Multi-Stage Optimization and Comparative Valorization of Agro-Industrial Residues for PHB/PHBV Production by Priestia megaterium SMI2: Structural Insights and Environmental Fate

Abstract

Growing environmental concerns linked to conventional petroleum-based plastics have accelerated the shift toward sustainable, biodegradable materials such as polyhydroxyalkanoates (PHAs), which offer the added advantage of being biologically derived and environmentally compatible. This study comparatively evaluated the ability of Priestia megaterium strain SMI2 to convert two abundant agricultural residues, wheat bran and banana peel, into value-added biopolymers. A two-stage optimization strategy was employed to maximize polymer production and assess material quality and degradability. Initial screening using a One-Factor-at-a-Time (OFAT) approach established baseline physiological conditions influencing polymer accumulation, including incubation time, temperature, pH, substrate concentration, and inoculum size. Optimal general conditions were identified at 35 °C and 4 g/L substrate concentration; however, substrate-specific differences were observed. Wheat bran supported faster polymer accumulation, achieving optimal production at pH 7.0 within 48 h using a 2% inoculum, whereas banana peel required 96 h at pH 7.5 with a 1% inoculum. Subsequent multivariable optimization using Response Surface Methodology (RSM) based on Central Composite Design (CCD) significantly enhanced yields and revealed distinct performance patterns. Wheat bran produced a maximum yield of 367.47 mg/L, outperforming banana peel, which yielded 225.8 mg/L. The banana peel model was statistically significant (F = 5.78, p = 0.0009) with an R² of 0.84, explaining 84% of variability, while the wheat bran model was highly significant (F = 14.92, p = 0.0003; R² = 0.783) with strong agreement between predicted and experimental values. FTIR and GC–MS analyses confirmed the formation of PHB/PHBV copolymers containing both 3-hydroxybutyrate and 3-hydroxyvalerate units. Biodegradability assessment showed up to 72.5% weight loss after 60 days of soil burial, indicating substantial environmental degradability. These findings provide a commercially viable pathway for industrial-scale biopolymer production that aligns with global mandates for circular bioeconomy.