Upcycling biodegradable PVA/starch film to a bacterial biopigment and biopolymer

Abstract

Meeting the challenge of circularity for plastics requires amenability to repurposing post-use, as equivalent or upcycled products. In a compelling advancement, complete circularity for a biodegradable polyvinyl alcohol/thermoplastic starch (PVA/TPS) food packaging film was demonstrated by bioconversion to high-market-value biopigments and polyhydroxybutyrate (PHB) polyesters. The PVA/TPS film mechanical properties (tensile strength (σu), 22.2 ± 4.3 MPa; strain at break (εu), 325 ± 73%; and Young’s modulus (E), 53–250 MPa) compared closely with low-density polyethylene (LDPE) grades used for food packaging. Strong solubility of the PVA/TPS film in water was a pertinent feature, facilitating suitability as a carbon source for bioprocessing and microbial degradation. Biodegradability of the film with greater than 50% weight loss occurred within 30 days of incubation at 37 °C in a model compost. Up to 22% of the PVA/TPS film substrate conversion to biomass was achieved using three bacterial strains, Ralstonia eutropha H16 (Cupriavidus necator ATCC 17699), Streptomyces sp. JS520, and Bacillus subtilis ATCC6633. For the first time, production of the valuable biopigment (undecylprodigiosin) by Streptomyces sp. JS520 of 5.3 mg/mL and the production of PHB biopolymer at 7.8% of cell dry weight by Ralstonia eutropha H16 from this substrate were reported. This low-energy, low-carbon post-use PVA/TPS film upcycling model approach to plastic circularity demonstrates marked progress in the quest for sustainable and circular plastic solutions.