This study evaluates the feasibility of microbial upcycling of thermoplastic starch-based bioplastics (TPS-BP) by converting them into lactic acid (LA). To enhance the accessibility of the starch fraction within the bioplastic matrix, microwave-assisted pretreatments were optimized using a 2^4⁻1 fractional factorial design. Optimal pretreatment conditions were identified as 150 °C with 0.1% H₂SO₄ for 30 min, which enabled the release of 17.5 g L⁻¹ glucose within 24 h of saccharification. Among the evaluated strains, Heyndrickxia coagulans was identified as the superior producer at pH 6.0, achieving a maximum volumetric productivity of 8.25 g L⁻¹ h⁻¹ and a yield of 0.91 g g⁻¹. To enhance carbon availability and overcome the low starch content (≈20%) of commercial TPS-BP, a co-fermentation strategy with rice straw was implemented. While separate hydrolysis and fermentation (SHF) were limited by carbon catabolite repression, simultaneous saccharification and fermentation (SSF) intensified production. By utilizing TPS-BP pretreatment slurry as the liquid phase for rice straw saccharification, a final LA concentration of 55.1 ± 0.7 g L⁻¹ was achieved. This co-valorization strategy demonstrates a feasible route for integrating bioplastic waste into circular biorefineries while effectively mitigating the seasonal availability of agricultural residues.

This study evaluates the feasibility of valorizing the starch fraction of thermoplastic starch-based bioplastics (TPS-BPs) by converting it into lactic acid (LA) through microbial fermentation. To enhance the accessibility of the starch fraction within the bioplastic matrix, microwave-assisted pretreatments were evaluated using a 2⁴⁻¹ fractional factorial design. The best pretreatment conditions within the tested design were identified as 150 °C with 0.1% H₂SO₄ for 30 min, which enabled the release of 17.5 g L⁻¹ glucose within 24 h of saccharification. Among the evaluated strains, Heyndrickxia coagulans showed the best kinetic performance in the preliminary comparison at pH 6.0, with a maximum volumetric productivity of 8.25 g L⁻¹ h⁻¹ and a yield of 0.91 g g⁻¹, and was selected for subsequent experiments. To enhance carbon availability and overcome the low starch content (≈20%) of commercial TPS-BP, a co-fermentation strategy with rice straw was implemented. While separate hydrolysis and fermentation (SHF) was limited by carbon catabolite repression, simultaneous saccharification and fermentation (SSF) intensified production. By utilizing TPS-BP pretreatment slurry as the liquid phase for rice straw saccharification, a final LA concentration of 55.1 ± 0.7 g L⁻¹ was achieved. This co-valorization strategy demonstrates a feasible route for integrating the starch fraction of bioplastic waste into circular biorefineries, which could mitigate the seasonal availability of agricultural residues.