1Department of Food Technology Research, National Nutrition and Food Technology Research Institute, Shaheed Beheshti University, M.C., P.O. Box 19395 Tehran, I.R. IRAN
2Department of Chemical Engineering, Faculty of Engineering, Azad Islamic University, Science and Technology Campus, P.O. Box 14115-775 Tehran, I.R. IRAN
3Department of Bioprocess Engineering, National Institute for Genetic Engineering and Biotechnology, P.O. Box 14965-161 Tehran, I.R. IRAN
A solid state fermentation method was used to utilize wheat straw as substrates for citric acid production by using Aspergillus niger ATCC 9142. The Plackett Burman design (PBD) of experiments was used to test the relative importance of the variables affecting production such as moisture content, age of spore, inoculum size, initial pH of substrate, methanol concentration, incubation temperature and time, as well as initial sugar concentration, steam time and type of solvent. In best condition, the microorganism produced 47 g citric acid per kg dry crude wheat straw, with yield of 93 % based on the amount of fermentable sugar consumed. The effect of pretreatment of wheat straw with HCl, NaOH and urea on the yield of production was also investigated by using PBD. Acid, alkaline and urea pretreatment of wheat straw increased citric acid concentration to 115, 85.0 and 109.5 g/kg of dry wheat straw and yield to 97, 97 and 96 % (respectively) based on available sugar consumed. Finally, up scaling was achieved to a 20-L solid state fermentor (tray bioreactor) in which moisture was constant in gas phase and acid pretreated wheat straw was selected as the most efficient pretreated substrate. The produced acid concentration and yield in fermentor was 60.53 g/kg of dry wheat straw and 2.11 g/kg.day, respectively.
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