CITRIC ACID FERMENTATION 221 



Although many organic substances may be fermented to citric 

 acid, sucrose and fructose have generally given the best results. 

 Doelger and Prescott found that in batches which were allowed to 

 ferment for 9 to 12 days, mashes containing 14 per cent sucrose were 

 found to give the highest yields. They recommended the use of 

 sucrose or technical glucose for industrial fermentations. These 

 sugars were preferable to maltose or molasses. Fructose was also 

 found to give high yields but its use would not be commercially 

 feasible. Increasing the sugar content, or replacing part of the su- 

 crose with glucose or fructose, or partially hydrolyzing sucrose dur- 

 ing the sterilization process lowered the yields. However, molasses 

 is used in present-day commercial practice. 



Currie and Doelger and Prescott have shown that the molds gen- 

 erally produce more citric acid when inorganic salts containing potas- 

 sium, phosphorus, magnesium, sulphur, and nitrogen are added to 

 the fermentation liquor. Conflicting reports are found in the litera- 

 ture concerning the addition of iron and zinc. There is a distinct 

 possibility that the strains of molds used react differently to the 

 additions of these metals. Doelger and Prescott noted that the 

 source of water used made a difference in the yields obtained. This 

 observation may be linked with the mineral requirements of the 

 molds. Where the water is deficient in trace amounts of these ele- 

 ments the addition of metals may help, and where there is already 

 an abundance they may exert a toxic effect. 



These men also found that the pH range of 1.6 to 2.2 was the most 

 suitable for carrying out the fermentation with their organism. They 

 recommend the use of hydrochloric acid in adjusting the pH to this 

 range. Sulphuric, nitric, acetic, and formic acids were found to be 

 inferior to this mineral acid. Whereas Wehmer and others have 

 advocated the use of calcium carbonate to neutralize the acid formed 

 during the fermentation, Prescott and Dunn ^* advise against its use. 

 They maintain that its absence favors higher yields, shortens the 

 fermentation periods, and decreases the possibilities of contamina- 

 tion. 



Doelger and Prescott also studied the influence of the ratio of 

 the surface area to the volume of the fermentation mash. They 

 advocated the use of shallow pans of aluminum of high grade of 

 purity for growing the mold and carrying out the fermentation. In 

 such pans, there would be large surface areas of mycehum exposed 

 to relatively shallow layers of medium. Agitation of the medium 

 was found to be undesirable. 



