Citric acid production front glucose by Aspergillus niger immobilized in polyurethane foam.

In this work the citric acid production from glucose by Aspergillus niger NRRL 2270 mycelia immobilized in polyurethane foam particles (density 0.021 g/cm(3)) was studied. A simple adsorption technique was used for immobilization, and allowed a complete colonization of the polyurethane foam particles. Citric acid yield and productivity by immobilized mycelia were close to those obtained by free mycelia provided that 10 g/l carrier particles were used. Higher levels of polyurethane foam resulted in a solid state fermentation with smaller yield values. The effect of nitrogen and phosphorous concentration and of initial pH was studied by factorial experiments and empirical modelling. The effect of initial pH was relatively unimportant. Maximum yield and productivity (respectively 0.42 g/g and 0.10 g/l h) were obtained with ammonium sulphate concentrations between 1.5 and 2 g/l and potassium dihydrogen phosphate concentrations lower than 0.5 g/l. The kinetics of citric acid production was studied in a substrate with 120 g/l glucose. Fermentation was completed in 29 days and involved a citric acid yield and a productivity of 0.63 g/g and 0.107 g/l h respectively.