and water-resistance properties of the PPI-based films were evaluated. Especially, PPI treated by heat alone for 2-96 h were investigated as positive controls, which allowed us to demonstrate the simultaneous effect of heating and MR. Results confirmed conjugation between PPI and pullulan based on free amino acid amount, browning index, MR products, and reduced/non-reduced SDS-PAGE. MR generated large-molecular-weight conjugates that cannot be cleaved by beta-mercaptoethanol. Heating alone induced protein aggregation, and significantly enhanced the tensile strength from 1.42 MPa for unheated PPI film to 2.58 MPa for film with 4 h heating, and decreased water vapor permeability by 58 %. Meanwhile, comparing to respective positive controls, MR produced films with outstanding elongation at break (353.21 % when heated for 4 h), and substantially improved water contact angle from 43.14 degrees for PPI-Pullulan physical blend to 104.35 degrees when heated for 6 h. Additionally, heat played a major role in improving the trans-parency of films. This study highlights the need of proper positive controls to validate the occurrence of MR, and our results demonstrate the simultaneous effect of heating and MR under wet-heating condition and its potential to improve properties of protein-based films.