Silver nanoparticles-essential oils combined treatments to enhance the antibacterial and antifungal properties against foodborne pathogens and spoilage microorganisms.

Plant-derived essential oils (EOs) and commercial silver nanoparticles (AgNPs) were tested to evaluate their antibacterial and antifungal efficiency against two pathogenic bacteria (Escherichia coli O157:H7 and Salmonella Typhimurium) and three spoilage fungi (Aspergillus niger, Penicillium chrysogenum, and Mucor circinelloides). A broth microdilution assay was used to determine the minimal inhibitory concentration (MIC) of EOs and AgNPs. In the MIC assay, the cinnamon EO, Mediterranean formulation, citrus EO and spherical-shaped silver nanoparticles (AgNPs) (AGC 1, AGC 0.5, AGPP and AGPPH) showed moderate to high antibacterial and antifungal properties, with MIC ranging from 7.8 to 62.5 ppm for AgNPs and 312.5-1250 ppm for EOs against the tested bacteria and fungi. The possible interaction between the EOs and the AgNPs was determined using a checkerboard method by evaluating fractional inhibitory concentration (FIC) values. The combination of two or more EOs and AgNPs (Active combination 1: AGPPH+cinnamon EO, Active combination 2: AGC 0.5+Mediterranean formulation+citrus EO, Active combination 3: AGPP+cinnamon EO+Asian formulation+lavang EO) showed synergistic effects (FIC <1.0) against all tested bacteria and fungi. A modified Gompertz model was used to evaluate growth parameters including maximum colony diameter (A), maximum growth rate (Vm), and lag phase (λ), under the three active combinations suggested by the checkerboard method using a vapor assay. The three active combinations 1, 2 and 3 reduced the growth rate and maximum colony diameter of E. coli, S. Typhimurium, A. niger, P. chrysogenum, and M. circinelloides, and extended their lag phase from 1 to 5 days. In in situ tests with inoculated rice, the three active combinations showed a significant reduction of all tested bacteria and fungi at 27 °C for 28 days.

Evaluation of bioactive low-density polyethylene (LDPE) nanocomposite films in combined treatment with irradiation on strawberry shelf-life extension.

A low-density polyethylene (LDPE) film reinforced with cellulose nanocrystals (CNCs) with an encapsulated bioactive formulation (cinnamon essential oil + silver nanoparticles) was developed for preservation of fresh strawberries. Antimicrobial activity of the active LDPE films was tested against Escherichia coli O157:H7, Salmonella typhimurium, Aspergillus niger, and Penicillium chrysogenum by agar volatilization assay. The optimal condition of the films showed ≥75% inhibitory capacity against the tested microbes. Strawberries were stored with different types of films: Group 1 (control): (LDPE + CNCs + Glycerol), Group 2: (LDPE + CNCs + Glycerol + AGPPH silver nanoparticles), Group 3: (LDPE + CNCs + Glycerol + cinnamon), Group 4: (LDPE + CNCs + Glycerol + active formulation), and Group 5: (LDPE + CNCs + Glycerol + active formulation + 0.5 kGy γ-radiation) at 4°C for 12 days. Weight loss (WL) (%), decay (%), firmness (N), color, and total phenolics and anthocyanin content of the strawberries were measured. Results showed that the most effective LDPE-nanocomposite film for reducing the microbial growth was LDPE + CNCs + Glycerol + active formulation film (Group 4). When combined with γ-irradiation (0.5 kGy), the LDPE + CNCs + Glycerol + active formulation (Group 5) significantly reduced both decay and WL by 94%, as compared to the control samples after 12 days of storage. Total phenols (from 952 to 1711 mg/kg) and anthocyanin content (from 185 to 287 mg/kg) increased with storage time under the different treatments. The mechanical properties, water vapor permeability (WVP), and surface color of the films were also tested. Though the WVP of the films were not influenced by the types of antimicrobial agents, they did significantly (p ≤ 0.05) change color and mechanical properties of the films. Therefore, combined treatment of active film and γ-irradiation has potential as an alternative method for extending the shelf-life of storage strawberries while maintaining fruit quality. PRACTICAL APPLICATION: Bioactive Low-density polyethylene (LDPE) nanocomposite film was developed in the study by incorporating active formulation (essential oil and silver nanoparticle) to extend the shelf life of stored strawberries. The bioactive LDPE-based nanocomposite film along with γ-irradiation could be used to preserve fruits for long-term storage by controlling the growth of foodborne pathogenic bacteria and spoilage fungi.

Mixture design methodology and predictive modeling for developing active formulations using essential oils and citrus extract against foodborne pathogens and spoilage microorganisms in rice.

The antibacterial and antifungal effects of six plant-derived essential oils (EOs) and two types of citrus extracts (CEs) were studied against two pathogenic bacteria (Salmonella Typhimurium and Escherichia coli O157:H7) and three fungi (Aspergillus niger, Penicillium chrysogenum, and Mucor circinelloides). A broth microdilution assay and checkerboard method were used to measure the minimal inhibitory concentration (MIC) of each extract and the possible interactions between them. The MIC assay showed that cinnamon EO, Mediterranean EO, Southern formulation, citrus EO, organic citrus extract (OCE), and natural citrus extract (NCE) had the highest antimicrobial and antifungal activity. The checkerboard method showed that the Mediterranean EO+OCE combination acted in synergy against all tested pathogens. A centroid mixture design was used to develop active formulations by predicting optimal concentrations of EO/CEs for increased antibacterial/antifungal activity. A mixture of four formulations (625 ppm OCE, 313 ppm Mediterranean EO, 625 ppm citrus EO, and 313 ppm cinnamon EO) named as active formulation 1, and the mixture from five formulations (625 ppm NCE, 625 ppm Asian formulation, 313 ppm Southern formulation, 625 ppm cinnamon EO, and 313 ppm savory thyme EO) named as active formulation 2, were formulated and tested because of their high microbicidal effectiveness. In situ tests with rice showed a significant reduction (P ≤ 0.05) of all tested pathogenic bacteria and fungi from the vapor of active formulations 1 and 2 after 28 days of storage. PRACTICAL APPLICATION: Active formulations (essential oils and citrus extracts) developed in the study are highly effective against foodborne pathogens. Active formulations in this study could be used as natural preservatives in the food industry for controlling foodborne diseases and spoilage organisms in stored foods.