Microbial contamination of most foods occurs primarily at the surface during postprocessing and handling; therefore, preventing cross-contamination by incorporation of antimicrobial substances in contact with the surface of the product is an efficient strategy in reducing food contamination risks. Zinc oxide nanoparticles (ZnONPs) have been used widely to achieve antimicrobial films in various applications including the food industry. This work describes the fabrication of antimicrobial polymeric films containing ZnONPs produced by the coextrusion and dip-coating techniques. Effects of skin layer thicknesses containing ZnONPs on the antimicrobial effectiveness of the film by their capability to inactivate Gram-positive and Gramnegative bacteria were studied for both methods. The antimicrobial properties of the coextruded multilayer LLDPE/ZnONP nanocomposite films evidenced antimicrobial activity in the range 0.5−1.5 log reductions, while in the case of a sandblasted multilayer film, it showed high antimicrobial properties as around 99.99%. The optical properties of coextruded multilayer films were measured and discussed. Furthermore, to achieve a thinner LLDPE thickness, ZnONPs were coated with different concentrations of LLDPE solution by the dip-coating method. TEM confirmed that a homogeneous layer is formed on the surface of ZnONPs. The thickness of the LLDPE layer estimated by TEM was about 2 nm and film produced 3 log and 4 log reductions for E. coli and S. aureus, respectively. The results show that developed films have the potential to be used as food packaging films and can extend shelf life, maintain quality, and assure the safety of food. The antimicrobial mechanisms of ZnONPs were also investigated. It was found that direct contact of particles with products is necessary to assure high antibacterial activity of the films.
The integration of essential oils (EOs) into polymers to endow antimicrobial properties has received a lot of attention. EOs are remarkable in that they have broad antimicrobial activity from a natural source while also being volatile. Their volatility, on the other hand, makes high-temperature processing techniques difficult to incorporate into polymers. In this study, active films based on low-density polyethylene (LDPE) and limonene essential oil (LEO) were prepared and characterized. Before incorporation of LEO into LDPE, vacuum pulling method was used to load the LEO into five different mineral carriers. All Carrier-LEO complexes were added into LDPE using melt compounding. The goal is to analyze the potential use of these formulations to achieve prolonged antimicrobial film packaging. The halloysite nanotubes (HNTs), kaolinite (Kao), mesoporous silica nanoparticles (MSNs), zinc oxide nanoparticles (ZnONPs), and molecular sieve type 4A (Z4A) were used as mineral carriers for limonene. The functional characterizations including mechanical, thermal, optical, barrier, and antimicrobial properties as well as limonene release behavior from the LDPE composite films were investigated. As expected, free limonene molecules acted as a plasticizer in the LDPE matrix. Thermogravimetric analysis (TGA) showed 20-25% of the initial limonene content was retained against thermal degradation in compounding and film making steps and its release from the films was efficiently delayed. A decrease in optical and oxygen barrier properties, as well as elastic modulus and tensile strength, was obtained for all developed films compared with neat LDPE. Significant antibacterial activities of the films were observed against Escherichia coli DH5-Alpha (E. coli) as a model gram-negative bacterial species. Moreover, the obtained results and the short-term and long-term release studies indicated that both HNTs and the MSNs due to their strong synergistic interactions with limonene exhibited sustained release profiles of limonene from LDPE films. Thus, these new active polymer composites present promising features in controlling microbial contamination, rendering them as excellent candidates in active packaging applications.
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