278 PHYSIOLOGY OF THE FUNGI 



same species. Acid production by fungi is discussed in detail by Foster 

 (1949), Prescott and Dunn (1949), and Wallvcr (1949). 



The meaning of the term fermentation has been expanded by most 

 authors to include aerobic as well as anaerobic processes. The produc- 

 tion of most organic acids and antibiotics Ijy fungi takes place in the 

 presence of oxygen, and these processes are not fermentations in the 

 restricted (anaerobic) sense of the term. Indeed, adequate aeration is 

 one of the salient features of such processes. Aeration may be achieved 

 by cultivating the fungi on the surface of shallow layers of medium in 

 pans or trays ; or the fungi may be cultivated in closed tanks, which may 

 contain as much as 15,000 gal. of medium. Aeration is provided by 

 mechanical stirring and blowing in sterile air under pressure. 



The organic acids discussed in this chapter are derived from carbo- 

 hydrates present in the medium. In general, media highly unbalanced 

 with respect to carbohydrates are used. The balanced medium devel- 

 oped by Steinberg for the cultivation of Aspergillus niger (Chap. 2) has 

 a carbon-to-nitrogen ratio of 29 to 1, while the medium recommended by 

 Currie (1917) for the production of citric acid by ^. niger has a carbon-to- 

 nitrogen ratio of 72 to 1. A fungus first utilizes the nutrients in the 

 unbalanced medium for the production of mycelium (growth phase). 

 The excess carbohydrate which remains when the nitrogen is exhausted 

 is dissimilated ("fermentation" phase). Advantage is taken of such 

 preformed mycelium, for if the original medium is replaced by fresh 

 medium, the mycelium continues to dissimilate carbohydrate. The 

 replacement medium is frequently more unbalanced than the growth 

 medium. For example, Karow and Waksman (1947) used for A. wentii 

 a growth medium with a carbon-to-nitrogen ratio of 135 to 1, while the 

 replacement medium had a carbon-to-nitrogen ratio of 270 to 1. 



Economic amounts of organic acids may accumulate in the medium 

 because the normal use of these compounds for the synthesis of mycelium 

 is prevented by the imposed experimental conditions. If the nitrogen 

 supply is exhausted, no more protoplasm can be formed. The mycelium 

 then dissimilates sugars enzymatically. Enough nutrients are supplied 

 in replacement media to repair and maintain the enzyme systems of the 

 fungus in a vigorously functioning state. The enzymes, other than those 

 concerned with certain phases of carbohydrate dissimilation, are largely 

 idle because of the lack of suitable substrates. 



A fungus commonly produces several organic acids at the same time. 

 Citric and oxalic acids are produced by many isolates of A . niger, and the 

 relative amounts of these acids may be varied by controlling the pH of 

 the medium. In general, a highly acid medium (pH 2.0 to 3.0) favors the 

 synthesis of citric acid, while less acid media favor the production o/ 

 oxalic acid. 



