Optimization of nutrient concentration for citric acid production by solid-state culture of Aspergillus niger on polyurethane foams

A screening of several strains gave different results than in submerged culture and a reduction in the incubation time. The joint effect of nitrogen and phosphorus initial concentrations was examined by means of orthogonal factorial designs in five selected strains. The results showed that different requirements of N and P for each individual strain were needed. Optimization was done using different experimental strategies for two selected strains. Following either the direction of the linear approximation gradient of the first-order empirical model obtained or with a new rotatable factorial design, an optimum value could be obtained.