CARBON SOURCES 139 



products. Carbon balances are especiall}^ useful in determining the effi- 

 ciency with which a fungus produces metabolic products of value, such as 

 alcohol and citric acid. For further examples see Raistrick ct al. (1931). 



Utilization ratios. The relations of the amounts of fungus metabolic 

 products to the amount of carbon soiu'ce (or other substance) used are 

 frequently expressed as ratios. However, these ratios are valid only for 

 the fungi and the experimental conditions used. These ratios should be 

 considered as absolute values only for the conditions under which they 

 were obtained. The various utilization ratios are of less value than 

 complete carbon balances, but the analytical determinations are fewer. 

 To be of most value, these ratios should be determined at various intervals 

 during incubation, because these ratios change with age. 



The most useful of these ratios is the economic coefficient, which is 

 obtained by dividing the weight of mycelium and spores by the weight of 

 sugar or other carbon source used. The residual carbon source in the 

 medium must be determined at the end of an experiment. In general an 

 efficient fungus will convert half the weight of sugar supplied in the 

 medium into cellular material. The efficiency of most fungi when grown 

 on laboratory media is much less. This is due in part to the use of 

 unbalanced media and to the type of carbon metabolism taking place. 

 The carbon which is not utilized for the synthesis of cellular material 

 appears either as carbon dioxide or as intermediate metabolic products, 

 such as alcohol and organic acids. In industrial applications it is desir- 

 able to employ cultural conditions which divert a large part of the carbon 

 used into the desired intermediate products, rather than into the produc- 

 tion of mycelium and carbon dioxide. 



The economic coefficient of Fusarium sambucinum under various 

 cultural conditions has been studied by Holzapfel (1925). This fungus 

 utilized sucrose (0.33) and fructose (0.36) more efficiently than glucose 

 (0.24). The economic coefficient varied with the concentration of the 

 carbon source and w^ith the source of nitrogen, as w'ell as with the age of 

 the cultures. 



For a discussion of other utilization ratios and examples, see Steinberg 

 (1942), Peterson et al. (1922), White and Willaman (1928), and Fries 

 (1938). 



Intermediary metabolism. The problem, to be considered here is the 

 way fungi utilize the various sources of carbon available to them. From 

 the data and discussion in the earlier part of this chapter it is clear that 

 structure and configuration play an important role in determining which 

 compounds may serve as a source of carbon for a given fungus. The 

 availability of complex natural compounds, such as the carbohydrates, 

 was found to depend upon the production of the necessary extracellular 

 hydrolytic enzymes. The utilization of simple compounds, such as the 



