Bioplastic production in air-lift bioreactors using a spent cooking oil-based method for sustainable recovery

Global concern over plastic pollution and fossil resource depletion has driven the development of biodegradable alternatives like polyhydroxybutyrate (PHB). PHB, synthesized by microorganisms, faces commercialization challenges due to high production and separation costs. This study evaluated an alternative platform for sustainable PHB production using air-lift bioreactors (ALRs) of up to 15 L. We achieved a high concentration, enrichment and specific productivity of PHB of 4.4 g ± 0.1 g·L⁻¹, 87.9 ± 0.3% (w/w) and 0.43 ± 0.0 g·L⁻¹·h^-1, in an Azotobacter vinelandii mutant strain deficient in alginate biosynthesis, in 28 h. Bacterial cells were cultured using air as the sole N source. PHB was recovered with 96% efficiency and 95% purity by osmotic shock using glycerol and soap produced from recycled spent cooking oil. This strategy not only replaces commonly used hazardous solvents for PHB extraction but also repurposes spent cooking oil, an otherwise difficult-to-manage waste, into a valuable resource for bioplastics production. Additionally, we demonstrate the feasibility of recycling glycerol recovered after PHB extraction, which partially substituted for sucrose as a carbon source in subsequent PHB production. The cells exhibited a preferential utilization of C sources in the following order: sucrose > analytical-grade glycerol > glycerol recovered from recycled spent cooking oil post-osmotic shock and PHB recovery > fresh glycerol recovered from recycled spent cooking oil. These findings provide practical opportunities for improving the environmental and economic sustainability of PHB production while aligning with circular economy objectives, supporting the gradual replacement of certain petrochemical plastics.