ALIPHATIC ORGANIC ACIDS 



139 



particular reactions shown in Figure 1 may not all occur in fungi, but 

 there is no doubt that fatty acid and amino acid skeletons are built 

 up from intermediates of carbohydrate oxidation. It is significant that 

 strains of Aspergillus Jiiger which do not accumulate citrate have a 

 high growth rate (185), and that highly acidogenic mutants are rela- 

 tively slow growing (218). 



5. Carbon may also be drawn off into gluconic or oxalic acid in 

 Aspergillus niger and into still other acids in other fungi. 



These biochemical relationships explain many of the known physio- 

 logical requirements for citric acid accumulation. In the first place, 

 the concentration of available carbohydrate must be high (354); usu- 

 ally glucose at 15 to 20 per cent is employed. Under these conditions 

 there is an excess of carbon over that required for energy and for the 

 synthesis of cellular material; this excess will then, we think, accumu- 

 late wherever genetic and environmental factors permit rapid forma- 

 tion but only slow breakdown of a metabolite. Such a metabolite 

 may be directly on a major pathway or on a "shunt," diversionary, 

 pathway (200). 



The requirement for a high carbohydrate concentration has its 

 counterpart in the usual finding that highest yields of citrate are ob- 

 tained if other major nutrients — nitrogen, phosphorus, magnesium, 

 sulfur— are at levels which limit growth (103, 155, 308, 443). Data 

 which contradict this generalization are, however, often reported, 

 and there are undoubtedly secondary physiological effects of nutrients 

 (472, 473). 



Minor elements affect citric acid production, but contradictory re- 

 sults have been reported. There is no doubt, on the one hand, that 

 zinc, iron, manganese, and copper are essential to growth and acid 

 formation (518) or, on the other, that high concentrations of metals 

 may lower yields (420). Between these two extremes, it appears from 

 recent work that the best yield of citric acid and other acids is obtained 

 at concentrations of the minor elements which allow some growth 

 but are at the same time limiting to growth (130). Increases of zinc, 

 for example, may result in a greater total yield but the percentage of 

 conversion of glucose to acid is lowered (129). Perhaps the clearest 

 example is the inhibitory effect of iron on citrate accumulation; the 

 inhibition disappears in a medium in which growth is limited by low 

 phosphate (472). 



The optimum level of any given nutrient, affected as it is by the 

 strain used, the conditions of culture, and the other nutrients present, 

 is a matter for individual determination. The important general 



