Limited PLA Mineralization Under Mesophilic Amycolatopsis orientalis Bioaugmentation and Skimmed Milk Powder Biostimulation

Polylactic acid (PLA) remains poorly mineralized under mesophilic conditions relevant to home and decentralized composting. This study assessed whether bioaugmentation with Amycolatopsis orientalis, protein-based biostimulation with skimmed milk powder, or their combined application could enhance the mineralization of compression-molded amorphous PLA fragments at 28 °C in activated vermiculite. Closed respirometric bioreactors were monitored for 90 days, and the PLA mineralization extent was calculated from the cumulative CO2 evolution after correction using treatment-specific blanks. The recovered PLA fragments were further analyzed by FTIR-ATR and DSC to provide complementary physicochemical monitoring. The final mineralization remained low, reaching 1.19 ± 1.88% for bioaugmentation, 3.49 ± 1.82% for biostimulation, and 8.75 ± 4.31% for the combined treatment. The combined treatment gave the highest mean value, which was significantly higher than bioaugmentation alone, but the individual biological replicates followed heterogeneous trajectories. In particular, BABS-3 reached 13.19% mineralization, indicating that higher responses can occur at the individual bioreactor level, although they were not consistently reproduced. FTIR-ATR and DSC revealed treatment- and replicate-dependent physicochemical changes but did not provide evidence of extensive bulk PLA transformation. These results contrast those of previous reports of higher PLA mineralization under warmer, mature compost conditions, emphasizing the complexity of the combined influence of temperature and matrix. Overall, the tested strategies were insufficient to achieve effective home compostability of PLA at 28 °C.