Biodegradable Films of PLA/PPC and Curcumin as Packaging Materials and Smart Indicators of Food Spoilage

Abstract

Bio-based and biodegradable packaging combined with chemical sensors and indicators has attracted great attention as they can provide protection combined with information on the actual freshness of foodstuffs. In this study, we present an effective, biodegradable, mostly bio-sourced material ideal for sustainable packaging that can also be used as a smart indicator of ammonia (NH₃) vapor and food spoilage. The developed material comprises a blend of poly(lactic acid) (PLA) and poly(propylene carbonate) (PPC) loaded with curcumin (CCM), which is fabricated via the scalable techniques of melt extrusion and compression molding. Due to the structural similarity of PLA and PPC, they exhibited good compatibility and formed hydrogen bonds within their blends, as proven by Fourier transform infrared (FTIR) and X-ray diffraction (XRD). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis confirmed that the blends were thermally stable at the used processing temperature (180 °C) with minimal crystallinity. The rheological and mechanical properties of the PLA/PPC blends were easily tuned by changing the ratio of the biopolymers. Supplementing the PLA/PCC samples with CCM resulted in efficient absorption of UV radiation, yet the transparency of the films was preserved (T₇₀₀ ∼ 68-84%). The investigation of CCM extract in ethanol with the DPPH^• assay demonstrated that the samples could also provide effective antioxidant action, due to the tunable release of the CCM. Analyses for water vapor and oxygen permeability showed that the PPC improved the barrier properties of the PLA/PPC blends, while the presence of CCM did not hinder barrier performance. The capacity for real-time detection of NH₃ vapor was quantified using the CIELab color space analysis. A change in color of the sample from a yellowish shade to red was observed by the naked eye. Finally, a film of PLA/PPC/CCM was successfully applied as a sticker indicator to monitor the spoilage of shrimps over time, demonstrating an evident color change from yellow to light orange, particularly for the PPC-containing blend. The developed system, therefore, has the potential to serve as a cost-effective, easy-to-use, nondestructive, smart indicator for food packaging, as well as a means for NH₃ gas monitoring in industrial and environmental applications.

Keywords: bioplastics; chemical sensor; curcumin; indicator; poly(propylene carbonate); polylactic acid; smart food packaging.

Figure 1

Schematic representation of the fabrication process, comprising methodologies suitable for large-scale production. Macroscopic views of the CCM-loaded PLA/PPC pellets and the final product.

Figure 2

(a) Chemical structure of the PLA, PPC, and CCM molecules. (b) FTIR spectra of the PLA/PPC blends (solid lines) and their analogues with CCM (dashed lines). (c, d) Magnified FTIR spectra at the wavenumbers areas of interest, highlighted with yellow in (b).

Figure 3

SEM micrographs of cross sections with magnified images, and of the surfaces, of PLA/PPC films loaded with CCM. The micro-phase separation of PPC in the sample containing 40 wt % PPC is denoted by the arrows.

Figure 4

(a) Tensile curves and (b, c) mechanical parameters for the PLA/PPC blends (solid lines/columns) and their CCM-loaded analogues (dashed lines/columns); *** indicates a significant difference in comparison with variable values at p < 0.05. The digital image shows a macroscopic view of the samples after tensile testing to the point of failure.

Figure 5

(a) UV–vis spectra and (b) WCA data for the PLA/PPC blends (solid lines/symbols) and their CCM-loaded analogues (dashed lines/open symbols).

Figure 6

(a) Water vapor permeability (WVP) and (b) oxygen permeability (OP) of the PLA/PPC blends (solid columns) and their CCM-loaded analogues (dashed columns).

Figure 7

(a) Release kinetics for CCM-loaded PLA/PPC films, followed by measuring the absorbance of the solution at 428 nm, the characteristic absorption peak for CCM. The kinetics are presented for the three films with different PPC concentrations. (b) Concentration of released CCM in ethanol over time, for the representative sample (PLA/PPC 60/40).

Figure 8

(a) Reaction mechanism of the DPPH^• with the natural antioxidant. (b) Antioxidant activity of CCM-loaded PLA/PPC films as a function of time. (c) Macroscopic picture showing the scavenging activity of the CCM released in ethanol.

Figure 9

(a) Schematic illustration of the testing assembly. (b) Digital photographs and (c) the corresponding color changes expressed as ΔE values of CCM-loaded PLA/PPC indicators at different times of exposure to NH₃ vapors. Shrimps sealed in a Petri dish at the beginning of the test (d) and on day 5 (e).