Abstract
Thymol and silver nanoparticles (Ag-NPs) were used to develop poly(lactic acid) (PLA)-based films with antioxidant and antibacterial performance. Different amounts of thymol (6 and 8 wt%) and 1 wt% Ag-NPs were added to PLA to produce the active films. Ag-NPs and thymol were successfully identified in the nanocomposite structures using spectroscopic techniques. A kinetic study was performed to evaluate the release of thymol and Ag-NPs from the nanocomposites to an aqueous food simulant (ethanol 10%, v/v) at 40 °C. The diffusion of thymol from the polymer matrix was affected by the presence of non-migrating Ag-NPs, which showed non-Fickian release behavior. The ternary system including 1 wt% Ag-NPs and 8 wt% thymol showed clear antibacterial performance by reducing the cell viability of Escherichia coli and Staphylococcus aureus by around 40% after 3 and 24 h of storage at 4, 25, and 37 °C compared to neat PLA. Significant antioxidant behavior of all active films was also confirmed using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) method. The obtained nanocomposite films based on PLA and the addition of Ag-NPs and thymol were proven to have combined antioxidant and antibacterial performance, with controlled release of thymol. These formulations have potential applications in the development of innovative and customized active packaging systems to increase the shelf-life of food products.
Highlights
The rising trend of the industrial use of environmentally friendly materials and stringent regulations have been the driving forces behind public demand for the of use of biopolymers.Antioxidants 2020, 9, 395; doi:10.3390/antiox9050395 www.mdpi.com/journal/antioxidantsMaterials such as poly(lactic acid) (PLA), poly(hydroxyl alkanoates) (PHA), polyethylene furanoate (PEF), thermoplastic starch (TPS), and biodegradable polyesters are interesting alternatives to polymers derived from fossil fuels
The FTIR spectrum of neat PLA is characterized by several absorption bands, such as those at 770 and 871 cm−1, which can be attributed to C-H bond stretching and the intense peak appearing at 1787 cm−1 due to the carbonyl group (−C=O) stretching vibration [28]
The FTIR spectra of nanocomposite films confirmed the presence of a significant amount of thymol remaining in the nanocomposite films after processing, since a broad absorption band at 3000–3500 cm−1 corresponding to the O-H stretching vibration and the flexion vibration of the methylene group (-CH2 -) at 806 cm−1 was observed for all formulations containing thymol, being a clear indication of the presence of this additive in the processed formulations
Summary
Materials such as poly(lactic acid) (PLA), poly(hydroxyl alkanoates) (PHA), polyethylene furanoate (PEF), thermoplastic starch (TPS), and biodegradable polyesters are interesting alternatives to polymers derived from fossil fuels. This is due to their renewable origin, biodegradability, and biocompatibility, making them a potential solution for the environmental problems caused by the accumulation of petrochemical-based plastic waste [1]. Several strategies have been developed to improve the structural and functional properties of PLA-based materials, including the incorporation of low amounts of nanoparticles (NPs) without relevant alteration of their migration behavior to ensure their suitability for food packaging applications [3]. In terms of innovative packaging concepts, the use of metallic-based NPs instead of antimicrobial organic agents offers some advantages, such as their high antimicrobial efficiency, lack of negative impacts on food sensory properties, and compatibility with harsh polymer processing conditions, making NPs suitable for food spoilage control [4,5,6,7,8]
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