Description

Incorporating nanoparticles into biodegradable films is a promising approach to enhance film qualities such as mechanical strength. These nano-sized materials can serve as fillers in film-forming matrices and thereby strengthening the films. Besides an improved mechanical strength, such bionanocomposite films may also exhibit good barrier properties and antimicrobial activities. The conventional nanoparticle synthesis techniques are often energy intensive or require toxic chemicals, which limits the application of nanoparticles in food products. Therefore, the goal of this study is to explore protein-mediated biosynthesis of ZnO nanoparticles (ZnO-NPs) and their applications in whey protein isolate (WPI) films. Biosynthesized ZnO-NPs were incorporated into WPI films at 1.35% and 2.7% of the protein weight by the solvent casting method. Percent light transmittance, color difference, mechanical strength, water vapor permeability, Fourier-transform infrared (FTIR) spectroscopy, and antimicrobial activity of the WPI films were assessed. Data were analyzed by one-way analysis of variance and Tukey’s honestly significant difference test. All the WPI films exhibited a transparent and colorless appearance except the WPI film incorporated with 2.7% ZnO-NPs, which resulted in visible aggregates and significantly decreased percent light transmittance (P < 0.05) over the visible light range. The addition of ZnO-NPs did not significantly affect tensile strength, elongation at break, elastic modulus, and water vapor permeability of the films (P >0.05). All films displayed similar FTIR profile including the characteristic amide A, amide I, amide II bands. A graduate increase in amide A signal was observed in response to an increased ZnO-NPs concentration suggesting that the ZnO-NPs likely interacted with WPI via hydrogen bonding. The addition of ZnO-NPs significantly increased the beta-sheet content while decreased random coils (P < 0.05). This indicated that the ZnO-NPs changed the native structure of the WPI. The WPI films loaded with ZnO-NPs presented improved resistance against the gram-negative E. coli. In summary, we successfully used WPI to catalyze biosynthesis of ZnO-NPs during the film forming process and observed enhanced antimicrobial activities. This presents a nontoxic and eco-friendly technique for synthesizing food grade nanoparticles.