Citric Acid and Gypsum Production from Wastes Biomass and Industrial By-products Using Microbial Fermentation A

Abstract

The economic potential of microbial citric acid (CA) production is substantial due to its wide-ranging applications in the beverage, food, detergent, cosmetic, and pharmaceutical industries. In Bangladesh, the majority of CA demand is met through imports, while annual consumption is increasing steadily at a rate of 3.5–4%, underscoring the need for sustainable domestic manufacturing alternatives. Large-scale production must therefore adopt eco-friendly processes that utilize readily available and inexpensive agro-industrial byproducts, while ensuring high production yields. CA is predominantly produced via microbial fermentation using Aspergillus niger. The present study aimed to optimize citric acid production from sugarcane molasses employing A. niger (F-81) strain under submerged fermentation, with gypsum as a byproduct. The concentration of CA in the fermentation broth was determined using the Marier–Boulet method. Maximum production was observed in the pretreated molasses sample (12.49 mg/ml), compared to the non-pretreated counterpart (7.3 mg/ml), under identical fermentation conditions (10 days, 2% inoculum size, and an initial pH of 5.6). These findings demonstrate the significance of substrate pretreatment in enhancing yield. The production rate of CA is influenced by multiple factors, including regulatory genes of A. niger, pH, temperature, inoculum size, substrate concentration, and the presence of heavy metals, which warrant further investigation. In addition, citric acid biosynthesis was evaluated using six indigenous fungal strains (FS1–FS6) isolated from soil, cultured under standardized fermentation conditions with 5% molasses as the carbon source, at an initial pH of 6.0 and ambient temperature (30ºC). The produced citric acid was isolated through precipitation method using calcium hydroxide and sulfuric acid, in which gypsum was found as a by-product. Citric acid and gypsum production was confirmed via FTIR analysis. Among the isolates, FS2 exhibited the highest and most consistent productivity, yielding 0.93 mg/ml 1.09 mg/ml and 1.11 mg/ml after 5 days, 7 days and 10 days of incubation, respectively. Moderate production was observed in FS5 (0.55–0.61 mg/ml) and FS3 (0.51–0.61 mg/ml), whereas FS1 (0.41–0.60 mg/ml) and FS4 (0.32–0.40 mg/ml) displayed comparatively lower yields. FS6 was the least productive, reaching only 0.14 mg/ml by day 10. Overall, the results identify A. niger (F-81) strain as the most promising candidate for industrial-scale citric acid production under the tested conditions in Bangladesh.