Co-Fermentation of the Starch Fraction from TPS-Based Bioplastics with Rice Straw for Lactic Acid Production

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 24⁻1 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.