METABOLIC PRODUCTS 



279 



Citric acid. Wehmer was the first to recognize the commercial possi- 

 bilities of citric acid synthesis by two species of Citromyces {Penicillium). 

 Selected isolates of Aspergillus niger appear to be used in industry, 

 although the propertj^ of producing citric acid is common to many fungi. 

 The following fungi have been suggested for commercial citric acid pro- 

 duction (Von Loesecke, 1945): Citromyces pfejferianus, C. glaber, C. 

 citricus, Aspergillus carhonarius, A. glaucus, A. clavatus, A. cinnamomeus, 

 A.fumaricus, A. awamori, A. aureus, Penicillium arenarium, P. olivaceum, 

 P. divaricatum, P. sanguifluus, P. glaucum, Mucor pyriformis. 



The production of citric acid in the United States increased from about 

 5 million to 26 million pounds between 1935 and 1945 (Von Loesecke. 

 1945). Presumably most of this was "fermentation" citric acid. At 

 present it is beheved that most citric acid is produced by surface cultures. 



Citric acid is formed from many sources of carbon. Sucrose is said 

 to be the best carbon source for the production of citric acid. There is 

 less agreement upon the value of other sugars. Different investigators 

 have found glucose, fructose, and maltose to vary from good to poor. In 

 part, this is to be attributed to the use of different isolates and different 

 experimental conditions. Beet molasses is used in industry. The 

 suitability of this substrate is said to vary with the source and year of 

 production (Bernhauer and Knobloch, 1941). The evaluation of carbon 

 sources is complicated by the metallic elements they contain, especially 

 iron and manganese. Methods of treating beet and cane molasses to 

 remove inhibiting impurities are described by Perlman et al. (1946), 

 Gerhardt et al. (1946), and Karow and Waksman (1947). The inhibiting 

 effect of metallic ions on the production of citric acid from sugars is illus- 

 trated by the data in Table 49. 



Table 49. The Effect of Removing Metallic Contaminants from Three 

 Sugars, by the Process of Cationic Exchange, on the Prodlction of 

 Citric Acid by Aspergillus niger, Wisconsin Strain 62 

 (Perlman et al., Arch. Biochem. 11, 1943. Published by permission of Academic 

 Press, Inc.) 



* Theoretical yield 123 per cent. 



The production of citric acid in submerged culture was tried at an early 

 date and abandoned in favor of surface culture. However, recent litera- 



