Banana peel Agro-waste is abundantly generated in Ethiopia but is commonly discarded without adequate disposal, causing notable environmental and aesthetic challenges. As part of a sustainable valorization strategy, this study employed banana peels as a low-cost, renewable substrate for citric acid production. The work focused on harnessing Aspergillus niger ATCC 10535 in solid-state fermentation, with process conditions optimized through Response Surface Methodology (RSM) to maximize yield and process efficiency. Citric acid production trials were conducted in a small-scale solid-state fermenter using banana peel agro-waste as the primary substrate. Process parameters were systematically varied, including substrate loading (18, 36, and 54 g), incubation time (3, 6, and 9 days), and fermentation temperature (25 °C, 30 °C, and 35 °C), to evaluate their individual and interactive effects on citric acid yield. Experimental design and statistical optimization were carried out using RSM, applying a Box–Behnken design to model the relationships between process variables and product yield and to determine the optimal operating conditions for maximum citric acid production. The minimum citric acid yield (28.39 g/L) was obtained at 36 g substrate loading, 3 days of incubation, and 25 °C, while the maximum yield (45.55 g/L) was achieved at 36 g, 6 days, and 30 °C. Statistical analysis confirmed the model’s significance (p < 0.01). Temperature and incubation time exhibited positive linear effects on yield, whereas increased substrate loading beyond the optimal range resulted in decreased production. The optimized process parameters determined by the Box–Behnken model were 35.014 g substrate loading, 5.811 days of incubation, and 31.054 °C fermentation temperature, yielding 45.563 g/L citric acid. The findings are demonstrated that banana peel agro-waste can be sustainably valorized into citric acid through RSM-optimized, low-level sucrose-supplemented solid-state fermentation by A. niger ATCC 10535, providing a cost-effective and environmentally friendly approach for waste management and value-added product generation.
Abraha et al. (Thu,) studied this question.