Effect of variation of process parameters on bioethanol production from sorghum straw using sequential co-cultures of Aspergillus niger and Scheffersomyces stipitis

The global quest for sustainable and environmentally friendly alternatives to fossil fuels has accelerated research into second-generation bioethanol production from agricultural residues. In this study, sorghum straw a lignocellulosic waste abundant in sub-saharan Africa was valorized for bioethanol production using a co-culture system of Aspergillus niger and Scheffersomyces stipitis. A. niger facilitated the saccharification of pretreated biomass by producing cellulases, while S. stipitis efficiently fermented the released hexose and pentose sugars into ethanol. Process parameters such as pH, temperature, inoculum size, substrate concentration, and fermentation time were systematically optimized. Optimal fermentation conditions (pH 4.5, 25°C, 10% inoculum, and 15% substrate concentration) yielded a maximum bioethanol concentration of 17.05 g/L with a conversion efficiency of 52.12% over 120 hours. Notably, a substrate concentration beyond 15% inhibited sugar release and ethanol yield, likely due to increased viscosity and mass transfer limitations. The study confirms that indigenous microbial strains, coupled with simple process optimization, can significantly enhance bioethanol yields from underutilized agro-wastes. Despite modest yields compared to integrated fermentation systems, the approach remains cost-effective and accessible for low-resource settings. The results hold promise for decentralized bioethanol production in rural communities, potentially transforming agricultural waste into a valuable energy source. This work not only advances local biofuel technologies but also contributes to global efforts in achieving renewable energy goals.