MICROBES man's MIGHTY MIDGETS 35 



Still the most important mold fermentation aside from the production of 

 penicillin. The production of citric acid by a mold was first discovered by 

 the German mycologist Wehmer, in 1893. He was investigating the metabolic 

 products of species of Penicillium, and being cognizant of the potential im- 

 portance of his discovery, he obtained patents covering this. The operation 

 of a factory for the manufacture of citric acid was attempted, but his molds 

 were not highly productive, contamination was a recurrent problem, and 

 technical difficulties of such magnitude were encountered that the operation 

 was soon abandoned. Two decades later Charles Thom, who had become much 

 interested in molds belonging to the genera Penicillium and Aspergillus as the 

 result of his cheese investigations, was joined by J. N. Currie in an investiga- 

 tion on the production of oxalic acid by different strains of Aspergillus niger. 

 They detected the presence of a considerable amount of acidity additional 

 to oxalic and identified this as citric acid. By careful attention to the com- 

 position of the nutrient solutions and to the selection of the strains of A . niger 

 employed for the fermentation, he and Currie soon found it possible to pro- 

 duce very substantial yields of citric acid from sugar solutions (1916-1917). 

 The only source of citric acid then available was to recover it from citrus 

 fruits and fruit wastes. Italy supplied about 90 per cent of the world's supply, 

 exporting this in the form of calcium citrate. Within a few years after publi- 

 cation of their researches a large factory was operating in New York City, 

 and by 1927 the United States had become virtually self-sufficient with re- 

 gard to this important chemical. Today, production in the United States and 

 its territories ranges in the neighborhood of 65 million pounds per year, and 

 of this amount ca. 90 per cent represents citric acid of microbiological origin. 

 The current price is in the neighborhood of 28 to 30 cents per pound. Citric 

 acid, particularly in the form of the calcium salt, is used in the preparation of 

 pharmaceuticals, while the uncombined acid is used in carbonated beverages, 

 fruit-flavored extracts, and confections. It also finds important uses in the 

 manufacture of engraving inks and certain textile dyes and as a plasticizer 

 of synthetic resins. 



The conversion of glucose to gluconic acid by bacteria was reported as early 

 as 1880. However, the success of the citric fermentation was required to 

 stimulate further investigations leading to a practical production process. 

 Molliard in 1922 found that molds, including Sterigmatocystis nigra, could 

 effect this oxidation. Soon thereafter a team of researchers, consisting of 

 mycologists and chemists in the Department of Agriculture, and including 

 Herrick, May, Moyer, and Wells, evolved a mycological method for the 

 production of gluconic acid from glucose. Molds were also employed in their 

 investigations, and early work was concentrated upon species of Penicillium, 

 including Penicillium chrysogenum, the species that is now universally used 

 for the production of the antibiotic penicillin. Later, they determined that 

 selected strains of Aspergillus niger produced even higher yields and were 



