Ill) 



CARBON METABOLISM III 



160 

 Time, hours 



360 



Figure 2. Growth and chemical changes in the course of citric acid synthesis by 

 Aspergillus niger in shake culture. Redrawn from P. Shu and M. J. Johnson, 

 Industrial and Engineering Chemistry, Vol. 40, p. 1202-1205, July 1948. Copyright 

 1948 by the American Chemical Society and reprinted by permission of the copy- 

 right owner. 



principle that growth and acid formation are to a degree competitive 

 should not be obscured by these secondary effects, important as they 

 may be for other reasons. 



The time course of citric acid formation in submerged aerated cul- 

 ture is illustrated in Figure 2, from which it is evident that there is a 

 phase of growth with little acid production, followed by a phase of 

 acid formation and slow growth. Maximum yields obviously require 

 that a balance be struck between growth and acid formation; at some 

 point there is enough cell material to convert the residual glucose 

 with maximum efficiency into product. These considerations apply 

 in principle to all acid syntheses, and in fact to all syntheses of non- 

 protoplasmic materials. 



Reference to Figure 1 makes it clear that many different carbon 

 compounds, entering the metabolic pathway at different points, may 

 be expected to provide carbon for citrate synthesis, and this proves 

 to be true (354). There are so many possibilities here that experi- 

 ments in which a carbon compound is added to a preformed mycelium 

 in an attempt to trace biochemical pathways generally fail — almost 

 any compound can be fed into the citric acid pool. 



