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. 2021 Oct 23;13(21):3652.
doi: 10.3390/polym13213652.

Development of Bio-Composites with Enhanced Antioxidant Activity Based on Poly(lactic acid) with Thymol, Carvacrol, Limonene, or Cinnamaldehyde for Active Food Packaging

Mohammad Nahid Siddiqui  1 , Halim Hamid Redhwi  2 , Ioannis Tsagkalias  3 , Evangelia C Vouvoudi  3 , Dimitris S Achilias  3
Affiliations

Development of Bio-Composites with Enhanced Antioxidant Activity Based on Poly(lactic acid) with Thymol, Carvacrol, Limonene, or Cinnamaldehyde for Active Food Packaging

Mohammad Nahid Siddiqui et al. Polymers (Basel). .

Abstract

The new trend in food packaging films is to use biodegradable or bio-based polymers, such as poly(lactic acid), PLA with additives such as thymol, carvacrol, limonene or cinnamaldehyde coming from natural resources (i.e., thyme, oregano, citrus fruits and cinnamon) in order to extent foodstuff shelf-life and improve consumers' safety. Single, triple and quadruple blends of these active compounds in PLA were prepared and studied using the solvent-casting technique. The successful incorporation of the active ingredients into the polymer matrix was verified by FTIR spectroscopy. XRD and DSC data revealed that the crystallinity of PLA was not significantly affected. However, the Tg of the polymer decreased, verifying the plasticization effect of all additives. Multicomponent mixtures resulted in more intense plasticization. Cinnamaldehyde was found to play a catalytic role in the thermal degradation of PLA shifting curves to slightly lower temperatures. Release of thymol or carvacrol from the composites takes place at low rates at temperatures below 100 °C. A combined diffusion-model was found to simulate the experimental release profiles very well. Higher antioxidant activity was noticed when carvacrol was added, followed by thymol and then cinnamaldehyde and limonene. From the triple-component composites, higher antioxidant activity measured in the materials with thymol, carvacrol and cinnamaldehyde.

Keywords: antioxidant properties; biobased polymers; carvacrol; cinnamaldehyde; food packaging; limonene; poly (lactic acid) PLA; thymol.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Chemical structure of (a) thymol, (b) carvacrol, (c) limonene and (d) cinnamaldehyde.
Figure 2
Figure 2
Chemical structure of the repeating unit of PLA.
Figure 3
Figure 3
FTIR spectra of neat PLA and the triple blend of PLA with thymol, carvacrol and limonene, PLA-ThCaLim, together with the corresponding spectra of PLA with each of the active ingredients (a) of neat PLA and the triple blend of PLA with carvacrol, limonene and cinnamaldehyde, PLA-CaLimCi, together with the corresponding spectra of PLA with each of the active ingredients (b) of neat PLA and the triple blend of PLA with thymol, limonene and cinnamaldehyde, PLA-ThLimCi, together with the corresponding spectra of PLA with each of the active ingredients (c) and of neat PLA and the triple blend of PLA with thymol, carvacrol and cinnamaldehyde, PLA-ThCaCi, together with the corresponding spectra of PLA with each of the active ingredients (d).
Figure 3
Figure 3
FTIR spectra of neat PLA and the triple blend of PLA with thymol, carvacrol and limonene, PLA-ThCaLim, together with the corresponding spectra of PLA with each of the active ingredients (a) of neat PLA and the triple blend of PLA with carvacrol, limonene and cinnamaldehyde, PLA-CaLimCi, together with the corresponding spectra of PLA with each of the active ingredients (b) of neat PLA and the triple blend of PLA with thymol, limonene and cinnamaldehyde, PLA-ThLimCi, together with the corresponding spectra of PLA with each of the active ingredients (c) and of neat PLA and the triple blend of PLA with thymol, carvacrol and cinnamaldehyde, PLA-ThCaCi, together with the corresponding spectra of PLA with each of the active ingredients (d).
Figure 3
Figure 3
FTIR spectra of neat PLA and the triple blend of PLA with thymol, carvacrol and limonene, PLA-ThCaLim, together with the corresponding spectra of PLA with each of the active ingredients (a) of neat PLA and the triple blend of PLA with carvacrol, limonene and cinnamaldehyde, PLA-CaLimCi, together with the corresponding spectra of PLA with each of the active ingredients (b) of neat PLA and the triple blend of PLA with thymol, limonene and cinnamaldehyde, PLA-ThLimCi, together with the corresponding spectra of PLA with each of the active ingredients (c) and of neat PLA and the triple blend of PLA with thymol, carvacrol and cinnamaldehyde, PLA-ThCaCi, together with the corresponding spectra of PLA with each of the active ingredients (d).
Figure 4
Figure 4
FTIR spectra of the PLA composites with all triple mixtures and the quadruple mixture of thymol, carvacrol, limonene or cinnamaldehyde.
Figure 5
Figure 5
X-ray diffraction (XRD) patterns of the pure PLA and their composites with 10% thymol, carvacrol, limonene or cinnamaldehyde.
Figure 6
Figure 6
DSC measurements of PLA and their composites with 10 wt% thymol, carvacrol, limonene or cinnamaldehyde recorded during the first (a) or the second heating (b).
Figure 7
Figure 7
DSC scans of PLA and their composites with the triple or quadruple blends.
Figure 8
Figure 8
Thermal degradation under an inert atmosphere of PLA and its composites with a single additive (a) or ternary or quaternary composites (b) obtained from a pyrolizer using the EGA method.
Figure 9
Figure 9
Release profiles of carvacrol (a) or thymol (b) from the polymer matrix at five different temperatures, 50, 100, 122, 144 and 165 °C. Measurements were carried out isothermally using TGA.
Figure 10
Figure 10
Comparison of different models in the simulation of the experimental data on the release profile of thymol at 142 °C. The diffusion model is based on Equation (6), the combined diffusion model on Equation (10) the power-law model on Equation (11), whereas in the last simulation (blue line) the same Equation (11) was used but setting the value of n fixed at 0.5.
Figure 11
Figure 11
Comparison of the simulation results obtained from the combined diffusion model (Equation (10)) and the best-fit parameters shown in Table 4, to the normalized isothermal TGA experimental data plotted in the form of Mt/M∞ and obtained at different temperatures, for carvacrol (a) and thymol (b).
Figure 11
Figure 11
Comparison of the simulation results obtained from the combined diffusion model (Equation (10)) and the best-fit parameters shown in Table 4, to the normalized isothermal TGA experimental data plotted in the form of Mt/M∞ and obtained at different temperatures, for carvacrol (a) and thymol (b).
Figure 12
Figure 12
Arrhenius-type plots of the fast and slow diffusion coefficients for the release of thymol and carvacrol in order to estimate the activation energies of the processes.
Figure 13
Figure 13
UV spectra of neat PLA and PLA composites with 10% of thymol, carvacrol, limonene and cinnamaldehyde after 1 h in DPPH solution.
Figure 14
Figure 14
Comparative UV spectra of all ternary composites of PLA and the quaternary after 1 h in DPPH solution.
Figure 15
Figure 15
Comparative UV spectra of neat PLA, each ternary composite and the components of this composite after 1 h in DPPH solution.
Figure 16
Figure 16
Effect of time stored in the DPPH solution of neat PLA and its composites with 10% of thymol carvacrol, limonene or cinnamaldehyde.
Figure 17
Figure 17
Antioxidant activity of all composites at several timepoints incubated in the DPPH solution.
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