14 BIOCHEMISTRY OF FUNGI 



Raistrick and Clark (1919) maintain that the hexose first be- 

 comes a-y-diketoadipic acid, that then acetic and oxalacetic acids 

 arise bv hydrolysis, and that finally they combine to form citric 

 acid. 



Optimum conditions for citric acid formation vary not only 

 with the substrate but also with the mold concerned and with the 

 pH. This variation is indicated by an optimum pH of 2.0 for 

 A. niger and of 3.0 to 4.0 for Penicillium glaber. Of the sub- 

 strates tested, the following carbohydrates have been found suit- 

 able for citric acid fermentation: starch, sucrose, glucose, fructose, 

 lactose, maltose, glycerol, and molasses. 



Whatever the mechanism, it has been found to be commercially 

 practicable to produce citric acid by mold fermentation, several 

 thousand tons being produced annually, in competition with citric 

 acid extracted from natural sources. It requires an initial concen- 

 tration of about 1 5 c o of sugar, a low concentration of ammonium 

 nitrate, and a pH of 3.5. 



J-Gluconic acid. Gluconic acid is of value when used as a cal- 

 cium salt in food and medicine. It was first isolated by Molliard 

 in 1922 from among the by-products in fermentations induced 

 by A. niger. Subsequent workers, notably Bernhauer (1924), 

 Herrick and May (1928), May, Herrick, Thorn, and Church 

 (1927), Mover, May, and Herrick (1936), and May, Herrick, 

 Mover, and Hellbach (1929) have shown that a variety of other 

 molds possess the ability to ferment this acid, among them being 

 Aspergillus cirmamomeus, Penicillium glaucum, P. purpurogemim 

 var. rubrisclerotvum, P. chrysogemim, and Finnago vagans. 



Gluconic acid arises from the fermentation of glucose as fol- 

 lows: 



CH 2 OH CH 2 OH 



HCOH HCOH 



HCOH HCOH 



I ^ I 



OHCH OHCH 



HCOH HCOH 



CHO COOH 



(f-Glucose <f-Gluconic 



acid 



