158 



PHYSIOLOGY OF BACTERIA 



A much clearer case is that of the citric acid fer- 

 mentation by Aspergillus niger. This Aspergillus forms 

 oxalic acid only when the sugar concentration is low, 

 and citric acid besides oxalic acid when the sugar con- 

 centration is high. Currie (1917) who was the first to 

 make this observation, gave, along with others, the data 

 compiled in the following table. The best medium for 

 a good citric acid formation is one high in sugar and low 

 in nitrogen. 



In butyric acid fermentation, a gradual change takes 

 place with time. Perdrix found that the formation of 

 acetic acid ceases after five days, while butyric acid 

 continues to be formed in large quantities. 



(d) SELECTION OF FOODS 



The selection of foods by microorganisms must be 

 discussed in two entirely separate chapters, one dealing 

 with food for energy purposes, and the other dealing 

 with food as construction material. 



