ORIGINAL ARTICLE Citric acid production by Aspergillus niger on wet corn distillers grains G. Xie 1 and T.P. West 2 1 Department of Chemistry and Biochemistry, South Dakota State University, Brookings, SD, USA 2 Department of Biology and Microbiology, South Dakota State University, Brookings, SD, USA Introduction Citric acid has a variety of commercial applications in foods, beverages and pharmaceuticals (Tran et al. 1998). Nearly 1 million tons of citric acid are produced globally every year (Karaffa et al. 2001). Previous investigations have studied the use of the fungus Aspergillus niger to produce citric acid from brewery wastes such as brewers spent grain liquor and from ground corn using surface fermentation (Moyer 1953; Hang et al. 1975, 1977; Rou- kas and Kotzekidou 1986). Using solid-state fermentation, A. niger strains were able to produce citric acid from sub- strates such as apple pomace (Hang and Woodams 1984), grape pomace (Hang and Woodams 1985), kiwifruit peel (Hang and Woodams 1987), pineapple waste (Tran et al. 1998), figs (Roukas 2000) or cassava bagasse (Van- denberghe et al. 2004). Wet corn distillers grains are a major co-product resulting from ethanol fermentation and it has not been determined whether the grains could be utilized as a substrate by A. niger strains to produce citric acid. For each bushel (35Æ24 l) of corn processed at ethanol plants, c. 18 pounds (8Æ2 kg) of dry corn distillers grains are produced and over a million tons (907 000 metric tons) of dry distillers grains are produced each year. The primary use of wet corn distillers grains is as a protein supplement in animal feeds (Ham et al. 1994). In this study, seven citric acid-producing A. niger strains were screened for their ability to synthesize citric Keywords Aspergillus niger, biomass, citric acid, wet corn distillers grains, yield. Correspondence Thomas P. West, Department of Biology and Microbiology, South Dakota State University, Box 2140D, Brookings, SD 57007, USA. E-mail: thomas.west@sdstate.edu 2005/1500: received 19 December 2005, revised 5 April 2006 and accepted 13 April 2006 doi:10.1111/j.1472-765X.2006.01958.x Abstract Aims: To determine which citric acid-producing strain of Aspergillus niger util- ized wet corn distillers grains most effectively to produce citric acid. Methods and Results: Citric acid and biomass production by the fungal strains were analysed on the untreated grains or autoclaved grains using an enzyme assay and a gravimetric method respectively. Fungal citric acid production on the grains was found to occur on the untreated or autoclaved grains. The high- est citric acid level on the grains was produced by A. niger ATCC 9142. The autoclaved grains supported less citric acid production by the majority of strains screened. Biomass production by the fungal strains on the untreated or autoclaved grains was quite similar. The highest citric acid yields for A. niger ATCC 9142, ATCC 10577, ATCC 11414, ATCC 12846 and ATCC 26550 were found on the untreated grains. Treatment of the grains had little effect on citric acid yields based on reducing sugars consumed by A. niger ATCC 9029 and ATCC 201122. Conclusions: It is feasible for citric acid-producing strains of A. niger to excrete citric acid on wet corn distillers grains whether the grains are treated or untreated. The most effective citric acid-producing strain of A. niger was ATCC 9142. Significance and Impact of the Study: The study shows that the ethanol pro- cessing co-product wet corn distillers grains could be utilized as a substrate for the commercial production of citric acid by A. niger without treatment of the grains. Letters in Applied Microbiology ISSN 0266-8254 ª 2006 The Authors Journal compilation ª 2006 The Society for Applied Microbiology, Letters in Applied Microbiology 43 (2006) 269–273 269