220 BIOLOGICAL ACTIVITIES OF MOLDS 



MOLD FERMENTATION 



Citric Acid Fermentation. Various molds produce citric acid from 

 sugars but strains of Aspergillus niger are apparently more active 

 than other species, and have been used more extensively for both 

 experimental and commercial purposes. Wehmer,^^ who was study- 

 ing the fermentations of molds, first noted that some produce citric 

 acid. He named these molds Citromyces, but they were later classi- 

 fied with the monoverticillate Penicillia. 



Currie and Thom ^^ found that with some strains there was a lag 

 between the curves for total acidity and oxalic acid, an acid then 

 known to be produced by some molds, during the fermentation. This 

 led to a search for another acid, which was subsequently identified 

 by Currie ^* as citric acid. This was considered an intermediary 

 product in the fermentation. According to this worker, the oxida- 

 tion of sugar by A. niger proceeded as follows. 



Carbohydrate -^ Citric acid — » Oxalic acid -^ CO2 



Although this view is no longer held, Currie made other contributions 

 to our knowledge of the citric acid fermentation. He showed that 

 the proportion of oxalic and citric acids could be controlled by pH 

 and the addition of inorganic salts. Low pH was found to favor the 

 production of citric acid and suppress the formation of oxalic acid. 

 Furthermore, it minimized the danger of contamination. 



A large number of fungi have since been found capable of produc- 

 ing citric acid. Strains of Aspergillus, Penicillium, Mucor have been 

 found to produce this acid, but strains of the A. niger group have 

 proved the most satisfactory in the production of citric acid. The 

 most desirable strains are those which efficiently convert sugar to the 

 acid, are easily cultivated, retain their biochemical characteristics, 

 and produce the least amount of other metabolic products. 



Doelger and Prescott,^*^ among others, corroborated the findings of 

 Currie and carried out further extensive studies on the techniques 

 of this fermentation. They found that the successive transfer of 

 spores in the same medium stimulates the mold to give high yields 

 of citric acid. They also observed that it was best to seed only one 

 fourth to one half of the surface area of the medium. Where high 

 yields were obtained, the molds produced very little if any spores. 

 Thus sporulation, or lack of it, could be used as an index of the 

 efficiency of a fermentation, according to these men. 



