ALIPHATIC ORGANIC ACIDS 141 



Oxygen is required for citrate synthesis, both in surface culture 

 (169) and in the more recently developed submerged processes (308, 

 172). Too vigorous aeration may, it has been suggested (186), sweep 

 out the carbon dioxide rapidly enough to reduce citrate formation. 

 The probably essential role of carbon dioxide, mentioned above, is 

 supported by experiments showing that increases in the carbon dioxide 

 of the atmosphere increase the yield of citrate (259). 



Measurements of the oxidation-reduction potential during the cul- 

 ture cycle suggest that there is an optimum potential for citrate forma- 

 tion; above the optimum, Aspergillus niger forms gluconic acid rather 

 than citric, and below it ethanol is formed (332). 



Virtually all studies of citric acid formation indicate that a low cul- 

 ture pH is essential to good yields; generally in surface culture an 

 initial pH of about 2.0 is recommended for Aspergillus niger, 3.0 — 

 4.0 for Penicillium spp. (169, 213). In aerated cultures of A. niger 

 a somewhat higher initial pH — about 4.5 — appears to be optimal 

 (472). At pH values above the optimum gluconic and oxalic acids 

 tend to replace citric (213, 383). The mechanism of this pH effect is 

 unknown; it has a counterpart in the effect of pH on the balance of 

 metabolic products formed by the lactic acid and the coliform bacteria. 



Metabolic poisons affect citric acid formation and have been used 

 in attempts to elucidate the biochemistry of acid formation. The 

 results are ambiguous, as is to be expected from the multiplicity of 

 reactions which compete with or supply carbon to the citrate pool 

 and from the fact that most poisons have several sites of action. The 

 effect of slightly growth-inhibitory concentrations of the lower ali- 

 phatic alcohols on the yield of citric acid is very striking (383, 455) and 

 may prove, on further investigation, to be specific. 



Fumaric Acid and Related Acids. Four of the 4-carbon dicarboxylic 

 acids — oxalacetic, malic, fumaric, and succinic — may be considered 

 together inasmuch as they are readily interconvertible (see Figure 1). 

 Oxalacetate is unstable and does not accumulate appreciably. Prob- 

 ably all the other acids of this group can be detected by sufficiently 

 sensitive methods in the cells or culture fluid of fungi during aerobic 

 metabolism. For this reason the listing of fungi which produce 

 these acids is somewhat deceptive; it is more a history of the interest of 

 investigators than a survey of acid formation. 



Fumaric acid is formed in small amounts by many fungi (Table 

 1); certain of the Mucorales, especially Rhizopus spp., are the only 

 fungi in which it accounts for a major fraction of the glucose con- 

 sumed. Aspergillus fumaricus as originally isolated formed fumaric 

 acid in large amounts (549), but the strain on examination ten years 



