Citric acid production by Aspergillus niger strains grown on corn substrates from ethanol fermentation

Citric acid is an important specialty chemical which can be synthesized biologically. It has a number of commercial applications including its use in foods, pharmaceuticals and other industries. In this study, the coproducts resulting from ethanol fermentation of corn were tested for their suitability to be utilized as substrates for citric acid production using solid-state fermentation or surface fermentation. These coproducts include dried corn distillers grains with solubles, wet corn distillers grains, thin stillage and condensed corn distillers solubles. Seven citric acid-producing strains of the fungus Aspergillus niger were selected and screened for their ability to produce citric acid from these corn-based substrates. The treatments of the substrates include autoclaving and mild-acid hydrolysis. In addition, the effects of 3% (v/v) methanol addition and 30 mM KH2 PO4 supplementation were also studied. The concentration of citric acid was analyzed by a coupled enzyme assay. It was found that A. niger ATCC 9142 produced the highest level of citric acid on solid substrates including dried distillers grains with solubles and wet distillers grains. On the other hand, A. niger ATCC 12846 and ATCC 26550 produced the highest biomass level on dried distillers grains with solubles and wet distillers grains, respectively. The effects of methanol and phosphate supplementation on citric acid and biomass production were strain-dependent. It was also found that A. niger ATCC 201122 was the most effective strain for citric acid production on liquid substrates including thin stillage and condensed distillers solubles. A. niger ATCC 201122 also produced the highest specific productivity and citric acid yield on the liquid substrates. Moreover, A. niger ATCC 9029 and ATCC 10577 produced the highest biomass level on thin stillage and condensed distillers solubles, respectively. It was concluded that A. niger strains could use corn-based coproducts from ethanol fermentation as substrates for citric acid production.