Solomon Kiros (PhD)Setty Teklehaimnot2024-09-032024-09-032024-04https://etd.aau.edu.et/handle/123456789/3450Citric acid is a versatile organic compound widely employed across various industries, with its primary application residing in the food sector. This organic acid exhibits remarkable attributes such as high solubility, palatability, and low toxicity, rendering it invaluable in the realms of food, biochemistry, and pharmaceuticals. The principal objective of this study is to synthesize citric acid using the fungus Aspergillus niger, utilizing water hyacinth as the primary substrate, and to assess the resulting citric acid's quality and concentration. In the laboratory setting, we harnessed the solid-state fermentation method within 250 mL Erlenmeyer flasks to transform water hyacinth into citric acid. To systematically analyze and optimize the synthesis process, Design Expert 13.0.5.0 was employed, applying Response Surface Methodology (RSM) to study the concurrent impact of key variables: incubation time (3 days, 6 days and 9 days), fermentation temperature (set at 25⁰C, 30⁰C and 35⁰C), and pH levels (2, 4 and 6). The study utilized a three-variable, three-level Box-Behnken design to establish a statistical model that elucidates the relationship between citric acid concentration and these independent variables while optimizing the production process. The statistical model proved significant (p < 0.0001) and demonstrated low standard deviation (0.3445). Notably, it exhibited no lack of fit (R2=0.9959), and the predicted R-Squared value of 0.9907 closely aligned with the Adj R-Squared of 0.9514. Citric acid production showcased significance under moderate levels of incubation time, fermentation temperature, and pH. Analysis of the model's regression equation highlighted the positive influence of temperature and time, while pH exhibited a negative effect on the yield of the desired response. Through rigorous experimentation, we achieved a maximum citric acid yield of 22.205 g/L under the optimal conditions, which included an incubation time of 7.375 days, a fermentation temperature of 31.071 ⁰C, and a pH level of 4.245. These settings yielded a combined desirability of high value. The validation process confirmed the statistical model's reliability, demonstrating an insignificant difference between experimental and model-predicted results.en-USCitric acidAspergillus nigerwater hyacinthSubstrateSolid State FermentationThesis