Production and characterization of PHAs by pure culture using protein hydrolysates as sole carbon source

Protein hydrolysates obtained from discarded biomass can be further upgraded into high market value products, in the optic of a circular bioeconomy. In this work, residues from the cultivation of alfalfa, soybean and rice, and bovine wet blue shavings were fermented with mixed microbial cultures obtaining high concentrations of volatile fatty acid (up to 50 times compared to the original hydrolysate), mainly butyric and acetic acid. This rich medium was used for growing the bacterium Thauera sp., a known producer of polyhydroxyalkanoates (PHAs), biodegradable polymers with potential to replace petrol-based plastics. The overall process resulted in the production of 1.4 gPHAs/L, with a conversion rate of 32% for the alfalfa hydrolysates when considering the COD given by the initial VFAs. The obtained biopolymer was poly(3-hydroxybutyrate-co-3-hydroxyvalerate), as confirmed by the presence of characteristic peaks and by the melting temperatures and thermo-oxidative degradation in the expected range; the polymer has a high degree of purity, being without inorganic residues. This work showed the feasibility of a process aimed at the valorisation of protein hydrolysates into high-market value products such as bioplastics.