Bioconversion of apple peels (Malus domestica) to polyhydroxybutyrate using statistical design to optimize process parameters through Bacillus thuringiensis via solid-state fermentation

Plastic, polypropylene, is the main cause of soil, water, and air pollution as it is non-degradable. Polyhydroxybutyrate, (PHB), serves as an alternative to polypropylene as it has the same physical and chemical properties but is biodegradable. The main hindrance in the production of PHB is the cost of substrate used for biosynthesis. Hence, the current study was designed to evaluate the potential of apple peels (Malus domestica) as a substrate for PHB biosynthesis using solid-state fermentation (SSF) by Bacillus thuringiensis strain of bacteria. Different process parameters were optimized using central composite design (CCD) of RSM (response surface methodology). The highest yield of PHB (12.2 mg/gds) was obtained by fermenting apple peels at 70% moisture content after incubation for 3 days at pH 7.5, temperature 30 oC, and addition of 0.3% peptone as nitrogen source. F-value (16.16) and p-value (0.00) ensure the higher significance of the planned model. The coefficient of determination (R-2) was calculated as 96.71% which confirms the goodness and accuracy of the statistical model. Incubation days, temperature, and concentration of peptone had significant while moisture percentage and pH had an insignificant effect on the production process. Identification of PHB was performed by FTIR analysis while the HPLC method indicated that PHB produced was 98.6% pure in comparison with the standard. The molecular weight (Mw) of PHB was observed as 3754 kDa, while the number-average molecular weight (Mn) was calculated as 2860 kDa. Hence, the polydispersity index (pI) was reported as 1.31. The experimental results specified that bioplastic can be efficiently biosynthesized using apple peels as the substrate through SSF. The optimized results can be further utilized for production on an industrial scale.