Cost effective and eco-friendly solid state fermentation method has been developed for citric acid production from citrus fruits waste by Aspergillus niger by shake culture method. The solid substrate supplemented with different concentration of Carbon, nitrogen, methanol, temperature and shaker speed were optimized. With 10 % nitrogen supplement, the maximum yields of citric acid obtained were 38.16 %, 33.60 %, 27.84 % and 27.60 % from four different citrus peels waste. With 15% carbon supplements, the yields were 32 %, 29 %, 22% and 24 %. With the minimum concentration of 3 % methanol, the yields of citric acid were 27.12 %, 22.56 %, 26.88 % and 38.88 % and this is due to the high effect of methanol as an enhancer. These results indicate the use of citrus peels as inexpensively available medium for the production of commercially valuable organic acid by using Aspergillus niger.
