Abstract
The use of conventional food packaging material poses a threat to the environment as they are non-biodegradable. Also, they may contain undesirable substances which may leach into packaged food and cause detrimental effects to human health. Biopolymers are presenting an alternative packaging material to conventional packaging materials. In this study, selenium nanoparticles (SeNPs) were fabricated using a green synthesis method. Moringa oleifera leaf extract was used as a reducing and stabilizing agent following the solvothermal technique. The SeNPs were characterized and further used to develop a nanocomposite edible film with improved physico-chemical properties. Lastly, the developed film was assessed for its migration behaviour as well as its food preservative properties.
Transmission Electron Microscopy (TEM) indicated that the SeNPs exhibited a polygonal-like shape, without agglomeration with an average particle size of 82.86 nm. Scanning Electron Microscopy (SEM) confirmed the polygonal structure of the nanoparticles. Energy Dispersive Spectroscopy (EDX) mapping indicated that SeNPs were dominant and uniformly distributed. X-ray Diffraction (XRD) studies on SeNPs indicated that the nanoparticles were amorphous in nature which agreed with the red brick colour observed after synthesis. Fourier transform infrared (FTIR) spectrometer analysis confirmed the involvement of biomolecules from Moringa oleifera leaf extract in the biosynthesis of SeNPs. The antibacterial property of SeNPs was evaluated against Escherichia coli, Salmonella typhimurium, Bacillus cereus and Listeria innocua by disk diffusion method. Bacterial growth inhibition was observed for E. coli (10.1 mm), S. typhimurium (12.5mm) and B. cereus (9.8 mm). No inhibition was observed for L. innocua. The SeNPs exhibited a dose-dependent antioxidant activity and up to 80% antioxidant activity was achieved.
Potato starch edible films incorporated with various SeNPs concentrations (0, 0.5, 1.0, 5.0 mg/mL) were developed. The microstructure analysis using SEM revealed that addition of SeNPs resulted in a slight increase of roughness and heterogeneity of the nanocomposite film structure with increase in SeNPs concentration. The film without SeNPs was intact without pores and cracks with
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smooth surface, however slightly heterogenous. The SeNPs based nanocomposite films displayed a smoother cross section with increase in SeNPs concentration and they also appeared continuous without any visible crack or separation. EDX analysis showed the presence of selenium in the potato starch nanocomposite film and this increased with increase in SeNPs concentration, while elemental mapping showed uniform distribution of the SeNPs. There were no characteristic selenium peaks observed with XRD analysis on the films. The FTIR spectra of the films with or without the SeNPs exhibited similar general features. TGA revealed that the addition of SeNPs did not negatively influence the thermal stability of the nanocomposite film, however, its residual mass increased with increase in SeNPs incorporated. The nanocomposite film density and thickness increased with increasing SeNPs concentration, while water content, film solubility, swelling degree as well as water vapour transmission rate decreased. Studies on colour parameters indicated that an increase in the SeNPs concentration incorporated into the potato starch film caused a significant change (p ˂ 0.05) of colour to red (a*) and decreased lightness (L-value). The tensile strength and elongation at break of the potato starch film improved with addition of SeNPs with 1 mg/mL concentration showing the best tensile strength.
A lower antioxidant activity was observed for the SeNPs/potato starch nanocomposite films (up to 12.56% DPPH and 8.55 ABTS) as compared to antioxidant activity for SeNPs (up to 80%). The overall release of potato starch films with or without SeNPs constituents was greater when they were immersed in hydrophilic simulants with the highest migration of 9.45 mg/dm2 and 5.38 mg/dm2 as compared to fatty food simulant (3.3 mg/dm2). The overall migration of starch nanocomposite film constituents to all food simulants was within the EU Overall Migration Limit of 10 mg/ dm2. Specific migration of selenium was studied by ICP-MS and found to be 0.25833, 0.23833 and 0.00233 mg.kg-1 for acidic, aqueous and fatty food simulants, respectively. The specific migration of selenium in all the food simulants was within the acceptable levels (60 mg. kg-1) stipulated in the EU regulation.
Antibacterial properties of the film were investigated against Gram-negative (S. typhi and E. coli) and Gram-positive bacteria (B. cereus and L. innocua) using the disk diffusion and shake flask culture method. The presence of SeNPs enhanced
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the antibacterial activity of the nanocomposite film as evidenced by the inhibition observed for S. typhi (12.00 mm), E. coli (11.00 mm) and B. cereus (6.67 mm). Again, no zone of inhibition was observed for L. innocua with the films. The percentage of total bacteria killing ratio ranged from 0.73 – 11.29% for all investigated bacteria.
To assess the applicability of the developed nanocomposite film on real food, processed meat and strawberry were inoculated with E. coli and the microbial count was observed over a 5-day period. Films with SeNPs decreased E. coli counts by 1.1 and 1.4 Log CFU/g after 3 days followed by a slight increase thereafter, on both processed meat and strawberry, respectively whilst there was 1.3 and 1.5 Log CFU increase in E. coli count in films without SeNPs both processed meat and strawberry after the same period. With yeast and mould, there was no growth observed on the processed meat, whereas on the strawberry samples, growth was observed after the third day of the study. Despite this, yeast and mould growth on strawberry coated with SeNPs containing films was significantly less (up 4.9 Log CFU/g) than uncoated samples (up to 5.8 Log CFU/g).
In this study a green, simple and cost-effective method for SeNPs synthesis was developed. Furthermore, SeNPs based potato starch nanocomposite film with improved physical and mechanical structure was developed. The SeNPs/potato starch nanocomposite film exhibited low levels of film constituents’ migration. The nanocomposite film also displayed some food preservative properties by retarding growth of foodborne pathogens. Furthermore, it displayed antioxidant activity, however, decreased activity as compared to the naked SeNPs. All these properties exhibited by the SeNPs/potato starch film present an alternative biodegradable biopolymer material that can be used as food packaging material in replacement of non-biodegradable synthetic polymer material. Its antioxidant activity and antimicrobial activity benefit may be improved in conjunction with other food preservative agents.