Bioconversion of Breadfruit Starch to Citric Acid by Fungus Aspergillus niger: A Microbial Fermentation Parameter Optimization Investigation

Abstract

Starch hydrolysate from breadfruit was used as the sole carbon source for citric acid (CA) biosynthesis by the filamentous fungus Aspergillus niger under surface fermentation conditions. The process was modeled and optimized by examining the influence of four critical factors: Starch hydrolysate concentration ranging from 50 to 100 g/L, medium pH between 3 and 6, nitrogen source comprising of (NH₄)₂HPO₄​ or NaNO₃, and fermentation time from 1 to 7 days, on CA concentration. The results demonstrated that A. niger efficiently metabolized the hydrolysate, achieving a maximum CA concentration of 14.7 g/L after 7 days of fermentation. Statistical modeling predicted the optimal production conditions as a starch hydrolysate concentration of 50 g/L, pH of 5.4, (NH₄)₂HPO₄​ as the nitrogen source, and a fermentation duration of 7 days. Under these conditions, the predicted CA concentration was 14.7 g/L, which was validated experimentally. Additionally, the process yielded 2.02 g/L of biomass and 15.2 g/L of reducing sugars. This study underscores the potential of breadfruit as a low-cost and sustainable substrate for CA biosynthesis. Applying response surface methodology with D-Optimal design proved effective in optimizing process variables and enhancing production efficiency. These findings provide a framework for developing cost-efficient and scalable fermentation processes, particularly in regions with abundant breadfruit resources.