PHB into PHB: Recycling of polyhydroxybutyrate by a tandem “thermolytic distillation-microbial fermentation” process

Highly efficient polyhydroxybutyrate conversion into crotonic acid was achieved.Low temperature and no catalyst were used in the crotonic acid production.Renewed polyhydroxybutyrate was obtained by using crotonic acid and C. necator.High overall yield and low energy demand make this strategy feasible and promising. The recycling of polyhydroxybutyrate (PHB) via depolymerization-polymerization is here proposed and assessed for the first time. Effective depolymerization of PHB to crotonic acid through a thermolytic distillation at mild conditions (170 °C and 150 mbar), and without the need of any catalyst, gives a crotonic acid-enriched condensate (94% yield and 98% selectivity towards trans-crotonic acid) that was used without any further purification as a substrate for producing renewed PHB with a culture of Cupriavidus necator. The yield of PHB (0.6 g PHB per g of crotonic acid), final PHB content (63%), and PHB-molecular weight (1.5 MDa) confirmed the technical feasibility of this tandem approach, with a 55% overall yield. A preliminary evaluation of the energy consumption of various depolymerization approaches indicated that the thermolytic distillation consumes 20–25% less energy than previously reported methods; the energy requirement for producing crotonic acid from PHBwaste with 50% moisture has a still acceptable energy demand (4 MJ/kgcrotonic acid), comparable to the energy required for obtaining fermentable sugars (4.4 MJ/kgsugar) to be used for feeding bacteria that accumulate PHB. [Display omitted]