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

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.