Production, Optimization and Characterization of Polyhdroxyalakanoates (PHA) Bioplastics Using Sweet Potato Peels as Renewable Carbon Source

Author

Benjamin P. C, D. A. Ajiya, A. A. Mahmoud, U.F Hassan, D.S John, B.C Okoli

Abstract

 Persistent petroleum-based plastics and the high cost of microbial polyhydroxyalkanoate (PHA) production have increased interest in low-cost renewable feedstocks. This study investigated the production, optimization and characterization of PHA from sweet potato (Ipomoea batatas L.) peel hydrolysate using Bacillus subtilis. Peels were dried, milled and hydrolysed with 1% (v/v) H₂SO₄ at 80 °C for 90 min, and the neutralized hydrolysate was used as the main carbon source in a medium supplemented with peptone and MgSO₄. A 13-run Box–Behnken design was used to evaluate the effects of peel concentration (60–100 g/L), incubation time (24–72 h) and temperature (30–37 °C) on PHA yield, expressed as a percentage of dry cell weight. Yields ranged from 10.9% to 45.0%, the highest being obtained at 100 g/L, 48 h and 37 °C. The full quadratic model was not significant (p = 0.264), whereas a reduced model was (p = 0.002; R² = 0.79, adjusted R² = 0.72). Peel concentration was the most influential variable (p = 0.0008), the effect of incubation time was not significant, and no significant interactions were detected. Because the design had a single centre point and no replicates, pure error and lack of fit could not be assessed. PHA recovered by chloroform extraction and ethanol precipitation was a brown, brittle solid. FTIR showed a strong ester carbonyl band at 1728 cm⁻¹ together with C–O–C, CH₂ and CH₃ bands typical of PHA. SEM revealed a heterogeneous, granular and aggregated morphology, and DSC gave melting temperatures of 176.5 °C and 173.6 °C and a crystallization temperature of 82.1 °C, consistent with a PHB-rich polyester. The polymer was insoluble in water, ethanol and n-hexane. These findings indicate that sweet potato peel is a promising substrate for PHA production by B. subtilis.

These findings demonstrated the potential of sweet potato peel biomass as an inexpensive and renewable substrate for microbial PHA production while highlighting the importance of optimizing fermentation conditions to enhance polymer yield. Characterization confirmed the successful production of biodegradable polyhydroxyalkanoates with physicochemical properties suitable for biodegradable plastic applications. The study concludes that the utilization of sweet potato peel biomass provides an environmentally sustainable and economically viable approach for PHA production while contributing to agricultural waste valorization and the development of eco-friendly bioplastics.

Keywords

Polyhydroxyalkanoate, Bioplastics, Sweet potato peels, Bioprocess optimization, Polymer characterization, Agricultural waste valorization, Biomaterials

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References

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