Fabrication of antibacterial food packaging from polyvinyl alcohol/sodium caseinate composite films cross-linked with lactic acid

In this study, Polyvinyl Alcohol (PVA)/Sodium Caseinate (NaCAS) were cross-linked by mixing with Lactic acid (LA) in different proportions (0.5%, 1%, 2%, and 4%). Composite films were obtained from the prepared solutions using the solution casting method. The obtained composite films were appeared as slightly opaque and lightly yellowish, but mostly transparent. As the amount of LA increased, the polarized microscope surface images of the composite films became rougher. It was explained that cross-linking occurred with the aldehyde groups (CHO) determined in the Fourier transform infrared spectroscopy (FTIR) analysis. The increasing 5% weight loss temperature of the films because of cross-linking also increased the thermal strength. In X-ray diffraction analysis (XRD), the 2θ peak intensity of the 2% LA-added film, which had the highest cross-link density, was indicated to be the lowest. It was reported that 4% and above the LA contribution rate negatively affects the physical and mechanical properties of the PVA/NaCAS matrix. The pure PVA/NaCAS film had 30.08 MPa tensile strength and 1.49% tensile strain. The film with 2% LA added showed the tensile strength value closest to the pure film with 23.89 MPa. With a 6.52% value, the film containing 2% LA showed the lowest moisture retention behavior. Pure PVA/NaCAS film, which has a hydrophilic structure with a contact angle of 57°, gained hydrophobic ability with the addition of LA. The 1% LA-added film exhibited the highest hydrophobic behavior with a contact angle value of 99°. With the LA additive, an increase in the contact angle values of the composite films was observed between approximately 61% and 74%. In addition, the water vapor permeability (WVTR) with 582.37 g/m² day value of pure PVA/NaCAS composite film decreased after the addition of LA. It was degraded to 322.72 g/m² day WVTR value in the 1% LA-added film. 100% antibacterial activity against both E. coli and S. aureus was observed in composite films containing 1% or more LA additive. The resulting films were suggested to be used as active food packaging with antibacterial effects.

Highlights

• LA can be used as a crosslinking agent for PVA/NaCAS blends.
• 2% LA contribution has shown the highest cross-linking density.
• 4% LA and higher LA contribution would negatively affect the mechanical properties of the matrix.
• High antibacterial activity was obtained against both E. coli and S. aureus in films containing 1% or more LA.