Citric acid production by the diploid strains of Aspergillus niger obtained by protoplast fusion

Diploid strains were obtained following protoplast fusion between two citric acid producers of Aspergillus niger, one for the solid culture and the other for the shaking culture. In the shaking culture, all the diploid strains exhibited lower productivities than one parental strain. However, in the solid culture, some diploid strains exhibited higher productivities than either parental strain; the best diploid strain produced 1.2 times as much citric acid as the parental strain in solid culture.