Valorization of custard apple (Annona squamosa) waste through polyhydroxybutyrate (PHB) production by Bacillus megaterium MAPCS4: optimization, characterization, and biodegradation studies

Polyhydroxybutyrate is a microbially synthesized biopolymer that has been shown to have incredible attributes for substituting conventional petro-based plastics and alleviating environmental blight on a global scale. However, owing to the increased use of carbon raw materials, the cost of manufacturing polyhydroxybutyrate (PHB) is higher than the cost of manufacturing synthetic plastic. The current study emphasizes harnessing non-edible parts of custard apple (peel and seed), underutilized agro-industrial food waste as a potential substrate for biopolymer PHB production through bacterial fermentation. Among the 10 bacterial strains isolated from petrol bunk soil, hyper-production of PHB was shown with strain MAPCS4 and was genotypically identified as Bacillus megaterium by 16S r RNA sequencing. The final optimum fermentation conditions in modified mineral salt medium with custard apple hydrolysate (5 % v/v) include pH (8), temperature (37°C), incubation period (72 h), inoculum level (10% v/v), NaCl % (0.10 g), agitation speed (120 rpm), carbon source (glucose), and nitrogen source (malt extract). The maximum PHB production of (7.92 g/L) was achieved under optimized conditions compared to un-optimized conditions (5.44 g/L). The FTIR and NMR results confirmed that obtained polymer was short-chain-length PHA (PHB). The polymer showed great thermal stability with TGA and DTA analysis. The PHB films showed degradation within 6 weeks and through microbial degradation complete deterioration of the film was observed within 5 weeks which appears promising. Thus, the current study could be a feasible way for producing environmentally safe biopolymer PHB utilizing custard apple (peel and seed) as an economically affordable feedstock.