222 BIOLOGICAL ACTIVITIES OF MOLDS 



The optimum temperature range was found to be from 26° to 28° C. 

 and the fermentation was generally completed in 7 to 10 days. The 

 optimum amount of air passed over the mold mycelial mats varies 

 with each installation of equipment, too low or too high supplies 

 decreasing the yields of citric acid. While 60 per cent of the sugar 

 may usually be recovered as citric acid, yields as high as 87, 90.7, 

 and even 100 per cent have been reported. 



After the fermentation is completed, the liquor is drained off and 

 the mat pressed to remove any residual citric acid. Calcium car- 

 bonate may be used to adjust the pH of the liquor to approximate 

 neutrality and calcium citrate is precipitated from a hot solution. 

 The addition of sulphuric acid removes the calcium, which settles out 

 as calcium sulphate, and citric acid is then recovered. • 



Cahn ' has recommended that cane or beet pulp impregnated with 

 molasses or sucrose be fermented at 20° to 35° C. He claims that 

 the production of citric acid with the use of solid material shortens 

 the fermentation period to 3 or 4 days and, because the fermentation 

 proceeds so rapidly, the deleterious effects of bacterial contamina- 

 tion are obviated. Yields of 45 per cent on the basis of sugar in the 

 molasses or 55 per cent on the basis of sucrose were claimed. 



The exact details employed in the commercial production of citric 

 acid have not been made available to the public as yet. The reader 

 is referred to the publications of Currie, Doelger and Prescott, and 

 to Industrial Microbiology by Prescott and Dunn for further gen- 

 erally known details on the techniques employed in this tricar- 

 boxylic acid fermentation. 



Various theories have been suggested for the mechanism of the 

 production of citric acid. Because the theory must be such as to 

 explain its formation from two up to seven and even twelve carbon 

 compounds, and because it must account for the high yields men- 

 tioned above, those which have been proposed up to now have been 

 considered untenable. 



In general there seem to be two schools of thought. One school 

 maintains that the hexose chain is not broken but becomes trans- 

 formed to citric acid with its forked chain. The other proposes that 

 the hexose is initially split to shorter carbon chain compounds and 

 subsequently built up to citric acid. 



Challenger and his associates ^° and Franzen and Schmitt ^^ pre- 

 sent evidence in support of the hypothesis that glucose is not broken 

 down but is converted to the forked tricarboxylic acid. They sug- 

 gest the following series of reactions. 



