Polyhydroxyalkanoates production from cocoa bean shell fat: Bioprocess optimization and polymer characterization

The search for low-cost and residual carbon sources is a key strategy to make polyhydroxyalkanoate (PHA) production economically viable. In this context, cocoa bean shell fat (CBS fat), a byproduct rich in saturated fatty acids, was evaluated as the sole carbon source for PHA synthesis by Cupriavidus necator. Firstly, the biosynthesis was optimized by adjusting the molar carbon-to‑nitrogen (C/N) ratio and the substrate-to-biomass (S/X) ratio, with higher PHA yields observed at C/N = 70 and an S/X = 150 g fat/g CDW. Further characterization of the synthesized polymer by GC and FTIR revealed a copolymeric structure predominantly composed of 3-hydroxybutyrate (3HB) with minor fractions of 3-hydroxyvalerate (3 HV) and medium-chain-length (HAmcl) units. This composition, influenced by the fatty acid profile of CBS_fat, resulted in a lower degree of crystallinity compared to commercial homopolymeric PHB, leading to reduced melting and glass transition temperatures and the presence of a cold crystallization peak. Mechanical testing further indicated that the synthesized PHA exhibits higher puncture strength and improved deformation before failure relative to commercial PHB. Overall, these findings support the potential of CBS_fat as a renewable and cost-effective feedstock for producing PHAs with tailored thermal and mechanical properties, making it a promising option for sustainable bioplastic applications.