Method for increasing citric acid production by Aspergillus niger fermentation

A method of expressing citric acid from Aspergillus niger, which comprises:

• incubating an A. niger strain in growth medium comprising malt extract and tryptone at 35° C. for seven days to generate spores, wherein the A. niger strain comprises a genome comprising a gamma-aminobutyric acid (GABA) pathway succinate semialdehyde dehydrogenase (SSD) gene, wherein the SSD gene is under the control of a Pgas promoter, wherein the SSD gene sequence is SEQ ID NO:1, and wherein the Pgas promoter sequence is SEQ ID NO:3,
• harvesting the spores,
• inoculating seed culture medium with the harvested spores at a density of 106 spores per mL to generate a seed culture, wherein the seed medium comprises corn starch medium comprising a total sugar concentration of 10% and a total nitrogen concentration of 0.2%,
• growing the spores in the seed culture medium at 37° C. for 24 hours at pH 3.5,
• inoculating a fermentation medium with the seed culture at 1/10 volume,
• incubating the fermentation medium for 72 hours at 35° C. at pH 2.0, and
• centrifuging the fermentation medium and discarding bacteria to obtain citric acid.