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.