The Koji process or solid-state fermentation,developed in Japan,is the simplest process for production of citric acid. this process is the solid-state equivalent of surface fermentation.The raw materials used are sweet potato fibrous residues,rice or wheat bran and fruit wastes.The carbohydrate is then sreamed for sterilization,placed in trays and inoculated using conidia of A.niger.The pH at the start of fermentation is5.5 the starch is hydrolysed by amylase produced by the fungus and subsequently converted to citric acid.The fermentation is complete in4-5day.The main problem with this process is the presence of trace elements, whic cannot be removed by standard method