Production of citric acid using immobilized conidia of Aspergillus niger

Conidia of Aspergillus niger were immobilized in calcium alginate gel for t he production of citric acid. First, the type of the preactivation medium, together with the preactivation period, was investigated. It was found that A. niger requires a 2-d preactivation period at a 0.05 g/L NH,NO, concentr ation. Second, preactivated cells were used to determine the effects of nit rogen concentration and the flow rate of oxygen and air on the production o f citric acid. Maximum citric acid production was attained with medium cont aining 0.01 g/L of NH4NO3. The rate of citric acid production in the nitrog enous medium was 33% higher when oxygen was used instead of air during the production phase. This corresponds to an increase of 85% when compared to p roduction when neither oxygen nor air was fed into the system. In the nonni trogenous medium citric acid concentration remained similar regardless of t he use of air or oxygen. However, in the nonnitrogenous production medium, citric acid production was not influenced considerably when oxygen was used instead of air. The advantage of using immobilized cells is that productio n is achieved easily in the continuous system. Therefore, citric acid produ ction was also tested using a packed-bed bioreactor, and an increase in pro ductivity by a factor of 22 was achieved compared to the batch system.