Maillard-Induced Cross-Linking in Soy Protein Film and Its Effect on Film Properties

• Glucose-modified SPI films showed better properties than lactose-modified films • Highest tensile strength (5.24 MPa) achieved at SPI: sugar ratio 8:1 without lysine • FTIR confirmed covalent cross-linking due to the Maillard reaction • Structural heterogeneity influenced water vapor permeability (WVP) • Lysine addition accelerated the browning of the SPI film Packaging films from biopolymers are often inferior, particularly in mechanical strength, to their plastic counterparts. This study investigated the effects of sugar type reduction, SPI-to-sugar ratio, and lysine concentration on structural modifications in soy protein isolate (SPI) induced by the Maillard reaction, and on the resulting film properties. SPI was Maillard-modified using either glucose or lactose at SPI-to-reducing sugar ratios of 4:1, 6:1, and 8:1, with an addition of lysine at 0, 5, or 10% of SPI by weight. Results confirmed the formation of advanced Maillard reaction products and increased protein crosslinking, as evidenced by reduced free amino group content, increased fluorescence intensity, and a higher browning index. Maillard modification increased the film's tensile strength, surface hydrophobicity, water vapor permeability, and yellowness, while decreasing its water solubility and transparency. At higher SPI-to-sugar ratios, the glucose-modified films exhibited greater TS and water barrier properties as compared to the lactose-modified films. The film with an SPI-to-glucose ratio of 8:1 and no added lysine exhibited the highest TS (5.24 MPa), approximately two-fold higher than that of the control (2.60 MPa). Water solubility decreased by approximately 47%, and hydrophobicity increased by 99% compared to the control SPI film. Lactose-modified films showed higher WVP than glucose-modified films. Lysine addition at 10% to either Maillard-modified film decreased TS, elongation, and water solubility. In conclusion, the Maillard modification could increase protein cross-linking and enhance SPI film properties, offering a practical and sustainable approach to developing protein-based biodegradable packaging materials for low-load and light-sensitive food products.