Supercritical processing of recycled PLA into antimicrobial foams and fibers: Modelling morphology-driven thymol release for active food packaging

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The widespread use of conventional plastics has caused significant environmental problems due to their persistence after disposal. Polylactic acid (PLA), which is derived from renewable sources, has attracted interest as an alternative material for food packaging. However, the management of PLA at the end of its life remains limited. In this context, reusing post-consumer PLA to create functional materials is a strategy that aligns with the principles of the circular economy. Supercritical CO 2 technology was employed to infuse bioactive compounds into recycled PLA matrices, resulting in the development of foamed structures and polymer fibers with potential applications in active packaging. The materials retained their morphology and thermal stability following recycling and functionalization. Fibers exhibited significantly higher thymol impregnation degree than foams, reaching approximately 23 wt.% and 13 wt.% (based on polymer mass), respectively. Release studies demonstrated the sustained release of bioactive compounds, with both systems showing similar partition coefficients in food Simulant A, although markedly different diffusion coefficients were observed. Foams exhibited diffusion coefficients of 2.7·10 −18 m 2 s -1 and 1.2·10 −18 m 2 s -1 for PLA and rPLA, respectively, while significantly higher values were obtained for fibers (1.5·10 −17 m 2 s -1 for PLA fiber-Thy, and 1.4·10 −17 m 2 s -1 for rPLA fiber-Thy). Antimicrobial tests revealed that active foams exhibited a bactericidal effect against Salmonella enterica , with 9 log CFU mL -1 microbial reductions, and complete inhibition of Penicillium expansum growth, whilst electrospun fibers displayed a more moderate antimicrobial activity. These results demonstrate the high potential of these materials for application in active food packaging systems.